THIS BRIEF REFERS TO:
Briggs, AA, HS Young, DJ McCauley, SA Hathaway, R Dirzo, RN Fisher. 2012. Effects of spatial subsidies and habitat structure on the foraging ecology and size of geckos. PLoS ONE 7(8): e41364. doi: 10.1371/journal.pone.0041364
Observations at the Palmyra Atoll National Wildlife Refuge are shedding light on how nonnative vegetation can alter nutrient subsidies and native food webs. In a study published in PLoS ONE
, researchers from Stanford, Harvard, University of California-Berkeley and USGS describe how coconut palms (Cocos nucifera
) affected body size, dietary composition and trophic position of two species of Lepidodactylus
geckos, which are the sole terrestrial vertebrates native to Palmyra.
The dominant tree species native to the Palmyra Atoll are dicots such as Pisonia grandis
and Tournefortia argentea
, whose branches offer physical structures favorable to nesting and roosting seabirds. Seabirds in turn deposit guano in the surrounding soil, and this source of marine-derived nutrients can be tracked due to unique carbon and nitrogen isotopes — a product of the seabirds’ diet of marine life.
However, introduced coconut palms dominate more than 75% of some Palmyra forests. Palms make poor roosts and are not favored by seabirds — limiting the flow of marine nutrients to nearby soil. Knowing this, researchers compared dicot and palm forests for gecko abundance, body condition, prey items, prey diversity, as well as for marine isotopes in gecko tissue, prey tissue and soils to establish trophic positions.
Dicot forests had ten times the soil marine nitrogen than palm forests. The forests also differed in prey diversity and abundance. Geckos in dicot forests were larger, had higher body condition, had more diverse diets and ate at a much higher trophic position.
Yet despite the evidence of altered trophic pathways, gecko abundance did not differ between forest types. Research on islands with more complex predator communities may offer clues on how changes in nutrient subsidies actually ripple through the food web.
The diet of Lepidodactylus sp. nov., pictured here, differed from L. lugubris in dicot forests, suggesting that native forests are more favorable to diet specialization. Image credit: Stacie Hathaway/USGS.