By: Alex Wegmann, Project Manager, Island Conservation
Initial results from an August 2011 monitoring trip to Palmyra Atoll (Palmyra) are available and are positive! Although we continue to see anecdotal observations of birds, plants and invertebrates thriving, ongoing scientific monitoring will determine the impacts of removing non-native rats from Palmyra and the overall success of the project.
Palmyra Atoll National Wildlife Refuge is a moist Central Pacific atoll that supports one of the best remaining habitats for the broadleaf tree, Pisonia grandis, as well as 10 species of breeding seabirds totaling more than 325,000 individuals and a robust population of the world’s largest terrestrial invertebrate, the coconut crab (Birgus latro). Despite these riches, the atoll’s ecosystem had been greatly compromised by introduced rats (Rattus rattus) that were inadvertently brought to Palmyra during WWII. In the presence of rats, several native tree species experienced limited to no recruitment, and it is likely that rats caused the extirpation of up to eight seabird species.
Between June 1 and 30, 2011, a partnership between the US Fish and Wildlife Service, The Nature Conservancy, and Island Conservation successfully implemented a project to remove rats from Palmyra while minimizing harm to the ecosystem. By removing rats from Palmyra, the partnership aims to safeguard the atoll’s indigenous flora and fauna, encourage the reestablishment of extirpated seabird species, and create an ecological refuge for species within the Central Pacific region that are at risk of extinction. This project is a monumental conservation milestone for the Refuge, and has established a benchmark for future eradication campaigns on other tropical islands.
In the seven months since we completed the bait application, no rats have been seen anywhere on Palmyra, and anecdotal signs of recovery abound. The University of California Santa Cruz Coastal Conservation Action Lab (UCSC-CCAL) is monitoring the response of Palmyra’s terrestrial ecosystem by comparing measures of seabird, shorebird, and plant populations taken before and after rat removal. They conducted a baseline analysis of the several factors to provide a point of reference that is compared to measures collected after threats are removed. The methods used for this before-after comparison are measures of presence and abundance, and measures of interactions (e.g., seedling predation). Acoustic sensors record bird calls, and people count plant seedlings across the atoll. The team assesses the abundance of several native tree species by counting individuals along lines (or transects), and in long-term vegetation monitoring plots, and by counting seedlings around adult trees.
Removing rats is a significant first step in the restoration of Palmyra and our project team anticipated that many facets of the terrestrial ecosystem would quickly respond to their removal. Results from the August 2011 surveys demonstrate that native trees are responding quickly. Key results from the baseline and August 2011 monitoring surveys include:
• Total counts of tree seedlings increased significantly on 56 transects between pre-removal surveys in 2007 and post-removal surveys in 2011.
• Counts of two native tree species (Pisonia grandis, Pandanus fischerianus) increased significantly on 56 transects between pre-removal surveys in 2007 and post-removal surveys in 2011. In particular, counts of P. grandis seedlings increased from no seedlings observed in pre-removal (2007) to a post-removal (2011) average of 12.3 seedlings per transect.
• P. grandis seedling density measured across 8 long-term vegetation plots showed measureable change. While no seedlings were observed within long-term plots in pre-removal surveys (2008), we found seedlings in 3 long-term plots in post-removal surveys (2011). This included one long-term plot that contained a total of 2,420 P. grandis seedlings.
• Counts conducted in 2004 and 2011 under individual trees of 6 rare tree species (5 native, 1 non-native) showed a significant increase in the total number of seedlings as well as a significant increase in the number of seedlings of two species – Barringtonia asiatica and Hernandia sonora. While we do not have a direct link between the rat removal and this measurement of increased seedling recruitment, it is safe to assume that in the absence of rats – voracious seed predators – Palmyra’s native tree species will flourish. For example, we observed two small Cordia subcordata seedlings during the post removal monitoring trip; this is the first documentation of seedling recruitment for this locally rare species.
Seabird abundance appears roughly unchanged from that measured in previous years. However, because this year’s post removal monitoring effort occurred just 2 months after rat removal, a measurable difference in seabird abundance was not expected.
The Fish and Wildlife Service and other researchers on the Atoll will gather anecdotal and measured information on seabirds, shorebirds, plants, insects, and other animals as they live and reproduce without rats. Eight acoustic sensors across the Atoll are measuring trends in the presence and abundance of seabirds and shorebirds.
The monitoring team from UCSC-CCAL will return in August 2012 to continue monitoring. With the removal of introduced rats from Palmyra we expected to see a positive response by the atoll’s native plant, bird, reptile, and invertebrate communities. While the preliminary results from last year’s monitoring only show a strong response by the plant community, we expect that the other groups (native birds, reptiles, and invertebrates) will show a similar, strong response in due time.