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Lag Effects: A Neglected Issue

The size and trend of Yellowstone's grizzly bear population are almost certainly lagging behind major declines in key foods, including whitebark pine, cutthroat trout, and elk. These lags are probably on the order of a decade or so. Which argues against any hasty moves to eliminate ESA protections, and for being patient. Lets see if and how the potential lags in population size and trend play out. The rest of this page is devoted to expounding on the nature of and evidence for population lag effects, and why managers should be taking these effects seriously.

Ecologists have long recognized the potential for lags in the response of a population of animals to changes in habitat. Dr. Dan Doak elaborated on this potential for bear populations in an article that he published in 1995 featuring data from Yellowstone's grizzly bears (follow this link to a copy). He argued that changes in population size (essentially, trend) could lag behind deterioration of habitat conditions by as much as 10 years. One factor that Dan featured was the extent of movements by individual animals between good and poor habitat. Greater movements would dampen the lag effects; lesser movements would amplify them. Of course, one key consideration is the magnitude of movements by the animal relative the scale of habitat degradation. The key point here is that habitat conditions could deteriorate, as is happening in the Yellowstone ecosystem, and effects on the size of the grizzly bear population might not be detected for as much as a decade. Given that a period of rapid change in habitat conditions culminated in 2007, we still have a few years before we would predictably see this in changes in the size of Yellowstone's population.

There is one key trend that increases the potential for a long lag effect playing out in Yellowstone: a well-documented substantial decline in survival of cubs and yearlings. Agency scientists claim this decline has happened simply because of increasing bear densities (i.e., a density-dependent response). The more likely explanation is that more cubs and yearlings are being killed by other bears, and even by wolves, because their mom's are increasingly dependent on meat. Eating meat poses all sorts of hazards (see Conflicts & Mortalities). Regardless of the explanation, it will take the consequences of poor cub and yearling survival a number of years to manifest in estimates of population size. There are two main reasons: (1) estimates of population size and trend in Yellowstone are driven by observations of females with cubs-of-the-year (see The Numbers Game). Given that most females don't start reproducing until age 4, and then only reproduce at 3-year intervals, we won't see much evidence of a failure to recruit reproductive females until 7 or more years has passed. And (2), even if lower cub survival is factored into the multiplier by which agency scientists estimate population size, cubs comprise such a small segment of the population that other factors will swamp their contribution.  And, of course, the effect of lower cub and yearling survival would be on top of others potentially driving a lag effect. 

The North Fork of the Flathead: An Example

Some people might dismiss concerns about lag effects as being hypothetical. Well...interestingly enough, there is a well-documented example of a lag effect for a grizzly bear population in the Rocky Mountains. This for the population of bears in the transboundary region of the North Fork of the Flathead. The data that I show here were presented by Bruce McLellan in a paper that he published in 2015 reporting the results of a long-term study (1979-2010) in this region.

The basic story is this: Production of huckleberries (a key food) crashed in roughly 1996 and stayed low for most of the following 14 years. Population growth rate began to decline almost immediately, but didn't turn negative until 5-years later. Population size, itself, didn't decline dramatically until 11 years after the start of the "berry famine."


The relevant trends are shown in the graph below. The series of figures to the right show estimates of berry production for huckleberry from two different study areas (including Bruce's) at top; for buffaloberry in the middle; and for serviceberry at bottom. The estimates for the Cabinet-Yaak (which is near the North Fork) come from annual reports for that recovery area published by the US Fish & Wildlife Serivce.

The example of the North Fork of the Flathead is clearly relevant to what's unfolding in Yellowstone. Here, as in the North Fork, we are on the lee side of some catastrophic losses of key foods. And, agency managers and scientists are relying heavily on population trends to justify removal of ESA protections. At the same time, these same managers and scientists are being utterly dismissive of negative trends in habitat.


Given the strong potential for lags in the response of population size to habitat deterioration in Yellowstone, why not wait a few more years before moving to delist Yellowstone's grizzly bears? What is the hurry? The answer to this last question can be found on other pages of this web site, and none of it is suggests particularly virtuous motives on the part of our wildlife managers and researchers.  

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