POPULATION ECOLOGY STUFF

POPULATION ECOLOGY

Community stability: A central tenet of classical ecology is that complex communities tend to be more stable (largely based on observation) theory recently challenged, argument that simple systems may be less subject to external disturbances. What has emerged is that:

1. Competitive interactions between species lead to instability unless dominated by negative feedbacks within self-loops, i.e.,  one species setting in motion cyclic oscillations in numbers of another.

2. The number of competing species increase the competitive interactions must be proportionately weaker or instability will result.

3. Interactions between tropic levels (predator/prey) tend to stabilize populations (community).

4. community stability is frequently interrupted by severe environmental disruptions, leading to a series of successional communities gradually evolving to climax associations. Frequent or continual disruptions may lead to persistent nonclimax community. Herbivores play an important role in plant succession by tending to harvest unthrifty members of a plant community, e.g., overmature trees.  They also recycle nutrients and increase productivity and vigor of community, - mutualistic?

Population dynamics/epidemiology theory is a vast and formidable subject. However, the synoptic model developed by T. R. E. Southwood (1975 and subsequent papers) is most instructive, and summarizes much of the theoretical concepts and empirical bases for contemporary model building. 

The model has 3 salient features:

1. Natality (birthrate) is low at low population densities because of problems associated with low densities, e.g., finding mates, increases as population increases, peaks and finally declines as intraspecific competition increases, e.g., competition for food.

2. Predation increases with increasing host (herbivore) density, then declines as host populations overwhelm (and escape) their regulating influence. This occurs because the characteristic pattern is for overall predator response to be sigmoid, based on a) functional response of the individual predator and b) numerical response of the population.

3. Intraspecific competition increases with increased population (prey) density. This produces additional mortality as natality shows a down-turn. Other density-dependent mortality and stress also comes into play producing a marked increase in combined mortality.

The NET result of these interacting factors is that changes in the population are subjected to dramatic changes in cumulative mortality rates. These are illustrated in the following figures.