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Despite few food options, abundant assortment of species

A female fly (Blepharoneura) on a male flower of Gurania spinulosa.
[Image courtesy of Marty Condon]

A close look at tropical flies and the parasitic wasps that lay eggs inside them reveals an incredibly complex web of interactions, including some that would have remained hidden without new and advanced molecular techniques.

The complex web, details of which are published in the 14 March issue of the journal Science, raises questions about how species elsewhere in the world interact.

Explaining the structure of interactions between species is challenging for scientists. Species tend to occupy a space, called a niche, which contains the resources they need to survive and reproduce. But within that niche, how far they can move or how much of a certain food source they can eat might be restricted by interactions with predators and competitors.

Species try to reduce competition for resources by specializing to even more specific niches (for example, by eating different parts of the same plant). And scientists have thought this kind of specialization changing bit by bit to adapt to dissimilar niches is what helps generate the world's vast species diversity.

Particularly challenging for scientists to explain though, some systems, notably those in the tropics, appear to contain an exceptionally diverse population more species than they'd think to find there based simply on the number of niches.

Understanding the interactions among species that help to define such niches is hard, and it's made harder still for species that are difficult to observe.

Using new molecular techniques, scientists have made progress understanding how communities of plant-eating insects and the parasitic wasps that lay eggs in them interact, creating unique niches within a single host plant. At a site in Peru, Cornell College's Marty Condon et al. studied thousands of plant-eating insects whose larvae feed on the juicy parts of climbing squash flowers.

Using molecular methods to expose just how many fly and wasp species were present at this particular site, and how they interacted (both before and after wasps hatched), Condon and colleagues found extreme diversity: 14 fly species and 18 wasp species, all occupying just two species of climbing squash.

By looking at fly larvae samples before wasps hatched, the scientists found that some of the wasps didn't make it, suggesting that wasp offspring die if laid in the wrong species of fly (the fly's defenses kill wasp larva). If the wasps lay their eggs in the right fly species, however, they are successful.

The complex wasp/fly interactions described here generate a bunch of unique niches, which leads to extreme specialization and co-existence of a large number of both flies and wasps in a simple plant system (something that would appear to be a single resource when simply seen from the outside).

The authors argue that these highly specific interactions may contribute to the high levels of species diversity observed in tropical systems.