Volume 348, 15 July 2015, Pages 164–173
- Open Access funded by Austrian Science Fund (FWF)
Open Access
Highlights
- •
- We model carpenter ant and brown bear selection on clearcut coarse woody debris.
- •
- Clearcuts seemed to increase the availability and bear selection of carpenter ants.
- •
- Carpenter ant selection depended most on coarse woody debris hardness/decay.
- •
- Brown bear selection depended most on the presence of carpenter ant galleries.
- •
- Silviculture likely affects the probability of CWD selection for both species.
Abstract
Forest
management alters habitat characteristics, resulting in various effects
among and within species. It is crucial to understand how habitat
alteration through forest management (e.g. clearcutting) affects animal
populations, particularly with unknown future conditions (e.g. climate
change). In Sweden, brown bears (Ursus arctos) forage on carpenter ants (Camponotus herculeanus)
during summer, and may select for this food source within clearcuts. To
assess carpenter ant occurrence and brown bear selection of carpenter
ants, we sampled 6999 coarse woody debris (CWD) items within 1019 plots,
of which 902 were within clearcuts (forests ⩽30 years of age) and 117
plots outside clearcuts (forests >30 years of age). We related
various CWD and site characteristics to the presence or absence of
carpenter ant galleries (nests) and bear foraging sign at three spatial
scales: the CWD, plot, and clearcut scale. We tested whether both
absolute and relative counts (the latter controlling for the number of
CWD items) of galleries and bear sign in plots were higher inside or
outside clearcuts. Absolute counts were higher inside than outside
clearcuts for galleries (mean counts; inside: 1.8, outside: 0.8). CWD
was also higher inside (mean: 6.8) than outside clearcuts (mean: 4.0).
However, even after controlling for more CWD inside clearcuts, relative
counts were higher inside than outside clearcuts for both galleries
(mean counts; inside: 0.3, outside: 0.2) and bear sign (mean counts;
inside: 0.03, outside: 0.01). Variables at the CWD scale best explained
gallery and bear sign presence than variables at the plot or clearcut
level, but bear selection was influenced by clearcut age. CWD
circumference was important for both carpenter ant and bear sign
presence. CWD hardness was most important for carpenter ant selection.
However, the most important predictor for bear sign was the presence or
absence of carpenter ant galleries. Bears had a high foraging “success”
rate (⩾88%) in foraging CWD where galleries also occurred, which was
assessed by summing CWD items with the concurrence of bear sign and
galleries, divided by the sum of all CWD with bear sign. Clearcuts
appeared to increase the occurrence of a relatively important summer
food item, the carpenter ant, on Swedish managed forests for the brown
bear. However, the potential benefit of this increase can only be
determined from a better understanding of the seasonal and interannual
variation of the availability and use of other important brown bear food
items, berries (e.g. Vaccinium myrtillus and Empetrum
spp.), as well as other primary needs for bears (e.g. secure habitat
and denning habitat), within the landscape mosaic of managed forests.
Keywords
- Brown bear;
- Camponotus herculeanus;
- Clearcuts;
- Coarse woody debris;
- Resource selection;
- Ursus arctos
