How can so similiar species co-exist?

Submitted by editor on 15 December 2015.Get the paper!

Some ecosystems contain assemblages of species that can be surprisingly similar in their morphology, feeding niche and ecological function, but whose diversity can facilitate the persistence of the entire ecosystems (Duffy 2006). For example, a high diversity of crustacean mesograzers in seagrass meadows (up to a dozen species within a system, Best and Stachowicz 2014) can increase seagrass production and survival by reducing overgrowth by epiphytes (Duffy 2006). However, classical theory predicts that within a given habitat the coexistence of competing species should be at least partly controlled by their differential use of available resources to reduce the likelihood of competition between species (Chesson 2000). In seagrass meadows, a system with relatively high structural complexity, small scale differences in microhabitat use may be an important axis of niche differentiation (Beermann and Boos 2015). Likewise the exposure of seagrass-associated epifauna to high predation pressure can be an additional determinant of grazer microhabitat use (Duffy and Hay 1991): microhabitats with high value as shelter often require a close matching with morphological features of grazers. Therefore, variability in traits could translate to fixed differences in microhabitat use among species.

Figure 1 - First (left) and second (right) author inspect a mesograzer specimen.

We investigated whether microhabitat preferences of mesograzer species are fixed, or whether they are formed depending on a specific ecological context (presence of heterospecifics or predators), and the role of traits in microhabitat use. The eight crustacean species we used account for over 60% of the mesograzers commonly found in this system (Best and Stachowicz 2014), which we believe is an unprecedentedly comprehensive test of microhabitat use of a specific functional community. We also tested whether generic traits (body size and tube building ability) can explain patterns of habitat use across the species pool. Our findings are presented in the paper “Microhabitat partitioning in seagrass mesograzers is driven by consistent species choices across multiple predator and competitor contexts” recently published in Oikos.

Figure 2 - Mesograzer species form Bodega Harbor using in the study, from top left to bottom right: Allorchestes angusta, Ampithoe lacertosa, Ampithoe sectimanus, Aoroides columbiae, Caprella californica, Idotea resecata, Ischyrocerus anguipes and Pontogeneia rostrata. Pictures by Rebecca J. Best.

We found distinct differences between species specific microhabitat use, and moreover, small scale microhabitat partitioning that was remarkably consistent across different ecological contexts. None of the observed patterns of microhabitat use was explained by body size, while the ability to build silk tubes seemed to be correlated with the use of some microhabitats. The presence of common predatory fish did not affect individual microhabitat choices or microhabitat partitioning, neither did increased density of conspecifics. The presence of heterospecifics lead to a shift in microhabitat use of some of the species, but did not affect overall microhabitat partitioning across all species. The consistency of species-specific microhabitat use, regardless of the presence of predators or competitors, should make coexistence most likely among species that differ in these choices. For these species, it appears that the benefits accrued from their selected microhabitats are not affected by species interactions, or that any benefits of alternative microhabitat use are outweighed by risks through predation associated with movement.


Beermann, J. and Boos, K. 2015. Flexible microhabitat partitioning between hemi-sessile congeners. - Mar. Ecol. Prog. Ser. 520: 143-151.

Best, R. J. and Stachowicz, J. J. 2014. Phenotypic and phylogenetic evidence for the role of food and habitat in the assembly of communities of marine amphipods. - Ecology 95: 775-786.

Chesson, P. 2000. Mechanisms of maintenance of species diversity. - Annu. Rev. Ecol. Syst. 31: 343-366.

Duffy, J. E. 2006. Biodiversity and the functioning of seagrass ecosystems. - Mar. Ecol. Prog. Ser. 311: 233-250.

Duffy, J. E. and Hay, M. E. 1991. Food and Shelter as Determinants of Food Choice by an Herbivorous Marine Amphipod. - Ecology 72: 1286-1298.

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