The biogeography of “puddle islands” in urban New Orleans

John C. Carlson, Mark S. Fox, and Kevin A. Caillouet

Background

Predators have become increasingly important for controlling pest arthropod species in both agriculture and medicine[1].  As in other places around the world, efforts have been made to reduce the number of mosquitoes in New Orleans by introducing a variety of predators into the aquatic habitats in which mosquito larvae develop.  Multiple studies on the effects of predator-introductions have been conducted in Louisiana and in New Orleans[2-5] and the New Orleans mosquito control board continues to provide organisms to the public (including turtles, fish, and copepods) that have been experimentally shown to feed on the aquatic mosquito larvae.  However, there has been difficulty in keeping many aquatic habitats free from mosquito larvae long term with either biological or chemical control methods.  Repeated predator introductions are required in habitats that undergo temporal changes, such as periodic drying out or flooding[3].  In cases were repeated introductions of predators are required, long-term control efforts have been unsustainable, a problem that also affects mosquito control programs in many developing countries. 

In the freshwater wetlands surrounding New Orleans, natural predators play a key role in suppressing mosquito populations by decreasing the number of larvae that survive to adulthood.  In preliminary research, many of these aquatic species have been found living within the city as well, having colonized a variety of man-made habitats.  An understanding of the underlying ecology of predator populations already living within cities and the methods by which they naturally disperse throughout the city would facilitate urban planning efforts to suppress mosquitoes in a more sustainable fashion by circumventing the need for repeated introduction of predators into temporal habitats.

In contrast to marsh habitat, the water used by mosquito larvae in cities is fragmented into small, scattered pools.  These shallow pools have a unique combination of characteristics in common with both terrestrial and aquatic habitats.  Understanding the distribution of the organisms that live in shallow pools is made more difficult by a lack of ecological models applicable to these unique environments.  In the research proposed below, the models of Island Biogeography will be applied to ground pools in New Orleans in an attempt to provide such a model to this medically important habitat type. 

The principals of Island Biogeography, first experimentally verified by Simberloff and Wilson in 1969[6] are that 1: the distance of an island from a continent is positively correlated with the number of species that will colonize that island over time (IS) (see Figure 1) and 2: the size of an island is negatively correlated with its extinction rate (ES) (see Figure 2).  Following the establishment of these principals, Island Biogeography has been successfully applied to a variety of figurative islands, including the distribution of cold-adapted species in tropical regions between “mountain top islands”[7] and fragmented forests[8].  Island Biogeography has been applied successfully to aquatic systems to explain the species composition of frogs inhabiting ponds in Ontario[9] and mussel distribution in the Great Lakes[10], but has not been applied to very small habitats the size of most urban aquatic habitats.

In this study, the principals of Island Biogeography will be tested for their applicability to aquatic insects in New Orleans, a city with a history of devastation by mosquito-borne disease.  Larger bodies of water (artificial lakes in parks, etc.) will be evaluated for their role as “continents” from which subsets of aquatic predators disperse, while smaller bodies of water (stagnant water in streets, etc.) will be evaluated for their role as “islands” in which size and distance from the “continents” determine the number of insect species living within them.  Typically, the effects of isolation predominate in islands that are very small, such as those to be evaluated in this study[6].  However, due to the rapid dispersal and developmental rates of the insects being studies, it is conceivable that island size will have a significant, if smaller, role in determining the diversity of predators in the ground pools of New Orleans[11]. 

Methods:
Street-side puddles of variable size were comprehensively sampled using a hand-held bilge pump.  Total volume of each habitat was measured after it passed through a fine mesh sieve to collect invertebrates from the water.  The samples were preserved in ethanol for subsequent laboratory sorting and identification[12].  GPS coordinates of each habitat were obtained using hand-held GPS units and imported into ArcView 3.0.  Distance from the habitat to potential continents was calculated using ArcView 3.0. 

The volume of the habitats and the distance of each puddle to the nearest lagoon was individually correlated with total animal diversity, and diversity of insects vs. other invertebrates using SPSS 11.0 to calculate Pearson’s correlation coefficients. 

Results:
Habitat size and proximity to lagoon were independent from one another.

[Table: community composition flooded vs. unflooded areas]

 [Figure of correlation species richness vs. habitat size, unflooded area]

 [Figure of correlation species richness vs. habitat size, flooded area]

 [Figure of correlation species richness vs. distance from source unflooded area]

 [Figure of correlation species richness vs. distance from source flooded area]

Conclusions
As predicted by island biogeography theory, both habitat size and proximity to a large source of dispersers significantly affected species richness in urban puddles regardless of flood-induced changes in species composition and resultant differences in community structure.  While these preliminary data are based on only two of the three sampled sites, the addition of data from our third site is expected to support the results described here.  The applicability of island biogeography to the urban ecosystem is supported by the prediction of species richness in areas with substantial differences brought about by differential flooding from Hurricane Katrina.

References cited

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6.Simberloff D, Wilson E: Experimental zoogeography of islands: The colonization of empty islands. Ecology 1969, 2:278-296.

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12.       Merritt R, Cummins K: An introduction to the aquatic insects of North America, 3rd edn. Dubuque: Kendall/Hunt Publishing Company; 1996.