Species Frequency Of Occurrence Research Articles

Effect of species pool size on species occurrence frequencies: Musical chairs on islands

If species interactions affect species distributions, then species occurrence frequencies (nu(i)), defined as the fraction of an archipelago's islands that species i inhabits, should vary with species pool size. A "natural experiment" approximating this test is provided by the Bismarck, Solomon, and New Hebrides archipelagoes, whose bird species pools decrease in that order, the species of each archipelago being mostly a subset of those of the next richer archipelago. The average nu for an archipelago's species decreases with archipelago pool size. In the archipelago with the largest pool, most species are on few islands and few species are on most islands, whereas the reverse is true in the archipelago with the smallest pool. For species shared between two or more archipelagoes, nu(i) decreases with pool size or number of species in the same guild. These interarchipelagal differences in nu(i) or average nu reflect differences in level of interspecific competition, which reduces nus in species-rich archipelagoes in two ways: usually, by reducing a species' incidence on small islands and restricting the species to larger islands; less often (for so-called supertramps), by restricting a species to small islands.

THE OCCURRENCE OF BROWNTOP, IN THE MANAWATU

Results are presented from a survey of the frequency of occurrence of pasture species in the different topographical and soil type areas of the Manawatu. It was carried out in the early spring of 1967 and again in the summer of 1968. The survey indicated a high frequency of occurrence of browntop on hill country (81 and 87% in early spring and summer, respectively) and terrace country (46 and 40%, respectively) and moderate amounts on flat land and sand country. Lowest occurrence was recorded on flat country dairy farms (15 and 17%, respectively). Marked differences in species frequency of occurrence were recorded between farms on hill country. These ranged from farms in which pastures were dominantly browntop, sweet vernal, and moss, to farms where relatively high amounts of white clover, ryegrass, and dogstail occurred in association with other less productive species. These differences have been related to produotivity levels possible from such extremes in species composition. Results are discussed in relation to current hill country farming practices. Factors considered to be significant in altering hill country botanical composition and hence levels of productivity are also discussed.