Wetland dynamics influence mid-continent duck recruitment

Abstract

Recruitment is a key factor influencing duck population dynamics. Understanding what regulates recruitment of ducks is a prerequisite to informed habitat and harvest management. Quantity of May ponds (MP) has been linked to recruitment and population size (Kaminski and Gluesing 1987, Raveling and Heitmeyer 1989). However, wetland productivity (quality) is driven by inter-annual hydrological fluctuations. Periodic drying of wetlands due to wet-dry climate cycles releases nutrients and increases invertebrate populations when wet conditions return (Euliss et al. 1999). Wetlands may also become wet or dry within a breeding season. Accordingly, inter-annual and intra-seasonal hydrologic variation potentially influence duck recruitment. Here, we examined influences of wetland quantity, quality, and intra-seasonal dynamics on recruitment of ducks. We indexed duck recruitment by vulnerability-corrected age ratios (juveniles/adult females) for mid-continent Gadwall (Anas strepera). We chose Gadwall because the majority of the continental population breeds in the Prairie Pothole Region (PPR), where annual estimates of MP exist since 1974. We indexed wetland quality by calculating change in MP (?MP) over the past two years (?MP = 0.6[MPt – MPt-1] + 0.4[MPt – MPt-2]). We indexed intra-seasonal change in number of ponds by dividing the PPR mean standardized precipitation index for July by MP (hereafter summer index). MP and ?MP were positively correlated (r = 0.65); therefore, we calculated residual ?MP (?MPr) with a simple linear regression using MP, creating orthogonal variables. Finally, we conducted a multiple regression to examine how MP, ?MPr, and summer index explained variation in recruitment of Gadwall from 1976–2010. Our model explained 67% of the variation in mid-continent Gadwall recruitment and all three hydrologic indices were positively correlated with recruitment (Figure 1). Type II semi-partial R2 estimates indicated that MP accounted for 41%, ?MPr accounted for an additional 22%, and summer index accounted for the remaining 4% of the variation in recruitment. Our results are consistent with previous findings that quantity of MP was important for explaining variation in recruitment of ducks. However, our results also indicated that considering hydrologic dynamics was important for explaining recruitment. Additionally, the index for retention of MP within breeding year also was important, despite its coarse resolution as an average of precipitation events that can vary greatly spatially and in intensity within the PPR. Our results support the idea that wetland ecosystems in the PPR are ultimately regulated through bottom-up process driven by inter- and intra-annual hydrological dynamics. However from the ducks' perspective, hydrological dynamics could influence recruitment proximately through both bottom-up and top-down processes. Specifically, hydrological fluctuations may influence predator populations, prey switching by predators, or duckling vulnerability to predators (Cox et al. 1998). We will propose a conceptual model for understanding the potential role of bottom-up and top-down regulation of duck recruitment based on different hydrological contexts. Clearly, a better understanding of ultimate and proximate factors regulating duck recruitment would improve the effectiveness and efficiency of habitat conservation for ducks. Lastly, our findings could be used to improve models that predict fall flights for the purposes of informing harvest regulations.