Landscapes in Canada are undergoing change due to resource and land use stressors and climate stressors. Understanding the cumulative effects of these stressors is challenging because of the complexity of ecosystems, the variability of stressors, and species response to individual or multiple stressors. A key challenge within the field of cumulative effects assessment (CEA) is guidance that describes and evaluates analytical methods. In this review we discuss four broad categories of methods with current or potential use for project-based and effects-based CEA for species in terrestrial systems: (i) qualitative review, (ii) habitat supply models, (iii) empirical species–stressor models, and (iv) decision support models. We describe each method and provide examples, highlight advantages and limitations, identify how methods address key science-based CEA questions, and provide direction on when and why to use specific CEA methods. Empirical species–stressor models and decision support models are the only analytical approaches that provide answers to many science-based CEA questions including how multiple stressors combine to affect an individual species and the certainty of multiple stressor effects. We provide recommendations for using one or more methods as complementary building blocks to fill data gaps, improve understanding and communication, engage diverse partner groups, and increase the quality and credibility of the CEA. Our review supports a move toward regional scale, effects-based CEA where partner collaboration to design, implement, and analyze comprehensive assessments of multiple stressors will (i) expand our knowledge of terrestrial species response to stressors and (ii) inform best management practices for resource industries and conservation and management actions for land managers.
Outbreaks of folivorous Lepidoptera caterpillars are common in temperate forests (Ives and Wong 1988, Butterworth 1990). The high fat content, low chitin content, and large size of these larvae make them an important component of the diet of many temperate birds (Betts 1955, Robinson and Holmes 1982, Holmes and Schultz 1988, Sample et. al. 1993). Outbreaks of most species should increase available food for breeding birds, improving their reproductive success. Some caterpillars, however, have morphological or chemical defenses that make them unpalatable to avian predators (Heinrich and Collins 1983). During outbreaks, these species have the potential to reduce the abundance of palatable caterpillars through severe defoliation. Previous studies have not considered the effects of outbreaks of unpalatable Lepidoptera species on the breeding biology of birds. This investigation determines the effects of forest tent caterpillar (Malacosoma disstria) outbreaks on the reproductive success of Black-capped Chickadees (Parus atricapillus) near Athabasca, Alberta, Canada. The tent caterpillar is a major defoliator of trembling aspen (Populus tremuloides) in Alberta, undergoing three to five year population outbreaks every five to ten years (Ives and Wong 1988, Butterworth 1990). Egg masses and very early instar larvae can be consumed by avian predators. Thus, for resident bird species, an outbreak of tent caterpillars could increase food supply in winter and early spring prior to and just after hatch of egg masses. Late instar tent caterpillars, however, are equipped with spiny hairs and bristles and are unpalatable to most avian predators (Smith 1991). Thus, outbreaks of tent caterpillars that lead to defoliation during the brood-rearing period may decrease reproductive success for chickadees. Tent caterpillar outbreaks and complete canopy tree defoliation occurred in our study area during 1986 and 1987, while few or no caterpillars and little or no defoliation were evident during 1989-1992. We tested two predictions. First, in years with tent caterpillar outbreaks, initiation of breeding would be earlier and clutch sizes larger in response to extra food when compared to years with no outbreaks. Second, tent caterpillar defoliation would reduce the reproductive suc-1
Nest predation has been used to explain aspects of avian ecology ranging from nest site selection to population declines. Many arguments rely on specific assumptions regarding how predators find nests, yet these predatory mechanisms remain largely untested. Here we combine artificial nest experiments with behavioural observations of individual red squirrels Tamiasciurus hudsonicus to differentiate between two common hypotheses: predation is incidental versus learned. Specifically, we tested: 1) whether nest survival could be explained solely by a squirrel's activity patterns or habitat use, as predicted if predation was incidental; or 2) if predation increased as a squirrel gained experience preying on a nest, as predicted if predation was learned. We also monitored squirrel activity after predation to test for evidence of two search mechanisms: area‐restricted searching and use of microhabitat search images. Contrary to incidental predation and in support of learning, squirrels did not find nests faster in areas with high use (e.g. forest edges). Instead, survival of artificial nests was strongly related to a squirrel's prior experience preying on artificial nests. Experience reduced nest survival times by over half and increased predation rates by 150–200%. Squirrels returned to and doubled their activity at the site of a previously preyed on nest. However, neither area‐restricted searching nor microhabitat search images can explain how squirrels located artificial nests more readily with experience. Instead, squirrels likely used cues associated with the nests or eggs themselves. Learning implies that squirrels could be increasingly effective predators as the density or profitability of nests increases. Our results add support to the view that nest predation is complex and broadly influenced (e.g. by predator experience, motivation), and is unlikely to be predicted consistently by simple relationships with predator activity, abundance or habitat.
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