Long-term population dynamics of wood frogs (Lithobates sylvaticus) on a dynamic Prairie Pothole landscape on the northern plains

Background/Question/Methods

Effects of environmental variation on ecological and evolutionary dynamics depend on the mix of conditions encountered by individuals and populations.   The range of conditions “sampled” depends on spatiotemporal patterns of environmental variation and organismal movement patterns.   Amphibians are sensitive to a wide range of environmental factors that affect individual fitness and population vital rates, making them useful models for investigating the impacts of environmental variation.  Because individuals are not capable of extremely long movements and tend to have short generation times, amphibians are also a practical model for measuring spatial and temporal patterns of variation.   I initiated a long-term study (now > 15 yrs) of wood frogs in northeastern North Dakota, at the eastern edge of the Prairie Pothole Region, with the aim of examining the effects of environmental variation on population dynamics, genetic structure, the potential for local adaptation and selection on phenotypic plasticity.   I used an array of methods including capture-recapture estimation of dispersal, molecular markers to estimate genetic structure, and monitoring of breeding activity and metamorph production.  Here I report on patterns of wood frog breeding pond use, recruitment and dispersal among wetlands that varied in size, hydroperiod (e.g., temporary versus seasonal versus semi-permanent) and other factors.

Results/Conclusions

The core study area (1 x 3 km) containing ~ 20 wetlands is embedded in a landscape dominated by crop production but also including pasture and ungrazed, untilled land.  Wetland availability varied with precipitation, and particular wetlands were used according to availability and demographic history.  In most wetlands something different happened every year!  Some wetlands were reliable demographic “hotspots,” whereas others were used or produced metamorphs only sporadically.  Even wetlands used intermittently were sometimes highly productive.   Dispersal rates were high (32% of adults, 67% of juveniles) but usually to adjacent wetlands (< 200m), although the maximum movement detected was > 1 km.   The picture that has emerged is one of a highly demographically-connected landscape, where local breeding populations and recruitment vary considerably over time and space and where recruitment failure is common but often buffered by dispersal.  Despite the duration of this study, the observed range of conditions is limited by recent patterns of land use and climate, and questions remain about truly long-term dynamics.  Even so, it is apparent that effects of environmental variation must be understood at the landscape level and on a long-term basis.  Short-term, spatially-limited studies are unlikely to capture a sufficient view of system behavior.