Beyond the browse line: complex cascade effects mediated by white-tailed deer (original) (raw)
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The impacts of introduced or overabundant large herbivores are a concern for the conservation of forest plant communities and the sustainability of ecosystem function. White-tailed deer (Odocoileus virginianus) are considered ecologically overabundant in much of North America. Previous work suggests that impacts of deer overabundance are broadly negative and are consequently degrading forests at multiple ecological and taxonomic levels. However, no quantitative synthesis currently exists to verify the generality or magnitude of these impacts. Here, we report the results of a meta-analysis quantifying the effects of deer exclusion on the diversity, cover and abundance of woody, herbaceous, and whole community components of forest understories in North America. In addition, we explore the relationships of environmental and experimental factors on the direction and magnitude of plant community outcomes using meta-regression. Using 119 calculated effect sizes sourced from 25 peer-reviewed articles, we constructed 10 community-specific datasets and found strongly positive diversity, cover and abundance responses of the woody community to deer exclusion, but no significant effects for the herbaceous or whole community components of forest understories. Local deer density and time since exclusion were significant moderators of both whole community and woody community richness. Local deer density also moderated the effects of deer exclusion on whole community cover. Plot area, in contrast, showed no relationship to any of the community response outcomes. We suggest that the use of inadequate diversity indices, non-native species replacement, or legacy effects of chronic deer overabundance might explain why the herbaceous and whole community components of forest understories showed no diversity or cover responses to deer exclusion. We also suggest some strategies to increase opportunities for future quantitative syntheses of deer impacts on forests, including providing better access to existing and future data. Ultimately, we show that white-tailed deer have strongly negative impacts on forest understory plant communities in North America, but these impacts are not ubiquitous for all components of the plant community.
Ecological Impacts of Deer Overabundance on Temperate and Boreal Forests
2000
Deer have expanded their range and increased dramatically in abundance worldwide in recent decades. They inflict major economic losses in forestry, agriculture, and transportation and contribute to the transmission of several animal and human diseases. Their impact on natural ecosystems is also dramatic but less quantified. By foraging selectively, deer affect the growth and survival of many herb, shrub, and
Structuring Effects of Deer in Boreal Forest Ecosystems
Many deer populations have recently increased worldwide leading to strong direct and indirect ecological and socioeconomical impacts on the composition, dynamic, and functions of forest ecosystems. Deer directly modify the composition and structure of vegetation communities, but they also indirectly affect other species of the ecosystem by modifying the structure of the vegetation. Here we review the results of a research program on overabundant white-tailed deer (Odocoileus virginianus) in the boreal forest of Anticosti Island (Qu´ebec, Canada) aimed at identifying deer densities compatible with forest regeneration. Various silvicultural systems and treatments failed to regenerate deer habitat at high deer densities, but planting size-adapted seedlings could be effective at moderate densities. Using a controlled deer density experiment, we found vegetation recovery at deer densities ≤ 15 deer/km2. The same experiment revealed that other groups of organisms such as insects and birds responded favorably to a reduction of deer density. We also found that alternative successional trajectories may occur after a certain period of heavy browsing during early succession. We conclude that one of the most important remaining research gaps is the need to identify habitat-specific threshold densities at which deer impacts occur and then to design effective wildlife and forest management strategies to limit deer impacts and sustain ecosystem integrity.
NUTRIENT ECOLOGY AND THE DYNAMICS OF DEER HERBIVORY IN A TEMPERATE ECOSYSTEM
If it is to fulfill its mandate, modern conservation must address three critical, and counter-intuitive, trends of recent environmental history: 1) the prolific loss of biodiversity, 2) the rapid alteration of biogeochemical processes, and 3) the consistent expansion of protected lands. With this in mind, we consider the value of integrating perspectives from nutrient ecology with conservation science to better understand the drivers and implications of herbivore-plant-soil interactions for temperate forest biodiversity on protected lands. We describe a case study of white-tailed deer (Odocoileus virginianus) herbivory and its effects on understory regeneration in a mixed-hardwood forest in northern Michigan. Species-rich stands and those hosting nitrogen-fixing flora successfully predicted the spatial patterns of herbivory, while the onset of seed production increased the probability of browsing by 368% at repeatedly-observed sites. Overstory metrics were less consistent and effective at predicting these patterns. Meanwhile, deer exclusion was shown to significantly increase the likelihood of seed production except at those sites hosting nitrogen-suppressing flora. Taken together, these findings are consistent with the expectation that temperate forests are nitrogen limited and spring herbivore ecology is largely driven by the spatial and temporal dynamics of this resource. However, the effects of herbivory are only realized in the context of other ecological processes (i.e., vegetation effects on nitrogen cycling) which will condition the effectiveness of deer management strategies to support forest regeneration and biodiversity conservation. Thus, understanding the nutrient ecology of deer-vegetation interactions can improve our ability to conceptualize and manage the implications of herbivory for temperate landscapes. In light of these findings, we propose several recommendations to conservation biologists and wildlife managers seeking to protect the ecological integrity of natural systems in an era of anthropogenic change and relatively high deer densities.