The August 2023 issue of Global Change Biology published a study on the state of the GYE grizzly population from 2000 to 2020. Among other things, the authors found that, at least until 2020, the bear’s foods were sufficient across the landscape, but also found that the bears’ population growth nevertheless slowed — did not stop, but slowed down — from 2010 to 2020.

What’s happened in the five years since 2020 was beyond the scope of the study. But we’re left with the fact of a slowing from 2010 to the study’s end in 2020.

So what? What, if anything, might this decade of slowing mean for recovery of the bear?

First, it’s been standard practice to point to the population’s increasing numbers as bedrock evidence that the grizzly has recovered and can be delisted. By this analysis, the state of the bear population is reduced to a numbers game. 

But there were already other lines of evidence available for assessing recovery, whether recovery of an ecosystem or a single species within it. In 2007, for example, American Naturalist published “Slow Recovery from Perturbations as a Generic Indicator of a Nearby Catastrophic Shift.”

These authors wrote, “Such recovery rates decrease as a catastrophic regime shift is approached, a phenomenon known in physics as “critical slowing down.” They add, “In all the models we analyzed, critical slowing down becomes apparent quite far from a threshold point, suggesting that it may indeed be of practical use as an early warning signal.” 

A record of evidence

The concept of critical slowing down as a predictive early warning has repeatedly been tested within the context of ecological systems. For example, authors of a 2014 PNAS analysis concluded that “critical slowing-down indicators may be used as early warnings for the collapse of ecological networks.”

For just one more example, a July 13 2022 article in Nature applied the concept to “signals of declining forest resilience under climate change.” The authors explain that they “integrate satellite-based vegetation indices with machine learning to show how forest resilience, quantified in terms of critical slowing down indicators, has changed during the period 2000–2020.”  

Despite such evidence, it may be possible that the slowing growth of the grizzly population is still growth, that the slowing is not an early warning of trouble ahead, and that the rationale for delisting the grizzly is intact. That said, any risk of a critical slowing as prediction of trouble head for recovery probably can’t be dismissed lightly.

 Whether critical or not, that leaves the question of why the slowing developed. The authors of the Global Change Biology article refer to a “pronounced environmental changes have occurred in recent decades due to increased human impacts from recreation, development, and climate change.” 

These three changes can help account for a slowing, they likely amount to a triple cumulative threat, and they have certainly not ended in the 5 years since 2020. 

Recreation 

Human recreation is well-established as an adverse impact across a variety of wildlife. A basic issue at stake here is every wild animal’s need to move around a landscape to find a bite to eat and water to drink. Interference with or disruption of this need can be consequential. 

Under the title, “Human disturbance causes widespread disruption of animal movement,” the February 2021 issue of Nature Ecology and Evolution published a compilation of 208 research studies spanning 167 species. In reviewing these studies, the authors discovered that “Disturbance from human activities, such as recreation and hunting, had stronger impacts on animal movement than habitat modification, such as logging and.”

The authors of a February 2025 article in the Journal of Applied Ecology tested this effect specifically on grizzlies and wolves. In their article, “Integrating human trail use in montane landscapes reveals larger zones of human influence for wary carnivores,” authors report that, “The negative effects of human use on wildlife declined steeply with distance such that 50% of the decrease in detection rates immediately adjacent to trails would be expected to occur at 267 m for grizzly bears and 576 m for wolves. Weak effects, 5% as strong as the effect adjacent to trails, extended up to 1.8 and 6.1 km for grizzly bears and wolves, revealing the importance of cumulative measures of human use.”

Development

The effects of trails recreation can arguably have cumulative effect alongside those from development, a.k.a. housing sprawl. In effect, once established, sprawl becomes an irreversible and irretrievable loss for the bear.

By 2012, a team of researchers were already able to review sprawl’s effect on Yellowstone grizzlies in a Wildlife Biology article, “Impacts of rural development on Yellowstone wildlife: linking grizzly bear Ursus arctos demographics with projected residential growth.”

The authors begin the abstract of the article by reporting that, “Exurban development is consuming wildlife habitat within the Greater Yellowstone Ecosystem with potential consequences to the long-term conservation of grizzly bears Ursus arctos.” To test the effect of this sprawl, researchers “portioned habitats into either source or sink, and projected the loss of source habitat associated with four different build out (new home construction) scenarios through 2020” 

They go on to say, “Our findings showed that extremely low densities of residential development created sink habitats.” Along with increased human presence on the trails, this additional threat to the bear’s recovery has clearly persisted since the Global Change Biology study came to its end 2020.

Climate 

Add climate change effects to those from increased human recreation and housing sprawl, and the cumulative, triple-threat effects on grizzly recovery come to a fuller reckoning.

Here, the effects can be direct and physical, across the Animal Kingdom. In its February 2019 issue, Physiology, a journal of the American Physiological Society,  published a review of the literature including this observation: “Of immediate importance is the increased frequency and severity of heat waves occurring around the planet, exposing life to elevated, and often stressful temperatures now more than ever before during the past 150 years.” 

The reviewer goes on to say that these changes “are a present threat to animal life<<h>>.”

A key issue here is an animal’s ability to shed or dissipate its own body heat before it builds up to endangering levels.

Grizzlies aren’t immune. As previously reported in MoJo, authors of an article in the February 2021 Functional Ecology report that “… the heat dissipation limit theory posits that allocation of energy to growth and reproduction by endotherms is governed more by their capacity to dissipate heat than by their ability to harvest energy from the environment.”

Furthermore, they say, “Our results suggest that the costs of heat dissipation, which are modulated by climate, may impose constraints on the behaviour and energetics of large endotherms like grizzly bears, and that access to water for cooling will likely be an increasingly important driver of grizzly bear distribution in Yellowstone as the climate continues to warm.”

These authors tried a look ahead. Based on what the modeling had to tell them, they conclude that, “the availability of water for thermoregulation increased the number of hours during which lactating females could be active by up to 60% under current climatic conditions and by up to 43% in the future climate scenario. Moreover, even in the future climate scenario, lactating bears were able to achieve heat balance 24 hr/day by thermoregulating behaviourally when water was available to facilitate cooling.“

In other words, if I understand correctly, a lactating grizzly female in a hotter and drier world, but with opportunity to cool off, will be functioning well enough to go looking for food if she has the opportunity to cool off. Distance to cooling water and, then, ready access to it may rise in urgency along with the food question.

Authors of the 2021 Functional Ecology article aren’t the only ones who’ve ventured a look ahead. Authors of the 2023 Global Change Biology article went past the limits of their 2000 to 2020 to warn that ”…synergistic effects of continued climate change and increased human impacts could lead to more extreme changes in food availability and affect observed population resilience mechanisms.”  Their reference to resilience is of close interest because resilience is synonymous with recovery.

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