An Interdisciplinary Approach to Restoration: Hawaiian Seabirds as a Case Study

Abstract

Seabirds are experiencing dramatic declines in both their ranges and populations, resulting in decreases in ecosystem services they provide. Seabird breeding islands were historically rodent and mostly predator free, allowing seabirds to nest colonially and deposit large quantities of guano and other organic material. On average seabirds can increase inputs of nitrogen (N) and phosphorus, the two most common limiting and co-limiting nutrients to primary productivity, by 100 and 400% respectively. The goal of this research was to improve understanding of the impacts of decreased seabird numbers on ecosystems and the challenges to restoration. To address how losses of seabirds affect island ecosystems, the objectives of this research were to gain a better understanding of the historic role that seabirds played in the past, how that compares to current nutrient deposition, and how current efforts to restore seabird populations affect the native ecosystems. Using historical data and species habitat density models, I determined that seabird deposition of nitrogen into Hawaiian ecosystems was likely three – four orders of magnitude higher than it is today. During the pre-human era, seabirds could have deposited 1,460 – 5,290 kg of N ha -1 year -1. Based on current population estimates, and historic habitat, seabirds are currently contributing 0.535 kg of N ha-1 year-1. To address the current impact of seabirds on montane systems in Hawai‘i, I measured inorganic labile soil nutrients δ15N of seabird and non-seabird plots to determine marine-sourced N in the soil and foliage of two dominant plants. More NH4+ was found in the soil of seabird colonies than non-seabird colonies, and 28% of foliar N in the dominant tree and 17% of foliar N in a dominant understory plant, were from marine source. However, plant species composition was similar between seabird and non-seabird areas, despite differences in nutrient availability. Finally, I determined that costs of management actions vary widely depending on terrain and accessibility of the site, but all actions have positive ecosystem services benefits. As restoration of native ecosystems continue to be a priority, understanding the role seabirds played in the past and how they currently contribute to the ecosystem are critical for effective restoration efforts.