Proximity among protected area networks promotes functional connectivity for wintering waterfowl

Abstract

The equilibrium theorem provided a fundamental framework for understanding species' distributions and movement in fragmented ecosystems. Wetland-dependent avian species are model organisms to test insular predictions within protected area networks because their mobility allows surveillance of isolated patches without landscape barriers. We hypothesized size and isolation would influence functional connectivity of sanctuaries by GPS-marked wintering mallards (Anas platyrhynchos) within a mesocosm protected sanctuary area network. We evaluated functional connectivity and sanctuary use, measured by movements between sanctuaries, using a multistate modeling framework. Proximity drove connectivity, underscoring that patch isolation-not size-influenced connectivity, even for an avian species with no ascertainable landscape resistance or barriers. We also found that sanctuary use increased overwintering survival by reducing harvest mortality. Our test of equilibrium theory predictions demonstrated that isolation of protected sanctuary areas supersedes their size in determining functional connectivity for mallards and access to these areas may have direct fitness consequences. Our findings could refine land acquisition, restoration, and management practices with equal or greater emphasis on adjacency in protected area network design, especially for wetland-dependent migratory gamebirds.

Keywords: Anas platyrhynchos; Functional connectivity; GPS telemetry; Island biogeography; Multistate modeling; National Wildlife Refuge System; Protected area networks; Sanctuary; Wintering waterfowl.

Figure 1

The daily transition probabilities ($\psi$) from one waterfowl sanctuary to another by a wintering mallard (Anas platyrhynchos) captured or arriving in the west Tennessee and surrounding sanctuary complexes (November through February 2019–2023) relative to distance between sanctuaries (a), individual characteristics including female or male (green and orange, respectively) and age (juvenile or adult; b), and sanctuary sizes including the size of the sanctuary an individual left (emigration; c) and the size the individual transitioned to (immigration; d). Transition probabilities are associated with 68%, 90%, and 95% credible intervals for (a), (c), and (d) (dark to light gray) and 68% and 90% credible intervals for (b) (thick and skinny line, respectively). Predictions are generated from posterior distributions with all other values held constant at their mean value. Predictions for (a), (c), and (d) are for juvenile males because these were categorical indicator variables. Note different y-axes for visual aesthetics; despite increases or differences visually, distance between sanctuary nodes (a) was the only biologically meaningful effect. All Figures were produced in R version 4.3.3. https://www.r-project.org/.

Figure 2

Predicted functional connectivity of mallards (Anas platyrhynchos) represented as daily sanctuary transition probabilities (p) among sanctuary nodes within the west Tennessee and surrounding sanctuary network of Arkansas, Kentucky, and Missouri. Individual mallards were captured and monitored with GPS transmitters from November through February 2019–2023. Sanctuary nodes included 4 National Wildlife Refuges: Big Lake National Wildlife Refuge (BLNWR) in Arkansas, Reelfoot Lake NWR north unit (RLNWR_N) in Kentucky and Tennessee, and Reelfoot Lake NWR south unit (RLNWR_S), Lake Isom NWR (LINWR), and Chickasaw NWRs in Tennessee. Additional smaller sanctuary nodes included state-owned waterfowl sanctuaries: Lake Lauderdale (LL), Horns Bluff (HB), White Lake (WL), Bean Switch (BS), Maness Swamp (M), Hop-in (HI), Black Bayou (BB), and Phillipy Waterfowl Refuges (P). Greatest functional connectivity was clearly within the Reelfoot Lake sanctuary complexes that included Black Bayou, Phillipy, and Reelfoot NWR north and south units. State sanctuary nodes in the upper Obion River Complex including Hop-in, Bean Switch, and Maness Swamp Waterfowl Refuges also were more connected nodes illustrating distance, not size, as a primary driver of functional connectivity for wintering mallards. Figure was produced in R version 4.3.3. https://www.r-project.org/.

Figure 3

Predicted number of waterfowl sanctuary nodes used by wintering mallards (Anas platyrhynchos) within the west Tennessee and surrounding wetland complex protected sanctuary network relative to the number of days in the study area. Plots are faceted by the month (columns) and by individuals using sanctuaries during the same winter they were captured and individuals returning to the study area (rows). Figure was produced in R version 4.3.3. https://www.r-project.org/.