Abstract
AbstractMany models used to estimate nutritional carrying capacity (NCC) for ungulates differ structurally, but the implications of those differences are frequently unclear. We present a comparative analysis of NCC estimates for a large herbivore in a dynamic landscape, using models that differ in structure and scope. We compared three model structures across three estimates of winter ranges under three winter-severity scenarios for an isolated, introduced moose Alces alces population on the Copper River Delta of south-central Alaska. Model estimates of NCC ranged from 205 to 4,592 moose, demonstrating the critical influences of model structure and assumptions when applying NCC. Furthermore, population estimates during recent severe winters suggest that past models underestimated NCC on the Copper River Delta. We conducted a sensitivity analysis of a preferred model and determined that model components with the highest and lowest sensitivity were snow depth and lignin- and tannin-caused reductions in forage nutritional quality, respectively. Our low sensitivity values for lignin and tannin influences on NCC contrast with results in other NCC estimates. Overall, our results reinforce the need for, and will hopefully assist, adaptive management in response to landscape, population, behavioral, and climatic changes on the Copper River Delta, and demonstrate the importance of understanding model assumptions and structure in application of NCC estimates in the management of large herbivores in variable ecosystems.
Highlights
Nutritional carrying capacity (NCC) is the largest stable population of a specified organism supported by the total available nutritional energy, assuming a stable ecosystem within a defined range and time (Caughley 1979; Monteith et al 2014)
Our objectives were to 1) calculate the digestible energy available to moose in different stand types and develop a set of energy-based NCC models, varying in structure and assumptions, in order to represent the diverse factors that managers on the Copper River Delta (CRD) may consider relevant to the moose population with their current knowledge of CRD moose ecology; 2) compare estimates from these models to demonstrate the differences resulting from NCC model structures and assumptions; 3) perform a sensitivity analysis to determine the relative influence of factors within one model, range, and winter scenario within that combination
We summed the total median stand digestible energies, and compared the total calculated winter scenario- and range-specific digestible energy with the total energy required by an average adult female moose (500 kg) for maintenance (Schwartz et al 1988) across a scenario-specific winter length to determine the total number of moose supported within a specified range
Summary
Nutritional carrying capacity (NCC) is the largest stable population of a specified organism supported by the total available nutritional energy, assuming a stable ecosystem within a defined range and time (Caughley 1979; Monteith et al 2014). Application of NCC requires close consideration of the ecology of the organism and its environment. NCC depends on the species’ foraging behavior relative to the ecology of plant species being consumed (Jefferies et al 1994; Hobbs et al 2003; Windels and Hewitt 2011). Within temperate and arctic ecozones, decreases in nutritional quality and availability of browse in winter increase the necessary digestion and foraging time relative to other seasons (Schwartz and Renecker 1997), and historically winter was considered the limiting period for temperate ungulates.
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