Robert J. Warren II, Itamar Giladi, Mark A. Bradford 2014. Competition as a mechanism structuring mutualisms. Journal of Ecology. DOI: 10.1111/1365-2745.12203.
As ecologists usually think about them, competition and mutualism are very different types of interactions. Competition has a negative effect on resource availability for a species, while mutualism should have a positive impact on resource availability. Mutualisms involve interactions between two or more species, and as such are biotic in nature. While the typical definition of the fundamental niche includes all (and only) abiotic conditions necessary for a population’s persistence, with the realized niche showing those areas that are suitable once biotic interactions are considered (Pulliam 2000), mutualisms are a reminder that the a niche is not as simple as we hope. Mutualisms may be necessary for a population’s persistence, as in the case of obligate pollinators, and so some biotic interactions might be “fundamental”. More complicated still, species may compete for mutualist partners – plant species for pollinators, for example. If the mutualist partner is considered a resource, mutualism and competition may not be so far apart after all.
The relation between competition and mutualism is probably most acknowledged in terms of pollinators – patterns of staggered flowering in a plant community arise in part to decrease simultaneous demand for limited pollinator resources. Another possibly fundamental biotic resource is dispersers, which may be necessary for population persistence of some species. In Warren, Giladi, and Bradford (2014), the authors attempt to expand this idea of competition for mutualist partners to ant-mediated seed dispersal or myrmecochory. Myrmecochorous plant species are common in a number of regions of the world. They rely on ant dispersal to move their seeds, helping to increase the distance between parent and offspring (and thus decrease competition), lower seed predation, and introduce seeds to novel habitats. Ant species that disperse these seeds benefit from the high-energy seed attachment (elaiosome) provided by the plant. While myrmecochorous plants are dependent on ants for successful dispersal, most ants do not rely solely on elaiosomes for food; further, there are fewer seed-dispersing ant species than there are ant-requiring plant species. As a result, competition for ants between myrmecochorous species is a reasonable hypothesis. If there is competition for mutualist partners, the predictions are that species either increase their attractiveness as a competitor by making their seeds most attractive, or else decrease the intensity of competition by staggering seed release.
Warren et al. tested this predictions for eastern North American woodland perennials: at least 50 plant species rely on ant dispersal in this region, but a much smaller number of ants actually disperse seeds. This dearth of mutualist partners implies that competition for ant dispersers should be particularly strong. One way to successfully monopolize a mutualist is to ensure that the timing of seed release is coordinated with ant availability and attraction: in fact comparisons between myrmecochorous and non-myrmecochorous plant species suggests that those requiring ants set seed earlier, when ant attraction to seeds is higher (insect prey become more attractive later in the season). To look at competition within myrmecochorous species, the authors as whether seed size (and thereby attractiveness to ants) was staggered through time. Smaller mymecochore seeds should, for example, become available when larger and more attractive seeds are not in competition. This prediction held – small, less attractive seeds were available earlier in the season than the larger, more attractive later seeds. The authors then experimentally tested whether small and large seeds were in competition for ants and differed in their success in attracting them. Using weigh boats secured to the forest floor, the researchers provided either i) only small myrmecochore seeds, ii) only large seeds, or iii) a combination of both seed sizes. Not that surprisingly, the presence of large seeds inhibits the removal of smaller less attractive seeds by as much as 100% (i.e. no small seeds were removed).
The authors do a nice job of showing that species differ in their success in attracting ant dispersers, and species with differing seed attractiveness appear to partition the season in such a way as to maximize their success. Whether or not this likely competition for dispersers extends to impact the species’ spatial distribution or whether species are prevented from co-occurring by competition for mutualists is less clear, and an interesting future direction. The authors also hypothesize that dispersers, rather than pollinators, may drive flowering/seed production in a system, which is an alternative the usual assumption that pollinators, not dispersers are more important drivers of evolution. More generally, the paper is a reminder that, at least for some species, biotic interactions are fundamental to the niche. Or even more likely, that the separation between the determinants of a fundamental and realized niche aren’t so very distinct. And that’s a reminder that has value for many sections of ecology, from species distribution models to invasive species research.
