Predation increases functional diversity in the tropics revealing latitudinal variation in community assembly
Diana Lopez
5/03/2018
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
Sample community
However, in our tropical region we found a marginal statistical difference in FDiv between full (predation excluded) and open treatments indicating that predation may be stronger at lower latitudes and contribute to community assembly.
## numDF denDF F-value p-value
## (Intercept) 1 26 6933.547 <.0001
## Treatment 1 26 4.662 0.04036 Conclusions
In the tropics, predation had an effect on functional diversity of communities that developed in the presence/absence of predation.
Communities exposed to predation were more dispersed in functional space, suggesting predators may promote the coexistence of species that differ in functional strategies.
Our data suggest that predation may have a lower influence on community assembly in temperate regions. Perhaps, alternative assembly mechanisms such as environmental variability are stronger at higher latitudes.
This report shows a basic preliminary analysis that provide evidence of stronger biotic interactions, specifically predation, in tropical regions when compared to temperate latitudes.
The underlying mechanisms that drive global patterns of biodiversity are still a topic under investigation (Brown, 2014). Stronger predation and its influence on assembly mechanisms would provide some evidence that species interactions contribute to the maintenance of diversity patterns.
7 References
Brown, J.H. (2014) Why are there so many species in the tropics? Journal of Biogeography, 41, 8-22.
Freestone, A.L. & Inouye, B.D. (2015) Nonrandom community assembly and high temporal turnover promote regional coexistence in tropics but not temperate zone. Ecology, 96, 264-273.
Freestone, A.L., Osman, R.W., Ruiz, G.M., & Torchin, M.E. (2011) Stronger predation in the tropics shapes species richness patterns in marine communities. Ecology, 92, 983-993.
Spasojevic, M.J. & Suding, K.N. (2012) Inferring community assembly mechanisms from functional diversity patterns: The importance of multiple assembly processes. Journal of Ecology, 100, 652-661.
Villéger, S., Mason, N.W.H., & Mouillot, D. (2008) New Multidimensional Functional Diversity Indices for a Multifaceted Framework in Functional Ecology. Ecology, 89, 2290-2301.
Weiher, E., Freund, D., Bunton, T., Stefanski, A., Lee, T., & Bentivenga, S. (2011) Advances, challenges and a developing synthesis of ecological community assembly theory. Philosophical Transactions of the Royal Society B: Biological Sciences, 366, 2403-2413.