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.

Geese mediate vegetation state changes with parallel effects on N cycling that leave nutritional legacies for offspring

“…CRAM and CSUB plots were sampled after 0.75, 6, 12, 24, and 168 h, while CRAM Ã and CSUB Ã plots were sampled after 0.75, 6, 12, and 24 h. Fig. However, heavy nest predation by arctic foxes in the early 2000s reduced grazing pressure during the growing season because failed breeders typically leave the colony, leading to a loss (reversion to CRAM) of grazing lawns and reduced growth rates of goslings in subsequent years (Sedinger et al 2016, Lohman et al 2019. The same data as in Fig.…”

“…Aerial extent of grazing lawns has declined substantially since 2000, which can be directly related to reduced grazing by geese (Lohman et al 2019). Previous experimental and observational evidence suggests that the recent conversion of grazing lawns to C. ramenskii meadows would be reversible if geese were released from predation pressures , Sedinger et al 2016.…”

“…For example, following an increase in numbers of Black Brant broods at the Tutakoke River colony in the late 1980s, there was a decline in areal coverage of C. ramenskii meadows and an increase in C. subspathacea grazing lawns, followed in the early 1990s by an increase in average gosling mass . However, heavy nest predation by arctic foxes in the early 2000s reduced grazing pressure during the growing season because failed breeders typically leave the colony, leading to a loss (reversion to CRAM) of grazing lawns and reduced growth rates of goslings in subsequent years (Sedinger et al 2016, Lohman et al 2019.…”

“…Indirect effects include changes to litter quality and decomposition rate, effects on soil compaction and disturbance from trampling, and influences on root dynamics that affect microbial processes including nitrogen (N) cycling (Ruess et al 1989, Pastor et al 1993, Kielland and Bryant 1998, Hamilton and Frank 2001, Kielland et al 2006b). Vegetation responses to grazing and browsing are coupled through these herbivore-mediated changes to biogeochemical cycling (Cargill and Jefferies 1984, Ruess et al 1989, Hik et al 1992, which ultimately translate to both positive and negative feedbacks to herbivore nutrition and population dynamics (McNaughton 1984, Sedinger et al 1995, 2016, Lake et al 2008, Christie and Ruess 2015, Paragi et al 2015, Lohman et al 2019.…”

Constraint versus restraint in the body mass dynamics during moult of a species with precocial young

“…Investment in breeding carries over into the next breeding season (Inger et al 2010, Leach et al 2019, which could be manifest in part through nutrients acquired following breeding when foods may contain higher concentrations of protein than is true of foods available during autumn and winter (Sedinger 1997). Breeding female geese must therefore restore nutrients depleted during nesting in preparation for autumn migration (Sedinger & Bollinger 1987) while competing with other family groups for food (Mulder et al 1995) and performing behaviours related to brood care (Sedinger et al 1995a, Lohman et al 2019. In contrast, failed and no3n-breeding geese often undertake a moult migration to areas where competition with families is reduced and food may be more abundant or nutritious (Derksen, Eldridge & Weller 1982).…”