Monday, November 28, 2016

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?

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)