Summary1 The invasion of habitats by non-native plant and animal species is a global phenomenon with potentially grave consequences for ecological, economic, and social systems. Unfortunately, to date, the study of invasions has been primarily anecdotal and resistant to generalization. 2 Here, we use insights from experiments and from long-term monitoring studies of vegetation to propose a new theory in which¯uctuation in resource availability is identi®ed as the key factor controlling invasibility, the susceptibility of an environment to invasion by non-resident species. The theory is mechanistic and quantitative in nature leading to a variety of testable predictions. 3 We conclude that the elusive nature of the invasion process arises from the fact that it depends upon conditions of resource enrichment or release that have a variety of causes but which occur only intermittently and, to result in invasion, must coincide with availability of invading propagules.

No abstract

No abstract

Recent theory has suggested a mechanistic relationship between resource availability, competition and invasibility. In a field experiment, in which we manipulated resources and competition, we confirmed that changes in resource availability affected competition intensity, which in turn affected invasibility. We found that fluctuations in resource availability of as short as a few weeks had a large impact on plant invasion success (survival and percentage cover), including up to 1 year following the fluctuations. If resource availability is a primary mechanism controlling invasibility, it may serve as a unifying concept that can integrate earlier ideas regarding invasibility. The results emphasize the important role of history in the invasion process, particularly the occurrence of stochastic, short‐lived events that temporarily reduce or suspend competition and increase invasibility. Therefore, it may be very difficult, or even impossible, to reconstruct the ecology of particular invasions after the fact.

Summary 0We measured competition intensity "CI# between herbaceous vegetation and tree seedlings "Quercus macrocarpa and Q[ ellipsoidalis# along an experimental moistureÐ light gradient[ Contrasting theories were tested by comparing variation in competition intensity to changes in neighbour biomass and resource supply and demand[ 1 CI based on survival was inversely correlated with net soil water supply "gross supply minus demand by herbaceous vegetation#[ CI was not positively correlated with either gross resource supply or neighbour biomass\ contrary to predictions of Grime|s triangular model for plant strategies[ 2 Many of the inconsistencies and con~icting results that have characterized the recent literature on plant competition could be eliminated if changes in competition intensity along a resource gradient are compared with changes in net resource supply rather than changes in productivity or neighbour biomass[ 3 Tree seedling success in savannas and grasslands may be strongly in~uenced by the intensity of competition from herbaceous vegetation[ Factors that reduce soil water content are likely to increase competition intensity "and reduce seedling success# in these environments\ while factors that increase soil water content will favour seedling success through decreased competition for water with herbaceous vegetation[ Keywords] competition\ grassland\ Quercus\ resource demand\ resource gradients\ resource supply\ savanna\ tree regeneration Journal of Ecology "0887# 75\ 541Ð550