Understanding the role of density dependent (endogenous) and density independent (exogenous) factors in controlling population dynamics is a central challenge in population and community ecology and has implications for management.  We used 12-year time series with five estimates per year (60 temporal samples total) from each of 20 locations scattered over a large southern U.S. wetland to estimate the relative contribution of density-dependent and density-independent factors in controlling the dynamics of eastern mosquitofish (Gambusia holbrooki), least killifish (Heterandria formosa), bluefin killifish (Lucania goodei), and sailfin mollies (Poecilia latipinna).  Density-dependent factors were resource limitation and predation and density-independent ones were disturbance by marsh drying and habitat area.  Migration is a fourth important factor in local population dynamics triggered by marsh drying whose impact varies among these species.  We used response surface modeling to document nonlinear components and time lags in dynamics of these species.  

Results/Conclusions

Following detrending for time passed following marsh drying, we found evidence for endogenous influences on population dynamics in all species examined.   Mosquitofish recolonize rapidly following droughts and their dynamics are little affected by periodic drying; their population dynamics are dominated by density-dependent factors over most of the study area most of the time.  In contrast, least killifish and bluefin killifish, and to a less extent sailfin mollies, take much longer to regain pre-drought densities following drought and their abundance was well below asymptotic densities over much of the study area at any point in time.  We partitioned the relative contribution of endogenous and exogenous controls on population dyanmics by comparing the relative contribution to each in time series models. Mosquitofish population dynamics were dominated by endogenous control in all populations (more than 50% of explained variance was endogenous and less than 30% was exogenous in most cases), while least killifish displayed a range of relative contributions (from 90% exogenous to 90% endogenous).  The relative contribution of exogenous control was inversely correlated with hydroperiod.  Differences among species in the relative role of endogenous control may be related to the dispersal rate and mixing of populations at the landscape scale; mosquitofish have the highest dispersal rate and least killifish and bluefin killifish have low dispersal rates.