Changes to plant species richness in forest fragments: fragment age, disturbance and fire history may be as important as area

Aim The impact of fragmentation on a eucalypt forest was investigated by examining the effects of fragment size, time since fragmentation, degree of anthropogenic disturbance to fragment interiors, and time since fire, on native and exotic plant species richness per unit area. Location Two areas of dry open-forest were studied on the central coast of New South Wales in south-eastern Australia. Fifty forest fragments were located at Tomago, an area progressively fragmented over the last 60 years, most recently by clearing for sand-mining. Also at Tomago were six very large blocks of forest that were used as reference sites. The second area at Myall Lakes National Park (50 km north of Tomago) had four very large areas of intact forest that were also used as reference sites. Methods Fragments were allocated into (1) three size classes: small (<1 ha), medium (1 to <10 ha) and large (10 to <100 ha); (2) two classes of age since fragmentation: young (up to 10 years) and old (10 years or more); (3) two classes of disturbance (minor, major) based on the degree and extent of human-induced disturbance to the fragment interior; and (4) two classes of time since fire (between 5 and 6 years; 10 years or more). Reference sites belonged to a very large size class (>100 ha). Mean plant species richness per 25 m(2) area was determined for each fragment and analysed separately for native and exotic species. Results The most significant effects observed resulted from anthropogenic disturbance. In fragments with major disturbance, native species richness per unit area was significantly reduced in small-sized young and medium-sized old fragments. A significant increase was also observed for exotic species richness in fragments with major disturbance. With minor disturbance, native species richness in small fragments declined significantly with time since fragmentation, in contrast to medium and large fragments. Among recently created fragments, those recently burned had significantly more native species per unit area than those burned 10 or more years ago. Main conclusions Anthropogenic disturbance coupled with fragmentation had a stronger and more immediate effect in reducing native species richness and increasing exotic species richness than did fragmentation alone. In the absence of major disturbance, small fragments had fewer native species than larger size classes, but only after 10 or more years since fragmentation, confirming the importance of controlling for age of fragments when examining species-area relationships. This study has not tested whether differences in area were the direct cause of this loss of species over time; other factors that are correlated with area (such as edge effects) may also be involved. The increase in native species richness following fire was consistent with other studies of fire in unfragmented eucalypt forest. This study thus shows that in addition to the factors emphasized in classical island biogeography models, fragment age, disturbance and fire history are important in explaining species richness in fragmented eucalypt forests.