Geoscience Reference
In-Depth Information
mals probably result in increased reproduction and survival, whereas bad years
result in poor reproduction and high mortality. Including a negative covari-
ance of birth and death rates (or a positive covariance between birth and sur-
vival rates) in the model results in an even bigger impact of temporal variation
on persistence. That is, the bad years are really bad because of both poor repro-
duction and high mortality, and the good years are really good. The net effect
of this negative covariance of birth and death rates is to decrease persistence.
SPATIAL VARIATION
Spatial variation is the variation across the landscape that is normally associ-
ated with populations. Factors causing geographic variation include geologic
differences that affect soil type, and thus habitat, and weather patterns (e.g.,
differences in rainfall across the landscape). If the immigration and emigration
rates are high across the landscape, so that subpopulations are depleted because
of local conditions, high spatial variation can lead to higher persistence. This
is because the probability of all the subpopulations of a population being
affected simultaneously by some catastrophe is low when high spatial variation
exists and spatial autocorrelation is low. High positive spatial autocorrelation
causes low levels of spatial variation, whereas high negative spatial autocorrela-
tion causes high levels of spatial variation, as low levels of spatial autocorrela-
tion generally do. In contrast, with low spatial variation (and hence high pos-
itive spatial autocorrelation), the likelihood of a bad year affecting the entire
population is high. Thus, in contrast to temporal variation, where increased
variation leads to lowered persistence, increased spatial variation and low spa-
tial autocorrelation lead to increased persistence, given that immigration and
emigration are effectively mixing the subpopulations. If immigration and emi-
gration are negligible, then spatial variation divides the population into
smaller subpopulations, which are more likely to suffer extinction from the
effect of demographic variation on small populations. The combination of
temporal and spatial variation is called
environmental
variation. Both dictate
the animal's environment, one in time, one in space.
INDIVIDUAL VARIATION
All the models examined so far assume that each animal in the population has
exactly the same chance of survival and reproducing, even though these rates
change with time. What happens if each animal in the population has a differ-
ent rate of survival and reproduction? Differences between the individuals in
