Ecoregion and community structure influences on the foliar elemental niche of balsam fir (Abies balsamea (L.) Mill.) and white birch (Betula papyrifera Marshall)

Changes in foliar elemental niche properties, defined by axes of carbon (C), nitrogen (N), and phosphorus (P) concentrations, reflect how species allocate resources under different environmental conditions. For instance, elemental niches may differ in response to large‐scale latitudinal temperature and precipitation regimes that occur between ecoregions and small‐scale differences in nutrient dynamics based on species co‐occurrences at a community level. At a species level, we compared foliar elemental niche hypervolumes for balsam fir (Abies balsamea (L.) Mill.) and white birch (Betula papyrifera Marshall) between a northern and southern ecoregion. At a community level, we grouped our focal species using plot data into conspecific (i.e., only one focal species is present) and heterospecific groups (i.e., both focal species are present) and compared their foliar elemental concentrations under these community conditions across, within, and between these ecoregions. Between ecoregions at the species and community level, we expected niche hypervolumes to be different and driven by regional biophysical effects on foliar N and P concentrations. At the community level, we expected niche hypervolume displacement and expansion patterns for fir and birch, respectively—patterns that reflect their resource strategy. At the species level, foliar elemental niche hypervolumes between ecoregions differed significantly for fir (F = 14.591, p‐value = .001) and birch (F = 75.998, p‐value = .001) with higher foliar N and P in the northern ecoregion. At the community level, across ecoregions, the foliar elemental niche hypervolume of birch differed significantly between heterospecific and conspecific groups (F = 4.075, p‐value = .021) but not for fir. However, both species displayed niche expansion patterns, indicated by niche hypervolume increases of 35.49% for fir and 68.92% for birch. Within the northern ecoregion, heterospecific conditions elicited niche expansion responses, indicated by niche hypervolume increases for fir of 29.04% and birch of 66.48%. In the southern ecoregion, we observed a contraction response for birch (niche hypervolume decreased by 3.66%) and no changes for fir niche hypervolume. Conspecific niche hypervolume comparisons between ecoregions yielded significant differences for fir and birch (F = 7.581, p‐value = .005 and F = 8.038, p‐value = .001) as did heterospecific comparisons (F = 6.943, p‐value = .004, and F = 68.702, p‐value = .001, respectively). Our results suggest species may exhibit biogeographical specific elemental niches—driven by biophysical differences such as those used to describe ecoregion characteristics. We also demonstrate how a species resource strategy may inform niche shift patterns in response to different community settings. Our study highlights how biogeographical differences may influence foliar elemental traits and how this may link to concepts of ecosystem and landscape functionality.

Individual snowshoe hares manage risk differently: integrating stoichiometric distribution models and foraging ecology

Herbivores making space use decisions must consider the trade-off between perceived predation risk and forage quality. Herbivores, specifically snowshoe hares (Lepus americanus), must constantly navigate landscapes that vary in predation risk and food quality, providing researchers with the opportunity to explore the factors that govern their foraging decisions. Herein, we tested predictions that intersect the risk allocation hypothesis (RAH) and optimal foraging theory (OFT) in a spatially explicit ecological stoichiometry framework to assess the trade-off between predation risk and forage quality. We used individual and population estimates of snowshoe hare (n = 29) space use derived from biotelemetry across three summers. We evaluated resource forage quality for lowbush blueberry (Vaccinium angustifolium), a common and readily available forage species within our system, using carbon:nitrogen and carbon:phosphorus ratios. We used habitat complexity to proxy perceived predation risk. We analyzed how forage quality of blueberry, perceived predation risk, and their interaction impact the intensity of herbivore space use. We used generalized mixed effects models, structured to enable us to make inferences at the population and individual home range level. We did not find support for RAH and OFT. However, variation in the individual-level reactions norms in our models showed that individual hares have unique responses to forage quality and perceived predation risk. Our finding of individual-level responses indicates that there is fine-scale decision-making by hares, although we did not identify the mechanism. Our approach illustrates spatially explicit empirical support for individual behavioral responses to the food quality–predation risk trade-off.

Integrating plant stoichiometry and feeding experiments: state-dependent forage choice and its implications on body mass

Intraspecific feeding choices comprise a large portion of herbivore foraging decisions. Plant resource quality is heterogeneously distributed, affected by nutrient availability and growing conditions. Herbivores navigate landscapes, foraging not only according to food qualities, but also energetic and nutritional demands. We test three non-exclusive foraging hypotheses using the snowshoe hare (Lepus americanus): (1) herbivore feeding choices and body conditions respond to intraspecific plant quality variation; (2) high energetic demands mitigate feeding responses; and (3) feeding responses are inflated when nutritional demands are high. We measured black spruce (Picea mariana) nitrogen, phosphorus and terpene compositions, as indicators of quality, within a snowshoe hare trapping grid and found plant growing conditions to explain spruce quality variation (R² < 0.36). We then offered two qualities of spruce (H1) from the trapping grid to hares in cafeteria-style experiments and measured their feeding and body condition responses (n = 75). We proxied energetic demands (H2) with ambient temperature and coat insulation (% white coat) and nutritional demands (H3) with the spruce quality (nitrogen and phosphorus content) in home ranges. Hares with the strongest preference for high-quality spruce lost on average 2.2% less weight than hares who ate the least high-quality spruce relative to low-quality spruce. The results supported our energetic predictions as follows: hares in colder temperatures and with less-insulative coats (lower % white) consumed more spruce and were less selective towards high-quality spruce. Collectively, we found variation in plant growing conditions within herbivore home ranges substantial enough to affect herbivore body conditions, but energetic stats mediate plant-herbivore interactions.

Forage stoichiometry predicts the home range size of a small terrestrial herbivore

Home range size of consumers varies with food quality, but the many ways of defining food quality hamper comparisons across studies. Ecological stoichiometry studies the elemental balance of ecological processes and offers a uniquely quantitative, transferrable way to assess food quality using elemental ratios, e.g., carbon (C):nitrogen (N). Here, we test whether snowshoe hares (Lepus americanus) vary their home range size in response to spatial patterns of C:N, C:phosphorus (P), and N:P ratios of two preferred boreal forage species, lowbush blueberry (Vaccinium angustifolium) and red maple (Acer rubrum), in summer months. Boreal forests are N- and P-limited ecosystems and access to N- and P-rich forage is paramount to snowshoe hares' survival. Accordingly, we consider forage with higher C content relative to N and P to be lower quality than forage with lower relative C content. We combine elemental distribution models with summer home range size estimates to test the hypothesis that home range size will be smaller in areas with access to high, homogeneous food quality compared to areas of low, heterogeneous food quality. Our results show snowshoe hares had smaller home ranges in areas where lowbush blueberry foliage quality was higher or more spatially homogenous than in areas of lower, more heterogeneous food quality. By responding to spatial patterns of food quality, consumers may influence community and ecosystem processes by, for example, varying nutrient recycling rates. Our reductionist biogeochemical approach to viewing resources leads us to holistic insights into consumer spatial ecology.

Quantity-quality trade-offs revealed using a multiscale test of herbivore resource selection on elemental landscapes

Herbivores consider the variation of forage qualities (nutritional content and digestibility) as well as quantities (biomass) when foraging. Such selection patterns may change based on the scale of foraging, particularly in the case of ungulates that forage at many scales.To test selection for quality and quantity in free-ranging herbivores across scales, however, we must first develop landscape-wide quantitative estimates of both forage quantity and quality. Stoichiometric distribution models (StDMs) bring opportunity to address this because they predict the elemental measures and stoichiometry of resources at landscape extents.Here, we use StDMs to predict elemental measures of understory white birch quality (% nitrogen) and quantity (g carbon/m2) across two boreal landscapes. We analyzed global positioning system (GPS) collared moose (n = 14) selection for forage quantity and quality at the landscape, home range, and patch extents using both individual and pooled resource selection analyses. We predicted that as the scale of resource selection decreased from the landscape to the patch, selection for white birch quantity would decrease and selection for quality would increase.Counter to our prediction, pooled-models showed selection for our estimates of quantity and quality to be neutral with low explanatory power and no scalar trends. At the individual-level, however, we found evidence for quality and quantity trade-offs, most notably at the home-range scale where resource selection models explain the largest amount of variation in selection. Furthermore, individuals did not follow the same trade-off tactic, with some preferring forage quantity over quality and vice versa.Such individual trade-offs show that moose may be flexible in attaining a limiting nutrient. Our findings suggest that herbivores may respond to forage elemental compositions and quantities, giving tools like StDMs merit toward animal ecology applications. The integration of StDMs and animal movement data represents a promising avenue for progress in the field of zoogeochemistry.

Patterns and potential drivers of intraspecific variability in the body C, N, and P composition of a terrestrial consumer, the snowshoe hare (Lepus americanus)

Intraspecific variability in ecological traits is widespread in nature. Recent evidence, mostly from aquatic ecosystems, shows individuals differing at the most fundamental level, that of their chemical composition. Age, sex, or body size and condition may be key drivers of intraspecific variability in the body concentrations of carbon (C), nitrogen (N), and phosphorus (P). However, we still have a rudimentary understanding of the patterns and drivers of intraspecific variability in chemical composition of terrestrial consumers, particularly vertebrates.Here, we investigate the elemental composition of the snowshoe hare Lepus americanus. Based on snowshoe hare ecology, we predicted older, larger individuals to have higher concentration of N or P and lower C content compared with younger, smaller individuals. We also predicted females to have higher concentrations of N, P, and lower C than males due to the higher reproductive costs they incur. Finally, we predicted that individuals in better body condition would have higher N and P than those in worse condition, irrespective of age.We obtained C, N, and P concentrations and ratios from a sample of 50 snowshoe hares. We then used general linear models to test our predictions on the relationship between age, sex, body size or condition and stoichiometric variability in hares.We found considerable variation in C, N, and P stoichiometry within our sample. Contrary to our predictions, we found weak evidence of N content decreasing with age. As well, sex appeared to have no relationship with hare body elemental composition. Conversely, as expected, P content increased with body size and condition. Finally, we found no relationship between variability in C content and any of our predictor variables.Snowshoe hare stoichiometry does not appear to vary with individual age, sex, body size, or condition. However, the weak relationship between body N concentration and age may suggest varying nutritional requirements of individuals at different ages. Conversely, body P's weak relationship to body size and condition appears in line with this limiting element's importance in terrestrial ecosystems. Snowshoe hares are keystone herbivores in the boreal forest of North America, and the substantial stoichiometric variability we find in our sample could have important implications for nutrient dynamics, in both boreal and adjacent ecosystems.