Sunday, October 7, 2007

Week 8 Background: how humans alter biogeographic patterns

Humans and extinction:
As mentioned by Lyons et al. and thoroughly reviewed by Surovell, their are two basic camps in the contentious argument over the role of humans in the disappearance of very large fauna at the end of the Pleistocene. The overkill hypothesis argues that humans caused the extinction by overhunting the large slow reproducing game that probably occupied niches where risks of predation had been extremely low before human arrival. The climate change hypothesis argues that changes in temperature and precipitation that coincide with the start of the Holocene were more than the large bodied animals could cope with and this lead to their demise.

You would think these scenarios would be easy to test and evaluate but for our major case studies the timing of human arrival is about the same as the timing of the major climate change in question, as was the case (more or less) for Eurasia and North and South America. This makes isolating the effects of either climate or humans somewhat challenging. Australia is an ideal test case, which is why it is stressed by the Lyons et al paper. One lingering problem with Australia is that the paleontological finds are not well dated. This makes for a somewhat tenuous link between the timing of the appearance of humans and the disappearance of large animals there. This will undoubtedly be resolved in time.

In one sense the real difference between scholars in this debate is whether or not humans played a role in the extinction. The camps should really be named anthropogenic and non-anthropogenic and then subdivided into groups based on the specifics of their proposed scenario under each heading. The readings mention the hyper-disease idea and Diamonds' Sitzkrieg hypothesis which states that the extinction was anthropogenic. However, for Diamond, the extinction had little to do with predation but with human land disturbance, fire, and their cascading effects on the ecosystem. I like this view and think it may be correct but how do we really test it? Consider the possibility that it was simply a huge increase in the frequency of fires, started by humans, that led to habitat fragmentation. This could have been a special disadvantage for large-bodied herbivores because they need large home ranges for feeding and can't afford added costs to finding mates and rearing young. While some have found significant increases in charcoal flecks in pond sediments at about the time of human arrival, this is difficult to link definitely to humans, the fires they may have started started, and to the mortality of large herbivores.

It is interesting to note that, from my perspective at least, the more comfortable one is with overkill the more divorced they are from any actual analysis of the empirical record for human predation. This is in part because testing of any of the proposed models is difficult (discusses by Surovell) but also because evidence for direct predation of the extinct species is extremely rare. Note that the paper by Lyons et al. essentially ignores both data for the occurrence of megafauna in archaeological sites from the period as well as data on hunting patterns of living hunter gatherers (which would strengthen their argument).

The most outspoken skeptics for the overkill hypothesis are Donald Grayson (U of Washington) and Dave Meltzer (SMU) who think human predation has in no way been linked to the extinction of an animal in any prehistoric context.

One of the few approaches to the debate that has attempted to consider combined roles of both climate and predation comes from a thorough review published by Barnosky and colleagues. They review models, empirical evidence, climatological data, and ecological arguments.
Here's a figure from their paper published by Science (2004).

Fig. 1. Summary of the numbers of megafaunal genera that went extinct on each continent (Table 1), the strength of the extinction chronology, and a comparison of the timing of extinction with the timing of human arrival and late Pleistocene climatic change. Extinction timing for individual genera was judged as robust or provisional based on previous publications that evaluated quality of dates. Sources are as follows: Europe (3, 14, 47), Siberia (48), North America (11, 29, 46, 57), and Australia (4, 7). For humans, the date is the earliest generally accepted arrival of Homo sapiens sapiens; pre-sapiens hominins were present in Eurasia and Africa much earlier.

The other two papers:
Sutherland establishes that by many standards human languages are at greater risk and disappearing at a faster rate than species of birds and mammals. I'd add that there is very little 'conservation effort' to control or manage or slow this loss, mitigate the impact, or even to reverse the pattern. This would certainly be the case if we looked at funding.

Evans and Gaston show a positive but decelerating relationship between human population density and bird species richness in Britain. A positive decelerating correlation means that the two variables are correlated at lower values and the strength of the correlation weakens as one, population density, becomes larger. This generates a curve that flattens out at high values.
They present some more speculative evidence that human density lowers the rate at which new species are found in highly productive environments. They do this by isolating the effect of human density on the slope of the relationship between energy and species richness. They present this result because in many cases human population density is negatively associated with richness of other taxa and therefore their finding of a positive (but decelerating) correlation is somewhat surprising. Their analysis is interesting because it points suggests the possibility that humans and animals may yet both map onto highly productive environments. Alternatively, could it be that humans in cities may provide energy/food/shelter to birds in ways that facilitate their richness? Humans don't prey on birds, presumably, in British cities, they may reduce the number of predators, and may provide easy food sources. This is pure speculation and lots of the implications of this paper need more investigation, but the relationships in this paper are interesting.

See you on Tuesday,


Senorita Myra said...

How different are these glacial- interglacial periods that Lyons mentions? What if the last one is significantly different than the previous 20? Even though knowing these variations may not explain the mammalian extinction, it should be more thoroughly explained if it is going to be a foundation to their argument.

M. Simon said...

You might find this of interest re: climate change.
Criminals And Moralists Working Together

Enron Carbon Trading And Hansen

Enron And Carbon Trading

Fred Whiteman said...

I noticed that in the Lyons paper, megafaunal extinctions in Eurasia aren't discussed, although they are in the Surovell paper. Could the gradual megafaunal extinction in Eurasia and the smaller, gradual extinction in Africa be the exceptions that prove the rule?

Also, to what extent could the extinctions be the result of a combination of several factors? For example, could hunting AND land use by humans AND climate change AND reduction in numbers of keystone herbivores have all acted together to lead to the extinctions in the Americas? Would this account for the size bias in extinction?

Dan said...

Taking predator-prey foraging models may help to examine megafauna extinctions during the late Pleistocene. Given that the handling time for megafauna is relatively high, at what encounter rate would it make sense to let the mammoth graze? Would the encounter rate eventually be so low that it would be better to go after a conejo? I guess this is not inconsistent with the Lyons et al paper, since they cover their butts by allowing for other secondary (climate) causes of extinction.

Steven M. said...


Given that the graphs on 347 plot log(mass) on the x axis, there appears to be a lower threshold for carnivores that went extinct. Observe that in both N. and S. the carnivore extinctions seem to be shifted to the right. This could be just coincidence, but if it isn't then it would indicate that larger carnivores went extinct than other orders. Is this due to the fact that the large carnivores were also eating the large game that went extinct because of the humans?

Posting here as I thought the first one was the one to put comments on, but this one is where the comments appear to be.

Wenyun said...

In Surovell's paper, if we have the data of Africa and Asia for figure 2, what should we expect?

tlvandeest said...

In the Lyons et al. paper, they discuss how patterns in modern hunting have been the main reason large mammals are either endangered or extinct and use this to support their argument that earlt human hunters would have also targeted large mammals. While this may be true, did the authors consider that modern hunting is done mainly for sport and not subsistence? Wouldn't you expect different patterns with this non-subsistence based goal in hunting? Did the authors restrict the hunters to those groups with hunting practices similar to those used by early human hunters?

dodegard said...

Lyons et al. are quick to dismiss life history theory on page 348 because smaller fauna with the same life history traits as large fauna who died off, survive. Wouldn't there be a difference in LH traits because of the size difference?

Deepta said...

The Lyons paper categorizes redlisted mammals into groups based on their risks of extinction, whether due to intrinsic factors, loss of habitat, or hunting. Isn't there the possibility that there were interactions between these risks, that animals may have already been dying off due to loss of habitat, and this made them easier to hunt?

The authors also refer to a higher return for hunting larger animals, which is why these are the targets. However, there is also significantly more risk inherent in taking out a large animal as opposed to something small, which would have a significant effect on the return.

Deepta said...

Sutherland mentions that as languages become rarer, they may be less appealing to learn. How much of this is an effect of globalization, and the simple fact that more and more schools are teaching certain languages to increase the students' opportunities?

Justin Smith said...

Why did some fauna go extinct on some continents but not others? The horse and camel were domesticated in the East but apparently wiped out in the West.

Michael said...

With regards to the Sutherland paper, it seems rather counterintuitive that the number of televisions per 1000 people didn’t correlate with the number of languages in that country. Perhaps were area and population density controlled for, a pattern may appear. Although some larger cities (e.g. London) may still remain linguistically rich, more remote and less wealthy cities may show less diversity despite similar population size (e.g. Tehran). Either way, this is nowhere near the richness linguistic of New Guinea and the surrounding area. A greater concentration of wealth (i.e. televisions) in the cities would also be expected compared with more rural populations where a greater linguistic richness would tend to exist.

paul said...

Lyons shows that the species distributions were bimodal before the extinctions. That’s interesting, eh? I don’t think they suggest an explanation, do they? The second mode gets eliminated in North and South America, making their current distributions significantly different from Africa, where it persisted.

paul said...

Why wasn’t Africa as affected? They suggest that since human hunting capacity grew gradually in Africa, whereas it exploded onto the other continents, African mammals had time to adapt to human predation before being totally wiped out. Is there something else that makes Africa different? The megafauna of Africa might have a latitudinal advantage over North America, being able to maintain larger, denser populations? And Africa might have had an area advantage over South America, which is at a similar latitude?

Human macroecology admin said...

Surovell provides a nice overview of hypotheses for Pleistocene megafaunal extinctions. One thing that struck me about Diamond's "sitzkrieg hypothesis" is that such anthropogenic impacts as forest clearing and secondary burning would probably expand grasslands, and many megafauna, esp in the Americas, were grassland species.
-- Bill

helen elizabeth said...
This comment has been removed by the author.
helen elizabeth said...

Humans have decimated almost every environment we have every inhabited. We completely wiped out all protohumans and still today quickly encroach on every environment we enter to extract resources. It seems difficult to ever doubt the role of humans in mammal extinctions, especially when the fossil records clearly indicate that we coexisted in many areas at the same time...but, then as surovell states, these timing events only occurred on the large landmasses....

helen elizabeth said...

Wouldn't the question of whether weather or humans caused patterns of megafaunal collapses be answered by looking at other factors as well? Lyons et al suggests that a variety of human behaviors contributed and that the overkill hypothesis is a subset of those behaviors. How difficult would it be to create a comprehensive data set that included overharvesting when a popualtion moved to a new region, changes in habitat and introduction of new diseases and hunting?

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