Factors related to condition and rare and threatened species occurrence in lowland, humid basalt remnants in northern Tasmania

Abstract

Almost all of the natural vegetation on the humid, lowland, basalt of northern Tasmania has been cleared for agriculture. Fifty-three remnants of native vegetation larger than 5 ha were surveyed. Vascular plant species occurrence and abundance was recorded from the warm edge, the cool edge and the centre of each remnant and a species list made for the remnant as a whole. Geometric, environmental and management variables for the remnant were related to vegetation variables indicating condition and the importance of each remnant for rare or threatened vascular plant species. There was no significant relationship between the presence of rare or threatened species and any of the six condition variables, and this index was generally significantly related to a different set of independent variables than the condition indices. Among the condition variables, only exotic species richness was significantly related to management variables. Among all the dependent variables, only exotic species richness, exotic cover and the ratio between exotic cover and native cover were not significantly related to at least one axis of a two-dimensional ordination of the remnant floristic data. The warm edge quadrats were significantly different from the centre quadrats on several variables, mainly related to exotic richness and cover, and the cool edge quadrats were significantly different from the centre quadrats on a lesser number. None of these differences related to soil fertility. Among the management-related variables, only the number of stumps varied between the centre and the edge. Microclimatic differences, rather than nutrient drift, might explain many of the differences between the centre and edge quadrats. The results of this study conform with previous work that has indicated that remnant area, age and patterning may not be major influences on the condition of remnants, and that planning for remnant conservation must independently consider rare or threatened species and condition. The present study also indicates that the relationships between the value of remnants for conservation and geometric, environmental and management variables vary strongly between different environments. Thus, management and planning recommendations derived from one area are not necessarily transportable to another.