Aquatic insect deposition onto land promotes grass dominance in subarctic heathlands
Mobile insects are a potential natural source of nitrogen (N), a limited nutrient in most ecosystems. Vegetation surrounding productive aquatic systems may be able to take advantage of emergent insect N. However, we do not know how plant communities respond to insect N inputs. Lakes in N.E. Iceland support large-scale insect (midge) emergences that result in significant N input to land as midge carcasses. Our objective was to quantify the relationship between midge inputs, and plant community composition and function. Multiple transects were set up perpendicular to lakeshore at 7 lakes with varying levels of midge emergence. We measured midge deposition and a series of plant variables at 5, 50, 150, and 300-500 m from the lake. We expected fast-growing plant species (e.g., graminoids) to be dominant when midge inputs were high and slow growing plant species (e.g., heath and woody) to be dominant when inputs were low.
Insect inputs affected plant community composition and function. Inputs had positive effects on graminoid and forb abundance, but negative effects on heath abundance. Midge inputs had positive effects on graminoid and woody aboveground biomass production and overall belowground productivity. When broken down by species, Vaccinium uliginosum (heath species) significantly decreased leaf C:N in response to high midge inputs. Overall, graminoids were positively influenced by midge deposition, as graminoid abundance and productivity increased in areas of high inputs (i.e., near the lake shore). The strong relationship between graminoids and midge inputs suggests that graminoids are able to take advantage of midge-derived N much faster than other functional groups. Overall, insect carcasses are a strong source of N in N-limited systems and alter heathland plant community structure and function.