While ecologists recognize that the abiotic environment plays a significant role in determining species abundances and distributions, we have a limited understanding of the extent to which abiotic factors indirectly structure communities by altering interspecific interactions, both through density- and trait-mediated effects. Thus, we used both observational and experimental approaches to evaluate whether elevation shapes the abundance of the aphid Pterocomma populifoliae feeding on quaking aspen (Populus tremuloides) by mediating its interactions with resources, natural enemies, and mutualistic ants. Based upon theory and empirical evidence for stronger species interactions at low elevations and latitudes, we predicted that ants would provide stronger mutualistic services at lower elevations, and as a result, aphids would be more abundant. Such variation in mutualism strength and aphid abundance might be due to changes in (i) ant abundance, (ii) community composition, or (iii) behavior, as well as (iv) host plant quality or (v) top-down control by natural enemies. To test these hypotheses, within replicate high (2880-3196 m) and low (2567-2758 m) elevation valleys (n = 3 each), we documented naturally occurring patterns of ant, aphid, and predator abundance, and measured aphid colony performance in response to experimental manipulations of aphid multi-trophic interactions.
The population growth rates of tended aphids were similar between high and low elevations, but ants increased colony survival 1.8-fold at low elevations while offering no detectable benefit at high elevations. As a result, naturally occurring aphid colonies were 1.6-fold larger at low elevations (although colonies were not more abundant). These results were due exclusively to trait-mediated effects of elevation on ant behavior; there were no elevational differences in ant abundance, ant community composition, bottom-up effects of host plant quality, or top-down effects of natural enemies. In contrast, low elevation ants were 44% more likely to discover and protect (tend) experimentally established aphid colonies, 81% more likely to discover baits, and 71% more likely to forage in aphid-free aspen trees. By elucidating the direct and indirect (both density- and trait-mediated) mechanisms shaping species abundances and interspecific interactions across environmental gradients, we can better predict when species interactions may play an important role in structuring communities along abiotic gradients and in response to climate change.