Short term transient population dynamics are common in vertebrates, particularly invasive vertebrates, and can fundamentally influence the ability of a species to colonize new areas. The role of transient processes may be a fundamental key to ecological understanding and often explains higher order processes that drive ecological systems.
Here we used relatively recent methodological advances in the analysis of transient population dynamics to examine how environmental conditions correlate with short-term transient population dynamics, particularly population growth rates. We used a novel database of wild swine (Sus scrofa) vital rates describing survival and fecundity for 16 native and invasive populations occurring on four continents. Using maximum likelihood methods, we determined the vital rates and age classes that contribute most to transient population growth under different environmental conditions experienced by native and invasive populations. We evaluated hypotheses about differences in invasive and native life-history strategies using transient sensitivity analysis to describe the relative contribution of age-specific survival rates and fecundities to variation in transient population growth.
We found consistent differences in the way vital rates and age structure in invasive and native populations contribute to transient dynamics. Native pig populations appear to be more sensitive to changes in environmental conditions having consistently larger changes in transient dynamics relative to invasive populations. Contrary to the demographic buffering hypothesis, the vital rates that had the largest influence on population growth rates where those that had the greatest variability, and those that contributed little to overall population growth rates had the lowest variability. Juveniles contributed most (92%) to population growth in invasive populations while adults contributed most (45%) in native populations. In native populations, vital rates declined in their contribution to population growth within the first five generations and age structure became most important, contributing 58% to population growth, supporting the concept of demographic buffering. The opposite occurred for invasive populations where age specific vital rates and age structure moved in the same direction indicating that invasive and native populations have different mechanisms for buffering environmental variability.
The observed trade-offs between juvenile age structure, survival, and fecundity may have unexpected consequences when managing invasive populations and implementing population control. In light of our findings for invasive wild pig populations, any changes to age structure (e.g. harvest, culling) that shift populations toward greater juvenile densities may unintentionally increase the population growth rate.