Seabirds are ecosystem engineers that drive the bottom-up control of many island food webs by depositing large amounts of nutrient-rich guano into the islands in which they breed. This important function is threatened by invasive species, such as rodents, which led to a widespread attempt to eradicate them from islands. Such eradications result in the slow return of breeding seabirds and recovery of soil, plant, and spider nutrient levels. However, little is known about this ecosystem response in the nearshore systems. The project examines the nearshore recovery of seabird islands within the Mercury Islands chain in northern New Zealand. These islands vary in invasion and eradication history from those never invaded to those previously eradicated in 1986 and recently eradicated in 2014, which is directly related to seabird density. The goal of this research is to determine if never invaded islands and eradicated islands differ in algal community structure and stable isotope enrichment. Sampling occurred during the dry and rainy season. Macroalgae surveys quantified species richness and diversity and the most common species were collected and analyzed for δ15N (ornithogenic Nitrogen) and δ13C stable isotope content. Isotopic differences between species, seasons, and islands were identified using linear mixed models.
Macroalgae surveys indicate significantly higher species richness and diversity on never invaded islands, followed by previously and recently eradicated islands (Shannon Index: 0.765±0.053, 0.562±0.049, and 0.342±0.054 respectively; ANOVA p<0.001). There was significant nitrogen isotope enrichment for five of the six species during the rainy season, which is expected since frequent and heavy rains would transport nitrogen rich guano from the terrestrial system into the nearshore environment. During the dry season, there was a noticeable trend of decreasing δ15N with increasing depth. While during the rainy season, most species maintained a relatively high and uniform δ15N value. It appears that during the dry season, seabird-derived nitrogen that enters the nearshore system is taken up by algae in the shallowest areas, with little available in the deeper waters. When comparing isotope values between islands, three of the six species showed significant isotope enrichment on the never invaded islands when compared to the eradicated islands (p<0.05). More research is needed at islands differing by eradication year and burrow density. If a similar trend to terrestrial isotope sampling is found, this research could validate a new cost- and time-effective method of monitoring seabird islands undergoing restoration, minimizing disturbance to the already fragile ecosystem.