PhD Candidate in Ecology and Evolutionary Biology, Cornell University
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Does pollinator sharing affect the distribution of floral traits in communities?

Are Clarkia communities structured by ecological sorting or character displacement?


Closely related species are often predicted to compete for the same resources, yet many communities contain closely related species.

Two processes that can promote the coexistence of closely related species are ecological sorting and character displacement. Ecological sorting occurs when strong competition during community assembly excludes inferior competitors from a community, while character displacement occurs when species experience selection in sympatry to evolve trait differences that reduce competition.

While both of these processes were first articulated as being driven by resource competition, shared interactions with predators, mutualists, or facilitators can also generate ecological sorting and character displacement.

Because pollinators are often shared among co-occurring plants and can be essential for reproduction, the indirect interactions among co-occurring plants that share pollinators can lead to ecological sorting and character displacement.

 
Clarkia unguiculata  blooming in the greenhouse common garden at Cornell University.

Clarkia unguiculata blooming in the greenhouse common garden at Cornell University.

 
Figure 1 in Eisen and Geber 2018 in JEB.

Figure 1 in Eisen and Geber 2018 in JEB.

This figure represents the expected distribution of traits if pollinator sharing generates ecological sorting or character displacement.

Trait aggregation can occur within a community due to habitat filtering, competition, or facilitation (A-C).

Trait segregation can occur within a community in response to competition (D-F).

Character displacement will occur when sharing pollinators does not lead to competitive exclusion but instead generates selection for phenotypic change. In response to either competition or facilitation for pollination, species phenotypes may diverge (H,I) or converge (J,K).

We explored how these interactions may affect communities during both community assembly (ecological sorting) and through ongoing, in situ evolution (character displacement), and how the effects of these interactions may change with community context.

To determine if communities display patterns consistent with ecological sorting, we assessed the frequency of co-occurrence of four species of Clarkia in the southern Sierra foothills (Kern County, CA, USA).

To investigate potential character displacement, we measured pollination-related traits on plants grown in a greenhouse common garden from seed collected in communities with one, two or four Clarkia species.

We found that among the four species of Clarkia in this region, the two species that are often found in multi-species communities also co-occur with one another more frequently than expected under a null model, which is consistent with ecological sorting.

Patterns of trait variation in a common garden suggest that these two species have diverged in floral traits and converged in flowering time where they co-occur, which is consistent with character displacement. Trait variation across community types also suggests that the process and outcome of character displacement may vary with community context.

This work was published in the Journal of Evolutionary Biology. Click here to check out the full text of the paper.