Ecosystem Engineering by Caterpillars Increases Insect Herbivore Diversity on White Oak

By creating or modifying habitats used by other organisms, physical ecosystem engineers can influence local patterns of biological diversity. However, there have been very few empirical studies quantifying engineering effects in different biological systems. In this study, we examined the effect of shelter-building caterpillars on the species richness and guild structure of leaf-chewing herbivores occupying individual white oak (Quercus alba) saplings. For each of two years, we disrupted leaf-tie formation on 93 white oak saplings during a three-week period in early summer that coincided with the peak in leaf-tie construction by the most common leaf-tying caterpillar, Pseudotelphusa sp. (Gelechiidae). We then created artificial leaf ties on 62 of these saplings by clipping together adjacent pairs of leaves (10% of the foliage) on each tree. For 31 of these trees, we also placed a single Pseudotelphusa caterpillar into each artificial tie to examine possible nonengineering effects associated with the caterpillar in addition to its shelter-building activity. One month later, removal of leaf ties had reduced the average density of leaf ties present on a tree by 61% (1999) and 54% (2000). This effect persisted throughout the season, indicating that no species assumed the engineering role of Pseudotelphusa sp. once it was removed. The decrease in shelter availability significantly decreased mean cumulative species richness of leaf-chewing insects by 38% (1999) and 14% (2000) compared to the two artificial tie treatments (with and without caterpillars initially), which did not differ from one another. Herbivore community composition also differed significantly between the removal and artificial tie treatments in both years, due to increased species richness of leaftying caterpillars (1999 and 2000), sawflies (1999 only), and beetles (2000 only) on trees with artificial ties. By creating leaf shelters early in the season, Pseudotelphusa sp. caterpillars have large and persistent effects on seasonal patterns of herbivore recruitment to and/or retention by white oak trees.