Effects of natural habitat fragmentation on the species richness, diversity, and composition of cliff vegetation

Plant species richness, diversity, and some aspects of species composition were measured on natural limestone cliff fragments of varying size within the Niagara Escarpment Biosphere Reserve, Canada. This information was collected because knowledge about how different components of community structure change in response to natural fragmentation may permit the prediction of the effects of future anthropogenic fragmentation. The number and relative abundance of vascular plant, bryophyte, and lichen species were determined on cliff fragments that varied in area from 185 to 126 000 m2. Latitude, aspect, percent available photosynthetically active radiation on the cliff face, distance from the nearest neighbouring cliff, and length of the nearest neighbouring cliff were also measured. Regression analysis was used to test for a significant relationship between fragment area and diversity of vascular plants, bryophytes, and lichens both separately and combined. Multiple regression with all subsets selection was used to find the best predictors of species richness from among all variables measured for the 21 cliff fragments. Multivariate analyses were used to study the effect of fragmentation on the structure of the vegetation as a whole. The results showed no significant relationship between cliff fragment area and richness or diversity for vascular plants and bryophytes, and only a marginally significant increase in richness with area for lichens. The multivariate analyses also showed that only one community type exists, and that its structure mainly varies as a function of latitude. These results indicate that very small fragments of cliff face can support a similar plant biodiversity as do large continuous portions of the Niagara Escarpment.Key words: habitat fragmentation, plant species richness, lichens, bryophytes, cliff vegetation.