April 17, 2007
Studying plant-animal interactions in Mauritius, an Indian Ocean island famous for its extinct dodo bird, researchers found that a rare plant, Trochetia blackburniana, benefits from its proximity to Pandanus plants because they house high densities of geckos responsible for pollination. The findings, which unusually identify a lizard as a key pollinator, are significant because they provide "valuable management insights for ongoing conservation efforts to save the highly endangered flora of Mauritius."
The researchers, led by Dennis M. Hansen of the Institute of Environmental Sciences at the University of Zurich in Switzerland, used a gecko exclusion experiment to determine the importance of the endemic blue-tailed day gecko (Phelsuma cepediana) in pollination of Trochetia blackburniana, a species that is now in decline due to the impact of introduced species and the disappearance of its key pollinator, the olive white-eye (Zosterops chloronothos), a bird, across much of its range. The authors found that unlike alien invasive wasps and birds that fed on Trochetia blackburniana nectar without collecting pollen, the blue-tailed day gecko was tagged with pollen "either just behind the head or on the gecko's throat and chest," making it a crucial pollinator of the plant species. Hansen and colleagues showed that gecko exclusion had a "highly significant negative effect" on fruiting of Trochetia blackburniana.
Nectar-feeding male P. cepediana day gecko approaching a flower of T. blackburniana. Photo by Dennis Hansen.
The researchers report that "dense patches of palmlike Pandanus plants (Pandanaceae) are favored microhabitats of this gecko... Even a small patch of Pandanus plants forms a dense, impenetrable matrix of spiky, serrated leaves. Hiding in such patches may protect P. cepediana from sudden attacks by its main predator, the Mauritian kestrel Falco punctatus, a bird feeding almost exclusively on Phelsuma geckos... and from other endemic Mauritian birds that prey on Phelsuma geckos... Furthermore, Pandanus patches provide good egg-laying sites, and the dense shade they offer may be important for Phelsuma thermoregulation."
Male P. cepediana day gecko courting a female. Photo by Dennis Hansen.
The researchers say their work may be applicable to conservation efforts in the neighboring islands of Reunion and Madagascar where there are also large populations of day geckos and Pandanus plants.
"Our results highlight the significance of the community context when considering conservation management of endangered plant species," they write.
"Given that lizard pollination is widespread on many islands, and given that islands harbor much of the world's endangered flora and herpetofauna, our results are relevant for the conservation of many endangered island plants and lizards. Our findings emphasize the importance of considering plant animal interactions such as pollination at relatively small spatial scales in both basic ecological studies and applied conservation management."
Hansen and colleagues say that while the role of geckos in plant pollination is interesting, a more important finding from their work is the nature of the relationships between species in the ecosystem.
"While it has recently turned out that Phelsuma geckos and other reptiles are very important in Mauritius and other island ecosystems, our study actually goes that one step further and says that these crucial mutualistic interactions are affected by habitat structure and heterogenity -- that is, indirect interactions that shape the direct interactions are as important, if not more, as the direct mutualism," Hansen told mongabay.com via email. "For conservation management purposes the major take-home message is the need to promote habitat structural diversity to provide the structural "backbone" for many lizard-mediated interactions."
Mauritian forests highly degraded
Today, less than 2% of Mauritius is covered with native forest, most of which is heavily degraded by invasive species; there is no pristine forest left. Forest loss on this island country occurred over a relatively brief span of time. Early Dutch sailors wiped out wildlife, including the famous dodo bird, but inflicted little actual damage on the forest. During the late 1700s and early 1800s, French colonists cleared large tracts of forest for sugar cane plantations, as a source of fuelwood to power mills, and as a source for timber. After the French, very little forest remained, mostly restricted to mountain areas. From 1948 to 1973 the entire landscape was doused with DDT which severely affected bird populations. Still, despite this, about 30 percent of the island's animal species and 43 percent of its plants are endemic.
Hansen said that while the forests of Mauritius have been degraded, there appear to be encouraging signs of slow recovery.
"Inside fenced and weeded conservation management areas of between one and 24 hectares, native forests have slowly started to regenerate," he said.
CITATION: Dennis M. Hansen, Heine C. Kiesbüy, Carl G. Jones, and Christine B. Müller (2007). Positive Indirect Interactions between Neighboring Plant Species via a Lizard Pollinator. The American Naturalist vol. 169, no. 4: 534-542 April 2007