Environmental rugosity, body size and access to food: a test of the size‐grain hypothesis in tropical litter ants

Abstract

2004. Environmental rugosity, body size and access to food: a test of the size-grain hypothesis in tropical litter ants. -Oikos 104: 165-171.In terrestrial walking organisms, long legs help to decrease the cost of running, allowing animals to step over environmental interstices rather than walking through them. However, long legs can complicate the infiltration of these interstices, which may contain food sources and refugia. Since the number of environmental interstices perceived by an organism (rugosity) increases as it body size decreases (size-grain hypothesis, SGH), natural selection should favor proportionally smaller legs with decreasing body size. Recent work demonstrated that ants fit this hypothesis. We experimentally tested the assumption of the SGH that small ants, which have proportionally smaller legs than larger ants, are more successful in exploring environmental interstices because they can easily penetrate them. We examined the ability of tropical litter ant species with different body sizes to access food baits in 'landscapes' (= plots) with different levels of rugosity and food exposure. In the first experiment, three levels of landscape rugosity were defined by manipulating the density of leaf litter placed on the ground plots: a) plain landscape: no litter fall, b) intermediate rugosity ( 0.5 kg of litter fall) and c) high rugosity ( 1 kg). In a second experiment, food baits were in plain landscapes, exposed or covered by leaf litter. The body lengths of ants that first accessed food baits ranged from 1.5 to 12 mm. Ants that first reached food baits in the most rugose landscapes were 40% smaller than ants that first found baits in plain landscapes. Smaller ants were also the first to access covered food. The application of a phylogenetic comparative method suggested the same patterns. We conclude that these results support the size grain hypothesis. Environmental rugosity might have operated as a selective force to shape the morphological characteristics of litter ant species.

Simple does not mean poor: grasslands and forests harbor similar ant species richness and distinct composition in highlands of southern Brazil

“…However, this relationship might be found even when only the ground stratum is considered (Andersen 1986). The ground of a forest can be considered more complex and heterogeneous (Farji-Brener et al 2004) than a grassland because of a litter layer with variable depth, more heterogeneous distribution of plants in the understory, and availability of tree-related resources (e.g. fallen fruits).…”

Assembling an ant community: species functional traits reflect environmental filtering

“…First, the size-grain hypothesis posits surface complexity drives an allometric relationship between ant size and leg length, with larger ants having proportionally longer legs than smaller ants (Kaspari and Weiser 1999;Parr et al 2003). This relationship may be a consequence of how ants move through their environment: smaller, shorterlegged ants more successfully forage through the leaf-litter while larger ants forage across it, as longer legs can enable faster or more eYcient movement (Farji-Brener et al 2004;Pearce-Duvet et al 2011a). As a result, ant assemblages in comparatively low surface complexity environments are expected to contain species of larger sizes and greater relative leg lengths (Parr et al 2003;Sarty et al 2006).…”

The size–grain hypothesis: do macroarthropods see a fractal world?

“…Subsequently, this allometry has been confirmed in a variety of native ant assemblages (Kaspari & Weiser, 1999;Espadaler & Gomez, 2000;Parr et al, 2003;Farji-Brener, 2004;Farji-Brener et al, 2004) but not an island assemblage recently assembled from invasives (Sarty et al, 2006). But is this allometry robust?…”