Concluding Comments

The mystery of the vanished giants is no easy one to solve. This is still very much a dynamic field where new research reveals more dimensions of the puzzle and throws up more questions than answers. After gaining a better understanding of the topic through keeping this blog, I am ever more aware that the answer is not a simplistic, uni-dimensional one.

I found the arguments where human impacts are the ‘last straw’ for already stressed populations struggling to adapt to climate change the most convincing. Critics have argued that megafauna  survived the previous glacial-interglacial transitions, citing anatomically advanced humans with sophisticated hunting technologies as the only differentiating factor. This is a simplistic view of past climates which assumes that all transitions are the same. There is evidence that the late Pleistocene was unusually, warmer rather than cooler than the Holocene and supported a rich mosaic of vegetation types which was in turn able to support a huge variety of megafauna. This disappeared during the Holocene.  Besides, ‘megafauna’ is not a static concept; Graham and Lundelius (1985) concept of ‘co-evolutionary disequilibrium’ suggests that species often co-evolve in unique and individualistic ways, meaning that the species composition of ecosystems change all the time. The late Pleistocene was a biotically unique period, limiting comparability with other glacial-interglacial transitions.

Besides, extinctions did not happen in the rapid ‘Blitzkrieg’ manner that some researchers have argued. Especially in Eurasia, where humans were not necessarily present in all areas, extinctions occurred gradually and even happened in areas where there were no humans, e.g. the Irish Elk. The African Anomaly also adds another piece to the puzzle – African hunters were also sophisticated but it is the continent which experienced the fewest extinctions, and there is evidence that climate change was less pronounced in Africa.

Whispers from Ghosts Past: Lessons for the Future

So where does this all leave us? I do not think it is possible to provide a conclusive answer to the mystery of the vanished giants, but I hope I have left room for more thought. I also hope that a better understanding of past climates and extinct species would lead us to be more aware of the importance of modern conservation – once species are lost, they are irrevocably gone.

I can see 3 major takeaways from this blog:

Our current level of biodiversity is already a depauperate version of what it once was, yet human impacts are resulting in ever more extinctions than before.

Today, our actions are themselves causing climate change. In the late Pleistocene, climate change was essentially separable from human impacts such as hunting. Today, we have entered what Nobel Prize winner Paul Crutzen (2002) calls  ‘The Anthropocene’ – human impacts have reached such unprecendented levels that they have become significant geological forces. 

The late Pleistocene megafauna extinction event has shown that the adverse impacts of humans and climate change are a lethal combination for species biodiversity, and the effects are further amplified today.    

References

Crutzen, P. J. (2002) The ‘anthropocene’, Journal de Physique IV, 12, 10, pp. 1-5

The Late Pleistocene Extinction Event and How It Stacks Up to Extinctions Today

The Big Five, and a Sixth?
This is my last blog post before I conclude and I thought it would be useful to compare the late Pleistocene megafauna extinction to modern extinction rates. Indeed, the latter is so severe that there has been concern that we are causing the ‘6th mass extinction’ through habitat destruction, killing of species, changing global climate and introducing non-native species. 

There have been 5 mass extinctions in the past: near the end of the Ordovician, Devonian, Permian, Triassic and Cretaceous Periods. A mass extinction is defined as ‘having extinction rates spikes higher than in any other geological interval of the last 540 million years and exhibiting a loss of over 75% of estimated species’ (Barnosky et al 2011: 51). 

A discussion on the evidence surrounding the 6th mass extinction is beyond the scope of this blog; Xijia has an interesting blog here on this topic.

Modern Extinction Rates 
This paper by Barnosky et al (2011) gives a bit more insight on how extinction rates are calculated. Although this paper is primarily about the 6th mass extinction, it does provide a comparison figure for the Pleistocene extinction event. A widely-used metric in this field is extinctions per million species-years (or E/MSY). The ‘natural’ background rate of extinction is 1 E/MSY, i.e. if there were 1 million species on Earth, one would go extinct per year. Rates are estimated using fossil extinctions that occurred in million-year time bins. Current rates are projected to a million years, as current extinctions have occurred within very short timespans (a few decades to hundreds of years). It is important to note that extrapolation may introduce inaccuracies, and the shorter the time intervals, maximum ESY and its variance increase. 

Using a paleontology database combined with lists of recently extinct species, the most complete set of which are available for mammals, the results are as follows:

The extinction rates observed for the past 1,000 years (24 E/MSY in 1,000 year time bins – 693 E/MSY in one-year intervals) is much higher than the maximum late Pleistocene extinction rate (9 E/MSY) and definitely higher than the background rate.

Clearly we are in the midst of a very severe biodiversity crisis and losing species much faster than they can be replaced. Given the already severe loss of biodiversity that was the late Pleistocene megafauna event (with the result that our planet is already a depauperate version of its former self in terms of biodiversity), this is a major cause for concern. While some have attempted to put an economic value on biodiversity (See Ali's blog), as we have seen biodiversity is intrinsically important in maintaining the balance of ecosystems, which are highly complex and interdependent systems.  

References

Barnosky, A. D. et al (2011) ‘Has the Earth’s sixth mass extinction already arrived?’, Nature, 571, pp. 51-57

Is Pleistocene Re-Wilding Viable for Conservation Today?

The mammoths have vanished, so what do we do now? An obvious answer is to realize the ecological and intrinsic importance of today’s surviving megafauna and do our best to protect them, but a group of scientists have gone one (controversial) step further: they want to ‘broaden the underlying premise of conservation from managing extinctionto encompass restoring ecological and evolutionary processes’ (Donlan et al 2006: 660). They argue for the introduction of Pleistocene rewilding in North America, which involves:

1. Reintroducing modern species descended from Pleistocene species that once lived in North America OR
2. Reintroducing ecological proxies for Pleistocene species if the above do not exist 

This paper by Donlan et al (2006) summarizes the key arguments supporting Pleistocene rewilding in North America.

Ecological Arguments

Megaherbivores and large carnivores play important roles in ecosystems and have been dominant in ecosystems for the past 200 million years, until their widespread extinction in the late Pleistocene. Given their instrumental roles (See also my previous blog post on the role of megaherbivores and extinction consequences), it must follow that restoring them would have positive benefits.

Large predators’ roles include the following:

•         Buffering against climate change: To cite a contemporary example, evidence from a Wilmers and Getz (2005) paper shows that the reintroduction of gray wolves have helped maintain carrion availability for the survival of other scavenger species, important since snow thaws earlier in Yellowstone due to climate warming.
• 
  
•        Controlling disease (some of which can spread to humans):  Another contemporary example concerns the lyme disease epidemic (spread to humans by ticks) which occurred in North America in the early 2000s was probably caused by peak populations of white-tailed deer, once kept under control by gray wolves.

Grey Wolf

Evolutionary and Conservation Benefits

Pleistocene rewilding is a way to transform conservation biology from mere preservation of existing species to reconstructing ecosystem processes and species interactions. Donlan et al (2005) argues that what is ‘natural’ must be challenged; we often think of the 1492 Columbian landfall on America and the state of the environment then as what is a ‘natural benchmark’ for conservation, but this fails to recognize the rich biodiversity of the late Pleistocene period. There are also additional positive benefits from maintaining large viable populations of target species to facilitate adaptation to climate change. North America could provide an additional refugia for conserving the genetic proxies of Pleistocene megafauna (such as Asian elephants, a proxy for mastodons), since these are endangered in Asia and Africa, the only 2 continents which preserve a large diversity of megafauna.  

Criticisms (Rubenstein et al 2006)

Effects on Ecosystems are Unpredictable

We do not really understand how Pleistocene ecosystems functioned and therefore should not attempt to reconstruct them. Rather, Pleistocene rewilding may disrupt contemporary ecosystems, e.g. by introducing new diseases, etc. Also, the effect of introducing ‘exotic’ species is unknown. Even when reintroducing native species, their effects on the ecosystem are unpredictable. For example, the introduction of one-humped camel in wreaked havoc on Australia’s desert ecosystems as they selectively ate rare plant species.

Reintroductions Do Not Always Work

Many modern-day examples show that even reintroducing native species within their original geographic regions is not always successful. The most successful examples (Przewalski’s horse and the Asian ass) are those where only a short time between extinction in the wild and reintroduction, as the ecosystem would not have changed much in that time frame. In other examples, problems such as unexpected changes in environmental conditions, naivete towards predators and diseases have rendered reintroductions unsuccessful.

Will Not Restore Evolutionary Potential

Most of the species which are supposedly to be introduced as part of rewilding are genetically distinct from their ancestors, e.g. cheetahs and lions. Thus, introducing them would not help restore the evolutionary potential that once was during Pleistocene times.

Anti-Conservation Backlash

Local and state governments in North America already face much trouble from people about fears over native predator attacks, e.g. cougar attacks on joggers. The introduction of exotic species such as elephants as proxies for mastodons for example would generate even more human-wildlife interactions and conflicts, such as those currently taking place in Africa.

My Thoughts

After reading all of this, I feel that the Rubenstein paper points out many pertinent problems that rewilding poses. Rewilding of native species, on the other hand, is a more promising aim for modern conservation. Pleistocene rewilding is much more difficult and prone to ecosystem-devastating error. The difficulties in even establishing the causes of late Pleistocene megafauna extinction reveals our lack of certainty about late Pleistocene environments. Rather than trying to enforce ‘revolutionary’ ideas, I think modern conservation should focus on preserving existing environments and rewilding (where appropriate) of native species.

I would like to end off with a link to an organization supporting the contemporary re-wilding agenda. Its ideas centre mostly around preserving ‘keystone’ species like large carnivores which play important roles in regulating ecosystems.

The Rewilding Institute: think-tank which supports rewilding programmes in America

Nevertheless, the controversy surrounding rewilding and the question of whether this is right for the environment goes a long way to highlight the fact that once these magnificent giants are lost, they have almost certainly vanished for good.

References

Donlan, J. et al (2005) Pleistocene rewilding: An optimistic agenda for twenty‐first century conservation, The American Naturalist, 168, 5, pp. 660-681

Rubenstein, D. R. et al (2006) Pleistocene park: Does re-wilding North America represent sound conservation for the 21st century?, Biological Conservation, 132, pp. 232-238

Dire Consequences

You may be wondering, what is the relevance of the Pleistocene megafauna extinction event to today’s world? Why does this topic remain so important to a whole plethora of researchers – paleo-climatologists, biologists and so on – when it happened in prehistoric times?

The answer is that it holds many important lessons for modern conservation and even the field of climate change. By studying the animals that once roamed the earth and their habitats, researchers can understand a great deal about past climates and in the process, gain a better understanding of both natural and human-facilitated climate change today. My last few posts will focus more on the implications which the Pleistocene megafauna extinction event holds for modern conservation. 

Consequences
Much attention has been focused on the causes of megafauna extinction, while the consequences have been much less studied. According to Rule et al (2012), herbivorous megafauna have a large role in the ecosystem by:

• maintaining vegetation openness and patchiness, removing material that would otherwise fuel landscape fire 
• dispersing seeds 
• physically disturbing soil 
• recycling nutrients via excrement

A comprehensive paper by Johnson (2009) details the vegetational changes that have happened in various continents following the extinctions. A general pattern emerges:

Changes in Vegetation Cover and Decreased Plant Biodiversity
Vegetation becomes more uniform (zonal patterns) as there is less pressure from herbivore feeding. For example, a cave site with records of Middle Pleistocene fauna in Australia (400-230 kyr ago) gives evidence of extremely rich biodiversity, consisting of a giant wombat and 18 extinct large kangaroos. The large diversity of feeding habits supported a more diverse vegetation than today, probably a mosaic of woodland, shrubland and grassland. Today, vegetation is a uniform shrub steppe. 

Increased Fire
Without herbivorous megafauna, plant material accumulates and fuels fire. Again, biodiversity decreases as only species with traits that allowed fire survival or post-fire regeneration would survive. For example, in Northeastern USA, burning increased several hundred years after the megafauna extinctions, as indicated by charcoal proxies. 

A Disclaimer and a Conclusion
The literature on the consequences of megafauna extinction on the ecosystem may be patch because of uncertainty over whether these vegetational changes were a cause or consequence of extinction. As mentioned in my earlier posts, climate change which led to vegetational changes has often been cited as a cause of extinction. Besides, there is also confusion as to whether humans played a major role in changing vegetation (e.g. increased burning) and hunting megafauna. Also, megafauna extinction was not associated with vegetational change in all places. 

Although there is a decided lack of clarity on this issue, one thing is clear – the rich assemblage of Pleistocene herbivorous megafauna had helped to maintain biodiversity and their loss was a major loss to the ecosystem as well. One need only consider the large impact today’s surviving megafauna have on the environment to understand this. For example, African elephants are heavy browsers and help maintain savannah conditions by breaking branches of trees while feeding. White rhinos maintain short-grass lawns within thickets, impeding fire and protecting woody areas from conflagarations. Shifting grazing by bison maintains high species diversity in tallgrass prairie. They all help to maintain the savannah ecosystem on which hundreds of other species depend for survival. We need to understand the importance of conservation as preserving not just one species, but an entire ecosystem – imbalances upset the whole system. This is why in the face of human-induced climate and habitat change today, keeping the ecosystem in balance is ever more important.   

References
Johnson, C. N. (2009) ‘Ecological consequences of Late Quaternary extinctions of megafauna’, Proceedings of the Royal Society of Biological Sciences, 276(1667), pp. 2509–2519

Rule, S. et al (2012) ‘The aftermath of megafaunal extinction: Ecosystem transformation in Pleistocene Australia’, Science, 335, pp. 1483-1486 

A Word on Methodology: The Sporormiella Proxy

In my previous post, I mentioned the Sporormiella proxy used to determine abundance of Pleistocene megafauna in Madagascar. This is an analytical technique that has recently gained prominence in the study of the late Pleistocene megafauna extinction. There is an interesting paper by Feranec et al (2011) about the Sporormiella proxy and the problems associated with using it.

The Sporormiella Proxy

Sporormiella is a fungus that is present on the dung of herbivores. Sporormiella sporulating on dung release spores which adhere to nearby objects (usually plant matter). Herbivores then eat this plant matter and the spores, which pass through their digestive tracts, are released in their dung. The spores of this fungus are preserved readily in lake sediments, and stratigraphic changes in the abundance of this fungus in Pleistocene and Holocene sediment sequences have been used as a proxy to define megafaunal presence, decline and extinction globally.

Sporormiella Spores 

Problems

The presence of Sporormiella is not exclusive to large herbivore dung and has been found in the dung of small herbivores as well, such as hares. Thus, it is difficult to use Sporormiella as a sole and direct proxy for megafauna abundance unless specific species of Sporormiella associated only with large herbivores can be identified.

A stratigraphic decline in Sporormiella does not necessarily indicate a decline in megafauna. For example, Sporormiella is more abundant near lake shores than in the middle of lakes, so a decrease could simply mean a rise in the lake level. Sporormiella may also be preserved to varying degrees depending on type of lake sediment, lake levels, etc. A related point is that the absence of Sporormiella does not indicate the absence of herbivores – some modern day sites with abundant livestock have been shown not to contain Sporormiella in Davis and Shafer’s (2006) study. Thus, Sporormiella needs to be calibrated to other indicators of large herbivore population and is non-conclusive on its own.

Some academic papers must be viewed with some scepticism due to methodological over-reliance on this particular proxy. For example, in a Gill et al (2009) paper, a decline in Sporormiella in a Lake Appleman core in Indiania which starts from 14,800 years ago and which pre-dates a major change in the pollen assemblage is used to conclude that the late Pleistocene megafauna extinction was not caused by (usually climate-linked) vegetation changes. They also show that charcoal frequency increased at that site, indicating that human factors (like vegetation burning) were probably behind the extinctions. However, the tail end of the Sporormiella decline is also associated with a change in lake sediment size, which may reflect changes in the sediment input and hence catchment area of the Sporormiella source, rather than megafauna decline.

Conclusion  

While this analytical technique is certainly promising in contributing to research on Pleistocene megafauna extinction, it still needs to be refined. What is also important is to avoid complete reliance on just one proxy; the conclusions drawn from using this proxy should be calibrated to other indicators of megafauna abundance.

References

Davis, O. K. and Shafer, D. S. (2006) ‘Sporormiella fungal spores, a palynological means of detecting herbivore density’, Palaeogeography, Palaeoclimatology, Palaeoecology, 237, 1, pp. 40-50.

Feranec, R. S. et al (2011) ‘The Sporormiella proxy and end-Pleistocene megafaunal extinction: A perspective’, Quarternary International, 245, 2, pp. 333-338

Gill, J. L. et al (2009) ‘Pleistocene megafaunal collapse, novel plant communities, and enhanced fire regimes in North America’, Science, 326, pp. 1100-1103

Island Extinctions and the Case of Madagascar

Island Extinctions

Islands which were only inhabited by humans after the late Pleistocene megafauna extinction event offer interesting ‘control experiments’. A compelling argument for human factors driving extinction is the time lag between continent extinctions and those of nearby islands (Martin and Steadman 1999). For example, New Zealand’s moas lasted 30,000 years longer than Australia’s extinct giant bird, the mihirung, which went extinct during a Australia’s wave of rapid megafauna extinction (before the late Pleistocene).

The argument for prey naiveté on early contact with human hunters finds support also in the remarkable tameness of wild birds in remote islands which were undiscovered by prehistoric explorers. These include the Galapagos, Christmas Islands, etc. Galapagos’ avifauna were unafraid of humans, as depicted in historic accounts from 17^th century sailors who discovered the island (Martin and Steadman 1999).

In this blog post I look closer at the island of Madagascar.

Madagascar

This paper by Burney et al (2004) discusses how islands can be used to better understand megafauna extinction. Madagascar is interesting because it is the last place on Earth where megafauna went extinct prehistorically – extinctions in which humans had some part to play in most other parts of the world occurred much earlier, during the late Pleistocene or even earlier in Australia. Madagascar offers a relatively fresh record of paleoecological change, since humans only arrived in the late Holocene, about 2,000 radiocarbon years ago. Very little is known about how or why this group of Iron Age people came to Madagascar. Evidence of the first humans can be shown by human-modified megafauna bones, such as cuts on the fossilized bones which show removal of flesh from bone by a sharp object.

Very little is known about the late Pleistocene biota in Madagascar. The amount of data increases greatly for megafauna in the mid-Holocene, where conditions for fossil formation probably became more favourable. For example, most lakes and swamps along the coastline formed only after around 5,000 radiocarbon years ago. Nevertheless, there were major climatic changes in in the late Pleistocene and pre-human Holocene, most of which were survived by most of the megafauna. Although there have been range shrinkages, there were no extinctions. Of the 9 genera of extinct lemurs dated, only one is not securely dated to the human period. Some examples of major climate change are as follows: 

·        

•      20,000 radiocarbon years ago (LGM): widespread dessication occurred. Lake Alaotra, a large lake in humid eastern Madagascar, was dramatically reduced in area if not completely dry during that period. 
•      10,000 calendar years BP: At another site called Trtrivakely, pollen evidence shows the nearly complete replacement of heath vegetation with wooded grassland.

A drastic decline in megafauna, as shown by a huge decrease in Sporormiella in sediments at 1700 radiocarbon years BP (within a few centuries of first human contact), was observed. Sporomiella is a fungus that grows in the dung of large plant-eating mammals, and it releases spores which are preserved in sediments. The presence of these spores is used as a proxy for the presence of megafauna. Humans could have hunted these megafauna or altered their habitat. Before humans arrived, these herbivores had very few predators other than large crocodiles. Although there is very scant evidence for direct human hunting of megafauna, as in many other continents where Pleistocene megafauna extinction has occurred, another way in which humans could have contributed to the decline is through altering the existing fire regime by further increasing fire incidence through burning for settlement and agriculture and through hunting of plant-eating megafauna. The decline of large herbivores such as giant hippos caused ground litter to accumulate, feeding more fires. This can be shown by charcoal peaks above background values, first occurring in the South West where humans first settled, and then spreading outward over Madagascar. Nevertheless, the extinction pattern on Madagascar does not support a Blitzkrieg hypothesis. There is an overlap of around 2,000 years from earliest human evidence to the last occurrence of extinct megafauna.

The chart (Burney et al 2004) shows a summary of events in Madagascar:

My Thoughts

The evidence from Madagascar is indeed intriguing and I feel it does make the argument for human factors in the extinction of megafauna more compelling. Madagascar’s physical geography and vegetation is very similar to Africa such that it is referred to as an ‘Africa in miniature’, and it is probably safe to assume it went through similar climate changes and vegetation responses as Africa. The megafauna on this island certainly survived all these before the humans came, after which they experienced dramatic decline and finally, extinction. The fact that it is an island is important; in a previous post I mentioned the reason for why Africa still has such a large diversity of megafauna left is that it is larger and probably provided more refugia for megafauna. Madagascar probably provided more limited refugia for the stressed populations of megafauna.     

References

Burney, D. A. et al (2004) ‘A chronology for late prehistoric Madagascar’, Journal of Human Evolution, 47, pp. 25-63.

Martin, P. S. and Steadman, D. W. (1999) ‘Prehistoric extinctions on islands and continents’ in MacPhee, R. D. E. (ed.) Extinctions in Near Time: Causes, Contexts and Consequences, New York: Kluwer Academic/Plenum, pp. 17-50

Dissecting the Hyperdisease Hypothesis

I decided to do a post on the disease hypothesis after Josh from http://no-mammoths.blogspot.co.uk/ suggested an interesting paper by Rothschild and Laub (2006). Here is the link to his post on this topic specifically. The hyperdisease hypothesis proposes that humans and their domesticates introduced novel hyperdisease to vulnerable populations of Pleistocene megafauna who had never encountered such diseases before and whose bodies were therefore unable to cope. Since migrations of animals from Europe to North America were not uncommon before the period we are studying, it is more likely that humans and their domesticates were the disease vectors (Lyons et al (2004).

Tuberculosis and the American Mastodon 
Rothschild and Laub (2006) have suggested that new evidence for the hyperdisease theory has surfaced in the form of bone alterations from infectious tuberculosis found in just over half of 113 mastodon skeletons in the Western Hemisphere. Since not all animals infected with tuberculosis develop this bone alteration, it must follow that probably almost all of the mastodon population must have been infected with tuberculosis. The disease thus qualifies as a pandemic in the sense that it had an extremely high infection rate. Besides, it has a persistent presence in the fossil record from around 34,000 – 10,000 years BP, establishing that it must have been present in the late Pleistocene period. 

However, there is a difference between infection and mortality – the disease was not necessarily fatal. Rothschild and Laub (2006) hypothesize that this disease may have weakened mastodons in the face of climate change and human impacts in the late Pleistocene, further stressing their populations. While the disease could have remained latent, the environmental stresses of that period could have resulted in a loss of latency, increasing mortality. However, it is unlikely that the hyperdisease could have been a major factor in the extinction event. 

The Modern Day West Nile Virus: A Proxy for the Mystery Hyperdisease?

I also found another paper by Lyons et al (2004) which proposes some criteria for the hyperdisease theory to be plausible. 

1. It must be able to survive in a carrier state in a ‘reservoir’ species when there are no susceptible hosts to infect.
2. It must have a very high infection rate.
3. It must be extremely deadly with a 50-75% mortality rate
4. It must be able to infect multiple host species without infecting humans

Lyons et al (2004) use the West Nile Virus in birds, a disease which has seen recent introduction and spread in North America’s bird population, as a proxy to test this hypothesis as it appears to fulfil all of the above criteria of a hyperdisease.

One of the unique features of the late Pleistocene megafauna extinction event was its size-selectivity – smaller and medium-sized animals were largely unaffected. Thus Lyons et al (2004) have tried to test if West Nile virus causes such size-selective infections in birds. It can be shown that it does not, as infection rate increases positively with body size (Fig. 1) and infection occurs across a range of body sizes. This contrasts with the pattern shown by late Pleistocene mammal extinctions. The x-axis of the graph shows the size category of the bird species infected by the West Nile virus and those of the mammals which went extinct during the late Pleistocene. It has been re-scaled for mammals since they contain a much larger range of body masses. Each filled square shows the percentage of species pool in each size category infected by the virus or that went extinct. 

Fig 1 (Lyons et al 2004)

Some have argued that large body size makes species inherently vulnerable to extinction because of life history factors, e.g. low reproduction rates which make it harder for populations to recover from mortality caused by disease. However, Lyons et al (2004) counter-argue that if this is true, then larger species should have high extinction rates relative to smaller species over evolutionary time, which is not the case. 

The Verdict?

I find the hyperdisease hypothesis unconvincing so far and I think it is only considered seriously as a factor in the extinction event because of the general lack of evidence surrounding even the exhaustively-researched hypotheses of climate change and human hunting (e.g. lack of kill sites). However, the even more severe lack of evidence in the hyperdisease hypothesis is even more disturbing. The only known modern disease which fulfils the 4 criteria of a hyperdisease capable of wiping out megafauna during Pleistocene times, the West Nile virus, itself cannot be shown to cause the size-selective extinction pattern in modern day bird populations in North America. Besides, the paper by Rothschild and Laub (2006) only shows a pandemic-scale disease in one type of animal, the American mastodon. It is difficult to find equivalent disease explanations for all other megafauna species killed during the late Pleistocene (mammoth, for example, were not affected by this disease and were close cousins of the mastodon). Therefore, I conclude that hyperdisease is an unlikely explanation for the megafauna extinction event we are studying here.

References

Lyons, S. K. et al (2004) ‘Was a hyperdisease responsible for the late Pleistocene megafaunal extinction?’, Ecology Letters, 7, pp. 859-868

Rothschild, B. M. and Laub, R. (2006) ‘Hyperdisease in the late Pleistocene: validation of an early 20th century hypothesis’, Naturwissenschaften, 93, 11, pp. 557-564