Terminal Pleistocene: the peopling of the Americas
Summary:Ecological consequences of Late Quaternary
extinctions of megafauna (C.N.
Johnson 2009)
by:
Sarah Glover
This
paper provides a review of the interactions between large herbivores and the
flora in habitats across the globe before and after the extinction of the large
herbivores, as well as providing evidence for the consequences the extinctions
had on the surrounding ecosystems. The majority of sites analyzed in this paper
focus on the comparison between ecosystems of the late Pleistocene and the
early Holocene. These time periods are significant because of the radiation of Homo sapiens across the globe that occurred
which corresponds to the time of extinction for many large herbivores. The
author makes sure to emphasize that he believes the extinction of the large
forms of herbivores was in fact due to the spread of Homo sapiens into their habitats. Figure 1 demonstrates that large
forms of herbivores went extinct at a higher rate than smaller sized ones
between the Pleistocene and Holocene.
The
effects herbivores have on vegetation as explained in this paper can be
summarized as:
1. Direct
effects on Physiology form & growth through grazing
2. Effects
on plant communities:
a. Control
of the density of vegetation
b. Manipulation
of plant species coexistence
c. Dispersal
of seeds
d. Suppression
of sensitive species
e. Reduction
of fire potential by eating dry plant
matter
f. Acceleration
of nutrient cycling due to digestion of plant material
3. Drives
evolution of plant defenses: thorns, poison, etc..
In
addition to these general effects all herbivores can have on vegetation in their
ecosystem, the impact increases with body size of the herbivores due to the
greater capacity to use fibrous material, higher total biomass over large
areas, and invulnerability to predators due to large size. Both the elephant
and the rhino are used as modern examples to demonstrate how vegetation density
and reduction of fire potential are achieved through the grazing of vegetation.
The
results of the mass extinctions of large herbivores as presented in the paper can
be summarized as:
1. . Loss
of open vegetation and habitat mosaics: Examples of these impacts are
demonstrated by certain studies throughout the world.
a. Europe:
Herbivores were thought to maintain mosaic grasslands through the grazing and thinning
of forests which allowed for grasslands to form and be inhabited by thorny
shrubs. Pollen records support that during the early Holocene only tall forests
persisted, but that in the previous time period there were open plains. Giant
deer in Ireland 16-13 ka changed landscape to grassland. After extinction
habitat changed back to forest.
b. Australia:
Before 50 ka Emu’s and Wombats ate C4 and C3 plants, but by 45 ka diet only consisted
of C3 because habitat had changed from mosaic to uniform shrubs corresponding
to the extinction of large herbivores.
c. North
America and Eurasia: The mammoth steppe was characterized by dry grasslands and
low vegetation and the trasnsition to moiser and more vegetation is consistent with
the mammoth extinction 13.6 ka. In addition, in North America black mats (dark
sediment) cover mammoth fossils. There is some debate over the source of the
black mats.
d. New
Zealand: Herbivores were thought to help conifers grow by weeding out extra
vegetation and after their extinction there has been a progressive long-term
conifer declines.
2. Increases
Fires: The southwestern portion of Madagascar provide an example of this. This
area previously had high herbivore biomass and after 2ka ( a time of herbivore
extinction) there was a significant increase in charcoal indicating greater occurrences
of fires.
3. . Decline
of coevolved plants: After the extinction of many of the large herbivores there
was an abundance of plants that had evolved certain characteristics or defenses
towards these herbivores. With the herbivores no longer in existence, these
plant species invested in these traits but received no benefit. This allowed
other types of vegetation to evolve into the new ecosystem leaving the once
coevolved plants at a disadvantage.
Questions:
1. Do
you think that the observed ecological changes of this time period were caused
solely by the extinction of herbivores due to humans or is there another
explanation i.e. changes in climate?
2. Think
about the black mats that were described in this paper. Some believe that the
black mats indicate the impact of an extraterrestrial object which caused the
extinction of the large herbivores. The author believes it was the extinction
of the large herbivores the caused the buildup of vegetation and eventually
concentrated sediment. What do you believe was the cause of these dark
sediments?
3. What
do you think caused the increase of fires in ecosystems after the extinction of
large herbivores? Was it a result of a lack of vegetation clearing once
performed by the herbivores or was it due to the usage of fire by humans?
Summary:
Of mice, mastodons and men: human-mediated extinctions on four continents (Lyons
et al. 2004)
by:
Andirhaman Thomas
There are many hypotheses for why several
megafauna went extinct 12-15 ka, ranging from environmental change to predation
by humans. Much of the research has focused on a specific taxon or continent.
This study has compiled quantitative data for Africa, Australia, North and
South America comparing body size of mammals, before and after human
colonization. The data for Australia reflects an extinction event that occurred
about 46 ka, which coincided with human immigration. Data for the other
continents represent extinctions occurring 12-15 ka. Comparisons were also made
between patterns of extinction during the terminal Pleistocene and contemporary
extinction patterns. Lyons et al. evaluated their data using the
Kolmogorov-Smirnov test and Mann-Whitney U-test, both of which compare data
sets to determine whether they differ significantly.In their comparison of extant species and extinct species (Fig. 1), Lyons et al. conclude that the terminal Pleistocene extinction (46 ka for Australia) was size biased. Larger animals were the victims of the extinction, and smaller ones survived. Additionally, they looked at carnivores and xenarthrans (anteaters, sloths, armadillos) in North and South America. The same pattern is observed. The larger members went extinct and the smaller survived. In their comparison of the late Pleistocene (46 ka) extinction with more contemporary extinctions (past 300 years) in Australia (Fig. 3), Lyons et al. show that victims of the event following the initial colonization of humans were significantly larger animals than those recently extinct. For North America (Fig. 4), species that are threatened from human hunting tend to be relatively large animals, whereas habitat loss affects animals throughout the entire spectrum of size distribution. This pattern is even more pronounced for South America and Africa (Fig. 5 and 6).
The researchers lead us to the conclusion that the extinction of the megafauna in the terminal Pleistocene was caused by human hunting. The extinctions took place on all four continents at a time when humans were colonizing. In Australia, the extinction occurred when glaciation would not have been a factor. Additionally, no extinction is observed in marine ecosystems, bringing glaciation as a causal agent into question. Although Lyons et al. do not discount the effects of climate change, for this extinction event, they do claim that the primary cause was human hunting.
Questions:
1. One of the hypotheses, for explaining the terminal Pleistocene extinction, contends that humans brought disease with them, and disease caused the extinctions. How do the researchers address this hypothesis? Do you agree or disagree? Why? How could we test this hypothesis?
2. To what degree do you think innate characteristics of the megafauna (reproductive rates, feeding requirements, etc.) contributed to their extinction?
What do the researchers say about it? Would innate characteristics contribute more or less to marsupial extinctions? Explain.
3. Africa appears to conform less to the researchers’ proposal. Humans were present in Africa for a longer period of time, and the largest of the land mammals live there today. How do the researchers explain this? Do you agree? Why or why not?
4. Why would humans hunt large animals to extinction? What rationale do the researchers give to explain this? Is it sufficient? Why or why not?
5. Compare and contrast the terminal Pleistocene extinction with contemporary anthropogenic extinctions.
Summary:Megafauna
in the Earth System
(Smith et al. 2016)
by:Miranda Buck
This
paper explains the issues of how megafauna have declined overtime and especially
in the Pleistocene Epoch. What causes the large bodied mammals to become
extinct is still not fully understood knowing it can include issues of climate,
habitat fragmentation, humans, and other factors like trophic downgrading; some
of which are explained in this paper. Figuring out how these large bodied
mammals have gone extinct and understanding their role in the ecosystem was
important in this paper because only so much is known that helps understand the
changing of the ecosystem and the mammal’s functions within it.
One idea that can be
confirmed from this paper and other papers in proving extinction of megafauna
is that of human migration around the globe. With humans migrating and
increasing in populations, the average body mass of mammals began to decrease.
This was a major point in the Late Pleistocene as it was seen that all mammals
over 600 kg were going extinct when humans entered the New World. With other
studies, it became known that humans had a major impact on these large bodied
mammals because with migration, they hunted and began changing the land which
therefore changed the habitats for many mammals.
Even though megafauna
decreased in the Late Pleistocene, some of the megafauna recovered to similar
levels before the extinctions, but there was a shift from wild animals becoming
domesticated because of the humans. This changed the impact that animals had on
the world because they couldn’t roam as wild animals as they did before they
were domesticated. Understanding how smaller body masses of animals have interacted
and changed the ecosystem is important after the Late Pleistocene extinction.
One interaction with the
ecosystem after the extinctions is a change in the interaction with vegetation
and habitat structure. Megafauna have changed the forest structure because their
large body masses have allowed them to knock down trees. With domesticated
animals that occurred after the extinction’s, their smaller body mass wouldn’t
allow for them to knock down trees and they had different impacts on
vegetation. Wildlife had an easier time dispersing seeds and other nutrients
than domesticated animals. This goes back to how wild animals were able to roam
everywhere and now domestic animals do not have that opportunity as much and in
turn stopping nutrients and vegetation from spreading as it had before the
megafauna went extinct.
Other impacts on the
ecosystem from megafauna extinctions include biogeochemical cycling and changes
in carbon and other elements and difference in community structure and species
interactions from top-down control and trophic downgrading of animals. These
factors that changed as megafauna decreased brought up rewilding: a
conservation effort to help restore trophic interactions as they were when
megafauna were abundant.
In one paper, megafauna
were found to be impacted mainly from humans and their colonization around the
world. 60% of variation in the extinction of megafauna was caused from humans.
Climate only showed about 25% of extinction variation. It was also found that
humans and large bodied mammals were at once in co-existence for thousands of
years making climate the main impact of extinction variation.
Humans and domestic
animals are dominant today rather than wild animals as they were before human
colonization and trophic cascades can still be applied to today’s ecosystem
structure. Rewilding is still thought about in trying to make the present large
bodied mammals and wildlife become more abundant, but more research must be
done to understand climate effects on habitats and other habitat limitations,
such as other animals.
Questions:
1. Based
off of knowing that megafaunal extinctions occurred due to humans,
biogeochemical cycling, habitat structure changes and trophic downgrading, what
do you think had the most impact on the ecosystem as megafauna went extinct?
2. What
are some ways that megafauna extinction could have had on atmospheric CO2?
3. Do
you think that trophic rewilding and introducing species to restore top-down
interactions to promote self-regulating biodiverse ecosystems is a better way
to conserve megafauna or is creating a system of experimental sites close to
urban centers that engage public a better way for conservation efforts?
Johnson 2009
ReplyDeleteI think that there may have been some other explanations for some of the extinctions that were seen but not enough to really be significant. I think that it was mainly due to humans because hunting large animals would have been beneficial to the humans to have more resources of food from just one animal. I also believe that the black mats described in the paper were unlikely to be caused by an extraterrestrial object because then the extinction would have affected a vaster population of body size instead of just knocking out the megafauna. I think that it was probably caused more by the aftermath of the extinction and less vegetation being eaten because there was a lot less large herbivore there to eat the vegetation which caused the buildup of vegetation leading to the concentrated sediment. I believe that the increase in fires in ecosystems was caused by both the increase in vegetation because there were less herbivores to eat it and because humans used fire. Without humans using fire, how would the fires have started? And with more vegetation to catch on fire, this would also cause more fires as well.
Lyons et al. 2004
The researchers talk about how man brought disease with them that would have likely spread to the animals and caused extinction. However, like they also discuss I think this would be highly unlikely to cause such a widespread extinction like the one that was seen. It is also strange to think that whatever disease that was spread only affected the large bodied animals of the time. I’m not sure how we could really test this without knowing what exact disease was being passed to the animals at the time. I also think that innate characteristics like reproductive rates and feeding requirements did not help the megafauna during their extinction. I think that because of the slow and small numbers of reproduction of larger animals would have allowed for the death rate to exceed the birth rate by a substantial amount. I also think that finding enough food to survive for a megafauna could be difficult especially when fighting with humans for the same resources. Humans would hunt large animals to extinction because they needed food and without having a way to track the numbers of animals left, they had no way of knowing that they were killing off these animals. They would have no way of knowing that they needed to stop or slow down to conserve these food resources and so that would lead them to killing all of them off to feed themselves.
Smith et al. 2016
I believe that humans had the largest impact on the ecosystem as megafauna went extinct. I think they had the largest impact just because of how many various places they got their resources to live from, they were hunter gatherers and would not only hunt the megafauna but could also use the vegetation that was there as well. Megafauna extinction could have led to a decrease in atmospheric CO2 because as the megafauna population decreases and eventually falls to zero, there would be an increase in vegetation because there were no more animals to eat it. With this increase in vegetation there would be more CO2 being taken in by the vegetation and more oxygen being given out to the atmosphere by the vegetation. This would lead to a complete change in the atmosphere gas levels. I believe that trophic rewilding and introducing species to restore top-down interactions is a better way to conserve megafauna because if we were to just use experimental sites close to urban centers then these animals would have no concept of living in a wild ecosystem. When reintroduced to a wild ecosystem later in life they would likely not survive because they would not be used to everything that comes with it such as other animals, the weather, or finding vegetation to eat instead of just being fed whenever it is needed.
Johnson 2009
ReplyDeleteI believe that the extinction of herbivores was not solely based on humans and their interactions. Just like with any other extinction, there are usually multiple factors that lead to these die offs of species. With the case of the herbivores, I do believe that climate change and continent reconstruction had major roles in their extinction. With a decrease in temperature or an increase in ice formation, species could see a loss in their plant/food supply leading to extinctions.
Lyons 2004
I think that innate characteristics of megafauna did have a big role in their extinction, but it was more based on the human impacts. If humans killed off most of the females in a population, they would shrink the rate at which that species could repopulate and produce more offspring. The same can be said for feeding requirements. If humans were destroying or changing environments as they moved along, megafauna would also have to adapt to the changes left to feed from.
Smith 2016
Humans by far had the most impact on ecosystems. It can be implied that humans were made for destruction and that can be seen anywhere a human has touched or set foot in. Within an ecosystem, humans were hunting down a lot of the main contributors to the ecosystems, the animals. They also burned down parts of ecosystems just to score a meal. They made lasting impacts that set hard adaptions for species to obtain just to survive.
This comment has been removed by the author.
DeleteJohnson Paper
ReplyDeleteI believe that both provided options (lack of herbivores and humans using fire) would lead to an increase of fires in some ecosystems, however I think the main perpetrator is the lack of herbivores to clear underbrush and whatnot. Large herbivores not only can consume huge amounts of plant material but also trample down the areas where they pass through so as the keep growth down in the future as well. When the world no longer has these grazers and herds to keep the plant growth in check the next wave of fires that pass through that area will be very bad and very easy to start due to no clearings and lots of dry dead plants that are easy to spark due to lightning or possibly a stray spark from a human started fire.
Lyons et al. Paper
Africa does not follow the general trend at the end of the Pleistocene in terms of megafauna extinctions, as it was hit the least hard by human hunting and is where today the largest terrestrial animals still live. This is likely due to the fact that Africa is where humans and in similar ways hominids originated from and so as our species grew and evolved so too did the animals that shared the same habitats with us. They grew and adapted to our ways as we made them which is why African animals suffered the least in our worldwide sweep, because they had been dodging us for as long as we have been around and got use to running from humans. The other continents and their animals however did not “grow up” with us and so when humans swept through those areas we tended to decimate their herds and caused many animals to go extinct as a result of our hunting.
Smith et al. Paper
I think that in terms of conservation efforts the system of experimental sites sites close to urban centers will be better, however at that point we are bringing back species we know almost nothing about so that we can look at them in zoos which is weird to me. I get why people want to bring megafauna back as they are so cool, and could possibly be an asset in restoring a good wildlife ecology to some areas. However I think if we immediately released them into the actual wild people would poach the hell out of them just because they could. So that leaves the urban centers option; however, like I said at the beginning, it seems we would be trying to re-brand a species so that we could reintroduce them to a wild and planet that they know nothing about and that the other species in the areas would know nothing about… I honestly don’t know what I would do if I had to make this choice, because I love megafauna and it’d be a dream come true to see them at all let alone in the wild thriving, but at the same time we don’t know what would happen to them other animals so I wouldn’t like knowing I was causing them pain or stress either.
Johnson
ReplyDeleteI think this extinction was based greatly on the appearance of humans in new habitats. I do believe other factors contributed to the extinction such as climate changes but human were the main factor that drove down animal populations. I think fires in vegetation were caused by both humans and lack of vegetation clearing by animals.
Lyons
I do not believe that a disease carried by human would have been able to effect mainly large megafuna. I think this theory would be hard to replicate because we do not have many viruses that can be transmitted between humans and animals nor are they size specific. I do believe that animals eating habitats did contribute to there extinction. If animals were obligated to eat specific things they are less likely to survive than opportunistic eater. I think since homo began in African megafuna was able to adapt to escape human predators therefore there extinctions were less.
Smith
I think that humans had the greatest impact on ecosystems. I think there would have been a decrease in atmospheric CO2 because as megafuna decreased the amount of vegetation wold have increased.
C.N. Johnson 2009
ReplyDelete1. I think the ecological changes could have been mostly due to the extinctions of herbivores, but I have a hard time believing it was JUST because of the extinction of herbivores. Of course, that might be because I'm a human and I'm always inclined to WANT to believe that humans aren't THAT bad at messing up ecological systems...anyway, it's probably a multitude of factors, but the extinctions of herbivores is probably a really big factor.
2. A bolide impact is probably not the case...it would be very odd for a bolide impact to have such size selectivity. Even if being larger is harder in nature, it just seems implausible to me that it specifically caused the extinctions of the megafauna. No, it's definitely a case of humans being humans and killing everything off, which it comes to the extinction part. As for the sediment...to be honest? I don't know. Possibly compression due to the build up of vegetation?
3. As with all things, there probably wasn't just one factor. There could've been a build up of vegetation when it wasn't being eaten, meaning there was plenty of vegetation left over to dry up at the end of the season. Because of this, a spark from a human fire, torch, cooking fire, etc could have easily started a fire. It happens all the time today, why not back then as well?
Lyons et al. 2004
1. The researchers do note that the extinction happened at the time that humans came to the land, but don't take into account that it was most likely due to hunting, not disease. There were signs of disease in some of the deceased, but it does not account for the wave pattern of the extinction or the size selectivity. So I disagree. As for testing...it's been noted that a lot of the bones of the extinct species had cuts and scratches on their bones to indicate injury from human tools, and that seems like a VERY big piece of evidence for being hunted.
2. Megafauna have to eat a lot, don't reproduce very fast, and usually don't worry about predation from other animals (very often, anyway). So these characteristics probably helped a lot with their extinctions. As for marsupials, I am unsure about the question.
3. Africa is definitely non conforming. I would guess that the largest land mammals are still there today because their ancestors were much more adapted to the land and to their predators in Africa, and generally just better off.
4. Large animals are slower and less used to predation, and to be glaringly assuming, they are large so they have more meat. Not to mention, they had no way to telling how many animals were in a population like we do now.
5. Both extinctions are human caused. That is the biggest similarity. Interestingly enough, the Pleistocene extinction focused mostly on large animals. Whereas in the present, no one is safe. Everything from fish to frogs to tigers are being decimated.
Smith et al. 2016
1. As megafauna went extinct, I presume there were a few predators who suffered from a lack of food resources, but not many. I also presume that a lot of land was overrun with vegetation with no megafauna to eat it. I think the greatest impact on the ecosystem when megafauna went extinct was most certainly humans.
2. I assume there would be less CO2 in the atmosphere with less megafauna to produce it. And as a result, there would be more O2 in the atmosphere. As for how many individuals needed to die in order for a major change to occur...that I do not know.
3. Well ideally, both. But if the need is financial, the sites close to urban centers- zoos? - would be a better way to get the public informed about these megafauna and as a result, raise funds to better fund the actual conservation efforts in the wild.
Johnson 2009
ReplyDelete1) I don’t think the ecological changes where caused solely by the extinction of herbivores from humans, but I do think that this played a major role in it as herbivores do have an effect on the environment and humans did impact herbivores. I think there might have been other things that also amplified this effect.
2) I would have to agree with the author since if these fauna did go extinct the build it would probably happen and overcrowding of plants and competition could have made an increased turnover rate.
3)I think the increase in fires was due to humans using it to hunt but also because with the herbivores gone fie preventing measures decreased, also with their extinction humans might have tried hunting smaller hard to find animals and to do this they used fire.
Lyons et al. 2004
1) The disease model is unlikely as it was seen that the extinction event was selective for large body animals, while a disease would have affected all sized mammals equally.
2) I think these had a huge impact on the extinction of megafauna as the death rate would be easily influenced causing an extinction to be inevitable as these individuals would not be able to recover their number quickly enough to keep up with human interactions.
Smith et al. 2016
1) I think the environmental changes most likely had the greatest effect on the ecosystem as the megafauna went extinct as this would be the most widespread effect.
2)One of the big differences would have been the change in plant diversity, if more forest are present there would be a shift in the carbon isotopes but also their disappearance would most likely correspond to a decrease in CO2 as the are no longer producing it and vegetatio possible increased.
3) I think engaging the public is a better method as if we re-introduce these species but the public does not know anything, there might be a repeat of what happened in the past, also the effects of the extinction would be more apparent.
Johnson: I believe that most of the observed ecological changes occurred because of the extinction of herbivores due to humans. At this point, there had been multiple Ice Ages, so the climate had changed before but the megafauna had not gone extinct like it did during this time. It’s sometimes fun to think about extraterrestrial objects impacting earth, but because of the lack of evidence, it’s hard to believe. I think the black mats were caused by the extinction of the large herbivores. After they went extinct, the vegetation was able to grow and flourish, which allowed for the buildup of organic matter. With more vegetation, this could have caused increases in fires. Because humans were killing the large herbivores, that left more foliage that could have caught on fire. Getting rid of the large herbivores would have greatly impacted the environment, because they were no longer cutting down the amount of vegetation. With more vegetation, fires created by humans would have had the potential to burn longer so both the extinction and increase in fires would have been enough to increase the amount of fires in the ecosystem.
ReplyDeleteLyons:
1) Humans bringing over diseases is most likely not the cause of this extinction. The authors state that most epidemic diseases affect a single order, have low transmission rates and low mortality rates. It is also unlikely that a disease would target only large animals. Testing this would be hard, but I think the best method would be to have mammals of varying sizes and expose them to a disease that is likely to infect all of them, although there is probably some ethics problems doing this.
2) I think the innate characteristics contributed to the extinction rates but only because of humans. Larger animals tend to have higher feeding requirements and low reproductive rates. Larger animals tend to give birth one at a time, unlike smaller animals that can give birth to multiple. Because of this, humans killed them faster than they were able to reproduce, causing them to go extinct.
3) Humans co-evolved with the African megafauna, making it possible for those animals to evolve anti-predator behaviors. I agree with this because we see co-evolution in other species as well. There were some extinctions in Africa, but just not as many as other continents.
4) Humans would hunt large animals to extinction because you can get more food in a shorter amount of time. It makes more sense to kill a mammoth than to kill 8 small rodents. As humans moved into other continents, they would be able to use those skills to kill more animals and gain more food.
Smith: I believe that humans had the most impact. If it weren’t for the humans killing the megafauna, the habitat and the ecosystem would not have changed as much. As humans killed the large herbivores, this would cause an increase in plants which would then cause a decrease in carbon dioxide in the air. With a decrease of carbon, this could have potentially caused the earth to cool. So humans killing has a domino effect. As we kill the megafauna, this allows plants to grow, which brings down carbon levels etc. The same can be seen today. As we keep pumping carbon into the air and cutting down trees, this causes the earth to warm, which is causing the bees to die which is potentially allowing a cascade of other things to happen.
Johnson:
ReplyDeleteLike the previous comments I don't believe that humans were the sole cause of extinction, but perhaps were a primary cause which severely impacted the population sizes, and climate change aided in the extinction rates of megafauna. Megafauna typically have more reproductive investment ( think elephants) therefore the ability to repopulate or adapt would take a much longer time than those of smaller body size ( rabbits and mice have lower reproductive investment and faster turn around). Therefore it was probably a combination of events that led to megafauna demise. Also, over browsing or under browsing in herbaceous areas can cause an area to become more susceptible to wildfires, so I do not think that humans may have had such a large impact on the increase in fires, but the dry or large litter layer could have caught fire due to things like lightning.
Lyons:
Like I mentioned previously, megafauna have lower reproductive rates because they have longer lives, reach sexual maturity later, have higher rates of parental care, and larger gestation periods (typically) which is why we are seeing a similar trend in today's mass extinction. We are not currently worried about mice or rabbits because they have quick reproduction rates and can fairly quickly recover whereas large land mammals , like elephants, don't reach sexual maturity for 10-14 years, and then only have one offspring every 5 years. The over hunting of these animals caused their decline, yes, however their innate characteristics made it impossible for them to recover from the over hunting.
Smith:
Megafauna would have contributed to the CO2 content of the air, so with their decline and increase in uncontrolled plant-life the CO2 concentrations would have declined, possibly causing a slight cooling due to the decrease of greenhouse gas.
I believe that the change seen in floral fauna can be directly attributed to the loss of megafauna but I also believe like with all things having to do with biology it was a combination of factors including climate. I come to the same conclusion for forest fires but we know in places like Australia humans had a greater impact on forest fires then in other places.
ReplyDeleteI think that the disease hypothesis is completely false and not based in reality because there is no modern disease that can cause a size selective extinction. I think it makes sense that traits of these megafauna would have led to their extinction because low fecundity and long gestation times would have only needed a small tip in the wrong direction to cause deaths to be greater than births and lead to an extinction. Humans would have hunted large megafauna because they were huge food sources and could be hunted in large groups and feed entire communities. The mega carnivores would have preyed on the same animals that humans did so that explains why they went extinct too.
Lyons:
ReplyDeleteClimate will always play a role, but I think the author of the paper did a compelling job of mitigating climate as a factor for a lot of these changes. As for the black mats, I would need to do more research on the impact idea, but again, Johnson proffered and supported a much more pragmatic explanation when he pointed out that the megafauna die-off happened right around the same time period and likely before, based on the black mats being found covering the fossils. This last question is a more intriguing one because I don't feel like there is enough information to come to a conclusive or even compelling answer.
Johnson:
The researchers address the disease hypothesis by looking for a disease vector that a) could transmit across a variety of species (a superdisease, of sorts) and b) that would be size-selective. I have a hard time coming up with a way to test the hypothesis, but based on my rudimentary understanding of zoonotic diseases and vectors, it doesn't seem a likely answer.
As for the African question, the authors' explanation was the co-evolution of hominims along with megafauna as well as the vast geographic size of the continent, however they do point out that Africa still faced some megafuanal extinctions that is ongoing today.
Smith:
At the risk of sounding self-evident, I would suggest the loss the megafauna itself had the biggest impact, based on this series of papers. The loss of grazers and their fecal byproducts reduced habitat diversity significantly. By changing the plant makeup from the loss of megaherbivores, it would prsumably alter CO2 production ad even sequestration, if soil and sediment loads were further altered by the change in local flora.
Public engagement would be a boon in either method, I would be wary of experimental sites close to urban centers because of increased human conflict with wildlife. It strikes me as dubious that the urban center idea could lead to a self-sustainable situation that wouldn't need frequent monitoring and intervention, keeping animals from ranging out of the designated area would be difficlt (particularly predatory animals that tend to have large ranges), and it could add further stress to species that are struggling.
A couple questions I had after reading these papers:
Would this mean hominims would be considered an invasive species globally? Once they (we) left Africa, there seems to conformation to all the hallmarks: extinctions of endemic species, significant ecosystem upheavel or engineering, etc. Or do we conveniently leave ourselves out of that analysis?
If the loss of megafauna led to a lack of ecosystem diversity (both in terms of plant life and mammals and other animals that would depend on the altered ecosystem), why wasn't there a significant loss of smaller species from the loss of diversity and niches?
Johnson (Glover)
ReplyDelete1. I don't think you could solely blame humans for the extinction of herbivores, but I definitely think that they were the main candidate. There is evidence that human activity caused a massive decline of species diversity, but it's difficult to disregard external environmental factors.
2. The black mats sound like they are due to the buildup of vegetation, or concentrated sediment. If it was an extraterrestrial force, the effects would be more than regional.
3. The fires could possibly be caused by the extraterrestrial force, but is most likely due to the lack of vegetative clearing from terrestrial species. Buildup of vegetation is more prone to large wild fires that could be due to environmental factors (lightning) or human activity.
Lyons (Thomas)
1. The disease model is less likely because the extinctions were size selective.
2. Innate characteristics probably have a huge part with extinctions because it determines their ability to survive various trauma.
3. African animals were used to human activity and knew to avoid them. This behavior is still visible in terrestrial animals and their encounters with humans.
4. Size-selective extinctions due to humans could be with hunting and prey competition.
Smith (Buck)
1. I believe there is solid evidence that human activity was the biggest source of trauma for terrestrial species.
2. Extinction of megafauna could have potentially lead to decreased atmospheric CO2. This could be due to the increased vegetation as well.
3. I believe that trophic rewilding and introducing species to restore top-down interactions is a better way to conserve megafauna because it would re-establish the ecosystem and restore environmental balance.
C.N. Johnson 2009
ReplyDeleteI think the observed ecological changes were mostly due to humans hunting many of the herbivores. As humans continue to hunt these herbivores, their tools that used become more advanced giving them an opportunity to successfully hunt the bigger game. It is difficult to believe that the black mats were caused by an extra terrestrial object due to the fact that if there was an extra terrestrial object that hit the earth, why would it only affect the large herbivores? The fact that it exclusively affected only large herbivores and with no evidence of an impact in the ground, this would lead me to agree with the author that the black mats were caused by buildup of vegetation in the sediments. I think an increase in fires would be caused by humans and lack of vegetation clearing. As the larger herbivores are being hunted they can no longer maintain vegetation’s. For instance, in dry conditions vegetation can easily produce fire when struck by lightening.
Lyons et al. 2004
Researchers in this paper suggest that it is feasible that man could have brought disease with them, which would in turn indirectly cause extinctions. The author quickly dismisses this idea, stating that if this were to occur it would most likely affect only a single order, and if it does attack more than one order it will have low transmission and mortality rates. Based on this information, I would agree with the authors in saying that it is not really plausible for disease to affect only large bodied mammals and cause major extinctions of more than one order. One major characteristic that affects mega fauna extinctions is their low recovery after being hunted. If it takes these mega fauna long periods of time to reach sexual maturity and they are being killed off quicker than they can reproduce, this is very bad and extinctions will occur quicker than normal. Mega fauna that co-evolved with humans in Africa had the most time to adapt to humans hunting them and therefore result in evolving traits to help increase their survival. Continents that were later habited by humans after Africa gave mega fauna less time to adapt to humans and therefore depending on when humans reached their habitats, determines how quickly they were hunted and drove the extinction rates up. Humans would hunt this mega fauna to obtain the most protein for their diet and could essentially feed many more humans. By hunting larger mega fauna, they ultimately in my opinion drove them to extinction. At the same time they may not have realized that they may be indeed causing extinction.
Smith et al. 2016
Humans definitely had the biggest impact on the ecosystem in regards to mega fauna extinction. In time humans were also able to upgrade their weapons they used to hunt this large bodied mega fauna allowing them to drive extinction rates very high. By losing mega fauna, this had a big impact on the ecosystem because they were not able to recover quickly enough to maintain vegetation. With that being said, more vegetation being around would potentially lead to more atmospheric CO2 being produced in the environment. I think trophic rewilding and introducing species to restore top-down interactions to promote self-regulating biodiverse ecosystems would be the better route to take to restore species. Of course this process would not happen instantaneously and these species would have to learn to adapt in their habitat. The gradual process over time could end up helping to preserve the species.
C.N. Johnson 2009
ReplyDeleteI think humans; changes in climate, and diseases all played a role in the extinction of herbivores. Horses, dogs, rates, and birds that moved alongside humans may have acted as a host for the disease that was spread. Climate change may have also had an affect on herbivores as well by losing ice bridges that connected the continents isolating them to one location. But ultimately it seems predators of herbivores, and especially humans created an increase in the decline of herbivores.
Lyons et al.2004
The researchers in this article suggest that disease extinction may not have occurred, because diseases are not size selective, and would have also affect humans, which I could agree with because diseases can rarely be transferred between animals, and humans, but there are some disease that are transferred by domesticated animals, that target certain disease such as black leg fever, and West Nile that target certain populations of animals if they share the same grazing range, or transferred by another host, e.g. birds, and insects.
I do not believe humans during this time realize they were hunting large animals to extinction, they may have been hunting these animals because they where not as skittish as smaller animals that were hunted by other carnivores, also most large animals are territorial, so there fight or flight reaction would be to defend themselves against predators.
Smith et al. 2016
I think megafauna extinction may have had a higher impact in habitat structure, because plants were not being consumed, or disturbed enough to be ‘managed’. Over growth would have occurred as well as a lost of nutrition needed from the droppings of megafauna. I believe both trophic rewilding, and experimental sites close to urban centers to help introduce megafauna back into the environment. Also interactions with farmers and ranchers would also help in rewilding megafauna, because they would be impacted the most due to their geographical range they may have to share with the megafauna.
I think, as has been mentioned before, that there is a combination of reasons that led to the extinction of megafauna and the subsequent ecological and environmental changes that occurred on the continents. I believe the hunting of megafauna by Homo sapiens contributed greatly to the loss of megafauna in the environment. Megafauna tend to take a long time to sexually mature and they have long gestation periods, so it only requires a slight increase in their death rates (a small increase due to hunting) to devastate their populations. In this sense, I believe that these innate characteristics gave megafauna a distinct advantage against maintaining their population in the face of human hunting. However, I also think any sort of environmental changes could add stress to a species’ population and help contribute to their demise. While I believe human hunting was the most significant reason for their extinction, I also think that climate changes would add additional stress to the megafauna.
ReplyDeleteAs for the black mats, the paper states that many people believe that they “indicate a brief interval of raised water tables”. However, the paper also mentions that it is odd that they are not a feature of “other similar climate events such as the Oldest Dryas.” I also don’t believe that they could be caused by an extraterrestrial object because there would likely be solid evidence. We would see evidence of regional fires and probably at least a slight change in atmospheric CO2, S, and N levels.
I tend to agree with the idea discussed in class that humans were opportunistic hunters, and did not intentionally seek out megafaunal game. I can imagine that it would take much more effort and coordination from a group of hunters to take down a mammoth than it would to take down a rabbit. Yes, the mammoth can feed more people for a longer period of time, but rabbits and other smaller game would have been more numerous and easier to come across. In addition, due to the innate characteristics of megafauna, it wouldn’t take more than a slight increase in death rates to eventually drive them to extinction.
Johnson 2009
ReplyDeleteAs with most extinctions, I believe the extinction of megafauna was caused by several mechanisms coming together in an unfortunate combination for the affected species. Certainly, human activity played a large role in the extinctions, and I believe it was likely the tipping point for several species, but environmental changes and increases in fire occurrence could also have had impacts on these populations. Regarding the black mats, I agree with Johnson, who does not support the proposed mechanism of an extra-terrestrial disturbance and instead believes the formation of the mats may be due to reduced nutrient recycling during a period in which many megafaunas were going extinct. It seems logical that the loss of so many large animals would have a corresponding impact on the nutrient uptake. Increases in fire occurrence would have been more related to the arrival of humans, but also may have been enhanced due to the increased vegetation present due to the lack of megafauna. Both contributed to the increase.
Lyons et al. 2004
Disease hypothesis supporters claim their hypothesis has credibility because of their disbelief that early man could hunt species to extinction, as well as the fact that there was a lack of extinctions into the Holocene. However, the hypothesis has several clear holes, such as the lack of existence of such as virulent and transmissible disease as well as its size selectiveness regarding host mortality. Considering our current knowledge of disease and infectious organisms, there is no likely candidate for such a disease. I do agree that megafauna’s life history characteristics made them particularly prone to extinction. Long reproductive waits and low population sizes are key characteristics of vulnerability in populations, and human impacts would have tipped vulnerable populations over the edge to extinction. Humans could have hunted megafauna to extinction with little consequences. As an adaptable and intelligent species, humans were capable to switching food sources when one became extinct, and was never truly dependent on one food source.
Smith et al. 2016
The loss of megafauna, and particularly mastodons, resulted in a lack of environmental engineering that has not been filled in modern times. The species interactions that megafauna also exert would have had significant influences once absent, and I believe would have resulted in the most substantial changes on ecosystems overall. The entire loss of an ecological niche which has yet to be replaced caused changes in the trophic and overall species interactions of the period which are still being influenced by modern human activities. Megafauna would have impacted CO2 emissions through their normal respiration, as well as through changes in vegetation. The woody encroachment which would have occurred following megafauna extinctions would have caused an overall carbon drawdown.
Johnson 2009
ReplyDelete1. I think that the observed ecological changes of this time period were not caused solely by the extinction of herbivores due to humans although I think this explanation explains why the majority of large herbivores went extinct. I do think that there were other factors in which contributed to the extinction of these large herbivores such as climate change, meteor impacts, anoxia, etc.
2. Thinking about the black mats described in this paper, I think that if these black mats indicated the impact of an extraterrestrial object, then there would be evidence of the impact (crater). I believe that the cause of these dark sediments was in relation to the buildup of vegetation (which occurred from extinction of large herbivores/not grazing) in which eventually lead to the concentrated sediment.
3. I think that humans were the ones that caused the increase of fires in ecosystems after the extinction of large herbivores because due to the lack of vegetation clearing, the vegetation grew more and was more prone to catching fire/drier. Humans used this to their advantage and were able to burn more vegetation/draw out smaller organisms.
Lyons et al. 2004
1. The researchers addressed the hypothesis that humans brought disease with them/caused the extinction that occurred at the end of the Pleistocene. I don't think that there is enough evidence to support this hypothesis especially since there is no known disease that could cause such an extinction to occur. We could test this hypothesis by analyzing other diseases and seeing how they affect smaller mammals as well as large mammals today.
2. I think the innate characteristics of the megafauna (reproductive rates, feeding requirements, etc.) did contribute to their extinction because these characteristics changed in response to the changes in their environment. I think innate characteristics would contribute more to marsupials because they are distinct/specialized in some characteristics (kangaroos w/pouch).
3. I think that the animals in Africa were exposed to humans for a long time and had the chance to co-evolve with them. I think that these land mammals were able to recognize humans as being a danger and developed strategies to avoid being hunted.
4. I think humans would hunt large animals to extinction because they would offer the most food source for a group of people and would last longer than hunting small animals. We know that this mass extinction was characterized as being size selective and evidence supports this because large mammals were the ones going extinct while small animals were able to survive.
5. The terminal Pleistocene extinction can be compared with contemporary anthropogenic extinctions in the sense that humans are contributing the most to animals going extinct although by different means: in the past, it was from predation/hunting while today, it is due to deforestation/habitat loss, climate change (increase in CO2), etc.
Smith et al. 2016
1. I think that all of these factors (humans, biogeochemical cycling, habitat structure changes and trophic downgrading) contributed to the impact on the ecosystem as megafauna went extinct.
2. Some ways that megafauna extinction could have had on atmospheric CO2 is due to the regulation of vegetation. With less megafauna herbivores to eat/graze the vegetation, more continues to grow which uptakes CO2 from the atmosphere.
3. I think that using both of these techniques would be able to help conservation efforts. However, I think that creating a system of experimental sites close to urban centers would be most effective because it would engage the public to see how important conservation efforts are for future survival of species.
Overall, human beings were a drastic contributor to the extinctions discussed in these papers. In most cases, they are the cause behind smaller factors that led species to extinction such as fires, population decline, and habitat loss, but I would agree with Dr. Smith’s statements that we shouldn’t have to rely on a singular theory to explain these extinctions. Ecosystems are incredibly complex and would have been altered by changing climates, other species, diseases, and a vast array of other factors in both positive and negative ways. We know from examining extinction that some species are more sensitive to change depending on which changes are occurring, and so extinction itself is complex. Human beings overhunted and set fires, but they were likely unaware of the drastic implications of doing so until after the fact, and I think they followed game because it simply made sense. Climate change may have occurred as a partial cause of extinction, but it also would have likely occurred afterward. A decrease in megafauna may lead to a change in vegetation of an area, which may increase photosynthesis in that area and therefore decrease the amount of CO2 in the atmosphere. Any increase in vegetation, depending on the type, may affect other species in the same area and create a cascading set of effects for different trophic levels and ecosystem components.
ReplyDelete. Do you think that the observed ecological changes of this time period were caused solely by the extinction of herbivores due to humans or is there another explanation i.e. changes in climate?
ReplyDeleteIts probably a bit of a combination of all three like all mass extinctions- the result of multiple factors- however the extent of human effects is unquestionable. Humans could of changed conditions that herbivores interacted with like habitat, thus reducing the ecological effects of herbivores as they went extinct.
2. One of the hypotheses, for explaining the terminal Pleistocene extinction, contends that humans brought disease with them, and disease caused the extinctions. How do the researchers address this hypothesis? Do you agree or disagree? Why? How could we test this hypothesis?
If disease is a possible explaination- this is a really hard theory to test. First- the mechanism of this kill-disease is not something very common to only effect megafauna. Additionally, its very unlikely a disease of antiquity like this existed. I don’t agree with it as a possibility and if it wanted to be tested, it could be compared to modern day diseases or examined within bones or any existing DNA.
2. To what degree do you think innate characteristics of the megafauna (reproductive rates, feeding requirements, etc.) contributed to their extinction?
The innate characteristics probably played a role for their extinction- not only where these possible upsides for human intervention but larger size comes with more restrictions that could of made survival harder like those mentioned.
2. Think about the black mats that were described in this paper. Some believe that the black mats indicate the impact of an extraterrestrial object which caused the extinction of the large herbivores. The author believes it was the extinction of the large herbivores the caused the buildup of vegetation and eventually concentrated sediment. What do you believe was the cause of these dark sediments?
3. What do you think caused the increase of fires in ecosystems after the extinction of large herbivores? Was it a result of a lack of vegetation clearing once performed by the herbivores or was it due to the usage of fire by humans?
. Based off of knowing that megafaunal extinctions occurred due to humans, biogeochemical cycling, habitat structure changes and trophic downgrading, what do you think had the most impact on the ecosystem as megafauna went extinct?
I believe the thing that had the biggest impact on the ecosystem as megafauna went extinct was the changing vegetation and the extra room for humans to expand their ecological niches.
2. What are some ways that megafauna extinction could have had on atmospheric CO2?
Megafuanal extinctions possibly effected the atmospheric CO2 by having less of an uptake of plant life.
3. Do you think that trophic rewilding and introducing species to restore top-down interactions to promote self-regulating biodiverse ecosystems is a better way to conserve megafauna or is creating a system of experimental sites close to urban centers that engage public a better way for conservation efforts?
This is an interesting question. I think that restoring the ecosystems is obviously essential to maintaining the diversity however with the unquestionable impact that humans have, its probably needed to create experimental sites that allow for different ecological systems to develop.
Johnson et al. 2009
ReplyDelete1.) I think it is important to realize that extinctions are not necessarily due to sole purposes. Humans are undoubtedly a major contributor to the changes in ecology of Earth's ecosystems, however, climate change, regional weather conditions, disease, and even changes in oceans currents must be provided attention when discussing plausible causes of change.
2.) I would agree with the author that the black mats were a result of accumulation of vegetation and concentrated sediment. Take for example coal and oil. These are remains from microorganisms and vegetation that are black in color. You can say dark material when you pull up areas covered in grass and vegetation.
3.) I think the increase in fires could have been attributed to both natural causes and human impact. Humans most likely killed off large game, that cleared fire vulnerable material. This would have created a kindle box on the landscape. A spark from human activity or natural conditions, such as lightning, would have ignited the material once cleared by the grazers. Lack of large bodies herbivores, natural weather conditions, and human fire activity may have all contributed to the increased fires in ecosystems.
Lyons et al. 2004
2.) I think the innate characteristics of megafauna, such as reproduction, contributed to a large degree of their extinction. Large animals, especially mammals, produce few young that take a very long time to develop and reach sexual maturity. If humans killed off the older, sexually mature adults, population sizes would have declined. If you kill the old adults, fewer and fewer babies can be produced. This surely drives species towards extinction.
3.) Animals had been surrounded by humans for a long time. They adapted and coevolved with humans and understood the importance of running away when humans were sighted. Ecosystems that had not previously been exposed to humans did not understand the dangers they possessed.
4.) The researchers showed that the extinction events were size selective. There evidence is sufficient because humans would have targeted/extirpated large bodied animals since they provided more food, posed as bigger targets, and may have been slower as opposed to smaller, quicker game.
5.) The terminal Pleistocene extinction and current anthropogenic extinctions are results from human activity. However, the terminal Pleistocene extinction occurred because of humans hunting large prey while the current extinctions are due to poaching, habitat destruction, and atmospheric alteration.
Smith et al. 2016
1.) I think vegetation regulation and distribution had the most impact on the ecosystem as megafauna went extinct. Many of the megafauna that went extinct were grazers that helped maintain and manage grasslands. By consuming the same vegetation, such megaherbivores were able to help distribute seed through digestion and excretion. Their extinction would have cause a change in vegetation composition throughout the landscape. This would also have affected other species.
2.) Atmospheric CO2 probably decreased. The reason being, the extinction of megafauna allowed forests to inhabit previous grasslands. Large forests act as sinks for CO2.
3.) I think both options should be explored. Restoring species back to their natural habitats should help restore balance. Allowing animals to be in close proximity to humans would also be beneficial for future human-species interaction. Humans and animals would both learn from each other if this option were to be implemented.
I personally think that the wide spread of homo sapiens was the cause in many of the ecological changes due to their massive extirpation of large bodied organisms (birds, mammals, and reptiles), with taking these large bodies away in the masses, causes for a niche to be filled by other organisms and animals. Also, taking these large bodies from the web of life tends to change an areas ecosystem. I think that it was mainly humans but I also I think that natural selection had some bit of a role to play in this event. As far as the increased fires are concerned I think it was a mixture of both. Humans would have hunted large bodied animals because they provided lots of food and in some cases other use such as hides and fur in order to stay warm and provide tools from teeth or husks.
ReplyDeletemany of the megafaunal extinctions during this time really impacted the landscapes and ecosystem as everything is connect to one another and each individual plays a specific role.
The extinction of herbivores would surely have had detrimental effects to ecosystem structure and community composition which would have had reverberating effects throughout the biosphere as one ecosystem is disrupted after another and so on. Changes in climate also would have effect on ecosystem change, but not as directly and rapidly as megaherbivore extinction because it would have happened more slowly and less specifically to herbivores over time.
ReplyDeleteI think the sediments would have been due to either case, but specifically only from a rapid event, such as a massive die off which would have left a thick layer of biomass or an impact that left thick residue from burning and dust settlement. It could easily be a combination of both and likely was.
An increase in fires, both severity and frequency, would have resulted from the reduced interaction of herbivores with vegetation. Large herbivores express significant control of vegetation and reduce the amount of detritus material by consuming live material. Without herbivores, plant populations would erupt and detritus would accumulate, meaning fires could start much more easily within the dead material and could persist longer.
Researchers can look at the species humans brought along with them during migration. Certain mustelid and other rodent species would act as effective disease vectors which could have caused extinction. This could be examined by looking at the magnitude of these species in the fossil record surrounding extinct species as well as their temporal and geographic proximity to humans.
Megafauna develop more slowly and reproduce fewer offspring less often due to energetic constraints. Large species would also require larger shelters and habitats that are less frequent than small ones.
Humans would not have been able to understand how large animals contribute to the environment and the importance of practicing sustainable foraging, just as other animals don’t understand this. Humans would have made use of whatever they could have of their environment in order to survive and they happened to have advantageous skills (tool-making, wearing versatile and protective clothing) that made them effective in this survivalist endeavor.