Palmyra Atoll has been free of invasive rats for more than three years and native plants and animals are steadily recovering! A team of four researchers recently returned from a 2014 trip to conduct scientific monitoring to determine the impacts of removing non-native rats. Since the last scientific monitoring trip in 2012, both scientific and anecdotal observations show positive early effects of removing invasive non-native rats and are important indicators of potential long-term recovery of native plants and animals.
Using protocols developed by the University of California – Santa Cruz, the team measured presence and abundance by counting seedlings in a specific area (along transects), in long-term vegetation plots, and by counting seedlings around adults trees. Without rats, several native tree species are rebounding. In particular, the naturally prolific Pisonia grandis trees, where many seabirds roost and nest, have increased. With rats removed, Pisonia grandis seedlings are abundant, and three-year old trees that sprouted in 2011 are now towering overhead (photos below). Before the rats were removed, no seedlings of these trees were observed. A recent study by researchers from Standford University found that Pisonia play an important role in both the terrestrial and marine health of the atoll. Seabirds roosting on these trees fertilize soils below, which increases coastal nutrients and the abundance of plankton, thus attracting manta rays to coastlines bordering stands of native forest (You can read the full paper here.)
-Increased numbers of seedlings of native tree species, Pisonia grandis and Pandanus fischerianus, on 56 transects.
-Increased numbers of seedlings of two species that were absent before rat removal, Guettarda speciosa and Cordia subcordata. The team found 11 C. subcordata seedlings where only two were found in 2012; none were observed before rat removal.
-With rats no longer present on the atoll, the numbers of invasive Coconut Palm (Cocos nucifera) seedlings are unfortunately increasing as well. The U.S. Fish and Wildlife Service (USFWS) and The Nature Conservancy (TNC) are investigating a strategy to control invasive Coconut Palms so that native tree species have room to spread and re-establish Palmyra’s native forest. This is an important next step in restoring the atoll.
Fluctuations in invasive ant populations
The team is also monitoring Palmyra’s terrestrial invertebrate response to rat removal. The 2012 survey found increases in the abundance of most of these insects; however, in 2014, the monitoring team found that this trend had reversed. For example, in 2009 the team found on average 6.6 ants per sample. In 2012, they observed an average of 77 ants per sample and, this year, they found 9.9 ants per sample. Palmyra has 10 species of ants, all introduced and most are invasive. Long-term monitoring will be key to understanding this trend and the mechanisms for these fluctuations.
Two new land crab species (Geograpsus grayi and Ocypode cordimanus, above) have been detected on the atoll since the removal of rats. It is possible these crabs were present before rat removal, but kept in such low numbers due to rat predation that they were not observed by scientists. Crabs such as these play an important role at Palmyra as predators and scavengers of other invertebrates.
Seabirds play critical roles on coral atolls by bringing marine nutrients ashore and feeding the plant community. Seabirds are typically longer-lived animals and expected to be slower in returning to the atoll because they need time to prospect for their ideal nesting sites. We are currently monitoring for seabirds using new technologies such as automated acoustic monitoring songmeters. These microphones can be deployed for months at a time, and when brought back to the lab can be analysed to detect any bird calls. In addition, the USFWS is continuing regular monitoring of seabirds and shorebirds throughout the year at Palmyra Atoll. The hope is that species not seen breeding there in many years, such as the Wedge-tailed Shearwater, return and their eerie cries will fill the sky. Some observations indicate that White Tern nests appear to be increasing. In the first round of scientific monitoring, we recorded just one nest along the vegetation transects. This year, we found 18 nests. Black and Brown Noddies are also beginning to use more of the available nesting habitat across the atoll. In 2014, nests were found on six islets, double what was seen on the previous monitoring trip.