Patterns of plant abundances in natural systems: is there value in modelling both species abundance and distribution?

摘要

In plant ecology it is common to use biophysical models to predict species distribution; however, spatial quantitative models of plant species remain rare. In practice, occupancy models are often assumed to indicate habitant quality and are used as surrogate abundance models. This study assessed the potential value of quantitative models of plants for ecosystem management applications by assessing patterns of occupancy and abundance within two closely related understorey plant species, Xanthorrhoea australis and X. caespitosa. Vegetation quadrats were surveyed in Eucalyptus woodland and cover-abundances were assessed using a metric pin intersection technique. A zero inflated generalised additive modelling process was used to assess the relationship of species occupancies and cover-abundances to environmental properties. The models were applied to mapped environmental data to create spatial predictions of occupancy and cover-abundance. Both species shared several predictor variables, but differing responses to these variables resulted in mutually exclusive distributions. No significant correlation was observed between occupancy and cover-abundance for X. australis, but strong correlation was evident for X. caespitosa. The strength of the occupancy and abundance relationship was found to differ greatly between the two species and is therefore likely to be species specific. Occupancy models have been used successfully as proxies for habitat quality models of plant species; however where occupancy and abundance of plants are driven by different influences occupancy will be a poor surrogate for abundance. Outcomes may be improved if occupancy models are validated for abundance or quantitative models are developed and tested for individual species.