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.
Bryant
John P
JP
Box 306, Cora, Wyoming 82825, USA. ffjpb@wyoming.com
Clausen
Thomas P
TP
Swihart
Robert K
RK
Landhäusser
Simon M
SM
Stevens
Michael T
MT
Hawkins
Christopher D B
CD
Carrière
Suzanne
S
Kirilenko
Andrei P
AP
Veitch
Alasdair M
AM
Popko
Richard A
RA
Cleland
David T
DT
Williams
Joseph H
JH
Jakubas
Walter J
WJ
Carlson
Michael R
MR
Bodony
Karin Lehmkuhl
KL
Cebrian
Merben
M
Paragi
Thomas F
TF
Picone
Peter M
PM
Moore
Jeffrey E
JE
Packee
Edmond C
EC
Malone
Thomas
T
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