Functional traits and prior abundance explain native plant extirpation in a fragmented woodland landscape

Abstract

Summary 1 Habitat destruction and the resulting fragmentation increase the probability of local extirpation of plants but the process of community disassembly may take many years. There are very little direct data on the rate of this process. Revisitation studies using historical data offer the potential to quantify the rate and direction of change in remnant vegetation and can contribute to an improved understanding of the long-term species responses to habitat fragmentation. This study examined the changes that occurred in 12 grassy woodland remnants in rural southern Australia over three decades by comparing comprehensive site-level species lists collected in 1975 and 2006. 2 The probability of local population extirpation was assessed in relation to functional- and abundance-based mechanisms (i.e. plant functional traits, initial population abundance) and extrinsic threats (i.e. remnant area, remnant shape) using chi-square analysis and Bayesian logistic regression modelling. 3 Of the 775 populations of 177 perennial native species present in 1975, 199 populations (26%) of 98 species were not relocated in 2006 and are presumed to be locally extinct. 4 Extrinsic factors such as habitat area and shape provided little predictive power to understanding local population extirpation. Intrinsic factors (i.e. initial population abundance, seed dispersal and plant height, but not the plant traits clonality and life form) were better predictors. Populations of short species at low abundance in 1975 were most likely to become locally extinct due to inter-specific competition in the absence of disturbance whereas tall, abundant species have the least likelihood of population extinction. 5 Synthesis. The sensitivity of populations to local extirpation in fragmented landscapes is likely driven by a number of factors. The lack of substantial influence of site area and shape and some plant life-history traits suggest that simple predictions about population extinction are unlikely. Although population size was an important determinant of population loss, it is likely that other factors will interact to determine such outcomes (e.g. disturbance regimes, non-native plant invasions, edge effects) by affecting habitat quality, and put populations at greater risk of extinction than the life-history traits of plants alone.