COS 76-9
Community structure and co-occurrence patterns of grasses along salinity gradients in Serengeti National Park, TZ
A central observation regarding the structure of ecological communities is that organizing processes such as environmental specialization and biotic interactions can cause species to be non-randomly assembled along environmental gradients. Ultimately, the realized niche breadths and positions of species are a result of evolution, history, and these biotic and abiotic filters. Our interest in niche breadths was piqued by a particularly intriguing aspect of sodium (Na) in some grazing ecosystems— that herbivores preferentially graze Na accumulating plants to meet their metabolic requirements. It is unclear how this Na-herbivore interaction might influence plant community structure—species inhabiting areas of high Na may be a nested subset of those in the species pool, or there might be species turnover associated with Na. We aimed at testing for patterns that might reflect how species distributions relate to Na in the Serengeti ecosystem of East Africa. We collected species level abundance measurements and soil properties data at 180 1-m2plots distributed among 12 sites across the Serengeti rainfall gradient. We used these data to test for patterns of community structure (e.g. nestedness), species co-occurrence, and niche breadth variation.
Results/Conclusions
We expected to find that Na would represent an important environmental driver for herbaceous plants across sites as Na decreases with distance from the Serengeti Plains. An ordination analysis (NMDS) supported Na, as well as fire, rainfall, and other drivers, as an important factor in our vegetation data, but further analysis also revealed that there is comparable Na variation within a site—i.e. local variation is large. We fit a mixed effect model which suggested that salinity restricts the richness of plots at the 1-m2 scale. Additionally, there was significant clustering of species niche boundaries, identifying species that were largely restricted to high Na soils. In our pairwise species co-occurrence analysis, one quarter of all combinations were classified as having positive (78 pairs) or negative (86 pairs) associations. Interestingly, many of the negative species associations revealed by our co-occurrence analysis were those correlated with higher soil Na. Furthermore, we found that species mean Na niche was negatively correlated with plant height—an indicator of herbivore tolerance. Altogether, these results suggest the possibility that certain species are specialized to areas of high Na and the additional herbivore pressure that can accompany it.