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A leaf tie and (inset) a Pseudotelphusa caterpillar. The paler patches in the main picture are where the leaf material has been consumed by the caterpillar within.
Ecology 84, 682­690 (2003)

Caterpillars make oak trees better environments for other insects. John T. Lill and Robert J. Marquis have found that, without one industrious species, the diversity of insect herbivores on trees falls.

Pseudotelphusa caterpillars build a tent by tying together a pair of adjacent leaves on white oaks (Quercus alba). They take refuge in this shelter, and slowly eat their home from within (see picture). 

Lill and Marquis removed caterpillars from 93 trees, and then made artificial shelters on 62 of them ‹ to make sure that it was this that made the difference, and not some other aspect of caterpillar biology.

On trees without caterpillars or artificial shelters there was a steep reduction in the number of species of leaf-chewing insects, including other caterpillars, sawflies and beetles. It seems that the shelters protect other species from inclement weather and predators, and many of the squatters seek them out to lay their eggs on. White oak is a dominant tree in the forests of eastern North America. So by acting as ecosystem engineers, the common shelter-building caterpillars may be having a significant effect on the environment as a whole.