Ecological Consequences of the Pleistocene Extinction in Hall's Cave, TX (Smith et. al. 2016)
Mammal communities in the Hall's Cave region in the Hill Country of Texas have changed significantly over the past 22ka. This includes the loss of the famed Pleistocene Megafauna. Smith et. al. 2016 discusses the dynamics of these changes in the mammal community structure over this time interval.
Smith et. al. looked at exactly how certain types of mammals based on their diet became present or absent over time. To achieve this, they binned the data into 1.3ka intervals, and looked at the presence/absence of each species between each time bin. To test for significance, they primarily employed the PAIRS program to analyze patters of species co-occurrence at each time bin. They used alpha (α) and beta (β) diversity to evaluate diversity in the area.
Smith et. al. found that there was a fundamental change in the mammal community structure of the Hall's Cave region during the last 22ka. One of the main takeaways is that there was a massive downsizing in the maximum body size of mammals in the region. This is most notable in the loss of the Pleistocene Megafauna, which was a downgrading roughly equivalent to two orders of magnitude! The one two of the main losses in terms of the Megafauna were large swaths of the apex predators like Arctodus and Smilodon, but also in the giant grazers like Mammuthus and Camelops that previously dominated the landscape.
While it is evident that the horizon of the megafaunal extinctions occurs soon after there is human involvement in the area, there is definitely a case to be made that climactic variables additionally altered the structure.
Questions:
1.) What are α & β diversity? How is each measured, and what does each of them tell us about the overall (gamma, γ) diversity in the region over time?
2.) What is significant about the Hall's Cave region as opposed to other late-Pleistocene/early-Holocene deposits?
3.) Why might frugivores and granivores be grouped together for the analysis?
4.) How might α & β diversity be coupled in the time lasting from the beginning of the NA Glacial Retreat to the Extinction Horizon? (Figs. 2b & 2c)
In this study the extant taxa have fewer associations on average than the extinct taxa (4.7 pairs on average for extant species, 9.9 pairs on average for extinct species). No data on insects or arthropods appears in this study, however, which is totally understandable, you can't just conjure insect fossils out of thin air - BUT, I suspect many of the remaining species in the Holocene who are rated as having low average species interactions (many of which are small) have more associations with non-mammalian species (arthropods, diapsids, birds) than did the on-average larger extinct species. I wonder if the number of species interactions for these fossil organisms can actually be determined, given that we lack a goodly amount of data relating to the majority of animal species that undoubtedly lived in this environment and undoubtedly interacted with mammals living in it. Though I realize it’s an analysis of mammal community assembly, it seems somewhat selective to only consider the interactions between mammals and other mammals. I mean, if I looked at the interactions between turtles and only turtles and concluded that the turtle communities are 'unconnected' you'd call me daft. Turtles don't just interact with turtles and mammals don't just interact with mammals.
ReplyDeleteI also had two general questions:
1) May I ask, what's the difference between an aggregated and a segregated pair?
2) What is the single species of insectivora recorded in the study (isn't insectivora a defunct clade?)
The article indicates that there are many aspect of Hall’s cave, and surrounding sites which make them suitable for this sort of research as compared to other deposits from this time period. Firstly, it offers a fossil record that is contained in rocks that are well-defined with respect to surrounding rocks and time. Secondly, the preserved specimens in the cave have been protected from disruption from living organisms (human or otherwise). This results in increased reliability of the fossils and resulting data. Frugivores and granivores may have been grouped together for this analysis, because they represent a type of herbivory that is more consistent with that of a browser rather than that of that of a grazer. Thus, they are distinctly different from grazers and have different dietary preference and methods for obtaining resources. This requires that they be grouped apart from grazers, and together as browsers.
ReplyDeleteGamma (γ) diversity is the diversity of a group of organism(ie: species or genera) of a region. It's the big picture approach to diversity.
ReplyDeleteAlpha (α) diversity is the richness and evenness of a group of individuals within site of that region.
Beta (β) diversity is the amount of species diversity at each site within each region.
Gamma is equal to alpha times beta diversity.
How I understand it is that alpha, beta, and gamma diversity are measurements of diversity as a certain spatial scale where alpha diversity refers to individuals within a habitat, beta diversity refers to the diversity between habitats and gamma diversity is the diversity of habitats within a larger landscape. So, by characterizing (in this case) changes in alpha and beta diversity, the authors are able to look at the level of potential for changes in community organization overtime. Within the paper, alpha diversity only changed (increased) during 16-11ka which the authors argue may be a result of an increasing heterogeneous environment causing accelerated faunal turnover. Two changes in beta diversity occurred (16-18 and 12-13ka); the second, the authors correlate to the reduction in body size that occurs at this time.
ReplyDeletePractically speaking, how do you interpret changes in alpha and beta diversity? Ie Are you constrained to looking at factors within that specific spatial scale? (within habitats for alpha and between for beta)
Also, how exactly are alpha and beta diversity being calculated? It is based on a presence/absence matrix?
Hall's Cave has previously been excavated, archived, and well dated allowing for insightful evaluations of findings. Besides the amount of data available to researchers (over a 22,000 year span) the diversity of the fossilized assemblages present at the site makes the region important when studying past mammal communities.
ReplyDeleteHall's Cave provides a great site where we can research preserved specimens to determine ecological and environmental factors from over 22,000 years ago, as well as how important the role of apex consumers are. My question is why is there a lack of diversity at 17.2 ka? I assume that either there just was a lack of species that wound up in the cave or maybe there was an environmental factor (like the cave being blocked) that could have influenced this?
ReplyDeleteI was at first a bit confused by the differences between alpha, beta and gamma diversity - however CHD did a good job clarifying these three. I think that the Hall's cave does a good job providing evidence for the consumers as well as diversity from the past 22,000 years. This led me to question how things were well preserved to this extent and where else possibly we could find such well preserved specimens and what other clues this could possibly give us.
ReplyDeleteI have to admit that alpha and beta diversity is a very confusing subject but like the person said before me it was explained well. I think that this kind of biology is one of my favorites at the moment. I would love to be crawling around a cave one day finding ancient mega fauna!
ReplyDeleteFrom what I'm reading and researching I assumed that beta diversity was basically just trying to figure out the turnover rate in communities and I'm still struggling to figure out alpha diversity. But in response to Raena I would have to assume that the diversity decreased possibly due to environmental factors changing as well as changes in the competition for resources.
ReplyDeleteThe only reason I somewhat understand alpha and beta diversity is due to biology 203, but the graphs in the paper are very difficult to understand in comparing the two diversities. As in beta diversity in this paper what are we looking at? Is it the differences, the similarities, or something else?
ReplyDeleteWhat's significant about hall cave after the glacier started to go away is that it opened up a lot of new area allowing for new species to come in and play a part in the environment so halls cave reflects that by having more alpha diversity on the cave but also a lower beta diversity since there are so many new species coming into this new environment.This trend in diversity I assume can be found through many other glacial periods or near glaciers as well not just in halls cave.
ReplyDeleteWhat's significant about hall cave after the glacier started to go away is that it opened up a lot of new area allowing for new species to come in and play a part in the environment so halls cave reflects that by having more alpha diversity on the cave but also a lower beta diversity since there are so many new species coming into this new environment.This trend in diversity I assume can be found through many other glacial periods or near glaciers as well not just in halls cave.
ReplyDeleteHerp derp, anonymous is me, btw, don't know why it didn't take my name.
ReplyDeleteAlpha and beta was hard to understand but after reading the comments above it really clarified it for me. Other than that confusion, I agree that Hall's Cave was a great site to conduct this research. It is my understanding that the cave also has more alpha diversity and lower beta diversity?
ReplyDeleteHall's Cave is a dwelling for studying the past as it was uninterrupted by humans etc for a long time. This cave provides a great record for what life was during the late Pleistocene. The way to differentiate alpha & beta diversity, alpha is just one habitat while beta compares & contrasts two habitats.
ReplyDeleteWas there a specific reason that the diversity at 17.2 ka could have been under sampled? With the rich fossil record of Hall's Cave and the good stratigraphic information, I'm confused as to why this specific time provided so much less diversity. I found it interesting that there was little change in the omnivores, though after thinking about it, it makes sense as they have more food sources available to them and are less likely to be impacted by competition or destruction of food sources.
ReplyDeleteI liked how this study showed the change in diversity with the comparison to body size over time. I thought it was interesting how alpha diversity started increasing with the retreating glacier while it started decreasing around the time of the younger dryas. This could be the diversification of species from new available niches but once the temperature increased to much competition selected for the species that could adapt the quickest which would decrease alpha diversity in that area since alpha diversity is for a specific region. To address the question why might frugivores and granivores be grouped together for the analysis because they essentially get their food source from similar sources; one being the fruit for frugivores and the other being the seeds that come from fruit or angiosperms which would be the granivores.
ReplyDeleteThis paper was really interesting. The main thing that I took away was that human involvement has major and far reaching effects on ecosystems. I was really confused about alpha and beta diversity but felicia did a good job of explaining it in class.
ReplyDelete2)I think the significant thing about Halls Cave is that is has better records than other sites because more studies have taken place there.