Ecosystem Trophic Structure Part 2 / Overkill v Keystone Hypotheses

Ecosystem Trophic Structure: Continuation

Last week we had a look at how megafaunal carnivores can affect ecosystem trophic structures, focusing on a case study of the reintroduction of wolves into Yellowstone National Park. Also readily apparent in that study were the crucial roles that megafaunal herbivores have, both on ecosystem physical structure (to be covered in greater detail later), as evinced by the direct limiting effects elk had on cottonwood, aspen, willows and other woody browse, and in terms of suppression of smaller herbivores through competition for resources, as evinced by the rise in beavers, ravens, magpies etc, subsequent to the fall in elk numbers. 

Interestingly, this seems to paint megafauna as both ecological hero and villain, with increases in wolf populations exerting trophic pressure to maintain a healthy, diverse and robust ecosystem while increases in elk populations had the opposite effect. This may be too simplistic a viewpoint, the general consensus being that loss of megafauna leads to increased abundances of smaller animals and thus simpler ecosystems with fewer interactions between species, shorter food chains and species occupying the same functional roles, and therefore less resilience to change and undue pressure (such as those exacted by human encroachment for example) Malhi et al 2015, but perhaps this aspect warrants further exploration in a future post. The magnitude of megafaunal influence on ecosystem structure is, however, undeniable.

The Overkill Hypothesis

Another interesting prospect indirectly raised links back to a couple of posts ago when the architects of late-Pleistocene megafaunal demise were being discussed. One prevalent explanation for human-caused megafaunal extinction is the overkill hypothesis, essentially postulating that the rate of human hunting of megafauna exceeded the megafaunal birth rate, leading to megafaunal extinction. This was once well thought of, and reasonably compelling when considering the evidence of megafauna going extinct in the presence of modern humans and largely only surviving to the Anthropocene in areas less inaccessible to humans. Nowadays this hypothesis is increasingly seen as outdated and unsophisticated, assuming, contrary to more modern zoological evidence, that naïve prey don’t avoid novel human predation, among other issues. 

In the case of the wolves of YNP, humans were in fact responsible for their localised extirpation, believing them to be undesirable predators and hunting them without legal restraint well into the 20^th century. However, this is an unrepresentative and localised case, and a more credible explanation for anthropogenic-linked megafaunal loss lies in the not dissimilar concept of loss of keystone species.

Loss of keystone species, also variously described as foundation species or ecosystem engineers, can induce collapse of trophic cascades, leading to habitat change, altering species abundance and leading to further extinctions. 

Keystone Case Study: Extinction of Stellar's Sea Cow

Estes et al convey a strong argument for this hypothesis in their exploration of the extirpation of Stellar’s sea cow from the Commander Islands. Occurring not long ago, between 1741 (when Stellar’s sea cows were abundant in the Commander Islands, although largely extinct in other areas previously peopled with human hunters) and 1768 (when they were completely extinct from the islands) in a thoroughly researched and well understood coastal kelp forest environment, and under the observation of modern humans, this extinction event potentially entails a representative model for past megafaunal extinctions occurring as a result of indirect human effects on keystone species.

Previous arguments for complete overkill of Stellar’s sea cow were not based on robust evidence (presence of fur-hunters spuriously used as indication of sea cow hunting instead of solid evidence) whereas kelp forest collapse (the main source of food for Stellar’s sea cow) after sea otter loss has been well proven. 

Estes et al instead propose that extinction of the sea cows was due to a myriad of factors rather than just human intervention. Their argument is backed by; contemporary parallels of sea-otter–sea urchin–kelp dynamics mirrored elsewhere in the north pacific oceans and robust evidence of extensive otter-hunting for the Pacific maritime fur trade beginning in 1743. There is also data on the responses of dugongs (the closest living relatives of Stellar’s sea cow) to food reduction showing that extinction of the sea cows would have occurred even without any human predation (although this likely did occur to a certain extent). The process proposed is as follows:

-> Hunting to virtual extinction of sea otter (keystone species) in the Commander Islands in 1700s 

-> Reduced otter predation on sea urchins 

-> Increased sea urchin densities 

-> Consumption of kelp forests above critical threshold 

-> Kelp forest collapse 

-> Replacement of kelp forests by urchin barrens ~ loss of food base for Stellar’s sea cow 

-> Complete extinction of Stellar’s sea cow within decades

Closing Thoughts 

Such keystone species trophic cascades and habitat changes resulting from the loss of an ecological engineer are likely to be a widespread feature of extinctions and co-extinctions both past and future. This has been demonstrated by with numerous other examples recently proposed, such as the extirpation of wolves in the continental United States as responsible for increased coyote numbers, thus decreasing snowshoe hare numbers and in turn leading to localised extinctions of lynx. It is clear that such interaction dynamics between species can be crucial enough to lead to significant changes in species abundance, even extinctions, if significantly disturbed. Detailed knowledge of such interactions in late-Pleistocene ecosystems is not readily available, but such chain effects seem as/more worthwhile to consider than overkill hypotheses when faced with evidence of megafaunal extinction in temporal proximity to modern human arrival.