Ecosystem Physical Structure: Part 2, Case Studies

Megafauna in Africa:

A prime example of a remaining ‘natural’ megafauna regime, consisting diverse bottom-up- (nutrient-limited) and top-down-regulated communities in which herbivores suppress the accumulation of woody vegetation (reducing woody species cover by between 15-95%), thereby opening habitat for grasslands and forbs is, as the avid reader would have guessed from previous posts introducing megafaunal prehistory and contemporary megafaunal abundance, Africa (reduce woody species cover by 15-95%) which gives (as mentioned in the previous post re African elephants) about as good an indication of how megafauna affect ecosystem physical structure as is available. As such many studies analysing such effects are based here, yielding some interesting results.

Asner et al2015 compared the effects of a variety of environmental and biotic factors on treefall rates and patterns in woody plant canopies in Kruger National Park using Light Detection and Ranging – linked a mean biennial treefall rate of 8 (or 12% of total) per hectare strongly to Elephant density, matched only by spatial variation in elevation and soil. This is a pretty astounding result, with megafauna effectively proving as strong a shaper of an ecosystem as their abiotic counterparts.

 Keesing & Young 2014 exhibited a modern example of the effects of megafaunal loss with direct negative impacts for humans in their study of the Kenya Long-Term Exclosure Experiment, where the effects of removal of different combinations of domestic (eg cattle) and native grazers was compared read megaherbivores). Removal of large grazing mammals led to increased abundance in a small grazing mammal, the pouched mouse, attracting venomous snakes (to prey upon them) and fleas (and thus transmission of flea-borne pathogens), and leading to the decimation acacia seedlings – all with potentially substantial and undesirable consequences for humans.

Megafauna in Australia:

Rule et al2012 used data derived from sediment cores dating back 130kya concerning pollen, Sporormiella and charcoal to reconstruct the ecological consequences of Australia’s megafaunal loss. Vegetation, fire, and climate conditions are reconstructed via the proxies of pollen and charcoal, herbivore activity from Sporormiella found in their dung (chiefly associated with large herbivores, thus good proxy for large-herbivore biomass). Analysis of these cores not only suggests human arrival as the cause of Australian megafaunal extinction (Sporomiella decreasing drastically shortly after the arrival of humans ca 40k years ago whereas levels at other substantial climate driven shifts showed no significant variation) but also that this loss triggered the replacement of mixed rainforests with sclerophyll vegetation through relaxed herbivory and increased incidence of fire (note: these factors are by no means independent, megaherbivores often work in competition with or in congress with fire regimes, to great effect) – together consisting an ecosystem shift as large as any ecological effect of climate change over the last glacial cycle in the region, exhibiting the magnitude of megafaunal extinction implications on ecosystem physical structure.

Johnson &Rule et al 2015 later revisited the above discussion in greater detail, arguing that such megafaunal extinctions of large herbivores in fact have effects of varying magnitudes on ecosystem physical structure, as demonstrated by contrasting how a different site reacted to the Australian megafaunal extinction event upon which the previous paper’s conclusions had been drawn. In stark contrast to the warmer, more humid site studied in 2012, another site showed no evidence that the decline of megafauna triggered a change in vegetation or increased fire.

The conclusion that the magnitude of ecological responses to Pleistocene megafaunal extinction varied geographically per regional differences in climate seems an immediately evident, but is well demonstrated here, with a proposed hypothesis that the lack of change was because climatic constraints of cold and low atmospheric CO2 placing severe limits on plant growth, prevented the vegetation from responding to relaxation of herbivory in a substantially measurable fashion. Furthermore, it crucially demonstrates the complexity of the problem of untangling the effects of megafaunal loss, just as with the complexity of untangling the causes of megafaunal loss (although these Sporormiella based studies do seem quite conclusively to point towards a human perpetrator of Australian megafaunal collapse).

These are but a selection of case studies evincing the roles megafauna play in shaping their physical surroundings and the implications of their removal, such effects have been shown all around the world and will continue to be revealed in further detail and clarity, likely even unfolding before our eyes in cases where extant megafauna are under threat.