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16 June 2010 Fine-scale disturbances shape space-use patterns of a boreal forest herbivore
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Abstract

Natural disturbance is a key determinant of ecosystem structure and function. Disturbances can create novel resource patches and modify habitat structure, thereby inducing spatial heterogeneity in the trade-off between food acquisition and predator avoidance by prey. We evaluated how canopy gap dynamics in eastern Canadian old-growth boreal forest alter the spatial distribution of food and cover for snowshoe hares (Lepus americanus) and how hares responded to these spatial patterns. We 1st compared browse availability within canopy gaps and the surrounding forest. We then examined fine-scale habitat selection, movement patterns, and foraging decisions by hares during winter. Perception of risk within canopy gaps was assessed using foraging experiments. We found that browse availability was 4 times higher within gaps than under forest cover. Although hares acquired most of their browse from gaps, their use of space during winter was influenced by a greater perception of predation risk within gaps. Hares selectively used areas of higher canopy closure suggesting that they restricted their use of gaps to foraging activities. Furthermore, hares biased their movements away from gaps or increased their speed of travel in areas of relatively low cover. Hares consumed experimental browse stems more intensively under forest cover than in canopy gaps, indicating a trade-off between food and safety. When foraging within canopy gaps, hares also were less likely to use both experimental and natural food patches located far away from cover. Our study demonstrates how gap dynamics in old-growth stands can structure the fine-scale spatial organization of a key prey species of the boreal forest by creating spatial heterogeneity in their landscapes of fear and food. Spatial variation in browse use in response to predation risk may in turn influence patterns of sapling growth and survival within canopy gaps. Gap dynamics therefore may be a fundamental process structuring predator–prey interactions in old-growth boreal forests.

Natural disturbances that vary in size, severity, and frequency play a fundamental role in structuring aquatic and terrestrial ecosystems by creating heterogeneity at multiple spatial and temporal scales (Pickett and White 1985; Sousa 1984). Habitat disturbance can affect animal distribution by altering the composition and structure of vegetation that provide food and cover, and many animals benefit from disturbances that create productive conditions associated with areas undergoing regeneration (Sousa 1984). Although infrequent broadscale disturbances such as forest fires and tropical storms can influence patterns of species occurrence at the landscape scale (Fisher and Wilkinson 2005; Willig et al. 2007), frequent microhabitat disturbances such as tree-fall gaps, blowouts, and wave action create fine-scale heterogeneity that also plays an important role in determining species distribution (Bouget and Duelli 2004; Cramer and Willig 2005; Paine and Levin 1981).

Habitat heterogeneity can have a profound influence on trophic interactions. For example, heterogeneity can promote the persistence of predator–prey populations by reducing predator foraging efficiency, by creating spatial refuges for prey, or by creating locally asynchronous population dynamics (Hastings 1977; Holt and Hassell 1993; Huffaker 1958). Recent investigations have shown that the functional response of both herbivores and carnivores to food availability can depend on the spatial distribution of these resources (Hobbs et al. 2003; Pitt and Ritchie 2002). Resource heterogeneity therefore can influence the functional link among trophic levels. For herbivores, variation in the spatial arrangement of plants can affect the rate at which they encounter food patches, thereby influencing their rate of energy intake and dietary choice (Fortin et al. 2002; Hobbs et al. 2003). To increase their intake rate in heterogeneous environments herbivores should concentrate on aggregations of food patches to reduce travel time between patches (Nonaka and Holme 2007), but the most profitable food patches often are also the most risky (Brown and Kotler 2004).

Fear of predation is a major force influencing movement and foraging decisions of prey (Lima and Dill 1990), and disturbances that increase food resources also can remove habitat structure that provides protection against predators. Given that predators may be more efficient at detecting and capturing prey in certain habitats (Rohner and Krebs 1996), prey often rely on habitat structure as a cue for risk (Brown and Kotler 2004). For example, they may trade off food for safety by foraging less intensively in open habitats or with increasing distance from protective cover (Hochman and Kotler 2007). During locomotion prey also may attempt to mitigate risk by moving in areas of greater cover (Fortin et al. 2005; Lagos et al. 1995), or by adjusting their speed to quickly traverse areas where they would be more conspicuous to predators (Vasquez et al. 2002). Slight variations in habitat structure can result in relatively large changes in the perception of risk (van der Merwe and Brown 2008). Therefore, microhabitat disturbances should shape prey distribution by continually changing the landscapes of food and fear (Laundré et al. 2001) around which prey species structure their home ranges.

Canopy gap dynamics in old-growth forests provide an interesting system in which to evaluate how fine-scale disturbances influence the distribution of resources, prey, and their interaction in the presence of predation risk. Old-growth boreal forests are characterized by high structural heterogeneity due to fine-scale canopy disturbances such as windthrow, insect outbreaks, disease, and tree senescence (McCarthy 2001). Because canopy closure in mature boreal forest generally limits the availability of food resources for browsing herbivores (Fisher and Wilkinson 2005), the establishment of early successional plants and the release of advanced regeneration within canopy gaps could create resource-rich patches within a matrix of low food availability. Gap disturbances also decrease the cover on which such herbivores rely for protection from predators. Predation risk should influence how far and intensively herbivores are willing to forage within canopy gaps. Foraging and movement behaviors of herbivores can reveal how balancing food acquisition and predator avoidance lead to their spatial distribution in forests structured by gap dynamics.

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