Tuesday, 1 January 2013

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. 

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.   

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 

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