Spatial variability in plant and soil properties on New Zealand seabird islands, and the effects of introduced rats

Abstract

Seabirds are ecosystem engineers with two major impacts on island ecosystems: they bring large quantities of marine nutrients to the terrestrial environment in the form of guano, carcasses, feathers, eggs, and spilled food, and they disturb the soil surface. Burrowing seabirds can denude the soil surface of all seedlings and leaf litter, plowing them under and loosening the soil. However, seabirds are colonial, and burrows are not evenly spaced over the surface of an island, producing spatial variability within a single island that might reveal how seabird activities control island ecosystem function. In this dissertation I review seabird island ecology in general, focusing on how introduced predators have reduced seabird populations, interrupting seabird activities and altering island ecology. I then describe three studies designed to quantify the effects of seabirds on soil and plant properties within individual islands and compare these patterns across islands varying in seabird density, especially where seabirds have declined because of invasive rats (Rattus spp.). I used geostatistics to quantify the spatial variance in seabird burrows and various soil and plant properties (including soil and leaf N) within six islands of low, intermediate, and high burrow density. I found that burrow density was not a good predictor at within-island scales, and though the variance of some soil properties (pH, soil δ¹⁵N, and soil compaction) peaked on intermediate islands as expected, variables reflecting the soil N cycle (net ammonification and net nitrification potential, NH₄⁺ and NO₃⁻) continued to increase in variability on very highdensity seabird islands. Ecosystem properties clearly responded to seabirds at different spatial scales, possibly because seabirds deposit guano at different spatial scales than they dig. Using data within three rat-invaded and three rat-free islands, I used structural equ tation models to examine seabird influences on N cycling. I found some mechanisms that were constant across islands, such as seabird-related decreases in soil water and pH, but other mechanisms differed between invaded and uninvaded islands, suggesting that rats alter seabird control over island N cycles, thus manifesting an alternative island state which may or may not be reversible. Finally, I investigated whether plants can use ammonia (NH₃ gas) volatilized from seabird islands, measuring NH₃ concentrations across 10 islands and within a single island where I also experimentally manipulated plant N demand. Both rat-invaded and rat-free islands produced meaningful concentrations of NH₃ gas, and multiple plant species including Melicytus ramiflorus and Coprosma macrocarpa used it for up to 20% and 30% (respectively) of their total leaf N. Plant N demand modified NH₃ uptake, suggesting that plants located not on seabird colonies, but downwind, may benefit the most from this gaseous N source. I suggest that future studies attempt to estimate thresholds of burrow density at which seabird-controlled ecosystem properties can recover from rat invasion.