Spatial Grain and the Causes of Regional Diversity Gradients in Ants

Abstract

Gradients of species richness (S; the number of species of a given taxon in a given area and time) are ubiquitous. A key goal in ecology is to understand whether and how the many processes that generate these gradients act at different spatial scales. Here we evaluate six hypotheses for diversity gradients with 49 New World ant communities, from tundra to rain forest. We contrast their performance at three spatial grains from S(plot), the average number of ant species nesting in a m2 plot, through Fisher's alpha, an index that treats our 30 1-m2 plots as subsamples of a locality's diversity. At the smallest grain, S(plot), was tightly correlated (r2 = 0.99) with colony abundance in a fashion indistinguishable from the packing of randomly selected individuals into a fixed space. As spatial grain increased, the coaction of two factors linked to high net rates of diversification--warm temperatures and large areas of uniform climate--accounted for 75% of the variation in Fisher's alpha. However, the mechanisms underlying these correlations (i.e., precisely how temperature and area shape the balance of speciation to extinction) remain elusive.

Species–energy relationships at the macroecological scale: a review of the mechanisms

“…Circumstantial evidence, the form of correlations between richness, energy and abundance, suggests that the mechanism may frequently, but not invariably, contribute to species-energy relationships. The one sufficient complete test supports this (Kaspari, Yuan & Alonso, 2003), but along with theoretical considerations and data on b diversity suggests that it is unlikely to contribute significantly to macro-scale species-energy relationships.…”

“…First, methods such as jack-knifing can be used to estimate species richness if subsampling was complete, thus removing any influence of the sampling effect (Colwell & Coddington, 1994). If these estimates of species richness do not exhibit a positive and decelerating relationship with abundance/energy, which is present in the original data, then support is provided for the sampling mechanism (Kaspari et al, 2003). Second, when species richness and abundance are recorded at two or more grain sizes, the species-abundance/energy distribution at the larger grain can be randomly sampled to generate the species richness predicted, by the sampling mechanism, at the smaller spatial grain -which can then be compared with real data (Gotelli & Graves, 1996;Kaspari et al, 2003).…”

“…The only study that has shown this (Kaspari et al, 2003) concerns relationships between species richness and total abundance in ant assemblages across a large NPP gradient in the Americas. Their result, however, only held at very fine spatial scales (1 m 2 ).…”

“…Whilst theory and data support the assumptions behind the increased population size mechanism few studies fully assess its contribution to species-energy relationships as most do not adequately discriminate between it and the sampling mechanism (but see Kaspari et al, 2003). Some evidence exists against the increased population size mechanism.…”

Abundance matters: a field experiment testing the more individuals hypothesis for richness–productivity relationships

“…This increase in persistence of rare species increases diversity. An alternative mechanism by which productivity may affect richness via abundance is the sampling effect (Kaspari et al 2003;Evans et al 2005). This mechanism assumes that more productive sites attract more individuals drawn at random from a species pool, resulting in more productive sites containing more species.…”