Ecosystem, community, and population ecology 89and mid stages, is a well-known goal in management of certain natural vegeta-
tion.Retrogressive succession, where human effects progressively degrade natural
vegetation is particularly prevalent in urban regions, and can be reversed by
removing the cause and in some cases planting species of the next successional
stage.
Formore than a decade ecologists have basically dropped ‘‘balance of nature”
and equilibrium community from their vocabulary. Instead they emphasize the
non-equilibriumnature of nature, since the scientific evidence overwhelmingly
highlights change as the norm.Disturbanceas something causing a rapid major
change in the community may come from inside, e.g., a pest outbreak or fire, or
from outside, e.g., overhunting or tornado blowdown. Thus, like the gap dynam-
ics at a fine scale, disturbance-inducedpatch dynamicsmeans that the vegetation
at a broad scale resembles a mosaic of successional stages (Pickett and White
1985). Patch dynamics maintains high biodiversity. Indeed the prevention of
disturbance, rather than disturbance itself, is the threat. Furthermore some
disturbance-adapted species,such as fire-adapted species, evolved with and essen-
tially depend on periodic disturbance. Without disturbance these species are
outcompeted and gradually disappear. Patch dynamics is particularly important
in natural landscapes in the outer portion of urban regions.
Natural populations
Forming the basic unit of a natural community, apopulationrefers to
all the individuals of a single species in an area (Smith1996,Ricklefs and Miller
2000,Townsendet al.2000). The pandas of a mountain range and ‘‘polkadot”
palms of a large swamp are populations.Exponential growthrefers to a popula-
tion that increases in proportion to its size. Populations differ in their intrinsic
genetically determined rate of growth, basically the difference between birth
rate(natality) and death rate (mortality). Sex ratios and age structure (e.g., the
proportion of pre-reproductive, reproductive, and post-reproductive individuals)
are internal attributes affecting growth rate.Doubling time,howlong it takes to
double the population size, is often a useful integrator of these variables.
External factors also affect population growth rate. The balance between
immigration and emigration rates is important for some populations.Carrying
capacityrefers to the maximum number of individuals an area can support. As
population size gradually approaches carrying capacity,environmental resistance
becomes stronger and growth rate decreases, eventually reaching zero when pop-
ulation size reaches carrying capacity. Environmental resistance thus gradually
changes the exponential J-shaped curve into an S-shaped curve. Environmen-
tal resistance includes weather and many human effects, as well as inherent
density-dependent factorssuch as increased mortality and hormonal limitation on