Trophic Downgrading of Planet Earth (James A. Estes et al. 2011)
The paper discusses
the long term effects of the loss of higher predators on the ecosystem. The
present high rate of extinction is largely effecting predators and the paper
suggests that the loss of these species has a significant long lasting effect
on the future evolution of surviving organisms. The loss of predators in an
ecosystem cascade down the food web effecting all of the species interactions
in an ecosystem. The result of this “trophic downgrade” is an alternative
stable state in an ecosystem.
Trophic downgrading
is cryptic because the resulting effect of losing the apex species is not
entirely clear until that species is gone. It is also difficult to conduct a
study of these effects as often the species studied inhabits large range spaces
and as such effects a larger area than can reasonably be controlled in any
study and influence the change in the ecosystem across impractical time scales.
The paper goes on to
discuss that despite difficulties in artificially testing these species
interactions strong evidence is present in the real world of the effects of
extinction or extirpation of apex species from an ecosystem. On short time
scales immediate evidence can be seen in some ecosystems for the reduction of
certain herbivores. The loss of these predators resulted in a increase in
herbivory which reduced the availability of food for herbivores which were
already free of predators.
The paper then goes
on to list the effects of ecosystem reorganization due to trophic downgrading. These
effects include increasing wildfires, increasing disease transmission, chemical
changes in soil and water, increased risk from invasive species and decrease in
overall biodiversity.
Questions:
1.
Can you think of a viable experiment
than could show a top down cascade on an ecosystem? What problems would you
face?
2.
Give an example of an ecosystem
effect caused by the loss of a predator species. is there an instance where the
loss of a native species could be a good thing for biodiversity?
3.
How do you think the loss of many predator
species will impact future evolution after the next mass extinction?
4.
Which do think is more important to
ecosystem stability? Bottom up effect or top down trophic effects?
I found the paper to be interesting and some of the evidence compelling, particularly Part C of Figure 1, the experiment with the large-mouth bass. I'm happy to be convinced that high trophic level organisms are just as important to some ecosystems as lower trophic level organisms. BUT, near the very end of the paper, they say this:
ReplyDelete"We argue that the burden of proof be shifted to show, for any ecosystem, that consumers do (or did) not exert strong cascading effects."
- Regardless of their findings, I would argue that this call for a shifted burden of evidence is wrong. If I say, "Jaguars have a strong effect on the lower trophic levels of the ecosystems they inhabit." then I have made a positive claim. By the basic rules of logic, I cannot simply make that claim, as it were, dixit ipsedixit. Positive claims require positive evidence. The burden of proof cannot be so eschewed.
The effect of losing top, apex consumers and trophic downgrading has major consequences on nature. I found this article very informative. According the the author, "Bottom-up forces are ubiquitous and fundamental, and they are necessary to account for the responses of ecosystems to perturbations, but they are not sufficient. Top-down forcing must be included in conceptual overviews if there is to be any real hope of understanding and managing the workings of nature." The ability to alter our perception on top-down processing seems to be essential in understanding ecological dynamics.
ReplyDelete4. Which do think is more important to ecosystem stability? Bottom up effect or top down trophic effects?
ReplyDeleteJust reading about "bottom up" vs. "top down" trophic effects, I'd argue that bottom up effects are more important for keeping every trophic level stable, but that doesn't negate the importance of top down effects - top down effects are keeping every trophic level stable in the sense that taking one level away makes the whole ecosystem stumble. Also, I'm not sure if it's too important to the paper, but when the authors were describing "alternative stable states" in the beginning of the paper, I wasn't sure what they meant. They used the term "basin of attraction" -- would a more experienced biologist define this or give an example?
I would think that an experiment regarding the removal (and then reintroduction) of the impacts of a trophic cascade has already been conducted with, first, the extirpation of wolves from Yellowstone and then their reintroduction (although this also falls into your second question). In terms of another modern apex predator introduction (such as the condor or another modern animals), some problems would be as follows: (1) the importance of have knowledge of past baseline(s) for the species that existed at a point where condor were also present, (2) an understanding of the modern ecosystem dynamics (including human impact), and (3) navigating political issues. Perhaps in this case, a condor would not be the best species because of how it is currently fed unless they then relied mainly wild marine species.
ReplyDeleteI personally thought this paper was very interesting. Examples like the ones we can see in Figure 4 really help solidify the effects of apex consumers being gone from an ecosystem. While reading this entire paper, I kept wondering what study could be conducted to further quantify the effects of apex consumer extinction or extirpation. It was mentioned in this article that short term studies don’t really measure the magnitude of impact that it would have on the ecosystem to lose an apex consumer so what kind of long-term studies could be conducted? If this means re-introducing an apex consumer, then how can we make sure that their environment is stable enough to thrive? This reminds me of the discussion we had a couple of weeks ago in class regarding re-introducing seals in California and how their re-introduction was challenging because they co-habited with humans. I definitely agree with the author that a multidisciplinary approach is needed to do more research I this area.
ReplyDeleteIt's difficult to say whether bottom up or top down tropic effects are more important to ecosystem stability. I would argue that neither one is less or more important than the other. It is equally important for an ecosystem to have multiple tropic levels and each contributes to the stability of the other. For instance, the article reiterated that top predators are important in keeping the number of herbivores down in order for flora to flourish. But there's also a great importance in not having too large a proliferation of producers.
ReplyDeleteThis was one of my favorite papers that we have read thus far because it was easy for me to understand unlike some of the other ones. We can see the importance of the ecosystem and how things can be skewed based on the removal of an apex species as well as the difference between top down and bottom up trophic effects. Bottom up effects seem to make a bigger difference as far as trophic stability is concerned. Both do seem to make an important difference in stabilizing the ecosystem.
ReplyDeleteI agree that this paper was super interesting, I liked that this paper was the easiest paper to understand this whole semester and it seemed to have a lot of information not only from concepts we talked about this semester in class but also in our two hundred level classes. I was honestly reminded of the reintroduction of wolves in yellow stone as a top down cascade which we even talked about in class, but something more local I feel like a good experiment or study would be to look at the effects of the loss of the jaguar in our area, while there are still coyotes and mountain lions, I feel like the loss of this big cat in our area had to had some ecological affect.
ReplyDeleteI should think that bottom-up trophic effects would, typically, play a larger role in the functioning of an ecosystem than would top-down trophic effects. However, this does not mean that I discount top-down trophic effects, by any means. With a loss of many predator species, I should think that, following the next mass extinction, we will see greater numbers of prey species given that predators are no longer available to control their population. Additionally, I would expect to see other organisms adapt to fill the niche once occupied by other predators.
ReplyDeleteTop predators are one of the most important pieces of ecosystems in my opinion. Often times they are also the fist organisms to go when humans are involved. An example of a top predator being lost from an ecosystem is the North American grey wolf. The wolf was the top predator in most of North America and I still don't think we have a full understanding on how much impact losing the wolves in most ecosystems has had on diversity and other environmental impacts such as river and stream flow. This is one of the more important concepts of paleoecology in my opinion.
ReplyDeleteIn response to Andrew, if there was an increase of prey species due to a mass extinction taking out the major predators I would think that we would see ano increase in prey species bUT as their food sources run out they would eventually decline in their population numbers. So their population numbers wouldn't be left unchecked, instead it would just move to a lower trophic level in my opinion.
ReplyDeleteI think a viable experiment would be to reintroduce apex predators into areas were they were once extirpated and record the results. The problem would be working with the public to get the reintroduction approved and the animals protected. In Yellowstone the loss of the wolf caused a change in plant and riparian communities when elk and moose were no longer being predated. I cannot think of any instance were biodiversity has benefited from removing a native species.The loss of major predators will likely result in smaller carnivores taking their place but not before ecosystems undergo a simplification. I believe that the importance of top-down or bottom-up trophic effects is dependent on the ecosystem in question and whether the ecosystem was complex or simple in terms of trophic levels. Layering of trophic layers can determine whether a pond ecosystem is cloudy or clear.
ReplyDeleteIt is hard to determine how removing a native species would affect the biodiversity. Removing one species may improve another species, but decrease another species and then determine which species those would affect in their own as well is hard to determine, it is just a chain reaction that would be very difficult to predict. In order to remove a native species there should be significant proof that it will improve biodiversity before doing so.
ReplyDeleteThe only viable experiment I can think of is to observe first hand in the environment since this would be hard to recreate in a lab setting, but one exams that I can think of for a top down cascade is the loss of sharks to some marine ecosystems and we see what is described in this paper where is fluctuats all the way down the food chain, so on the other hand a bottom up effect would be much more devastating because you are removing the essential part of the ecosystem, the primary producers. With the loss of major predators it effects everything else in the chain as well as the environment increasing such things as herbivorous animals which in part could be bad for the environment and potentially make a bad situation worse.
ReplyDeleteI would have to argue that both top down and bottom up effect are both very important. However, I believe the bottom up effect would have more severe consequences. For example, the bottom keeps every trophic level above it in a stable condition. Once the bottom is altered or removed, then every level above it will suffer in some way.
ReplyDeleteThis paper was an interesting read and the biggest takeaway from it was inter connectivity of every organism regardless of trophic level affects their habitat. As soon in several examples in the paper the presence / absence of a predator affects everything. Both the bottom up and top down models have cons involved. The bottom up model seem to have more of an effect as the bottom is the foundation and this is tampered with how stable can the pyramid be. It will be of great significance if studies can be conducted on other organisms from different trophic levels and specialties, I see what effect their presence/ absence will have of the ecosystem.
ReplyDeleteI found this paper to be interesting in that it shows how important top predators are to trophic systems of the ecosystem. I think this paper would be of good use to help show how beneficial the reintroduction of predators can be for conservation biology. I think a possible experiment to show top down cascade could be a fence off of deer mice in its natural environment with a net over the top of the range to detour avian predators and the fence to fend off terrestrial predators and see how the deer mice affect the vegetation and possible other species in the area over a short time frame. This alludes to how a loss of a predator can lead to a possible over consumption of the primary producer leading to loss of vegetation and thus reduction in population size or it can pull a 180 and be beneficial maximizing population size while keeping the vegetation stable. In my opinion I think bottom up effect is essential for the ecosystem but top down cascade is where the fitness is either increased or decreased.
ReplyDelete2) An ecosystem effect that would be caused by the loss of a predator species would be an overpopulation of prey animals since more are surviving because of the lack of predation. This could cause the prey animals to become overabundant and deplete their resources. For example an over abundance of herbivores, that used to be prey, could cause severe depletion of the plant life in that area. According to the paper this causes less productivity within the whole system. However, in the paper the opposite of this seemed to happen when the fox was absent so that could be an example of when taking out the apex consumer is good for biodiversity.
ReplyDelete