A Change To Capturing One Data Point Shows Conservative Grizzly Bear Population Numbers
The Interagency Grizzly Bear Study Team has changed a data point in their calculations for the Greater Yellowstone Ecosystem grizzly bear population. This one adjustment shows how past population estimates on the number of grizzlies have been very conservative. Wyoming Public Radio's Kamila Kudelska asked study team leader Frank Van Manen why make this change now.
Frank Van Manen: One of the first things that the study team looked at when it was first established in 1973 was, 'How can we best monitor this population?' At the time research tools were still somewhat limited. And so one of the things they immediately honed in on was to keep track of the number of unique females with cubs, because females with cubs are a good indication of what the population is doing overall. They're the backbone of the population, so to speak. And, so keeping track of females with cubs as a reproductive segment of the population made sense. And so a lot of effort went into that early on, that led to what we will now refer to as the Knight et al. rule set. This was basically a rule set to identify sightings from females with cubs, as belonging to unique individuals in unique family groups, so to speak. There's a number of criteria that goes into that determination, into that rule set. But the biggest one, the most important one is a distance criteria. So if you have clusters of observations, you can separate them out based on distance criteria, simply because we know what your typical movements and home range sizes are, etc, that allowed researchers at the time to identify how many unique females with cubs there were. And that number was then extrapolated to the total population estimate. Now, at the time, it was still a recovering population. So there was an implicit conservative mechanism built into it. And essentially at the time, they said, you know, if we use 15 kilometers as a distance criterion to separate observations of females with cubs, that is what the data indicated, but they said, 'Let's double that just to be sure that we're not counting paper bears.' So they just doubled it, which was very conservative but justified at the time, simply because the last thing you want to do when you're dealing with a small recovering population is to overestimate how many pairs there might be.
Kamila Kudelska: Okay, so how did you realize or you as scientists realize that this estimate of population is possibly too biased?
FVM: If you look at the number of conflicts, they are increasing throughout the ecosystem. If you look at the expansion of occupied range that we've seen over the last decade, whereas our population estimate started leveling off, we started wondering, maybe, because we have reached high densities, and at least in the core of the ecosystem, maybe we're not really fully capturing that trend over time, because we're seeing all kinds of other signs that the population might have increased a little bit more than than we actually estimated.
KK: And what are those signs?
FVM: An increase in mortality. That's, of course, directly linked to conflicts as well. I think those combined, I would almost say auxiliary data that told us that that was probably going on, but we didn't know exactly how to what degree we were under estimating and how we could best correct for that. That's where the simulations with the actual empirical data, but you know that they're kind of re-scrambled to mimic the real population on the ground. That work was really essential for us to move forward with this, because we had to demonstrate to ourselves that this is really, first of all, that there is a problem that we do have an underestimation bias. And then secondly, how do we best correct for this bias without going the other direction without the risk of overestimation?
KK: What does this change for the population, if anything at all?
FVM: Just really two main implications for the population estimates. Because the population on the ground again, this is still the same. What it does mean is in terms of the population, as total population estimate is derived from the estimate of the number of females with cubs, so that estimate will go up, given that we've been under estimating by somewhere around 45 percent, maybe even more, that number is going to go up quite a bit.
KK: So you just said that the past population numbers were probably underestimated by 40 percent or so. So last year's population numbers came out at about 700 something. So that might be 1,000 something now with this new population calculation?
FVM: Correct. Yeah. And so one part of this is that the technique itself hasn't changed that much. It's just that distance criteria and the rule set. So the nice thing about this is that we can apply this retroactively and kind of update numbers from the past and see what numbers might have actually been like in the past, compared to what we had calculated in the past. So it also gives us a little bit of a view of...to make those corrections. In past numbers, what are we potentially looking at in terms of how it changes the population estimates.
KK: And it doesn't change anything on the ground right now?
FVM: No, I mean, obviously for managers there's some, they'll probably have some discussion on how to interpret this, this new data. But I think everyone is well aware that on the ground nothing has actually changed. And so it's just an issue of, okay, you know, these are different numbers, we're basically scaling up from the 727 bears, we estimated to be there last year, to whatever the new number would be. But everything scales up. And an important aspect of this is that our mortality estimates will likely actually be lower than we have presented in the past. And are lower simply because we will have a larger population size. We always tally up all mortality records that we get from throughout the ecosystem, we estimate how many of those might have been unreported or unknown to get at a total population or sorry, total estimate of mortality. And then we compare that to the total population size for males, for females, and dependent young to determine how close they are to the thresholds that have been established. And that allows us to gauge, are we below the thresholds that would allow a population to be stable or increasing or not? And, and those numbers would see with a larger population estimate those numbers would likely go down.