Restoration activities in the Everglades will include alteration of the present system of levees and canals, flow patterns, and hydrological dynamics of the ecosystem. The effect of these ecosystem changes on aquatic animals will, in part, be dictated by their patterns of movement and dispersal at present and in response to change. However, prior to this research, there existed little foundation of knowledge of aquatic animal population structure and dispersal patterns. In modeling the dynamics of aquatic animal populations in southern Florida wetlands, it has become apparent that the dispersal distances, rates of movements, and timing of movement are important but poorly understood. Migration is a critical parameter for understanding the response of organisms to environmental change. Migration rate and pattern can be examined using direct or indirect techniques. For animals, direct methods generally involve following the movements of marked individuals. Indirect methods include examination of patterns of genetic diversity and patterns of stable-isotopic markers. The small size of most of the wetland animals precludes the use of standard marking techniques for studying movement, and the large, open system of these wetlands makes recapturing marked animals improbable. Likewise, the short lifespans of these animals makes recapture difficult. Use of genetic markers in determining the presence of structure in populations offers an indirect way of assessing the degree of movement and mixing in these populations, and a cost-effective approach to examine landscape-level patterns of animal movement.

Populations of a given species that do not frequently interbreed and exchange genetic information will show larger differences in allele frequencies than two populations that often interbreed. When many populations are sampled spatially, the genetic signatures of the populations provide information on the degree and frequency of interbreeding and, therefore, dispersal. Mathematical algorithms have been developed to estimate migration rate from such data, given known assumptions. The analyses proposed use already proven methods of assessment that employ allozymes and DNA.

In this series of studies, our objectives are to identify population structure of selected aquatic species in the Everglades, estimate migration rate from genetic data on gene flow for incorporation in the "Across Trophic Level System Simulation" (ATLSS) model, and to test the hypothesis that levee and canal systems act as barriers to dispersal of aquatic animals in the Everglades. Future funding will support the expansion of this line of study by incorporating: (1) the use of DNA markers to study the genetic relationships of common Everglades aquatic animals; (2) the continued use of allozyme analyses with additional species; and (3) the physical tagging and marking of large and small animal species to directly measure movement, dispersal distance and seasonality, and residency in refuges. DNA analyses will expand the resolution of genetic analysis provided by allozymes. The scale of population isolation, and thus migration, resolvable by genetic markers depends on their rate of mutation and microevolutionary change. DNA markers can be identified with mutation rates covering a wider range than allozymic markers. In this case, DNA markers that evolve at as rapid a rate as possible to produce DNA fingerprints will be used to compare different regions of the Everglades, and across water-management structures.

The research to date has focused on the mosquitofish (Gambusia holbrooki), grass shrimp (Palaemonetes paludosus), and spotted sunfish (Lepomis punctatus). Our future plans include studies involving crayfish and large-bodied fish that inhabit alligator ponds. We will use mark-recapture-release techniques for large-bodies species at alligator holes for comparisons of dispersal results with estimates of population structure from genetic data. An important data gap in the system is information on the inter-and intra-annual variability in large-bodied fish populations in the wetlands. An ongoing U.S. Army Corps of Engineers-funded project is now examining seasonal changes in the use of marshes and ponds by large fishes. Tagging the fishes in ponds followed by recapture attempts in the ponds and nearby marshes would provide data on relative habitat use, population size estimates, and habitat fidelity. Similarly, at critical times of dry-down, marking large numbers of small fishes as they enter refuges will give the data needed to determine the effects of predation by larger fishes on survival and mortality of those numerically dominant species. Those data will directly support other ongoing and proposed studies of aquatic animals in the Rocky Glades, in alligator holes and headwater creeks, and in the Big Cypress Swamp. The data are essential to refine and test the ATLSS fish model.

With 1997 funding, we made 2 years of field collections of mosquitofish, grass shrimp, and spotted sunfish. Over 50 populations each of mosquitofish and shrimp, and 25 of spotted sunfish, were analyzed in 1996-97. The examination of changes in allele frequencies across generations at a particular location provides additional information about population demography. In 1997, a subset of those populations was resampled to compare interyear estimates of genetic variability. A paper describing the results of these efforts is in the peer-review process. We found clear evidence that water-management units (Shark River Slough, Taylor Slough, Water Conservation Areas-3B and -3A) display different patterns of genetic diversity in mosquitofish and possibly spotted sunfish. This indicates that the levees and canals separating them are acting both as barriers to mixing, and create unique hydrological environments within the management units that influence the population dynamics of mosquitofish differently. For example, genetic diversity is quite low for mosquitofish in Water Conservation Area-3B, which is isolated from adjacent canals by levees. This suggests that fish populations there have gone through a severe reduction in population size in the recent past, and experience little or no mixing from nearby areas. Mosquitofish from canals have consistently high levels of genetic diversity compared to nearby marshes, indicating that canals are sites of genetic interchange for this species. There was no evidence for structure in populations of grass shrimp.

Funding for this research was provided from the U.S. Department of the Interior, South Florida Ecosystem Restoration Program "Critical Ecosystems Studies Initiative" (administered through the National Park Service); and, in part, from the U.S. Geological Survey, Florida Caribbean Science Center, and the South Florida Water Management District.

REFERENCE

Trexler, J.C., Kandl, K.L., and Jordan, C.F., in review, Metapopulations or patchy populations: Population structure of three species of aquatic animals from the Everglades: Evolution.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)