Habitat fragmentation caused by woody plant encroachment inhibits the spread of an invasive grass.
Abstract
Although habitat fragmentation and species invasions are widely recognized threats to biodiversity, few empirical studies have examined both threats together. Here we test the hypothesis that habitat fragmentation may limit the spread of invasive species across landscapes. In central Texas, patches of herbaceous vegetation become increasingly fragmented (e.g. smaller and more isolated) as landscapes undergo woody plant encroachment. However, during this process, fragmentation and loss of herbaceous habitat are not completely correlated, which allowed us to separate the effects of fragmentation from the effects of habitat loss. At three sites in central Texas, we recorded the occurrence of the invasive bunchgrass Bothriochloa ischaemum in randomly-located plots in herbaceous patches. We measured fragmentation of the herbaceous habitat around these plots at five scales, using aerial photographs. Bothriochloa ischaemum occurrence was significantly negatively related to most aspects of fragmentation. These negative relationships were stronger after the effects of habitat loss were removed from the effects of fragmentation per se. The strength of the negative relationship between B. ischaemum occurrence and fragmentation varied among sites, probably reflecting the influence of grazing and other management practices. Synthesis and applications. Our results indicate that fragmentation, separated from habitat loss, can have positive as well as negative conservation effects. Fragmentation, if wisely used, could be a useful tool in the management of invasive plant species like B. ischaemum. Increasing the degree of habitat fragmentation may slow the spread of invasive species across landscapes.