92 Natural systems and greenspaces
whereby one component stimulates a second which inhibits the first, resulting
in some stability for both components. In this case, more prey leads to more
predators, which leads to fewer prey, which leads to fewer predators, which leads
tomore prey, which leads to more predators, and on and on. This predator--prey
cycle does not produce constancy in either component, but rather each cycles up
and down in numbers over time, and neither goes extinct. The persistence and
regular cycling is a form ofregulationorstabilityforboth populations. Many other
factors in an ecosystem affect predator and prey populations, so the relative
importance of predator--prey cycles varies widely by species and location.
Herbivores and plants are somewhat analogous to a predator--prey cycle. In
addition natural selection plays a key role. Over many generations a plant species
adapts to the chewing and sucking of herbivores by changing the chemistry of
leaves and stems. Chemicals either unpalatable or toxic to the herbivore species,
i.e.,defensive chemicals,accumulate, which effectively protects the plants from
being completely defoliated. Indeed leaf chemistry varies greatly from species to
species. Tree species planted along streets help determine what and how many
herbivore insects (and seed-eating birds) are present, and thus what and how
many insect-eating birds are present. When periodic explosions of an insect pop-
ulation occur (Robinson 1996 ), certain plant species are extensively defoliated,
while others with the appropriate chemical defenses are barely touched.
Public health issues in urban regions often depend on the growth rate and dis-
persal of certainspecies vectorswhich rapidly spread disease. In medieval Europe,
there were rats that carried the fleas that carried the plague bacteria that killed
thepeople walled up in cities. Urban wetlands and community gardens (allot-
ments) in the tropics may support mosquito populations that carry malaria
protozoa that kill people (e.g., Harare, Zimbabwe). Understanding life cycles of
thedifferent species, and especially the spatial patterns affecting dispersal or
transmission rates, are keys to public health solutions.
Finally,species movement from site to site by walking, flying, water trans-
port, or wind transport is a key ecological characteristic. It is usually difficult for
seedlings to grow to maturity close to their parent, and mostplant dispersalis by
seeds carried and dropped elsewhere by animals or wind. Many vertebrates have
aterritoryaround the den or nest that is defended, mainly against other individ-
uals of the same species. Thehome rangeis a larger area used in daily movements,
especially foraging for food. When young reach subadult stage,animal dispersal
normally occurs, where the individual looks for a mate and establishes its own
home range at some distance. Also,migration,acyclic movement between loca-
tions, helps some animals avoid difficult environmental conditions and access
beneficial conditions. Large semi-natural areas in an extensive built area tend to
be valuable feeding and resting spots for migrating birds. However, home range