Environmental Engineering Reference
In-Depth Information
provided ideal conditions for smaller grazers, which lived constantly in a landscape
of fear (Riginos and Grace 2008) and minimized their vulnerability to carnivores
by feeding in the open, where they could better detect the approach of predators.
In turn, grazers reduced i re hazard by preventing the accumulation of dry phyto-
mass, and their wastes intensii ed nutrient recycling. Nonglaciated temperate Pleis-
tocene ecosystems were thus mostly covered by grasses, forbs, and sedges. As a result
of megafaunal extinction, these mammoth steppes (relatively diverse in terms of
herbaceous species and supporting a high diversity of grazers and associated preda-
tors) were largely replaced by mossy or shrub tundra and coniferous taiga at higher
latitudes, and by deciduous forests further south.
In many areas the former savanna or parkland was replaced by transitory com-
munities of spruce and broadleaved trees that have no modern counterpart (Johnson
2009a). These forest ecosystems were less diverse, in terms of both plant and animal
species, than the open landscapes they replaced, and their primary productivity
(typically no more than 10 t/ha) was only moderately higher than that of the grass-
lands that used to grow in the same environment. But the new forests stored an
order of magnitude more phytomass than did the grasslands: even in a relatively
sparse temperate or boreal forest there would be about 100 t/ha, and typical rates
would be around 200 t/ha, while even rich grasslands would store only on the order
of 20 t/ha.
And with a much higher accumulation of phytomass and often thick litter layer,
the new forests were also much more prone to i res than the open landscapes
that preceded them (Johnson 2009b). But it would be simplistic to claim that the
elimination of megafauna led directly to a more l ammable world. Marlon et al.
(2009) used charcoal and pollen records to appraise the changes in North America's
i re regimes during the last glacial-interglacial transition of 15,000-10,000 years
ago and found that the timing of large increases in i re activity was not coincident
either with the changes in human population density or with the extinction of the
megafauna.
Obviously, this generalization (based on 35 samples for the entire continent)
leaves room for attributing some of the increase to human actions, but again, any
such attribution would be an indefensible guess, and we must conclude that there
is no way to quantify the net impact of the most likely combination of anthropo-
genically driven megafaunal extinction, vegetation change, and increased i re fre-
quency. Perhaps the only defensible conclusion is that the net impact of human
actions on the Earth's prehistoric biomass—contributing to the megafaunal extinc-
tion (hence reducing the global terrestrial zoomass stores) that led to the expansion
