Shete Boca National Park, Curaçao

Disentangling the impact of invasive species and urbanization on population declines

Why are native geckos vanishing in Curaçao?

Over the past 100 years, we have altered the landscape of vast portions of our planet. As someone who grew up mostly in cities, how animals respond to urbanization is something I find really fascinating. Urban habitats are overlooked as containing “nature” in some idealistic sense by many people. However lots of animals do very well in cities and suburbs.

We spend a lot of time diving at night when we conduct fieldwork in Curaçao. Whether we are in the city or small towns, there is one group of animals that is ubiquitous when we get out of the water: geckos.

Think about it. Numerous Geckos are adapted to climbing shear walls to hunt insects. We build sheer walls and attach lights to the outside that attract large numbers of insects. This should be gecko paradise. However there is a problem. The walls of tropical city streets provide a great habitat for many gecko species. Why then are the urban centers of Curaçao devoid of native gecko species?

In Curaçao native geckos are disappearing from the urban landscape and being replaced by a species of gecko that has invaded much of the Western Hemisphere: Hemidactylus mabouia. The prevailing hypothesis is that native geckos cannot compete with this newcomer. By day the streets of Willemstad mobbed with tourists searching for a meal, by night these alleys transform into the feeding dens of Hemidactylus mabouia. In the main urban centers of Curaçao, native geckos have been displaced entirely.

This is a pretty solid hypothesis. Hemidactylus mabouia is a highly aggressive and territorial species of gecko. In fact, this species is so aggressive that a study in Florida found that the arrival of H. mabouia led to the rapid displacement of other species of highly invasive geckos.

However, despite the intuitive appeal of this hypothesis, any hypothesis requires testing to rule out alternative potential drivers of displacement.

We recently published a manuscript doing just that. We tested the possibility that urbanization may disrupt the prey base of the native leaf toed gecko Phyllodactylus martini, and therefore leave P. Martini with insufficient prey resources to persist by assessing differences in fedding ecology between developed and undeveloped areas.
Overview of study area. A, Map of sites sampled in this study (redrawn from Dornburg et al. 2011). B, Photograph characterizing the habitat in undeveloped sites. C, Photograph characterizing the habitat in devel- oped sites. Abbreviation: CARMABI, Caribbean Research and Management of Biodiversity.

We found that in undeveloped habitats Phyllodactylus martini subsisted primarily on grasshoppers and their relatives. However, P. martini will readily shift their diet to consume roaches, beetles, and moths in urbanized settings. These are the usual insects around lights, suggesting P. martini to be readily able to shift prey types and foraging behaviour to use urban habitats.
Visualizations of the stomach content data from all individuals (A) and various sites (B-D). Colors correspond to prey items in the legend, and slices in each sphere correspond to the stomach contents of an individual specimen. Code to generate plots like this is provided here

While our study supports the expectations of the competition hypothesis, we also provide direct evidence for an additional driver of extirpation: predation.

We witnessed first hand how a Hemidactylus mabouia darted out from behind a poster on a wall of our fieldstation to consume a juvenile Phyllodactylus martini. We previously documented the remains of another native gecko, Gonatodes antillensis in the stomach contesnts of H. mabouia. This additional finding lends further support that H. mabouia may be opportunistically preying upon native species.

Luckily for the geckos of Curaçao, Hemidactylus mabouia is restricted to suburban and urban areas.Why remains unknown, though this is something we hope to find out. Other parts of the world are not as lucky, and Hemidactylus invasions are so pervasive that the group had been referred to as a complex of weedy species. Many areas in the North of Curaçao remain undeveloped, leaving native geckos with a refuge. However, the surrounding suburbs are already heavily colonized by H. mabouia.

While animals we have brought with us around the world such as cats and rats have been repeatedly demonstrated as direct drivers or extirpation and even extinction , our work in Curaçao underscores that other more clandestine species such as Hemidactylus geckos also have the potential to be highly destructive.

Dialogue & Discussion