Three-way interactions mediate spatial patterns of carnivorous pitcher plants, herbivorous moths, and ground-foraging ants in New England bogs

Northern pitcher plants (Sarracenia purpurea) are at the center of a tritrophic interaction web. Ants (especially Myrmica lobifrons, Tapinoma sessile, and Dolichoderus pustulatus) forage for sugars at the plant's extrafloral nectaries, occasionally are captured by this carnivorous plant, and use dead pitchers as nest sites. Larvae of two noctuid moths (Exyra fax and Papaipema appassionata) are obligate herbivores of S. purpurea. Exyra larvae drain and kill individual S. purpurea pitchers, whereas Papaipema larvae feed on roots and kill entire plants. These feeding activities simultaneously reduce the amount of available sugars, nest sites, and ant predators. In the summer of 2006, we mapped the small-scale spatial distribution of pitcher plants, moth larvae, and ant nests in 100-m² quadrats at 20 pitcher-plant bogs in Massachusetts and Vermont. Among bogs, the frequency of moth attack, the size and density of pitcher plants, and the density of ant nests and occurrences were related to site latitude, longitude, elevation, and cover of woody vegetation. Within sites, the spatial distribution of plants attacked by moths was patchy and highly localized. Ant occurrences were also patchy, but were related more to microtopography than to the occurrence of nesting sites from moth attacks. Positive and negative interactions between ants, plants, and moths may preclude simple correlations at small spatial scales.