Seabirds as agents of spatial heterogeneity on New Zealand's offshore islands

AimsThis study investigates how burrow-nesting, co-lonial seabirds structure the spatial patterns of soil andplant properties (including soil and leaf N) and testswhether burrow density drives these spatial patternswithin each of six individual islands that vary greatlyin burrow density.MethodsWithin individual islands, we comparedsemivariograms (SVs) with and without burrows as aspatial trend. We also used SVs to describe and comparethe spatial patterns among islands for each of 16 soil andplant variables.ResultsBurrow density within a single island was onlyimportant in determining spatial structuring in one-fifthof the island-variable combinations tested. Amongislands, some variables (i.e., soil pH,δ15N, and com-paction; microbial biomass and activity) achieved peakspatial variance on intermediate-density islands, whileothers (i.e., net ammonification, net nitrification, NH4+,NO3-) became increasingly variable on denselyburrowed islands.ConclusionsBurrow density at the within-island scalewas far less important than expected. Seabirds and otherecosystem engineers whose activities (e.g., nutrient sub-sidies, soil disturbance) influence multiple spatial scalescan increase spatial heterogeneity even at high densities,inconsistent with a"hump-shaped"relationship be-tween resource availability and heterogeneity.