Parasitism rates are expected to increase in fragmented landscapes, but it is unclear how much of this is due to a reduction in habitat area versus increased fragmentation per se, as both increase the amount of edge habitat on the landscape. Habitat fragmentation, a landscape-wide property, is typically only studied at the patch scale, further complicating efforts to tease apart the relative effects of patch vs. landscape effects on parasitism rates. To address this, we used a novel experimental design in which landscape patterns were created from the “top down” by independently adjusting habitat amount and degree of fragmentation at the scale of the landscape, rather than from the “bottom up” by adjusting patch properties to create fragmented landscapes. Our experimental system consisted of 36 plots (16 m x 16 m) consisting of different amounts (10-80%) of red clover arrayed as either a clumped or fragmented distribution. We sampled pea-aphid populations within 10% of the habitat cells at random within each plot, and quantified parasitism rates as a fraction of aphids parasitized (aphid “mummies”).
Because 75% of interior cells came from clumped landscapes, and 68% of edge cells came from fragmented landscapes, the potential effect of habitat edge on parasitism rates was nested within fragmentation. Overall, there was a significant interaction between habitat amount and the nested fragmentation effect of habitat edges (whether habitat cells were at the edge or interior of a patch) on aphid parasitism rates. In clumped landscapes, parasitism rates were generally greater in edge than interior cells, but the difference was only significant in 40% and 80% landscapes. Conversely in fragmented landscapes, parasitism rates were generally greater in interior cells, especially within 40% and 60% fragmented landscapes. The biggest difference in parasitism rates was seen for interior cells within 40% fragmented versus 40% clumped landscapes (0.49 + 0.01 vs. 0.27 + 0.05, respectively). We thus conclude that the degree of habitat fragmentation (a landscape-scale property) affects parasitism rates, but only at intermediate habitat levels where edge effects (a patch-scale property) are neither so great nor so low as to render landscape effects unimportant.