Importance of Pine Seeds to Grizzlies
There are several ways to reckon the importance of a food to a bear--as a proportion of ingested volumes, as a fraction of metabolized energy, or as a fractional contribution to different key nutrients. Unfortunately, all of the methods for estimating such contributions are fraught with uncertainty, regardless of the often grandiose claims that often herald the introduction of new techniques.
For a long time, the contributions of individual diet items were reckoned in terms of frequencies or percent volumes in fecal samples, AKA scats. Yet because different items are subject to widely varied digestibilities, such representations are prone to grossly inflate the contributions of poorly digested foods, such as grasses and forbs, relative to more thoroughly digested foods, such as meat, berries, and even pine seeds. However, this bias can be overcome by, first, correcting for differential translation of ingested to defecated volumes to estimate original ingested mass, and then, second, accounting for the differential digestibilities of ingested food, to finally come up with estimated of contributions of each item to the digested diet.
However, there is an additional chronically-overlooked additional step in estimating total dietary contributions from scats. Almost invariably, dietary contributions are estimated as fractions for a given time period, typically by month or bi-weekly step. The problem is that overall rates of ingestion, i.e., feeding activity, vary widely with time, from a typical nadir during spring and estrus, to a pronounced peak during hyperphagia, which usually begins in mid-July and lasts through October. Moreover, the proportion of bears out of their dens and feeding varies during the spring moths. If results are not adjusted to (approximately) account for this seasonal variability in population-level rates of ingestion, then the contributions of foods eaten predominately early in the active season will be considerably exaggerated.
Fraction of Ingested Diet: The Story of Scats
The graphs at left are the result of going through the requisite steps outlined immediately above to derive an estimate of dietary contributions to ingested diet from analysis of multitudinous scats. The two graphs represent diets from the two areas in North America where grizzly bears are known to eat substantial amounts of whitebark pine seeds--the East Front of the Rocky Mountains in northwestern Montana, at top, and the Yellowstone ecosystem of Wyoming, Montana, and Idaho, at bottom (see the map in the Introduction). The red-colored band represents the contribution of whitebark pine seeds.
In each case, the estimates are based on analysis of thousands of scats collected over numerous years by along the East Front and the Interagency Grizzly Bear Study Team around Yellowstone. Scat correction factors and food-specific digestibilities were developed in the lab of Charlie Robbins at Washington State University (see this entry on Bear Nutrition).
The upshot is that pine seeds accounted for roughly 14% of the digested diet for bears living along the East Front, and a larger 20% fraction for bears living in the Yellowstone ecosystem. Most of the pine seeds were ingested during fall, after the seeds had matured, and cones been harvested and cached by red squirrels (see Introduction). Consumption earlier in the year was primarily of seeds that had overwintered from the previous fall in cones cached by squirrels in middens (for more on this, see Temporal variations).
The tendency of stable isotopes to differentially concentrate as a function of differences in diet has allowed bear researchers to increasingly reconstruct diets simply from analyzing body tissues obtained from bears. The entailed technology is sexy and you only need to capture bears (which most researchers do anyway for other reasons) or otherwise collect tissues such as hair without the muss and fuss of wandering around picking up hundreds if not thousands of scats, and then spending the requisite hours poking through them. One premise, of course, is that different diet items contains different concentrations of stable isotopes that create a unique signature. To date, the isotopes most commonly employed are those of Carbon (C), Nitrogen (N), Sulfur (S), and Mercury (Hg).
This methodological paradigm was, of course, billed from the start as definitive and comparatively straight-forward. Such claims are a necessary basis for advancement of scientific careers. And, then, of course, complications and fierce debates ensued. Isotopic concentrations varied among body tissues and with time, so sampling considerations have gotten increasingly complicated and weighty. Perhaps more important, the ways that isotopic fractions are accounted for, one relative to other, via diet mixing models, and the statistical methods by which that is done potentially introduce all sorts of artifacts and other uncertainties. As a bottom line, stable isotopes are far from a silver bullet when it comes to estimating ursid diets.
All of that being said, the results of isotopic analyses done for Yellowstone grizzly bears are broadly consistent with results obtained by correcting the analyses of scats. Back is 2003, Laura Felicetti estimated that around 67% (plus or minus 19%) of all bears obtained >50% of their assimilated N and S (and even more of their energy) from pine seeds when seed crops were good, and that during such years around 97% of all bears made substantial use of this food. By contrast, during poor seed crops, as few 19% of all bears consumed substantial amounts of pine seeds (for more, see Temporal Patterns). More recently, in 2017, John Hopkins estimated that whitebark pine seeds accounted for an average 35% of the digested diet among grizzly bears using an area just northeast of Yellowstone Park, around Cooke City, where whitebark pine is still relatively abundant--and always has been.
Fraction of Digested Diet: The Story of Isotopes
Laura's results get at an important point. Whitebark pine is not uniformly abundant in space, nor are seed crops uniformly abundant one year to the next; nor are pine seeds equally important to all bears. The graph at right speaks to these fundamentals.
The graph at top (A) shows the percent contribution of pine seeds to fecal volumes in Yellowstone on a monthly basis, averaged over all years 1976-1996, and as a maximum observed for any single year. Consistent with the graph above, fecal fractions of pine seeds increase during the bears' active season to a maximum in the fall coincidence with ripening and harvest of seed crops. But, the maximum amounts consumed any given month can potentially account for the (near) majority of fecal volumes every month except April, and be 2- to 10-times the inter-annual average, depending on the month. The inset diagram (C) merely makes the point that, when a collected scat contained pine seeds, the seeds comprised essentially all of the scat material, regardless of the month--which emphasizes that, when bears feed on pine seeds, they eat virtually nothing else.
But that is not the whole story. The graph at bottom right shows the frequency with which male versus female grizzly bears were observed to consume pine seeds in the Yellowstone ecosystem--this based on following dozens of radio-collared grizzlies around and documenting their behaviors. The important point here is that females were roughly twice as likely as males to eat pine seeds and, when the estimated number of seeds eaten is factored in, consumed roughly twice the total volumes. In other words, pine seeds were a particularly important food for females compared to males, who parenthetically tended to eat more meat.
Contribution to Digested & Ingested Nutrients
But, in the end, contributions of foods to specific nutrients (including energy) are more important to understanding "importance" than is a simple reckoning of contributions to ingested volumes. It matters to the health and reproductive success of bears whether a food is rich in protein versus simple sugars versus fats (for more, see this entry on nutrition).
The figure at left shows the relative contributions of different foods or food groups to digested energy (top), ingested protein (middle), and ingested fat (bottom) based, again, on analysis of fecal samples collected in the Yellowstone ecosystem between 1976 and 1996. Pine seeds, high-lighted in red, made major contributions to digested energy and ingested fat, but especially the latter. This follows from the fact that pine seeds are one of the fattiest foods eaten by bears in the northern US Rocky Mountains. By contrast, pine seeds are not rich in protein, especially when compared to cutthroat trout and meat from large herbivores such as elk, bison, and moose.
Putting this all together, whitebark pine seeds clearly comprise a substantial part of ingested diets in areas where there is abundant whitebark pine--historically, second only to meat from large mammals in the Yellowstone ecosystem, and third only to berries and meat along the East Front. In such places, pine seeds are, moreover, a disproportionately important source of dietary fat, preferentially consumed by female grizzlies. This greater consumption of pine seeds by females makes sense given that their reproductive success is highly contingent on adipose reserves, and that accumulation of such reserves is predictably facilitated by consumption of a high-fat diet. And because growth or decline of bear populations is largely dictated by the survival and reproductive success of females, it is not surprising that access to pine seeds has historically been strongly correlated with growth rates of the Yellowstone grizzly bear population (see Yellowstone Demography).
I deliberately use the past-tense in many places, here, for good reason. As I elaborate under Recent History and Future Prospects, whitebark pine has undergone dramatic declines in the Rocky Mountains during the last 30 plus years because of disease and unprecedented outbreaks of mountain pine beetles, with almost certain functional extirpation from the direct and indirect effects of climate warming during the next 50-100 years.