Ecologists often think of organisms in terms of their niche – the environment and resources used by an organism, or alternatively, the functional role an organism plays. A fish’s niche, for example, could be “underwater habitat”, or “algae consumer”. We generally think that all available niches are filled, so that all available resources and habitat are used (otherwise new species would presumably invade the community and take advantage of these unused resources). Examples of empty niches are rare, but it is also very difficult to prove something is missing – or for that matter, define the exact niche of any given species. (How many different ways of describing an organism is sufficient to characterize the niche? Does this niche depend on the presence of other species?) Ecology remains an imprecise science.
Puerto Rico is an example of an island where several niches appear to be unfilled. One of the most striking examples is the lack of leaf herbivory – most forests are filled with leaves that have been eaten away by a wide range of insects. Here, nearly all the leaves we collect are intact, with no signs of damage at all.
There is also a dense population of land snails – more than thirty species in this forest. Here is one, Caracolus caracolla. It is six feet above the ground, hanging from a sierra palm leaflet – a testament to the ‘slow and steady’ approach to life. Nothing seems to eat these snails, despite their high population density and thin shells.
And finally, there are many species of small lizards, but nothing (aside from a few birds) that eat them. Here is an anole that caught a ride on a headband after a day of fieldwork.
What’s happening here? Why are there apparently missing links in food webs, and unexploited resources? Islands have two unique properties – first, their distance from mainlands means that all species must have immigrated to the island at some point. Perhaps the missing herbivores and predators couldn’t reach it. But insects can disperse over very long distances, and this island is not very far from other parts of the West Indies and North America. Another possibility is human impacts – perhaps our activities have caused the local extinction of all the relevant species. This idea is plausible, because there is fossil evidence for several mammals (several rodents and shrews) that were on-island before human colonization but that disappeared shortly after the arrival of Europeans (Turvey et al. 2007).
We don’t have the answer, but ponder it each day we walk through this strangely empty island!