Population dynamics: patterns of growth and extinction risks

Learning goals:
* understand the consequences of changes in population size for conservation issues
* be able to describe the general cycles or patterns of population change, and their possible underlying causes

Populations exhibit various patterns of growth, including exponential, logistic,  and fluctuations.  Fluctuations in growth may be regular cycles, or irregular changes in population size.
Exponential usually occurs for short periods, during colonization of new habitat, or when some population control has been removed (e.g. predation)
Birth and death rates will vary, in response to environmental variation and variation in other species (prey and predators), leading to changes in the carrying capacity

Population cycles
Some species populations exhibit increases and decreases in size at regular time intervals
Possible mechanisms causing population cycles:
* regular climate cycles affecting food resources
* behavioral responses to increased density/ crowding: hormonal response causing lower birth rates and higher death rates, or emigration from the population
* influence of predation; suggested by synchronous changes in predator abundance

Delayed density dependent effects on population growth
Theoretically a lag in the response of a population control to a cue results in cycles of population size with time
abundant food in one year would result in increases in the birth rates the following year in mammalian herbivores and carnivores;
likewise lower food availability would have a delayed response due to an animals ability to store energy as fat deposits;
similar feedbacks might exist between predator and prey populations through delayed effects of birth and death rates on population sizes
Research indicates multiple factors contribute to population cycling through delayed density dependence
Modifying the logistic equation with a time lag term gives cyclic behavior to populations; the greater the lag, the larger the amplitude; short time lags give a “damped oscillation”; larger time lags give a “stable limit cycle”

Population irruptions and crashes
Populations of some organisms periodically increase (irruption) or decrease (crash) at high rates
Triggers can be climatic, biotic interactions,  or some combination of these

Crashes- potentially lead to extinction of populations
Variation in the amount of population cycling can increase potential of a population going extinct- i.e. the greater the range in population size during cycling, the more likely a population may go extinct.  This risk increases in small populations
small populations more prone to extinction
* Density independent factors more likely to push the population to extinction- e.g. climatic catastrophes (e.g. hurricanes)
* Small populations have low genetic diversity- potential for detrimental genetic traits to increase in abundance, due to loss of individuals, increases (inbreeding depression)
* Fewer individuals in a population lessen the chances of finding a mate for reproduction
* Inherent, unpredictable (stochastic) environmental variation has a larger effect on small populations