Temporal niche complementarity in recruitment contributes to coexistence across latitudes

Mariana Bonfim, Biology, Temple University, Philadelphia, PA, Carmen Schlöder, Smithsonian Tropical Research Institute, Panama city, Panama and Amy L. Freestone, Department of Biology, Temple University, Philadelphia, PA; Smithsonian Tropical Research Institute, Panama; Smithsonian Environmental Research Center, Edgewater, MD

Background/Question/Methods
Competitive interactions in a community determines the opportunity for coexistence or competitive exclusion and therefore the diversity of a system. Niche overlap can be reduced through resource partitioning, alleviating intra- and especially interspecific competition. Shifts in niche complementarity, however, can be constrained in ecological time by forces shaping species fitness. Therefore, compensatory mechanisms can emerge during the recruitment stage of organisms to reduce competition and optimize survival, especially for species with non-overlapping niches at different life stages. While strong niche differentiation has long been considered a central mechanism of species’ coexistence in the diverse tropics, studies that explore niche overlap and differentiation at early life stages across biogeographic scales, however, are rare. To test the hypothesis that niche complementarity in recruitment varies with latitude, we measured recruitment rates of sessile marine invertebrates over one year at four regions of the northeastern Pacific coastline spanning 47 degrees of latitude. Newly recruited communities were assessed biweekly and the number of recruits and morpho-functional taxa were recorded. We hypothesized that i) niche overlap would be non-random at all latitudes, given the central role of niche partitioning in natural systems, and ii) greater niche complementarity among recruiting taxa would reduce niche overlap more in the tropics relative to higher latitudes. In addition, we expected iii) taxa to differentiate their niches through time, to maximize the likelihood of recruiting when total recruitment in the system was low and competition was likely weaker.
Results/Conclusions
While strong niche complementarity occurred in all regions across the latitudinal gradient, our results indicate that niche overlap was greater at lower latitudes. Further, the spacing of niches along niche axes suggested strong differentiation at all latitudes. Niches were spaced evenly across time in all regions, which allowed taxa to recruit during times when recruitment density in the system was low, particularly at high latitudes. Our findings suggest that competitive avoidance has shaped niches across the latitudinal gradient likely over ecological and evolutionary time, particularly at higher latitudes. Greater niche overlap at lower latitudes, however, contradicts classic theories about the contribution of niche differentiation to coexistence in the most species rich places on Earth.