Vegetation structure modulates ecosystem and community responses to
Ecosystem responses to external inputs of nutrients and organisms are highly variable. Theory predicts that ecosystem traits will determine the responses to spatial subsidies, but evidence for how vegetation structure can modulate those effects is lacking. We investigated how vegetation structure (i.e., leaf area index [LAI] and vegetation height) influenced the ecosystem and community responses to insect spatial subsidies in a subarctic grassland. Our experiment consisted of a 2 x 2 manipulation where in one treatment we either blocked flying insects over a 2-year period in 1-m2 plots near the shore of Lake Mývatn, Iceland where deposition of aquatic adult midges (Diptera: Chironomidae) to land is high, or we left control plots accessible to flying midges. In the second treatment, grassland vegetation was cut (Tall vs. Short) at the start of each season and then allowed to regrow. Within each plot (n = 6 replicates x 4 treatments), we measured litter decomposition and arthropod composition and density. Midge-exclusion cages reduced midge deposition by 81% relative to the open plots. Vegetation cutting initially reduced LAI and vegetation height by 3x and 1.5x, respectively, but these were not different by the end of the second growing season. We found that vegetation structure modulated the effects of midge subsides on litter decomposition, with taller canopies intercepting more insect subsidies than shorter ones, leading to 18% faster litter decomposition. In contrast, the short-vegetation plots intercepted fewer subsidies, had higher temperatures and sunlight, and thus resulted in no effects of midges on decomposition. However, by the end of the experiment when all vegetation structure characteristics had converged across all plots, we found no differences in decomposition between treatments. The effects of midge subsidies on arthropod composition depended on vegetation structure in the last year, suggesting that arthropod predators might also be responding to vegetation structure effects on insect subsidies. Our findings indicate that vegetation structure can modify the quantity of subsidies entering a recipient ecosystem as aerial insects, resulting in ecosystem- and community-level responses. Thus, changing vegetation structure via habitat disturbances will likely have important implications for ecosystem functions that depend on spatial subsidies.