The impact of propagule pressure on invasion success

Limiting the establishment and spread of invaders depends on understanding factors that facilitate invasions.  These factors include characteristics related to the invading organism, like its initial population density, and characteristics related to the environment, such as its productivity and history of disturbance. Using protist microcosms as a model system, I investigated the effects of disturbance, productivity, and propagule pressure on the invasibility of a community of competitors that coexist in a natural pond.  One month prior to invasion, I disturbed an original community of eight competitors at two levels of disturbance frequency, three levels of disturbance intensity, and two levels of productivity (nutrient availability). Native community diversity decreased with increasing disturbance intensity and frequency and with disturbance-induced loss of nutrients. To these established communities, I added four non-native competitors at two levels of propagule pressure. Three of the four invaders did not show the expected increase in invasion success with higher propagule pressure, while the fourth invader established more frequently in low propagule pressure treatments. Low productivity inhibited invasion for all species, while disturbance positively affected the invasion of only one species. The ability of disturbance to increase invasibility may depend on the life history characteristics of the invader, while the effectiveness of propagule pressure in increasing invasion success may depend on its interaction with disturbance and species’ traits.