Coral reef food web II

Trophic link distribution

What sort of network is the coral reef food web? In other words, how are the links or interactions between nodes in a food web distributed? Food webs have been modelled variously as everything from random (Poisson) networks to networks based on exponential, power law or mixed distributions, with or without hierarchical structure. Empirical measures suggest that link distributions in real world food webs follow exponential or power law distributions, perhaps a mixture of both (differentiated by scale). One of my worries with those measures is that they are based on food webs of varying sizes, and more importantly, levels of taxonomic and ecological resolution. So, for example, how much does it matter if your food web covers only a small part of the community’s taxonomic diversity, or only part of the trophic diversity? What about the level of aggregation of species into more inclusive groups? The high resolution of the coral food web presents an opportunity to address some of these questions, and here’s the first one: How are trophic in-links distributed at the guild level? Recall that guilds here are groups of species with potentially the same prey and predators. I say potentially, for while we have very specific trophic data for some species, e.g. heavily studied fish, data are less certain for many smaller or less well known species. Still, there are 265 guilds in this dataset, and 4,756 links (see previous post). The histogram is a basic frequency histogram of the number of links per guild. As predicted on the basis of previously studied food webs, the distribution is a (right-skewed) decay distribution, with a greater number of species possessing fewer prey, i.e. being relative specialists, and a few species having a broad repetoire of prey, i.e. relative generalists. The extreme generalists (to the right or tail of the distribution) are all large sharks, the most extreme being the tiger shark, Galeocerdo cuvier. These species range from microscopic, single-celled dinoflagellates to large carcharhinid sharks!

What type of distribution is this? A simple logarithmic transform of the data is shown in the second figure, and regression of the data yields the following function: y = 17238x^-1.9496 (r-squared=0.95). The significant and extremely good fit of a linear function to the transformed data suggests that the underlying link distribution is a power law distribution of the form , where is the link probability, is the number of prey available, and is the power law exponent. An exponent of ~1.95 is tantalizingly close to other empirical measures. Even more exciting, for me at least, is the fact that we have predicted on the basis of previous work that power law exponents that promote resistance or robustness to secondary extinctions should lie in the range 2-2.5. That work was based on terrestrial food webs from the Late Permian, 250+ million years ago!

Coral reef food web I

Caribbean coral reef food web

I’ve been compiling data for a Caribbean coral reef food web. This is intended to be a “typical” coral reef of the Greater Antilles region, focusing on Jamaica. Data are drawn, however, from a more general region encompassing the Cayman Islands, Jamaica, Cuba, Hispaniola, Puerto Rico and the U.S. Virgin Islands. The U.S.V.I. were included because of the large amount of data available for the reefs there, particularly fish. It has been a rather large task to assemble species lists for this region because of the tremendous species richness of the reefs, as well as the scattered nature of the literature. Most major animal groups have been included, with notable exceptions being barnacles, sea stars, and some minor but probably important groups, such as sipuncula, echiura, crinoids and brachiopods. All major producer groups are also specified at the species level, including nannoplankton, diatoms, macroalgae, etc. The community comprises the reef habitat and adjoining seagrass beds.

The current compilation includes a total of 905 species, for which trophic data are available for 761 (84%). The 761 species are further collapsed into 265 guilds. Guilds range in size from 1 species up to 54 species (symbiont-bearing scleractinian corals). There are 4756 links among guilds, yielding a guild-based connectance of 0.068, well within the range of connectances for published, lower resolution communities. That’s a lot of stuff happening on the reef! It is surprising to realize, though, how little we know about many familiar species, or perhaps how poorly documented that knowledge is. The situation would be far worse if I accounted to the true diversity of the reef, which must range into several thousand species. One can only imagine the difficulties in attempting this with an Indo-Pacific reef or a tropical rain forest. Sadly, the current condition of many Caribbean reefs means that my compilation is an overestimate, being based on accounts dating back to the 1950’s, when the reefs were still in reasonably good shape, by 20th century standards anyway.