Biodiversity assessment and conservation planning : the gradsect survey method and the use of local scale surrogate information

Abstract

Biological surveys are necessary to gather species distribution data for the identification of priority conservation areas. The rationale of the gradsect method is that sampling (transects) oriented along the steepest environmental gradient should detect the maximum number of species in an area. The efficiency of the gradsect survey method was evaluated by comparing it to random, systematic and habitat-specific survey methods, during faunal field surveys (target groups: birds and dung beetles). Three gradsects were positioned within the study area to follow the major physiographical characteristics, incorporate all environmental strata (land facets) and yet be as logistically convenient as possible. The efficiency of survey methods was expressed as the number of species recorded per sampling unit effort and illustrated using bootstrap estimations to plot species accumulation curves. The gradsect method proved to be as efficient as the habitat-specific survey method and consistently more efficient than the systematic and random surveys for both taxa sampled. The present study, therefore illustrates that the gradsect survey method provides a cost-effective and swift, representative sample of regional fauna. Where species distribution data are inadequate, surrogate measures of biodiversity have to be established. Surrogate classes at higher levels of the biological hierarchy (e.g. communities, landscape types, or environmental domains) can be used as attributes of candidate conservation areas during reserve selection. Information on the distribution of surrogate classes is easier and cheaper to acquire than species distribution data. The present study investigated the informativeness of land facets as a biodiversity surrogate in the Venetia-Limpopo Nature Reserve. Land facets are defined as the simplest units of a landscape with uniform slope, soils and hydrological conditions. Multivariate analysis (MDS) and analysis of similarity (ANOSIM) revealed that the land facets adequately represent distinct bird and dung beetle assemblages and are therefore useful surrogates. These land facets were subsequently used as attributes in the following reserve selection procedures: (i) Percentage Area Representation (PAR - represent a nominated percentage area of each assemblage); (ii) Species-Assemblage Representation (SAR - represent each species within the smallest number of assemblages); (iii) Assemblage Diversity (AD - maximising diversity by first selecting areas containing most dissimilar assemblages, i.e. cover the largest distances within a hierarchical classification). The influence of grid cell size (selection units), target representation percentages and a "over-representation constraint" on the efficiency of the algorithms were illustrated. The efficiency of the three procedures were compared. The SAR procedure did not represent two of the seven assemblages, since they lacked distinguishing species. The SAR procedure subsequently selected a total area that was 50% smaller than that selected using the PAR approach. The AD procedure selected a slightly larger area than the PAR procedure, but was highly effective at rapidly increasing the diversity of the reserve network. Where economic constraints restrict the total conservation area that can be currently acquired, the AD approach provides a method that first secures the most diverse priority areas.