Abstract
The forest canopy offers a vertical gradient across which variation in predation pressure implies variation in refuge quality for arthropods. Direct and indirect experimental approaches were combined to assess whether canopy strata differ in ability to offer refuge to various arthropod groups. Vertical heterogeneity in impact of avian predators was quantified using exclosure cages in the understory, lower, mid, and upper canopy of a north-temperate deciduous forest near Montreal, Quebec. Bait trials were completed in the same strata to investigate the effects of invertebrate predators. Exclusion of birds yielded higher arthropod densities across all strata, although treatment effects were small for some taxa. Observed gradients in predation pressure were similar for both birds and invertebrate predators; the highest predation pressure was observed in the understory and decreased with height. Our findings support a view of the forest canopy that is heterogeneous with respect to arthropod refuge from natural enemies.
Highlights
Temperate forest canopies are heterogeneous environments, where variation in resources, structure, and abiotic conditions exists at even small scales (Parker, 1995)
An important resource type that is often overlooked in ecological studies is the refuge – space within a habitat that allows organisms to escape from their natural enemies (Berryman & Hawkins, 2006)
Excluding birds and other vertebrates in our cage treatment resulted in increased density of all study groups, indicating that vertebrate predation has a significant negative impact on arthropod density
Summary
Temperate forest canopies are heterogeneous environments, where variation in resources, structure, and abiotic conditions exists at even small scales (Parker, 1995). Predation can play an important role in shaping the niches of arthropods, insect herbivores, by affecting the choice of feeding location, resource use, and fitness (Jeffries & Lawton, 1984; Bernays & Graham, 1988; Stamp & Bowers, 1990). These relationships, are poorly understood as a function of vertical stratification, even though it is well established that arthropods are structured along vertical gradients in forest (e.g., Larrivee & Buddle, 2009; Pinzon, Spence & Langor, 2013). We predict that the upper canopy crown represents a refuge for arthropods and that the relative impact of predators will be reduced with increasing canopy height
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