Geoscience Reference
In-Depth Information
Plate 22.4
Muskoxen on Devon Island, Arctic Canada. Hunting nearly made this arctic herbivore extinct, until a law protected
them in 1916. Now the Canadian populations are used to restock extirpated herds in Alaska and Russia.
Photo: Bill Barr
stable (
Plate 22.4
)
and species-rich communities which
are unstable. Some species-poor communities can be very
resilient, with the plants recovering quickly from an
unusual drought, for example, as in the species-poor
heather moorland of upland Britain, in contrast to nearby
species-rich limestone grassland communities, which can
show very low resilience.
Other ecologists emphasize the value of links which
have evolved over time (
co-evolutionary links
); human-
modified systems like agro-ecosystems have no coevolu-
tionary links between the interacting species. Farms and
gardens are not really ecosystems but, rather, haphazard
collections of species selected by the farmer and gardener.
It should also be borne in mind that, among natural com-
munities, stable complex systems have survived whilst
unstable complex systems have disappeared. Other work-
ers have discovered that in some ecosystems the more con-
nected the ecosystem, and the larger the connectance, small
variations can become amplified as they propagate
through the well connected system. This of course is
counter to the hypothesis of MacArthur.
The picture regarding diversity and stability is complex
and as yet unclear. Recent work has been concentrating
on the structures of food webs (Montoya
et al.
2006). The
question has been asked: are ecosystems like the internet,
or are they more like galaxies, with clusters of highly
connected species? 'Yes' superficially might be the answer,
but ecosystems are unique in being constrained by
processes of predation, competition and mutualism.
Ecological networks link many species together, and the
links tend to be nested, or in 'blocks', as in May's models.
Within the pattern of links there are denser clusters of
links, most likely the result of co-evolution, which give
mutualistic specialization between plants and their
pollinators or seed dispersers. If some highly connected
species disappear, it may trigger the loss of a cascade of
species dependent upon them. These highly connected
nodes are
keystone species
in the ecological network.
Like keystones in buildings, the whole structure collapses
without them. For example, the loss of a top predator,
the jaguar, in the South American rain forest, allows the
dominant herbivore to outcompete other herbivores that
used to coexist with it in the presence of the predator.
Computer models of temperate forests show that the
extinction of just 7 per cent of the most highly connected
and general-feeding insects results in the extinction of 50
per cent of the remaining species (Montoya 2007).
Figure
22.11
is a graphical representation of the food web of the