A Multi-scale Test Oe the Eorage Maturation Hypothesis in a Partiaeey Migratory Ungueate Popueation

The forage m atu ra tion hypothesis (FM H ) proposes th a t ungulate m igration is driven by selection for high forage quality. Because quality declines w ith p lan t m atu ra tion , bu t intake declines a t low biom ass, ungulates are predicted to select for interm ediate forage biom ass to maximize energy intake by following phonological gradients during the growing season. W e tested the F M H in the C anadian Rocky M ountains by com paring forage availability and selection by bo th m igran t and nonm igratory resident elk (Cervus elaphus) during three growing seasons from 2002-2004. First, we confirm ed th a t the expected trade-off between forage quality and quantity occurred across vegetation comm unities. N ext, we m odeled forage biom ass and phenology during the growing season by com bining ground and rem ote-sensing approaches. The growing season started 2.2 days earlier every 1 km east o f the continental divide, was delayed by 50 days for every 1000-m increase in elevation, and occurred 8 days earlier on south aspects. M igran t and resident selection for forage biom ass was then com pared across three spatial scales (across the study area, w ithin sum mer home ranges, and along m ovem ent paths) using V H F and GPS telem etry locations from 119 female elk. M igran t hom e ranges occurred closer to the continental divide in areas o f higher topographical diversity, resulting in m igrants consistently selecting for interm ediate biom ass at the two largest scales, bu t no t a t the finest scale along m ovem ent paths. In contrast, residents selected m axim um forage biom ass across all spatial scales. To evaluate the consequences of selection, we com pared exposure a t telem etry locations o f m igran t and resident elk to expected forage biom ass and digestibility. The expected digestibility for m igrant elk in sum m er was 6.5% higher th an for residents, w hich was co rroborated w ith higher fecal nitrogen levels for m igrants. The observed differences in digestibility should increase m igran t elk body mass, pregnancy rates, and adu lt and calf survival rates. W hether bo ttom -up effects o f im proved forage quality are realized will ultim ately depend on trade-offs between forage and predation. Nevertheless, this study provides comprehensive evidence th a t m ontane ungulate m igration leads to greater access to higher-quality forage relative to nonm igratory congeners, as predicted by the forage m atu ra tion hypothesis, resulting prim arily from large-scale selection patterns. K ey words: Cervus elaphus; digestibility o f forage; elk; forage maturation; fo rage quality; forage selection; migration; M O D IS ; N D V I; partia l m igration; phenology; R ocky M ountains, Alberta, Canada. I n t r o d u c t i o n scales than nonm igratory residents. This is because even 4u 1 j 4 4 4 m odest increases in forage quality can increase nu trien t M igration is though t to have evolved as a strategy to & u j . . n. ■ 4 1 . V V 1 j 4 1 in take for rum inants because o f the m ultiplier effects o f maximize htness m the lace ol seasonal and spatial ^ . . . . 4J4 im n c • 1 j j h igher n u tr ien ts an d accom pany ing reductions in variation m resources (Boyce 1979, Swmgland and c j t> j moos T 4 , 4 , , 1 rum ination and passage time (W hite 1983). Forage G reenw ood 1983). Farge vertebrate herbivores, such as u o v > o , ̂ C4 ■ 4 444,4,,, quality is highest in new p lan t grow th because o f high rum inant ungulates, are o lten m igratory (Berger 2004). o u o o W hile m igration can also reduce predation risk (Fryxell et al. 1988), selection for forage quality is proposed as the prim ary m echanism driving m igration in ungulates (M cN aughton 1985, Fryxell et al. 1988, A lbon and F angvatn 1992). M igration allows ungulates to exploit forage quality to maximize intake rate over larger spatial cell soluble content, w hich declines as plants m ature and fiber accum ulates (Van Soest 1982). Thus, by following spatio tem poral patterns in new p lan t grow th, m igratory ungulates are expected to maximize energy intake rates (e.g., Fryxell et al. 2004). Recent studies, however, suggest energy intake is no t simply a function o f quality, bu t o f trade-offs between M anuscript received 10 October 2006; revised 30 April 2007; quality and quantity (Fryxell 1991). D aily intake rates accepted 23 M ay 2007. Corresponding Editor: J. M. Fryxell. a re co n stra in ed by e ith e r p la n t c ro p p in g o r h an d lin g ^ Present addres^ Wildlife Biology Program , D epartm ent (Spalinger and H obbs 1992, G ross et al. 1993), ot Ecosystem and Conservation Science, College ot Eorestry and Conservation, University o f M ontana, M issoula, M onw hich change m im portance as biom ass increases. As tana 59812 USA. E-mail: m ark.hebblewhite@ cfc.um t.edu p lan t biom ass increases, encounters w ith potential bites