Bear Foraging on Fruit I.
Fruit consumption is the result of circumstances and choices that place bears in a given landscape at a given time, and, within such a spatial-temporal context, dictate which of all the available foods and microsites they exploit. The dynamics are multiscale, entailing both conscious choice as well as environmental and spatial constraints beyond any individual bear's control, notably weather, upbringing, and location of birth. Given a specific birthplace, a bear's first-order foraging decision is selection of habitats to use, followed by a choice of microsites, followed by a choice of individual bushes and aggregations of fruit. Without being exhaustive, what follows below is a brief overview of these multiscale choices made by bears both in the wild and in a laboratory setting.
Keith Aune, then of Montana Fish, Wildlife, and Parks, undertook one of the most comprehensive investigations of grizzly bear ecology in the Northern Continental Divide ecosystem of northwestern Montana. Keith not only documented the habitats within which bears seasonally foraged, but also the foods they ate, and relevant characteristics of the habitats they exploited. These latter measures included estimates of tree overstory and shrub cover, as well as cover estimates for individual bear foods.
The figures at left summarize use of habitat types exploited by East Front grizzly bears while consuming fruit, along with, in the inset diagrams, some seminal features of the vegetation in each. Bear activity is shown by elevation, from lowest at bottom to highest at top, with activity in any given habitat type denoted by a different shade of green. Habitat types associated with lower-elevation fruit-bearing shrubs (i.e., Prunus virginiana [Chokecherry] and Amelanchier alnifolia [Serviceberry]) are shown in the top diagram, along with percent cover of trees, shrubs, and fruit-producing species for each type in the inset. Habitat types associated with higher-elevation fruit-bearing shrubs (i.e., Vaccinium membranaceum [Huckleberry] and Shepherdia canadensis [Buffaloberry]) are shown in the bottom diagram, along with percent cover of trees, tree regeneration, and fruit-producing shrubs.
Not surprisingly, lower-elevation aspen forest and riparian shrub were used most heavily by bears during spring and summer, with consumption of fruits here concentrated during summer in aspen forests, which also supported the greatest abundance of fruit-producing shrubs. By contrast, grasslands forming the typical context for aspen and riparian vegetation had a dearth of fruit.
At higher elevations, fruit-producing shrubs were concentrated in shrubfields and comparably open timbered shrub in which most tree cover was comprised of saplings and poles. Both vegetation types are a product of wildfires, with timbered shrub representing a more advanced stage of forest succession. Grizzly bear use of shrubfields and timbered shrub peaked in lower subalpine habitats during summer, coincident with peak availability of huckleberry and buffaloberry. The fall peak in bear use of closed timber at higher elevations coincided with exploitation of high-elevation whitebark pine (Pinus albicaulis) seeds.
As a bottom line, habitat use and selection by grizzly bears along the East Front logically followed from seasonal concentrations of fruit, as well as the distribution of key fruit-producing species among habitat types.
Efficiencies of Fruit Consumption
Perhaps it goes without saying that different types of fruit are easier for grizzly bears to consume simply because of larger size, denser clustering, and presentation at the end of branches--this in addition to traits intrinsic to individual fruits such as energy content and digestibility. Even so, it is no easy task to quantify the effects of these differences on foraging efficiency, which has not prevented some enterprising researchers from doing precisely that.
One of the earliest studies by John Craighead was as ingenious as it was simple. He sent students out to find patches of fruit representing different species and densities, and then tasked them with tracking the time it took to collect a pint. The results are shown by the bar graph at right. The height of each bar is proportional to time, with species arrayed along the right-hand x-axis, and densities from left (low) to right (high).
The species producing largish fruit in dense clusters at the end of branches (i.e., Mountain ash [Sorbus] and Chokecherry [Prunus]) were most efficiently harvested at high densities, although overall differences among species were minor. But big differences among species manifest with diminishing densities. The time to collect a pint of Buffaloberry (Shepherdia) skyrocketed as densities declined, in marked contrast to the more modest declines in efficiencies for Huckleberry (Vaccinium) and Kinnikinnick (Arctostaphylos). Accounting for fruit quality, the upshot is that Huckleberry is a more beneficial food over a wider-range of berry densities compared to most of the other species, perhaps barring Serviceberry (Amelanchier; see below).
Christy Welch, a graduate student of Charlie Robbins at Washington State University, undertook investigations of foraging efficiencies under more controlled laboratory conditions using captive bears. I've summarized some of her key results at left in the form of x-y graphs with bite rate (top) and (bite size) as a function of fruit density. Bite rate times bite size self-evidently yields total rate of ingestion. The pink dots represent results for Serviceberry and the blue dots results for Huckleberry. Notably, Serviceberry produces fruit born in clusters at the ends of branches whereas Huckleberry produces fruit born singley intermixed with foliage.
The functional responses of bear foraging are clearly different for these two species. Bite rate is relatively unresponsive to densities of Serviceberry, whereas bite size increases dramatically, yielding substantial increases in rates of ingestion with increases in fruit density, almost wholly as a result of changes in bite size. By contrast, neither bite rate nor bite size change much as densities of Huckleberry change, although bite rate tends to be more reactive compared to bite size. The result is only modest increases in rates of ingestion as densities of Huckleberry increase, which is precisely what John Craighead found using humans and pint containers under field conditions, above.
All of this substantiates what grizzly bears tell us observing what they do. Huckleberry and Serviceberry are consistently important sources of fruit for bears wherever abundant, not only because of foraging efficiencies sustained over a wide range of fruit densities, but also because of intrinsic fruit quality. That said, the case for Buffaloberry is less clear. Grizzlies clearly eat lots of it wherever abundant, especially at higher latitudes. But this occurs despite the fact that foraging efficiencies drop dramatically with declining fruit densities, and despite the small size of its fruits born interspersed with foliage. Hmm...