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.
I never got to see these beasts when I went to Australia, think I was a bit too late... But your lyrical prose and magical descriptions are taking me there <3
ReplyDelete