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