Lecture 8 (Feb 12) Summary

Propagule pressure is very important in invasion success for several reasons. Simplistically put, the larger a is and/or, the larger the area it occupies, the less likely it is to go extinct.

Propagule pressure influences and can overcome several ecological processes that are important in the success or failure of small .

Both components of propagule pressure, (number of individuals introduced and the number of independent introductions) may influence population characteristics

o Number of individuals introduced can overcome demographic stochasticity and other density-dependent forces such as Allee effects. o Number of independent introductions can reduce the negative influence of spatially patterned forces (e.g. environmentally stochasticity – climate) or biotic interactions that are spatially heterogeneous. o Both propagule size and number of introductions affect genetic variation.

The Biology of Small Populations

Small populations are very susceptible to because several intrinsic and extrinsic processes become incrementally more important as population size decreases. These processes can act independently or synergistically on small populations and can drive population dynamics toward extinction

Environmental Stochasticity (extrinsic – imposed by the environment)

• Unpredictable (stochastic) changes in external forces acting on a population (climate, natural enemies, resources, etc). Temporal changes that affect all individuals of a given age or stage similarly; the variance in the vital rates are nearly independent of population size = largely density independent • Small populations may be more vulnerable because they are spatially constrained. • Catastrophe – special class of environmental stochasticity – occur very infrequently but have dramatic, large scale effects. • Bonanza – favorable wide-spread environmentally stochastic event.

Demographic Stochasticity

• Internal (intrinsic) properties of the population such as births, deaths, sex ratio, etc. that fluctuate from generation to generation. • Density dependent – smaller the population, the more important they become. In other words, these occur in all populations but are only a concern once populations become small. • Can result in chance extinction because they increase the variability around a mean birth or death rate. Increased variability means that the probability of all individuals failing to reproduce or all dying in a given year becomes quite high.

Genetic Stochasticity

• Random fluctuation in allele frequencies can lead to loss of particular genotypes and fixation of particular traits, independent of selection. is a key part of genetic stochasticity. • Density dependent • Because many newly established undergo a founder effect, genetic drift can exacerbate the already limited . In turn this can amplify the effects of inbreeding in some taxa.

Small Population Paradigm

• Small populations must contend with a series of forces that acting alone or in synchrony can push the population into an extinction vortex. • Vortex is a feedback loop: as the population gets smaller, these forces become stronger, further reducing population size, in turn increasing the influence of these factors until the population cannot persist.in • Small populations are where and invasion meet. In conservation we try to prevent populations from becoming small or are trying to assist small populations in growing. In invasion ecology we would like to prevent small populations from growing. leading to greater e