Griz 101: How many bears are there?

You know the old joke: where does a 600 lb grizzly sit down? Anywhere he wants to, and that’s why it is so challenging to study their population. Grizzlies are a long-lived, socially complex animal that vary widely in both their habitat and habits. Since the mid-1970s, the model for estimating bears in the Greater Yellowstone Ecosystem (GYE) has been primarily based upon yearly counts of females with cubs, in addition to the data from a radio-collared sample of animals and annual estimates of mortality rates. Recently, this model has been criticized for its inaccuracy.

Bear Biology

Grizzly bears are a mammal species that can live over 20 years. Over that lifetime, their yearly survival is dependent on building up enough fat and protein reserves to make it through hibernation. For female bears, they have the added burden of pregnancy and lactation. Thus a bear spends most of its life led by its nose and stomach to seek as much food as possible so it can recover from last year’s hibernation (spring) and fatten up for the next one (summer/fall). They are opportunistic omnivores that eat hundreds of species of animals, plants, and fungi over the course of one year. These food sources can vary in annual productivity, as anyone who has seen their favorite huckleberry patch have a productive year followed by a dismal year.  

Grizzlies are nomadic creatures that go where the food is. A bear may range over an area of less than 4 square miles to over 700 square miles. Over the course of a male bear’s life, he may range over an area greater than 1500 square miles, larger than the state of Rhode Island. Grizzlies don’t defend territory as wolves or even your local house wren will, but they do have a big personal space bubble, and the movement of bears is influenced by the presence of other bears. For example, if bear A is a subadult 3 year old, and bear B is a massive 15 year old male, bear A will have to look for food somewhere else that’s not already temporarily claimed by bear B. There are other factors that influence bear movements including human activity and forest fires.  

The unpredictable, wandering lifestyle of grizzly bears would make them hard to count as it is, but there are several other complications. Many surveys of large animals in the Greater Yellowstone Ecosystem are made in winter; the deep snow forces wildlife into greater concentrations at lower elevations. Wildlife are easier to spot against a snowy, leafless environment compared to the rest of the year. But while scientists are counting moose and elk by plane in January, grizzlies are uncountable in their dens. When they do come out, grizzlies split their time between open and forested habitats, so are harder to count than an animal that prefers open habitat like sandhill cranes and pronghorn. In fact the forest-dwelling black bear is so hard to count that there is currently no science-based population estimate for black bears in the GYE.

Current Research Techniques

But we do have a number for grizzlies, provided by the Interagency Grizzly Bear Study Team (IGBST). Coordinated by the USGS, the IGBST is a group of federal, state, and tribal representatives that monitor and research the grizzly bears of the Greater Yellowstone Ecosystem.  The IGBST has used counts of females with cubs to estimate population size since 1983. A female with cubs is the easiest class of bears to recognize, and correlates with growth in a population. The number of unique observed bears is plugged into a formula that estimates the total female with cub population, and from there the remaining segments (males, independent females, dependent offspring). This provides an estimate of the total bear population within the known monitoring area (often referred to as the demographic monitoring area). This data analysis method is called Chao2. The annual count is a comparatively simple and inexpensive method of collecting bear data.  

Data from ear-tagging and radio-collaring bears is also used by the IGBST in estimating population size. A minimum of 25 female bears, plus quite a few males, from across the ecosystem are collared annually. Collared bears are located at least two times a month. From this technique (called mark-recapture), scientists can gather data on a bear’s age, reproduction, cub production and mortality, annual survival rates, and ultimate cause of mortality. Known mortalities from non-collared bears also help estimate population size. Mark-recapture produces a lot of useful data compared to a simple count, but is also more expensive in both cost and resource use. It also provides a population estimate, though with a large degree of uncertainty.

…So How Many Bears Are There?

No one knows how many bears lived in the region historically, but we do know that bear numbers in Yellowstone were reduced by hunting before and immediately after it became a national park. Once bears became a tourist attraction, garbage dumps provided the majority of calories in a grizzly’s summer diet and the population may have grown, until it crashed again after the garbage dumps were closed in the 1970s. The IGBST gives us an estimate of less than 250 bears in the recovery area at that time. Grizzly bears were listed as an endangered species in 1975 and the numbers gradually began rising to over 500 in 2000, and seem to have apparently leveled off within the demographic monitoring area in the last ten years. 

A preliminary 2019 estimate provided at a recent virtual public meeting is 737 bears in the demographic monitoring area surveyed by the IGBST. There are additional bears beyond the demographic monitoring area, but their numbers have not been documented other than simply tracking the expansion of their range – and the ensuing human conflict.

…You said there was controversy?

While the status of grizzlies rests upon its population estimate, everyone – scientists, bear advocates, and bear opponents – all agree that the actual number of bears may not be the number provided by the IGBST. The reason for this goes back to that data analysis method: Chao2. Chao2 is a conservative estimate of grizzly numbers that correspondingly decreases in accuracy as population rises. The IGBST says that this method may underestimate bears by as much as 50%, leading to a population closer to 1000 in the demographic monitoring area (with additional bears outside the area).  

Meanwhile, the same method has been accused of overestimating bear numbers. In 2013, a study by scientists Daniel Doak and Kerry Cutler pointed out apparent flaws in the current approach to estimating bear numbers. Their argument was Chao2 is too unreliable to use as an indicator for a total bear population from sightings of females with cubs because it does not account for the variable sightability of bears over time. Not only do bears use different places and food resources from year to year depending on the seasonal availability of food, but some of their food sources have been declining (whitebark pine, cutthroat trout) – causing bears to look for other sources in other places. Additionally, Chao2 does not account for the effort, time, and skill put into the aerial search for bears. Doak and Cutler concluded that “While our most basic conclusion is that we cannot confidently assess the past or future trends of this population without further and more careful work, our analyses show that trends in Fcoy and Chao2 are consistent with a population that has grown little, or perhaps not at all, in the recent past, but also that was higher in the past than was realized.” 

The reintroduction of the wolves in 1995 also resulted in bears spending more time in the open as they feed upon carcasses of elk and bison killed by wolves. Additionally, the amount of time spent looking for bears increased over the years, and the accumulated knowledge of a grizzly’s biology and sheer practical skill at finding bears probably drove up sightings. Supporters of this theory argue that it was these factors that are responsible for the rise in sightings of females with cubs, not that the population was increasing.

The following year, the IGBST published a rebuttal refuting the claims of Doak and Cutler, pointing out other positive data trends do not support a decline in numbers. Excluding flights over the changing whitebark pine habitat portions of the demographic area, hours flown over non-whitebark habitat have remained roughly stable while sightings of bears have increased. Studies from other recovering wildlife populations show that they need a robust core population to create range expansion as grizzlies are doing, and indications from the mark-recapture study show there are a steady number of un-tagged bears being caught (50-70%) as well as more bears caught per year in general.

Doaks and Cutler then published a rebuttal to the rebuttal, but we’ll stop here to point out that both parties agreed the current method of estimating grizzlies numbers from sightings of females with cubs is problematic and unreliable. An alternative method titled the Integrated Population Model has recently been developed to more accurately estimate bear numbers from populations across the world. This method connects several types of grizzly data – including counts, mark-recapture, and litter numbers – to our knowledge of bear ecology, much of that knowledge acquired within the last few decades. It shows great promise to deliver a more accurate estimate of grizzly numbers here in the GYE.  

…And Why Does It Matter?

Scientists, grizzly opponents, and grizzly advocates can probably all agree that more accuracy in counting grizzlies is a good thing. But another controversy arises from the politics and policies that will come from that data. A minimum of 674 bears (as estimated by Chao2) will be maintained in the recovery area according to the agreement between the states of Idaho, Montana, and Wyoming – but this is an agreement that is now being redone. We don’t know whether the Integrated Population Model will result in a lower, higher, or identical number than the number we have today – but most likely, it will be higher. Many grizzly advocates fear that a larger population estimate derived from the Integrated Population Model may cause the states of Wyoming, Idaho, and Montana to allow a larger hunter harvest of grizzly bears then would be allowed under the more conservative Chao2 estimate.

Why would a larger hunter harvest be detrimental given that dozens of bears are already killed annually as a result of human conflict? As tourist visitation to the GYE increases, the majority of visitors are interested in seeing wildlife, and a free-roaming grizzly bear holds the top spot on their wish list. In addition to seeking out the charisma of a bear, grizzlies represent an opportunity for an increasingly urbanized population to connect with wilderness. This is not only from personal encounters on hikes and in bear-jams, but also through social media, such as the large followings of famous GYE bears on Facebook. Aside from its natural, entertainment, and spiritual value, bear viewing alone contributes $10 million dollars to the local economy of the GYE and supports over 150 local jobs. There’s a biological line driven by science of how much mortality a grizzly population can tolerate without decreasing, but there’s also a changing line of tolerance of hunting predators and trophy hunting in general. Images of hunters posing with dead grizzlies in 2020 are finding a different reception today than they would have in the days of Teddy Roosevelt.

Conclusion 

The complex life history of grizzlies, pressure of threats to their population, Due to the wandering nature of grizzlies as they find food, mate, and raise cubs on the GYE landscape, and the controversy about the statistical techniques used to estimate grizzly numbers, there is no easy way to count a grizzly. But the accumulated knowledge of grizzly research has created an opportunity to abandon the problems of the Chao2 method for a more accurate estimate, the Integrated Population Model. The policy and management decisions made from this new estimate will be influenced by the values of the surrounding human communities, from local gateway communities all the way to international visitors. No matter what path we use in the future to try to understand and manage this wildest of wild creatures, we must be thorough in our data collection, rigorous in our analysis of the data, cautious in our conclusions, and conservative in the management decisions made from those conclusions.

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