Geographic variation in plant species richness patterns within temperate eucalypt woodlands of eastern Australia

Abstract

Species density, pattern diversity and species pool are often studied in isolation and correlated individually to environmental gradients. However analysis of how these three measures interrelate can give insights into the interpretation of local and regional processes. In addition, an understanding of how these diversity measures change across the natural distribution of a community may help in decision making processes regarding reservation. Temperate eucalypt woodlands in eastern Australia are one of the most visible and ubiquitous communities in eastern Australia, but have undergone one of the most significant modification and fragmentation processes due to past and current pressure to clear for agriculture. Data from 176 vascular plant survey sites sampled across 14 woodland assemblages are used here to analyse geographic gradients in species density, pattern diversity and species pool size. It was discovered that species density was significantly correlated to pattern diversity and species pool size but that pattern diversity and species pool size were uncorrelated. There was a significant relationship whereby species density increased as pattern diversity decreased. These patterns may be explained by the maintenance of interconnectedness, dispersal and rescue effects at this scale of investigation but local interactions cannot be ruled out as important. Generally species density and species pool size increased from west to east in the study area and pattern diversity was strongly correlated to the coldest minimum winter temperatures. It is suggested that if local woodland richness is maintained by low pattern diversity and greater habitat connectedness then larger reserves are required in order to maintain the largest area of contiguous habitat. In such situations small isolated patches, which are increasingly fragmented by the pressure to clear for agriculture will accumulate larger extinction debts.