Global energy gradients and size in colonial organisms: Worker mass and worker number in ant colonies

Abstract

Body mass shapes processes from cell metabolism to community dynamics. Little is known, however, about how the average body mass of individuals varies among ecological communities. Ants alter colony mass by independently changing worker mass and͞or worker number. In a survey of 49 ecosystems from tundra to tropical rainforest, average worker mass and worker number were uncorrelated (rs ‫؍‬ 0.2, P > 0.14) and varied 100-fold. Data supported the hypothesis that higher mean monthly temperatures, T, reduce worker mass by increasing metabolic costs during worker development. In contrast, worker number was unimodal over a 1,000-fold gradient of net primary productivity (NPP, g of carbon per m 2 per yr), a measure of organic carbon available to consumers. At the lowest levels of NPP colonies appeared to be carbon-limited; above 60 g of carbon per m 2 per yr average worker number decreased to a global low. This decline in worker number with increasing NPP supports the hypothesis that abundant carbon ameliorates the Achilles heel of small taxa in competition with large taxa: their relatively high metabolic demands. Higher predation rates in resource-rich environments may also play a role in limiting worker number. In all, about half the global variation in worker mass and number was accounted for by gradients of NPP and T. Changes in global temperature and rainfall may thus mold gradients of ectotherm size, with consequences for the structure and function of the ecosystems.biogeography ͉ life history ͉ productivity ͉ temperature B ody size can fundamentally shape an organism's ecological niche (1-6), a population's rate of evolution (7-10), and an ecological community's structure and function (4,(11)(12)(13)(14). Given its role as a linchpin of functional biology, understanding how and why body size varies globally is a key challenge to ecology. Solar energy may shape gradients of body size by its effects on local temperature and net primary productivity † (16-20). Rising global temperatures (21,22) give new import to understanding these effects. Progress toward that goal has been hampered by the lack of global size data from a set of communities experiencing the full range of solar energy.Ants (family Formicidae) are colonial ectotherms that are important players in every terrestrial biome from tundra to rainforest (19, 23) and whose distribution thus spans global gradients of NPP and T. Studies of body size regulation in unitary organisms typically focus on the number and size of cells in an individual (18). Studies of size in colonial organisms such as bryozoans, corals, and the social insects, in contrast, focus on the mass and number of a colony's subunits (24, 25). Ant colonies have two life history options, under separate control, for increasing their mass: adding workers and increasing worker mass.Here I quantify the average worker mass and worker number of colonies in 49 New World ant communities. I explore how four energy-based hypotheses that sometimes make opposite predictions account for geographical gradien...

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