Predation increases functional diversity in the tropics revealing latitudinal variation in community assembly

1 Aim

Increasing evidence suggests that species interactions are stronger at low latitudes, and serve as biotic filters in community assembly processes while contributing to the maintenance of contemporary patterns of diversity (Freestone et al., 2011; Weiher et al., 2011; Freestone & Inouye, 2015). Functional diversity measures the distribution of species in trait space such as the level of clustering, or functional divergence, and can help differentiate community assembly processes (Spasojevic & Suding, 2012). Theory predicts that traits will over-disperse and fill more functional space when biotic filters such as predation limit the similarity of traits among species (Weiher et al., 2011). Therefore, we tested the hypothesis that predation would result in greater functional divergence in the tropics, while predation would have no effect on functional divergence at higher latitudes.

2 Question and problem

Does predation differentially influence the level of clustering of trait space occupation (i.e., FDiv) between temperate and tropical regions?

We expect predation to be stronger in the tropics affecting clustering levels between open and excluded communities, while having no effect in temperate regions.

3 Location and grain

• We used predator exclusion experiments on sessile marine invertebrate communities in four regions of the Pacific coast, across 47-degrees of latitude (i.e., Alaska, California, Mexico, and Panama). For this report, I am excluding Mexico since data collection is still ongoing.

Study locations

• Our focal communities grow in replicated 14 X 14 cm PVC panels placed one meter below the water surface and allowed to grow undisturbed for three months. The grain of our focal communities is the size of the panel, however each replicated community represents a community from a region.

Experimental units and treatments

4 Methods

Three data types, richness, abundance, and trait information, are needed to quantify community functional diversity indices. Richness and abundance were collected from panels deployed for three months in caged or uncaged (no cage and partial cage control) treatments to exclude and allow predation respectively (5 panels x 3 treatments x 3 sites x 4 regions = 180 panels).

Data collection

Trait data is being collected from three different sources: literature, field, and preserved vouchers. The degree of trait clustering will be measured with the functional divergence index (FDiv)(Villéger et al., 2008), and will be quantified for each community/panel.

Functional trait space

5 Results

We expect to find that predation increases functional divergence in comparison to communities that assembled in predator exclusions in the tropics, with no difference in functional divergence among treatments observed at higher latitudes.

ALASKA

Sample community

As expected, we found no difference in FDiv between full (predation excluded) and open treatments in Alaska

CALIFORNIA

Sample community

Also as expected, we found no difference in FDiv between full (predation excluded) and open treatments in California

PANAMA