Testing foraging optimization models in brown bears: Time for a paradigm shift in nutritional ecology?

How organisms obtain energy to survive and reproduce is fundamental to ecology, yet researchers use theoretical concepts represented by simplified models to estimate diet and predict community interactions. Such simplistic models can sometimes limit our understanding of ecological principles. We used a polyphagous species with a wide distribution, the brown bear (Ursus arctos), to illustrate how disparate theoretical frameworks in ecology can affect conclusions regarding ecological communities. We used stable isotope measurements (δ13 C, δ15 N) from hairs of individually monitored bears in Sweden and Bayesian mixing models to estimate dietary proportions of ants, moose, and three berry species to compare with other brown bear populations. We also developed three hypotheses based on predominant foraging literature, and then compared predicted diets to field estimates. Our three models assumed (1) bears forage to optimize caloric efficiency (optimum foraging model), predicting bears predominately eat berries (~70% of diet) and opportunistically feed on moose (Alces alces) and ants (Formica spp. and Camponotus spp; ~15% each); (2) bears maximize meat intake (maximizing fitness model), predicting a diet of 35%-50% moose, followed by ants (~30%), and berries (~15%); (3) bears forage to optimize macronutrient balance (macronutrient model), predicting a diet of ~22% (dry weight) or 17% metabolizable energy from proteins, with the rest made up of carbohydrates and lipids (~49% and 29% dry matter or 53% and 30% metabolizable energy, respectively). Bears primarily consumed bilberries (Vaccinium myrtillus; 50%-55%), followed by lingonberries (V. vitis-idaea; 22%-30%), crowberries (Empetrum nigrum; 8%-15%), ants (5%-8%), and moose (3%-4%). Dry matter dietary protein was lower than predicted by the maximizing fitness model and the macronutrient balancing model, but protein made up a larger proportion of the metabolizable energy than predicted. While diets most closely resembled predictions from optimal foraging theory, none of the foraging hypotheses fully described the relationship between foraging and ecological niches in brown bears. Acknowledging and broadening models based on foraging theories is more likely to foster novel discoveries and insights into the role of polyphagous species in ecosystems and we encourage this approach.

Black Bear Behavior and Movements Are Not Definitive Measures of Anthropogenic Food Use

Increasing human-bear conflicts are a growing concern, and managers often assume bears in developed areas are food-conditioned. We examined the relationship between human-bear conflicts and food conditioning by analyzing isotopic values of hair from black bears (Ursus americanus floridanus) involved in research (n = 34) and conflicts (n = 45). We separated research bears into wild and developed subgroups based on the impervious surface within their home ranges and separated conflict bears based on observations of human food consumption (anthropogenic = observations; management = no observations). We initially assumed wild bears were not food conditioned and anthropogenic bears were. However, using isotopic values, we classified 79% of anthropogenic bears and 8% of wild bears as food conditioned. Next, we assigned these bears to the appropriate food conditioned category and used the categorizations as a training set to classify developed and management bears. We estimated that 53% of management bears and 20% of developed bears were food conditioned. Only 60% of bears captured within or using developed areas showed evidence of food conditioning. We also found that δ13C values were a better predictor of anthropogenic foods in a bear's diet than δ15N values. Our results indicate that bears in developed areas are not necessarily food conditioned and caution against management actions based on limited observations of bear behavior.

The influence of human disturbance on Pacific salmon (Oncorhynchus spp.) in the diet of American black bears (Ursus americanus) in two areas of coastal British Columbia, Canada

Recent studies have highlighted the importance of salmon (genus Oncorhynchus Suckley, 1861) in the diet of bears, and of bears as consumers and key agents supporting the transport of salmon-derived nutrients to riparian ecosystems. Salmon abundance and human disturbance are known influences on bear ecology and behaviour, though few studies have quantified shifts in bear diet due to these effects. We used stable isotope analysis to investigate how salmon escapement and human presence influenced the proportion of salmon in the diet of American black bears ( Ursus americanus Pallas, 1780) in two locations in coastal British Columbia, Canada. We found that salmon constituted a small proportion of black bear diet across sexes and ecosystems, while bears appeared to gain a similar amount of energy and lean mass from terrestrial sources. Salmon consumption was not related to the total annual abundance of salmon in a watershed but was significantly lower in large streams with regular human presence, suggesting that human disturbance can cause a dietary shift in bears that could have important consequences to their fitness. We also observed that the isotopic signatures of key bear foods did not vary between foliage and fruit, simplifying data collection for future isotopic studies on bear diet.

Gaining insight into the assimilated diet of small bear populations by stable isotope analysis

Apennine brown bears (Ursus arctos marsicanus) survive in an isolated and critically endangered population, and their food habits have been studied using traditional scat analysis. To complement current dietary knowledge, we applied Stable Isotope Analysis (SIA) to non-invasively collected bear hairs that had been individually recognized through multilocus genotyping. We analysed carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes of hair sections and bear key foods in a Bayesian mixing models framework to reconstruct the assimilated diet on a seasonal basis and to assess gender and management status effects. In total, we analysed 34 different seasonal bear key foods and 35 hair samples belonging to 27 different bears (16 females and 11 males) collected during a population survey in 2014. Most bears showed wide δ¹⁵N and δ¹³C ranges and individual differences in seasonal isotopic patterns. Vegetable matter (herbs, fleshy fruits and hard mast) represented the major component of the assimilated diet across the dietary seasons, whereas vegetable crops were rarely and C4 plants (i.e., corn) never consumed. We confirmed an overall low consumption of large mammals by Apennine bears consistently between sexes, with highest values in spring followed by early summer but null in the other seasons. We also confirmed that consumption of fleshy fruits peaked in late summer, when wild predominated over cultivated fleshy fruits, even though the latter tended to be consumed in higher proportion in autumn. Male bears had higher δ ¹⁵N values than females in spring and autumn. Our findings also hint at additional differences in the assimilated diet between sexes, with females likely consuming more herbs during spring, ants during early summer, and hard mast during fall compared to males. In addition, although effect sizes were small and credibility intervals overlapped considerably, management bears on average were 0.9‰ lower in δ ¹³C and 2.9‰ higher in δ ¹⁵N compared to non-management bears, with differences in isotopic values between the two bear categories peaking in autumn. While non-management bears consumed more herbs, wild fleshy fruits, and hard mast, management bears tended to consume higher proportions of cultivated fruits, ants, and large mammals, possibly including livestock. Although multi-year sampling and larger sample sizes are needed to support our findings, our application confirms that SIA can effectively integrate previous knowledge and be efficiently conducted using samples non-invasively collected during population surveys.