Fire drives transcontinental variation in tree birch defense against browsing by snowshoe hares.
Fire has been the dominant disturbance in boreal America since the Pleistocene, resulting in a spatial mosaic in which the most fire occurs in the continental northwest. Spatial variation in snowshoe hare (Lepus americanus) density reflects the fire mosaic. Because fire initiates secondary forest succession, a fire mosaic creates variation in the abundance of early successional plants that snowshoe hares eat in winter, leading to geographic variation in hare density. We hypothesize that fire is the template for a geographic mosaic of natural selection: where fire is greatest and hares are most abundant, hare browsing has most strongly selected juvenile-phase woody plants for defense. We tested the hypothesis at multiple spatial scales using Alaska birch (Betula neoalaskana) and white birch (Betula papyrifera). We also examined five alternative hypotheses for geographic variation in antibrowsing defense. The fire-hare-defense hypothesis was supported at transcontinental, regional, and local scales; alternative hypotheses were rejected. Our results link transcontinental variation in species interactions to an abiotic environmental driver, fire. Intakes of defense toxins by Alaskan hares exceed those by Wisconsin hares, suggesting that the proposed selection mosaic may coincide with a geographic mosaic of coevolution.
Box 306, Cora, Wyoming 82825, USA. ffjpb@wyoming.com
John P
JP
Bryant
Thomas P
TP
Clausen
Robert K
RK
Swihart
Simon M
SM
Landhäusser
Michael T
MT
Stevens
Christopher D B
CD
Hawkins
Suzanne
S
Carrière
Andrei P
AP
Kirilenko
Alasdair M
AM
Veitch
Richard A
RA
Popko
David T
DT
Cleland
Joseph H
JH
Williams
Walter J
WJ
Jakubas
Michael R
MR
Carlson
Karin Lehmkuhl
KL
Bodony
Merben
M
Cebrian
Thomas F
TF
Paragi
Peter M
PM
Picone
Jeffrey E
JE
Moore
Edmond C
EC
Packee
Thomas
T
Malone
D000818:Animals; D029662:Betula / Q000235:genetics* / Q000378:metabolism*; D005075:Biological Evolution; D017753:Ecosystem; D005390:Fires; D034621:Hares / Q000235:genetics* / Q000378:metabolism; D014118:Toxins, Biological; D014197:Trees; D014315:Triterpenes / Q000378:metabolism
D014118:Toxins, Biological; D014315:Triterpenes