Both host plant and ecosystem engineer identity influence leaf-tie impacts on the arthropod community of Quercus.

Many insect herbivores build shelters on plants, which are then colonized by other arthropod species. To understand the impacts of such ecosystem engineering on associated species, the contributions of ecosystem engineer and host-plant identities must be understood. We investigated these contingencies at the patch scale using two species of leaf-tying caterpillars, which vary in size and tie construction mode, on eight species of oak (Quercus) trees, which vary in leaf size and leaf chemistry. We created three types of artificial leaf ties by clipping together pairs of adjacent leaves using metal hair clips. We left the first type of leaf tie empty while adding individuals of the leaf-tying caterpillars of either Pseudotelphusa quercinigracella or Psilocorsis cryptolechiella to the other two. We also created a control treatment of untied leaves by affixing clips to single leaves. Leaf ties increased occupancy in the early season and arthropod alpha diversity throughout the experiment, on average fourfold. Furthermore, the presence of leaf ties increased arthropod species density on average three times and abundance 10-35 times, depending on the plant species. The mean phenolic content of the leaves of each oak species was positively correlated with the leaf-tie effect on abundance and negatively correlated with the leaf-tie effect on species diversity. Species diversity, but not abundance, was affected by the identity of the tie-maker. Arthropod species composition differed between untied leaves and artificial leaf ties, and between ties made by the two leaf-tier species. Our results demonstrate that the presence of leaf ties adds to habitat diversity within the oak-herbivore system, not only by creating a new kind of microhabitat (the leaf tie) within trees, but also by exacerbating differences among the eight oak species in apparent habitat quality. The identity of the leaf-tying caterpillar adds to this heterogeneity by creating leaf ties of different size, thus influencing subsequent colonization by other leaf-tying caterpillars of different sizes.