Changes in behavior are unable to disrupt a trophic cascade involving a specialist herbivore and its food plant

Abstract

Changes in ecological conditions can induce changes in behavior and demography of wild organisms, which in turn may influence population dynamics. Black brant (Branta bernicla nigricans) nesting in colonies on the Yukon-Kuskokwim Delta (YKD) in western Alaska have declined substantially (~50%) since the turn of the century. Black brant are herbivores that rely heavily on Carex subspathacea (Hoppner's sedge) during growth and development. The availability of C. subspathacea affects gosling growth rates, which subsequently affect pre- and postfledging survival, as well as size and breeding probability as an adult. We predicted that long-term declines in C. subspathacea have affected gosling growth rates, despite the potential of behavior to buffer changes in food availability during brood rearing. We used Bayesian hierarchical mixed-effects models to examine long-term (1987-2015) shifts in brant behavior during brood rearing, forage availability, and gosling growth rates at the Tutakoke River colony. We showed that locomotion behaviors have increased (β = 0.05, 95% CRI: 0.032-0.068) while resting behaviors have decreased (β = -0.024, 95% CRI: -0.041 to -0.007), potentially in response to long-term shifts in forage availability and brood density. Concurrently, gosling growth rates have decreased substantially (β = -0.100, 95% CRI: -0.191 to -0.016) despite shifts in behavior, mirroring long-term declines in the abundance of C. subspathacea (β = -0.191, 95% CRI: -0.355 to -0.032). These results have important implications for individual fitness and population viability, where shifts in gosling behavior putatively fail to mitigate long-term declines in forage availability.

Keywords: Carex subspathacea; behavior; black brant; growth; population dynamics; survival.

Figure 1

Annual estimates and 95% credible intervals (dashed) of model‐predicted mass at 30 days black brant goslings captured on brood‐rearing areas near the Tutakoke River colony, 1987–2015. Linear trend (solid) and 95% credible intervals for linear trend (dashed) are also shown. No estimates were available in 2001

Figure 2

(a) Annual estimates and 95% credible intervals (dashed) of model‐predicted proportion of the colony covered by Carex subspathacea, the primary forage of juvenile brant, 1991–2016. Effects of annual proportion of C. subspathacea extent on annual estimates of black brant gosling (b) the predicted mass at 30 days, (c) the proportion of time resting, and (d) the proportion of time walking. Dotted lines on graphs indicated 95% credible intervals for estimates of both x‐ and y‐axis variables. No estimates were available for 1992, 1996, 2000, 2002, 2003, 2005, or 2006

Figure 3

Annual estimates and 95% credible intervals (dashed lines) of model‐predicted proportion of time spent (a, b, c) foraging, (d, e, f) resting, and (g, h, i) walking for (a, d, g) juvenile, and adult (b, e, f) female, and (c, h, i) male brant on the Tutakoke River colony, 1987–2015. Linear trends (solid) and 95% credible intervals for linear trend (dashed) are also shown. No estimates were available in 1989, 2006, or 2012

Figure 4

Annual estimates and 95% credible intervals (dashed lines) of model‐predicted proportion of time spent on aggression and alert behaviors for adult female and male brant at the Tutakoke River colony, 1987–2015. Linear trends (solid) and 95% credible intervals for linear trend (dashed) are also shown. No estimates were available in 1989, 2006, or 2012