84 Natural systems and greenspaces
food chain or pyramid has several important implications in urban regions.
First, rarely are there more than four or five trophic levels in any terrestrial
ecosystem. Most animals, and normally most animal species, present are her-
bivores. Second, not much energy remains for top predators, which are often
scarce.
Third,biological magnification,theprogressive increase and concentration of a
particular material through the food chain, means that predators (and especially
top predators) get a concentrated dose. If the material is toxic, predators and top
predators therefore are impacted the worst. Some heavy metals and long-lived
pesticides in urban regions biomagnify to toxic levels, just as some radioisotopes
from the periodic ‘‘tiny” releases of radioactivity from nuclear power plants
accumulate through the food chain to lethal levels in predators.
Afood webcombines the food chains in an ecosystem and indicates where each
species fits, i.e., what it eats and who eats it (Morin1999). That permits evaluation
of the vulnerability of a species, as well as the stability of the ecosystem. A bird
species that only eats seeds of one tree species disappears if the tree does not
flower or iskilledbyapest,whereasabirdthatfeedsonthreetypesofseeds
may readily persist in a park. Food webs also often have many feeding links
lower and fewer links higher in the web, which, overall, results in less stability
for higher trophic levels.
In view of the several preceding threats to predators and top predators, a final
principle is especially poignant and important. Some species at higher trophic
levels are calledkeystone predatorsbecause they exert an influence or control (far
in excess of their limited biomass or abundance) over many species lower in the
food web, as well as the ecosystem as a whole (Figure4.1). For example, in San
Diego (California) parks, the keystone predator, coyote (Canis latrans), controls
thepopulations of mid-size mammals (house cats, skunks, opossums) which
feed on the young of, and tend to eliminate, many native bird and mammal
species (Soule 1991 ). Thus the presence of the top predator, even at a low den-
sity, helps maintain a diverse native fauna and a stable natural community or
ecosystem.
The decomposers in the ecosystem primarily break down dead leaves and
wood into mineral nutrients and heat. A wholedecomposerordetritus food web
operates in dead wood and particularly in the leaf litter and humus on the
ground. The species include bacteria, fungi, and numerous types of tiny ani-
mals, which greatly affect urban soil conditions, from pH to water and humus
availability. The urban heat-island effect, as well as global warming, accelerates
decomposition, thus decreasing the leaf litter and humus cover that protects
and enriches soil.
The final big subject of ecosystems is the material ormineral nutrient (biogeo-
chemical) cycles,especiallyofsulfur,carbon, phosphorus, and nitrogen, all major