Interactions between demographic rates, population density and the environment : the spatial structure of the range of the holly leaf-miner
Generalisations about the abundance structure of species' geographic ranges may have implications within a number of areas of applied ecology. However, empirical data is largely restricted to a single taxonomic group within one geographic region. One approach to the study of range structure and its ecological consequences is to examine the relationship between demographic rates and environmental conditions. However, most studies of population processes are at small spatial scales and it is not known to what extent patterns found at these scales may be extrapolated. This thesis addresses these issues using surveying techniques to measure spatial structure in both the densities and demographic rates of the holly leaf-miner (Phytomyza ilicis Curt.) at a wide variety of spatial scales. Geostatistical tools are used to analyse the data collected. At a regional scale, spatial structure in leaf-miner densities is apparent despite considerable variation between hosts within individual habitat patches. This structure can in part be accounted for by variation in habitat and altitude. Over the entire range, broad scale trends in population density can be detected which can also, be correlated with environmental variation.' Many demographic rates components exhibit spatial structure. However, their relationships both with population density and environmental variation are more complex. Per capita mortality rates did not correlate well with population density at any spatial scale. The difficulties inherent in relating population density with demographic rates and regulation by natural enemies are discussed. The demographic approach has also been used to explain.the positive interspecific abundance-distribution relationship. Computer simulation techniques are employed to explore this model. The results indicate that the positive relationship should be extremely robust under a high degree of variation between the demographic rates. However, data from the holly leaf-miner surveys suggest that current. models of range structure make unrealistic assumptions about environmental variation. The key to understanding range structure and its consequences may lie in our ability to make. generalisations about environmental structure.