Vertical heterogeneity in predation pressure in a temperate forest canopy

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.

Keywords: Arthropods; Avian predation; Canopy; Predator exclusion; Refuge.

Figure 1. Exclusion cage design and placement.

(A) Example of a wire mesh cage used to exclude vertebrate predators, (B) location of cage in the middle canopy of a tree.

Figure 2. Arthropod density by treatment and stratum.

Mean density ( ± SE) on control and caged branches from 20 sugar maple (Acer saccharum Marsh.) trees across four vertical strata: understory (UN); lower canopy (LC); middle canopy (MC); and upper canopy (UC). Means and standard errors are back-transformed predicted values from mixed effects models with tree identity as a random effect.

Figure 3. Effect sizes by stratum for predator exclusion treatment.

Points represent the magnitude of the effects of a predator exclusion treatment on arthropod density surveyed from June–September 2007 on branches from 20 sugar maple (Acer saccharum Marsh.) trees in four vertical strata: understory (UN); lower canopy (LC); middle canopy (MC); and upper canopy (UC). Effect sizes and 95% CIs were calculated using predicted values from mixed effects models with tree identity as a random effect.

Figure 4. Proportion of mealworm bait predated by stratum.

Proportion of mealworm larvae bait predated ( ± SE) on sugar maple (Acer saccharum Marsh.) branches (A) during daytime observations, and (B) removed after an overnight period, in four vertical strata: understory (UN); lower canopy (LC); middle canopy (MC); and upper canopy (UC).