Are trade-offs among species' ecological interactions scale dependent? A test using pitcher-plant inquiline species

Abstract

Trade-offs among species' ecological interactions is a pervasive explanation for species coexistence. The traits associated with trade-offs are typically measured to mechanistically explain species coexistence at a single spatial scale. However, species potentially interact at multiple scales and this may be reflected in the traits among coexisting species. I quantified species' ecological traits associated with the trade-offs expected at both local (competitive ability and predator tolerance) and regional (competitive ability and colonization rate) community scales. The most common species (four protozoa and a rotifer) from the middle trophic level of a pitcher plant (Sarracenia purpurea) inquiline community were used to link species traits to previously observed patterns of species diversity and abundance. Traits associated with trade-offs (competitive ability, predator tolerance, and colonization rate) and other ecological traits (size, growth rate, and carrying capacity) were measured for each of the focal species. Traits were correlated with one another with a negative relationship indicative of a trade-off. Protozoan and rotifer species exhibited a negative relationship between competitive ability and predator tolerance, indicative of coexistence at the local community scale. There was no relationship between competitive ability and colonization rate. Size, growth rate, and carrying capacity were correlated with each other and the trade-off traits: Size was related to both competitive ability and predator tolerance, but growth rate and carrying capacity were correlated with predator tolerance. When partial correlations were conducted controlling for size, growth rate and carrying capacity, the trade-offs largely disappeared. These results imply that body size is the trait that provides the basis for ecological interactions and trade-offs. Altogether, this study showed that the examination of species' traits in the context of coexistence at different scales can contribute to our understanding of the mechanisms underlying community structure.

Figure 1. Mean interactions strength (± SE) of each species’ competitive ability (white bars) and predator tolerance (grey bars).

Higher values represent increased strength of that ecological trait. Species sharing a letter were not significantly different from each other based on Bonferroni post hoc tests (P = 0.05). The upper case letters are associated with competitive ability and the lower case letters are associated with predator tolerance. Species abbreviations are: Bo, Bodo sp.; Po, Poterioochromonas sp.; Ci, Colpidium sp.; Co, Colpoda sp.; Hr, Habrotrocha rosa.

Figure 2. Scatterplots of measured species’ traits (Competitive Ability [CA], Predator Tolerance [PT], Colonization Rate [Colonization], Size, Growth rate [r], and Carrying capacity [K]).

Bold borders indicate a significant relationship (P<0.05) from Spearman rank correlation, see Results for details. Correlations with competitive ability and predator tolerance address the local scale, and those including colonization address the regional scale. Species abbreviations are: Bo, Bodo sp.; Po, Poterioochromonas sp.; Ci, Colpidium sp.; Co, Colpoda sp.; Hr, Habrotrocha rosa.