Diversity Changes During Pioneer Stages in a Subhumid Pampean Grassland Succession

Abstract

The hypothesis that species richness and evenness increase during early succession is tested in a postagricultural grassland succession in the Argentine Pampa. The relationship between species diversity and spatial heterogeneity of the community is also examined. Species heterogeneity (H'), evenness (J'), species richness (N'), spatial heterogeneity (Db), and relative extinction (ER) and invasion (IR) rates were calculated. Db did not increase with succession nor was it positively correlated with species diversity. Diversity increased during the 1st year and a half and decreased during the last 2 years. The initial diversity increase is attributed to high initial IR, that decreased as the species richness increased. Later in the succession, increased interference of the dominant grass, Lolium multiflorum, caused high ER and low evenness. We suggest that these changes were encouraged by the lack of disturbance. Comparison of the reported results with similar studies by other authors shows that the disturbance regime influences diversity patterns during early successional stages in herb communities. INTRODUCTION Species diversity has often been presented as a measure of the organization and maturity achieved by the ecosystem (McNaughton, 1977; Margalef and Gutierrez, 1983), and it has been asserted that it increases during ecological succession, especially during pioneer stages (Odum, 1969; Margalef, 1974). However, lack of consistent empirical data to support this assertion has been claimed and its application to communities generally has been rejected (Drury and Nisbet, 1973; MacMahon, 1980; Tramer, 1975). Despite recent efforts devoted to the study of secondary succession, little information about diversity changes during successional processes in grasslands is available. However, diversity indices have been proposed for range monitoring (Ares and Leon, 1972; see also Odum, 1985). Therefore, clear comprehension of pattern and of processes that affect species diversity during grassland succession would be useful. Most studies of changes in diversity during grassland succession encompass long periods without close consecutive observation, and processes that drive species diversity changes during early stages are poorly understood. Moreover, divergent results have been reported when close consecutive observations in grasslands or herbaceous communities have been conducted. Leon and Oesterheld (1982), working on different-aged pastures in the Salado River Basin (Buenos Aires Province, Argentina), found that diversity increased steadily during 5 years. Similar results were reported for plowed grasslands in the Sierra de Guadarrama (Spain) during 5 years following the disturbance (Pineda et al., 1981). However, contrasting results were reported by Bazzaz (1975). Working in Southern Illinois old fields, he found that species diversity increased during the 1st 2 years and then diminished until the 5th year. He concluded that the decrease in diversity was due to the allelopathic effect of Andropogon virginicus, the dominant grass. In Ohio old fields, Tramer (1975) studied a permanent plot during 4 years and found the same diversity pattern. He concluded that both evenness and species richness diminished because r-selected species were displaced by k-selected species. The purpose of this paper is to test two main hypotheses in postagricultural succession in grasslands. The first hypothesis is that during early pioneer stages species diversity increases (Odum, 1969). The second postulates that the increase of species diversity is related to a more heterogeneous arrangement of the populations within the community (Margalef, 1974).