Response of Grizzly Bears to Changing Food Resources in the Greater Yellowstone Ecosystem

Frank T. van Manen, Cecily M. Costello, Mark A. Haroldson, Dan D. Bjornlie, Michael R. Ebinger, Kerry A. Gunther, Dan J. Thompson, Megan D. Higgs, Dan B. Tyers, Steven L. Cain, Kevin L. Frey, Bryan Aber, & Charles C. Schwartz

From Yellowstone Science 23(2): 2015, pages 26-31.

"The only thing that is constant is change."

- Heraclitus

 

This simple idea put forward by a Greek philosopher about 2,500 years ago beautifully captures a key essence of nature, in which stability is often the exception rather than the norm. Indeed, how organisms respond to changes in biotic composition and structure-or ecosystem dynamics-is a central topic in ecology. As ecologists, when it comes to studying how animals respond to change, we often refer to "generalists" and "specialists." This division is based on how natural selection shapes a species' diet and habitat preference, and each has its own advantages.

Generalists are animals that tend to forage on a wide variety of food items or are able to live in a variety of habitats. Conversely, as their name implies, specialists are more selective in what they eat and the places they live. Although the specialists do not have as much flexibility in what they eat, they usually have less competition when obtaining foods;whereas, generalists must compete with other generalists in the environment. But being a generalist has its advantages too, most notably the ability to better cope with change in the availability of a particular food item or habitat type. Heraclitus' famous quote about change is particularly relevant to grizzly bears during the past few decades, as scientists began to observe notable changes in their food resources. These changes provided the Interagency Grizzly Bear Study Team with a unique opportunity to investigate how well one of nature's most iconic generalists fit ecological predictions: Did grizzly bears cope with these changes in their food base and, if so, how?

How Are Food Resources of Grizzly Bears Changing?

Grizzly bears are opportunistic omnivores and feed on a wide array of animals and plants. Although grizzly bears commonly consume herbaceous vegetation, larger bears have difficulty meeting their energy requirements unless they also consume high-calorie foods (Schwartz et al. 2003). Similarly, high-calorie foods also tend to play an important role during fall hyperphagia -a period prior to entering hibernation in which grizzly bears gain substantial weight, mostly in the form of body fat. Depending on where they live, bears in the Greater Yellowstone Ecosystem (GYE) typically have access to at least one of five high-calorie food items that have experienced various levels of change: bison (Bison bison), elk (Cervus elaphus), cutthroat trout (Oncorhynchus clarkii), whitebark pine (Pinus albicaulis), and army cutworm moths (Euxoa auxiliaris).

Yellowstone grizzly bears have been identified as one of the most carnivorous interior populations in North America (Jacoby et al. 1999, Mowat and Heard 2006). The GYE contains large populations of ungulates, and winter-killed elk and bison are important spring foods to bears (Green et al. 1997, Mattson 1997). Some ungulate populations in the GYE have experienced significant changes during the past decade;whereas, others have not. The Yellowstone bison population has fluctuated between 2,500 and 5,000 in recent decades, largely because of a removal program directed at limiting population growth and regulating numbers due to limited tolerance for bison in surrounding states (White et al. 2015). During 2014, numbers approached the record high count of 5,000 (White et al. 2015). Elk numbers on the northern range, in the Madison headwaters, and Gallatin Canyon have substantially declined;but elk numbers from some herds east of Yellowstone either remained constant or increased (Creel 2010, Cross et al. 2010). Competition for the ungulate resource likely has increased due to an approximate three-fold increase in grizzly numbers since the 1970s and growth of the reintroduced wolf population from 31 individuals in 1995 to a minimum of 463 in the GYE in 2012 (USFWS et al. 2013).

Prior to the 1990s, spawning cutthroat trout were a valuable food for grizzly bears residing near the tributary streams to Yellowstone Lake from mid-May through July (Reinhart and Mattson 1990), but this fish population has declined due to non-native lake trout (Salvelinus namaycush) predation, whirling disease (Myxobolus cerebralis), and prolonged droughts (Koel et al. 2003, 2005). The cutthroat trout population is estimated to be less than 10% of historical numbers (Koel et al. 2005), and biomass of cutthroat trout consumed by grizzly bears in this region declined by 70% between 1997 and 2007 (Fortin et al. 2013).

Seeds from whitebark pine are a frequent food for grizzly bears during mid-August through late September and, occasionally, in spring when seed production in the previous fall was high (Mattson et al. 1991). Grizzly bears typically consume whitebark pine seeds by raiding seed caches (middens) made by red squirrels (Tamiasciurus hudsonicus). Whitebark pine is a masting species;and grizzly bear consumption of seeds is associated with this natural cycle of good and poor years of cone production. Whitebark pine has experienced widespread tree mortality because of mountain pine beetle (Dendroctonus ponderosae), wildland fire, and white pine blister rust (Cronartium ribicola), with mountain pine beetle having caused the greatest mortality since the early 2000s (Gibson 2007; see "How Important is Whitebark Pine to Grizzly Bears?," this issue).

Finally, army cutworm moths provide the richest food source (8 kilocalories per gram) for grizzly bears in the GYE (French et al. 1994). Use of this food resource is unique, not only because bears can obtain substantial energy by consuming thousands of these tiny insects in a day, but also because this foraging occurs on very remote, high alpine talus slopes found mostly in the Absaroka Mountains east of Yellowstone (see "Shorts: Grizzly Bears and Army Cutworm Moths," this issue). We know relatively little about the variability of this food resource and whether the long-term availability of army cutworm moths is changing.

Grizzly Bear Responses to Changing Food Resources:Summary of Research Findings

The overall goal for this research project was to leave "no stone unturned," so multiple research questions were asked to determine if and how grizzly bears have responded to changes in food resources. If all investigations point to a similar interpretation, the overall findings would be more reliable. Eight research topics were explored to determine different types of responses to changing food resources, ranging from the individual to population level:

1. Diet diversity;
2. Grizzly bear selection of whitebark pine habitat;
3. Body condition;
4. Animal matter as alternative food sources;
5. Changes in movements and home ranges;
6. Changing mortality risk due to changing food resources;
7. Home-range size as an indicator of density versus resource effects;and
8. Relationship among changing vital rates, resource changes, and density dependence.

Yellowstone grizzly bears exhibit substantial variation in their diets. Based on a review of existing literature from 1891 to 2013, Gunther et al. (2014) documented that grizzly bears consumed 266 different food species (see "Grizzly Bears: Ultimate Omnivores of the Greater Yellowstone Ecosystem," this issue). These findings of a diverse diet are supported by numerous other studies that showed considerable adaptability in foraging strategies among bears in general (e.g., Stirling and Derocher 1990, Yeakel et al. 2013) and brown bears in particular (Van Daele et al. 2012). Diet flexibility is central to the evolutionary strategy of grizzly bears, which allows them to occupy a wide range of the world's biomes (Schwartz et al. 2013) and may partly explain why grizzly bears occupy the greatest diversity of habitats of the eight bear species in the world (Schwartz et al. 2003).

Results of this research indicate that grizzly bears in the GYE gradually shifted several major food items in their diets over a period of four decades as availability changed, while other food items were relatively constant (Gunther et al. 2014). For example, graminoids and ants were documented in all years for which bear scat data were available (37 years during 1943–2009).In response to the reduced availability of whitebark pine, Yellowstone grizzly bears exhibited reduced selection of whitebark pine habitat over the past decade in addition to a shorter and delayed duration of use during poor cone production years (Costello et al. 2014;see "How Important is Whitebark Pine to Grizzly Bears?," this issue). This response presumably reflects a reduction in midden excavation by grizzly bears, which was also documented after the extensive 1988 fires (Podruzny et al. 1999). As an alternative to whitebark pine consumption, grizzly bears seem to have increased consumption of animal matter and other foods. A new technique called stable isotope analysis can be used to determine the proportion of the diet that is assimilated from animal (isotopic nitrogen 15N) versus plant resources (isotopic carbon 13C). Stable isotope analysis is much more accurate than traditional techniques for diet analysis, which are typically based on food items found in bear scats. Also, it can be performed on various samples and reflect different time periods. For example, hair samples provide an estimate of assimilated diet over the previous year;whereas, a blood serum sample reflects the diet from the previous 10 to 14 days. Stable isotope analyses of samples collected during 2000–2010 (Schwartz et al. 2014) and analyses of carcass use (IGBST 2013) support an increase in consumption of animal matter coinciding with the period of reduced use and selection of whitebark pine habitat.

Interestingly, in the apparent transition of grizzly bears reducing use of whitebark pine seeds and shifting their diets to other foods, movements did not increase (Costello et al. 2014). We predicted home-range sizes would increase for those bears that lost most whitebark in their home range, but we found no such relationship.In fact, home-range size was more closely linked with bear density, showing much less variation where bear densities are higher. This finding suggests social pressures in those areas may be confining bears to smaller home ranges (Bjornlie et al. 2014b).Schwartz et al. (2014) and IGBST (2013) showed that foods available in the GYE appear adequate to maintain body condition (body mass, percent body fat) at levels prior to the whitebark pine decline.Schwartz et al. (2014) also demonstrated that body condition did not change during years of poor whitebark pine seed production, likely because grizzly bears compensated by consuming more animal matter in those years. Because body condition may influence reproduction (Robbins et al. 2012), we also investigated whether reproduction declined. However, analyses by the study team did not indicate this was the case (IGBST 2012). Thus, this body of new research conducted in the GYE suggests grizzly bears continue to access sufficient food resources to maintain individual productivity.

At the population level, we addressed whether bears may become more vulnerable to mortality in less secure habitat areas due to whitebark pine decline. Based on observations by Schwartz et al. (2010) that grizzly bears move to lower elevations during poor whitebark pine years. Our analyses (IGBST 2013) indicate the benefits of good whitebark pine cone crops are still associated with reduced human-caused mortalities in fall for independent-aged (two years or older) grizzly bears. However, these findings also showed that mortalities inside the Grizzly Bear Recovery Zone did not increase much during the period of whitebark pine decline (2002–2011), which was confirmed by survival estimates of independent-aged bears. The annual survival rate for subadults (2–4 years old;both sexes) and adult females (more than 5 years old) was 0.95, showing no change for three decades. Also, adult male survival rates actually increased from 0.87 during 1983–2001 to 0.95 during 2002–2011 (Haroldson et al. 2006, IGBST 2012). We did observe an increase in mortality rate outside the Grizzly Bear Recovery Zone. We suspect these mortalities are more a function of expansion of occupied grizzly bear range (Bjornlie et al. 2014a) into locales where landscapes are less suitable for long-term occupancy and conflicts are more likely ("Shorts: Grizzly Bears and Army Cutworm Moths," this issue). Our final inquiry for this research project involved another population-level assessment, which is detailed elsewhere in this issue (see "Demographic Changes in Yellowstone's Grizzly Bear Population," this issue). Those analyses supported the notion that slowing of population growth that started in the early 2000s may potentially be associated with the population reaching carrying capacity rather than whitebark pine decline (van Manen et al. 2015).

Conclusions

Collectively, findings of these research projects do not suggest that changes in food resources have had profound negative effects on grizzly bears at the individual or population level. Grizzly bears obtained sufficient alternative foods through diet shifts and have maintained body mass and percent body fat over time. The picture that emerges is that grizzly bear diets are highly dynamic, changing daily, seasonally, annually, and even by decade, and vary by area within the GYE. Equally fascinating is to what degree diet specialization may be influenced by where a bear is raised and behaviors learned from the mother. With DNA techniques that allow us to determine parentage and new analytical approaches, we hope to address these new research questions.

Based on our extensive demographic analyses, we have not observed a decline in the Yellowstone grizzly bear population but only a slowing of population growth since the early 2000s (IGBST 2012, Higgs et al. 2013). Demographic analyses indicate increased bear density, rather than a decline in food resources, may be associated with this change in population trajectory, possibly indicating the population is nearing carrying capacity. These are key concepts in wildlife ecology that are often difficult to ascertain because most studies lack the long-term, detailed data needed to investigate them.

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