Results from an earlier post, based on REEF data only, suggested that the Jamaican reef food web is more connected than either the Caymans or Cuba. Our augmented data set (see previous post) now confirms this. Jamaica, of intermediate vertebrate species richness (S=160) has a connectance, C=0.06032, while the Caymans are S=156, C=0.05949, and Cuba, S=176 and C=0.05972. Are these differences in any way significant? That’s a difficult question to answer, since we really don’t know if there is a distribution underlying food web connectance. We tested it, again as reported earlier, but asking the following question: “Given a regional species pool comprising all species observed on at least one of our islands, what is the expected C for a system of size S?” To answer the question, we generated numerous food webs with random draws from that regional species pool (1,000 food webs per S shown in the figure). There is an expected regular relationship between C and S (blue regression line), but Jamaica is well above the curve. Random draws for S equalling exactly the richnesses of the island food webs confirms that Jamaica is more significantly connected than would be expected if it was a random draw from the regional species pool (10,000 randomizations, p=0.0054; p=0.2296 and 0.0548 for the Caymans and Cuba respectively).
Recall that the formula for connectance is
Two questions now come to mind. First, why does C increase with S? And second, how can Jamaica be more connected than Cuba? The answer for the first question lies in the shape of the link distribution curves. They are right-skewed and long-tailed. This means that a random draw from one of those distributions is likely to yield a species of intermediate to low in-degree, that is, a species with a low density of links. But as the number of species drawn approaches the maximum number of species available, the probability of drawing species out on the long tail, that is, link dense species, increases. So as far as C is concerned, you’re getting more bang for you buck. Okay, then why is Jamaica’s connectance greater than Cuba’s? Because Jamaica is not an unbiased or random draw from the pool. Somehow, Jamaica has an unusually high component of link-dense or high degree species. It’s drawing preferentially from the long tail! A better way to view this, is that Jamaica is lacking in trophic specialists.
A little data exploration shows this to indeed be the case. If we compare species present in one of our communities to species absent., but present in one or both of the other two communities, it is clear that Jamaica has a lower than expected number of specialized reef foragers. In other words, if you’re looking for species that forage only on the reef (and not seagrass beds), and that have a specialized diet, don’t expect to have as much luck finding them in Jamaica as you would in the Caymans or Cuba. Why? Well, that’s a very difficult question to answer. Perhaps specialists have a more difficult time establishing themselves as new colonists; but they don’t seem to have a problem other than in Jamaica. Maybe specialists, with their smaller number of resources, are more prone to extinction if the system is disturbed in some way. Or, maybe us Jamaicans find specialists to be tastier than generalists! We’ll explore these possibilities in future posts.