There are a finite number of species in the world and thus centuries of historical movement and establishment of invading species may have depleted the supply of species available for future invasions. However, the extent to which this depletion has occurred and how it may be affecting current and future invasions remains unclear. Previous attempts to model this depletion phenomenon confirm that it is real but the non-mechanistic nature of these models limits their use for predicting invasions in the future. Here we describe a process-based model that captures the simultaneous effects of depletion of source species pools along with increases in pathway rates (e.g., import rates) to predict future numbers of new invasions in the future. We assume that within species pools, there is a highly skewed distribution of species abundance. Given their high propagule pressure, the few abundant species are most likely to invade, while the many rare species are only likely to invade under very high pathway volumes. We apply this model to predict past and future invasions of true bark beetles, Scolytinae, to North America.
Results/Conclusions
Fitting a lognormal distribution of species abundance in the pathway (proxied by port inspection frequencies) indicates a highly skewed distribution of species abundance, explaining why only a small fraction of species has historically invaded. Forecasts from the model indicate that with increasing volumes of imports, more species from these regions are likely to invade in the future despite the depletion of the most abundant species from source species pools. Even though depletion of European and Asian Scolytinae species would cause attenuation of invasion rates under constant import rates, more realistic increases in imports will result in a near-steady accumulation of invasions in the future.