Biotic interactions underlie ecosystem structure and function, but predicting interaction outcomes is difficult. We tested the hypothesis that biotic interaction strength increases toward the equator, using a global experiment with model caterpillars to measure predation risk. Across an 11,660-kilometer latitudinal gradient spanning six continents, we found increasing predation toward the equator, with a parallel pattern of increasing predation toward lower elevations. Patterns across both latitude and elevation were driven by arthropod predators, with no systematic trend in attack rates by birds or mammals. These matching gradients at global and regional scales suggest consistent drivers of biotic interaction strength, a finding that needs to be integrated into general theories of herbivory, community organization, and life-history evolution.

No abstract

Summary 1.We describe the pattern of colonization of suitable, but currently empty, habitat by a host butterfly and two specialist parasitoids living in a highly fragmented landscape. 2. Using survey data collected over 8 years, field sampling and small-scale experiments we show that the ability of the Glanville fritillary butterfly (Melitaea cinxia) to colonize new habitat is intermediate between that of its two larval primary parasitoids. 3. The butterfly forms a classic metapopulation, which the parasitoid Hyposoter horticola experiences as a single patchily distributed host population because of its high rate of dispersal and long colonization distances. In contrast, most of the local butterfly populations are presently inaccessible to the parasitoid Cotesia melitaearum, which has a limited dispersal range and therefore persists only in tightly clustered networks of host populations. 4. At the regional scale, the butterfly may escape C. melitaearum by colonizing empty habitat, but host dispersal does not limit parasitism by H. horticola, which consequently must be limited by local interaction. 5. The parasitoid H. horticola mostly avoids direct competition with C. melitaearum because the majority of H. horticola populations are outside the range of dispersal by current C. melitaearum populations. In contrast, all C. melitaearum populations experience competition with H. horticola.

Studies in crop species show that the effect of plant allelochemicals is not necessarily restricted to herbivores, but can extend to (positive as well as negative) effects on performance at higher trophic levels, including the predators and parasitoids of herbivores. We examined how quantitative variation in allelochemicals (iridoid glycosides) in ribwort plantain, Plantago lanceolata, affects the development of a specialist and a generalist herbivore and their respective specialist and generalist endoparasitoids. Plants were grown from two selection lines that differed ca. 5-fold in the concentration of leaf iridoid glycosides. Development time of the specialist herbivore, Melitaea cinxia, and its solitary endoparasitoid, Hyposoter horticola, proceeded most rapidly when reared on the high iridoid line, whereas pupal mass in M. cinxia and adult mass in H. horticola were unaffected by plant line. Cotesia melitaearum, a gregarious endoparasitoid of M. cinxia, performed equally well on hosts feeding on the two lines of P. lanceolata. In contrast, the pupal mass of the generalist herbivore, Spodoptera exigua, and the emerging adult mass of its solitary endoparasitoid, C. marginiventris, were significantly lower when reared on the high line, whereas development time was unaffected. The results are discussed with regards to (1) differences between specialist and generalist herbivores and their natural enemies to quantitative variation in plant secondary chemistry, and (2) potentially differing selection pressures on plant defense.