90 Natural systems and greenspaces
birth rate, which tend to regulate or dampen fluctuations in population size.
Sometimes a population like rabbits, deer, and kangaroos exceeds the carrying
capacity, causing severe vegetation damage and soil erosion, so the population
crashes and subsequently rebuilds slowly. An unusually high immigration rate or
proportion of pre-reproductive individuals is an early indicator of a population
that may overshoot the carrying capacity and damage the environment.
Consider one large population of your favorite species in a large area; then
consider four small populations on four separate patches with individuals never
moving between patches (McCullough1996,Hanskiand Gilpin1997). Ametapopu-
lationis the intermediate case: its individuals are distributed in separate patches,
but organisms move from patch to patch. This pattern is particularly impor-
tant in urban regions where the land is fragmented by highways and residen-
tial areas, which convert large natural populations into interacting small sepa-
ratesubpopulations. Small populations are more likely to disappear (go locally
extinct) due to demographic fluctuations and inbreeding genetic deficiencies.
Metapopulation dynamicsrefers to the rate of loss and recolonization of subpopu-
lations on the patches. Characteristics of the patches largely determine species
loss, whereas characteristics of the matrix and corridors between patches largely
determine species recolonization. Metapopulation formation, species loss from
patches, and species recolonization are all readily affected by land-use pattern
and planning. For example, maintaining at least one large patch is particularly
effective at reducing metapopulation dynamics, because individuals are likely
tocontinually disperse outward from it to the small patches.
Natural selectionis the genetically based change in a population over time, a
process with four key components: (1) overpopulation (more individuals than can
survive in the next generation); (2) variation (individuals that differ genetically
and in their use of resources and environmental responses); (3) competition
(individuals compete for limited resources); (4) and survival of the fittest (the best
adapted or most genetically fit individuals survive and pass their genes to the
next generation). So-calledK-selected specieshave large individuals and reproduce
slowly, whereasr-selected specieshave small individuals and reproduce rapidly.
The former tend to dominate a site for a long time, while the latter rapidly
colonize disturbed sites. Natural selection is a central process in evolution and
in forming new species, includingendemic specieswhich only naturally exist in
one area. The Capetown (South Africa), Perth (Australia), and Concepcion (Chile)
areas are rich in endemic species.
Adaptationsare attributes genetically determined over generations that pro-
vide an advantage or increased fitness to an individual or population. For
instance, when industrial pollution blackened tree trunks in Britain, light-
colored pepper moths on the bark were increasingly visible and subject to bird