characteristic groups that define that community. Communities exist within the
abiotic physical and chemical environment, and these together form the ecosystem.
All ecosystems are maintained by inputs of energy and some nutrients from outside
the system, but some receive most of their nutrients from outside (allochthonous
supply), such as leaf debris from riverbanks for stream communities, or the rain
of detritus to the abyssal zone of the ocean; while other ecosystems are more self-
contained, depending on nutrient supplies within them (autochthonoussupply).
Ecosystems can be small or large, ranging from a stream complex or watershed to
the 25,000 km^2 Serengeti, or even segments of the vast boreal forest of Canada or
Russia. Although ecosystems are open, they are identified by some form of bound-
ary across which the combination of abiotic and biotic factors changes. The study of
the large-scale ecology of ecosystems is called landscape ecology.The early classic studies of plant communities suggested that communities of plants
existed as discrete units, something like a superorganism. These units had sharp bound-
aries. It was generally agreed that sharp divisions in the environment – different
geology, soils, or other environmental factors – caused discrete boundaries between
communities. However, it was also suggested that such boundaries could be caused
by groups of coevolved species occurring together. Another school of thought sug-
gested that plants generally existed independently of others so that a gradual change
of species took place along a gradient in the abiotic environment – for example,
gradients of moisture, or of altitude and temperature on mountains, or of exposure
and salinity on the seashore (Whittaker 1967). Present understanding of gradients
(Austin 1985) suggests that species do form a continuum along gradients, but not
uniformly. Groups of species appear and disappear together for two reasons. First,
some species depend on each other. Where plants go so do their associated animals,
and therefore these groups are found in similar places. Second, the abiotic gradient
is not usually uniform: there is often a break or rapid change in geology, soil sub-
strate, or exposure, and at these points there are rapid changes in species complexes.Where a species is found is determined by its tolerance and adaptation to the abi-
otic environment. This is the fundamental niche, which is constrained by the biotic
processes of competition and predation to form the realized niche. Sections 9.4 –9.6
give a more detailed explanation of niches, and how species may divide up the niche
space along gradients.A community can be divided into trophic levels, defined as the location of energy
or nutrients within a food chain or food web (Fig. 21.1). In terrestrial systems we
describe these as plants that capture energy from the sun (autotrophs), herbivores
that feed on plants, carnivores that feed on herbivores or other carnivores (the last
two levels being called heterotrophs), and detritivores that receive dead products from
all other trophic levels. Energy and nutrients enter a system from outside, and flow
through the system. Energy is progressively lost through the levels but much of the
nutrients cycle back via the detritivores into the plants. A fraction is lost by water
leaching out of soils, runoff into rivers, burning, and wind transport, etc. In a stable
system, inputs and outputs at all levels must balance.
Indirect effects are those where one trophic level affects components below or above
it. There are two forms of this: trophic cascades where linear predation influences366 Chapter 21