Ant Abundance along a Productivity Gradient: Addressing Two Conflicting Hypotheses

Abstract

The number of individuals within a population or community and their body size can be associated with changes in resource supply. While these relationships may provide a key to better understand the role of abiotic vs. biotic constraints in animal communities, little is known about the way size and abundance of organisms change along resource gradients. Here, we studied this interplay in ants, addressing two hypotheses with opposite predictions regarding variation in population densities along resource gradients- the 'productivity hypothesis' and the 'productivity-based thinning hypothesis'. The hypotheses were tested in two functional groups of ground-dwelling ants that are directly primary consumers feeding on seeds: specialized seed-eaters and generalist species. We examined variations in colony density and foraging activity (a size measurement of the forager caste) in six ant assemblages along a steep productivity gradient in a semi-arid region, where precipitation and plant biomass vary 6-fold over a distance of 250km. An increase in the density or foraging activity of ant colonies along productivity gradients is also likely to affect competitive interactions among colonies, and consequently clinal changes in competition intensity were also examined. Ant foraging activity increased with productivity for both functional groups. However, colony density revealed opposing patterns: it increased with productivity for the specialized seed-eaters, but decreased for the generalist species. Competition intensity, evaluated by spatial partitioning of species at food baits and distribution of colonies, was uncorrelated with productivity in the specialized seed-eaters, but decreased with increasing productivity in the generalists. Our results provide support for two contrasting hypotheses regarding the effect of resource availability on the abundance of colonial organisms- the 'productivity hypothesis' for specialized seed-eaters and the 'productivity-based thinning hypothesis' for generalist species. These results also stress the importance of considering the role of functional groups in studies of community structure.

Fig 1. Relationships between colony density or forager number per species and mean annual precipitation in 2007 (a,c) and 2008 (b,d) of the generalist species (open circles, dashed line) or both functional groups when monitored together (plus sign, solid line) in the 20 × 15 m plots.

The same relationships were also obtained for the specialized seed eaters (filled circles, dotted line) in the 100 × 100 m plots in 2008 (b,d) and 2009 (a,c) and standardized according to the 20 × 15 m plots.

Fig 2. The relationship between Standardized Effect Size values of co-occurrence patterns between generalist species and mean annual precipitation in 2007 (open triangles) and 2008 (filled triangles).

SES values between -1.96 and 1.96 indicate random species co-occurrence patterns in the experimental baits. Values above 1.96 and below -1.96 indicate non-random species segregation and aggregation, respectively.

Fig 3. The relationship between the standard score (Z) of Nearest Neighbor Distance and mean annual precipitation for the generalist species (a) and specialized seed eaters (b) in 2007 (open triangles) and 2008 (filled triangles, solid line).

Z values between -1.96 and 1.96 indicate random distribution of colonies. Values above 1.96 and below -1.96 indicate colony segregation and aggregation, respectively.