Invasive species can reduce the ability of Department of Defense (DoD) land to support threatened and endangered species and recover from disturbance by negatively impacting important ecological functions. In many systems, invasive species cannot be eradicated, therefore, ecological function must be recovered in the presence of invasive species to make DoD land resilient to disturbance and able to sustain biodiversity. This project focuses on the island of Guam, where the DoD controls 27% of the land area including habitat for endangered plant and animal species. The brown treesnake was introduced to Guam in the 1940’s, and caused the functional extirpation of birds and bats which dispersed seeds of about 90% of the forest trees. This project addresses four objectives:
Objective 1: Determine how invasive species have impacted seed dispersal.
Objective 2: Identify the ecological roles of the native frugivores that have been lost and existing non-native frugivores.
Objective 3: Identify the challenges associated with using native to restore seed dispersal.
Objective 4: Synthesize results to identify feasible restoration options.
To understand the impact of frugivore loss on the forest, this project 1) measured how the focal tree species depend on seed dispersal for successful regeneration, 2) assessed the impact of disperser loss on forest regeneration in degraded habitat, and 3) modeled the cumulative impact of disperser loss on forest diversity, structure, and regeneration. To identify the roles of native and non-native frugivores as dispersers the project team 1) observed fruiting trees to identify dispersers, 2) conducted feeding and germination trials with candidate frugivores, and 3) tracked native frugivores to identify movement and dispersal patterns. To identify limitations associated with each candidate frugivore, the project team 1) monitored night roost fidelity, as species that have high roost fidelity are likely to be more successful in fenced areas and 2) monitored movement, diet and survival of the remnant Micronesian Starling population on Guam. Finally, the project team created a model to identify priority areas for restoring frugivores, an economic model to assess the cost of snake control, and a board game to use with decision-makers.
The project team found that most plant species benefit from gut passage by frugivores, whereas few species require dispersers to escape high mortality near the parent tree. A forest without dispersers will have lower diversity and slower recovery from disturbance. The most effective dispersers are the large-bodied birds and bats, as they have the broadest diets and move seeds the furthest. Non-native pigs and rats are less effective than most native species, although they do disperse some native seeds. Spatially, the project team recommends several areas on Anderson Air Force Base for restoring bird and bat populations. Given current technology and pricing, fencing plus toxicant drops becomes less expensive than unbounded toxicant drops after approximately 34 years. Reducing the cost of fence upkeep or baits significantly influences the long-term cost.
Seed dispersal is a critical ecological function, and forests without dispersers will decrease in diversity and be less capable of recovering from disturbance. Larger-bodied frugivores tend to have broader diets and move seeds long distances, and therefore are a good immediate choice for restoring function to as many plant species as possible. The Micronesian Starling is still present on Guam and is an effective seed disperser, but is limited by the brown tree snake and cats, emphasizing the need for predator prior to bird recovery. The project team recommends that snake control and bird and bat recovery be prioritized on lands with intact limestone forest. Overall, this project provides a roadmap for recovering ecological function in a highly degraded ecosystem.