Animals can experience rapid environmental changes that often involve habitat degradation (MacMahon et al. 1989; Feary et al. 2007). As habitat quality declines, individuals may alter their behaviour and density to better exploit degraded habitat (Kohlmann & Risenhoover 1994; Pezzanite et al. 2005), they may exploit improved formerly lower-quality habitat (Vickery et al. 1995; Nolet et al. 2002), or they may modify when they use habitats of different qualities to best satisfy their requirements which may change during, growth or productive periods (McWilliams & Leafloor 2005). Such responses can lessen the negative impacts of environmental change on survival and fitness (Jefferies, Rockwell & Abraham 2004; Inger et al. 2006), but they may also reduce growth rates, increase mortality and ultimately lead to population decline (Holt & Kimbrell 2007). Insufficient response of animals to environmental change can lead to temporal and spatial mismatches between when resources are available and when they are required (Post & Forchhammer 2008). Keystone herbivores in Arctic ecosystems are especially susceptible to the effects of environmental change because timing of reproduction coincides with maximal availability of high-quality plant tissue that is most nutritious (Sedinger & Raveling 1986; Lepage, Gauthier & Reed 1998).
We investigated the resource use of lesser snow goose goslings (Chen caerulescens caerulescens Linnaeus 1758; hereafter LSG) along the south-west coast of Hudson Bay, Canada, where foraging by overabundant geese has caused large-scale degradation of tidal marsh (hereafter TM; Jefferies, Rockwell & Abraham 2004). This trophic cascade provides the opportunity to determine how resource use of extant geese has changed with the degradation of TM and its implications for growth rates of goslings. Prior to the degradation of the TM, LSG adults and goslings foraged almost exclusively in TM especially during early gosling growth (Cooke, Rockwell & Lank 1995). The two sward-forming graminoids in the TM, Carex subspathacea and Puccinellia phryganodes, are preferred by goslings because they contain more nutrients and less fibre than forage species common in the surrounding freshwater marsh (hereafter FM), such as Carex aquatilis (Gadallah & Jefferies 1995a). The importance of the TM graminoids to rapid LSG gosling growth was further revealed in captive trials where goslings fed the dominant FM sedge, C. aquatilis, lost or maintained body mass, whereas they gained body mass when fed the preferred P. phryganodes and C. subspathacea (Gadallah & Jefferies 1995b). Because this population of geese has been monitored for decades, we can compare gosling resource use over time and determine their response to this environmental change.
Recent observations indicate that the reductions in plant quality and quantity in the TM is coincident with family groups increasingly foraging in nearby, FM, although the extent to which this occurs during gosling growth and its consequences for growth is not well understood (Jefferies, Rockwell & Abraham 2004). Given that TM and FM plants eaten by LSG goslings differ in quality, the observed changes may negatively affect gosling growth. In this study, we compared the size of c. 28-day-old goslings in 2005 to that of goslings in the 1980s which foraged in high-quality TM prior to the environmental change associated with goose overabundance. We also compared resource use and body size of extant goslings that inhabited TM and FM to determine how current foraging strategies influence gosling growth and to test the hypothesis that during early stages of growth goslings require high-quality TM plants, but that during later stages of growth they may switch to foraging on lower-quality FM plants (Gadallah & Jefferies 1995b).
We used stable isotopes in forage plants and in gosling tissues to determine the extent to which gosling resource use changed with age, and how foraging history and habitat use affected their growth rate and body size. The primary advantages of using stable isotopes in this context are (i) stable isotope values of an individual gosling’s tissue are the product of what that individual ate and assimilated and (ii) stable isotope values of tissues with different turnover rates provide a record of an individual’s past and present resource use over a range of timescales (Bauchinger & McWilliams 2009; Bauchinger et al. 2010).