Interactive effects of landscape fragmentation and resource heterogeneity on consumers

Landscape fragmentation, the dissection of landscapes into isolated parts, represents a pervasive source of environmental change worldwide.  Although impacts of fragmentation on populations and communities have been widely described, far less is known about effects of fragmentation on individuals.  We hypothesize that the isolating effects of fragmentation can limit the ability of consumers to respond to heterogeneity in food resources, and in so doing, can harm their nutritional status.  By influencing food consumption of individuals, fragmentation may shape dynamics of populations.  We conducted a manipulative experiment to determine how fragmentation and spatial  heterogeneity in food resources influence consumers.  We imposed three levels of fragmentation (1, 4, and 16 fragments) and two levels of food resource heterogeneity in a 3x2 randomized complete block design.  Each landscape was uniformly stocked with 400 cabbage looper (Trichoplusia ni) larvae.  After 17 days, larvae were counted and weighed.  Increasing fragmentation reduced average mass of survivors (0.163g low fragmentation, 0.156g high fragmentation; p<0.05). Resource heterogeneity decreased survivorship at high levels of fragmentation (74.0% homogeneous, 67.8% heterogeneous; p<0.05), but not at low fragmentation.  No effects of heterogeneity on survivorship or average mass were detected between unfragmented landscapes.  These results suggest that patchiness of resource distribution can exacerbate negative effects of habitat fragmentation on populations and that fragmentation can decrease nutritional status of individual consumers. We clearly demonstrate that isolation created by habitat fragmentation can amplify the effects of habitat loss.