Differential and interacting impacts of invasive plants and white-tailed deer in eastern U.S. forests.
Forests in eastern North America are experiencing high densities of white-tailed deer (Odocoileus virginianus) and encroachment by invasive plants, both of which threaten native biodiversity. We review the literature on deer and invasive plant impacts focusing on studies that simultaneously evaluate the consequences of both. Deer have more frequent and more consistently negative effects than invasive plants. Widespread deer impacts now threaten many native plant species through much of their range. In contrast, invasive plant effects currently remain more localized and/or of smaller extent within forests. Deer impacts are also cumulative, hitting preferred plant species especially hard as they decline in density. This generates difficult-to-reverse legacy effects. Invasive plant effects, in contrast, tend to be more diffuse and may be more readily reversed. High deer populations also shift physical and chemical conditions in soils promoting "invasion cascades" involving non-native earthworms and certain introduced plants. Removing invasive plants without reducing deer populations can increase deer impacts on native species. Management should be integrated to address both deer and invasive plants. To safeguard and restore native biota when resources are limited, however, it may be most effective for managers to first reduce deer populations before investing in efforts to reduce invasive populations (except when invasions are at an early stage). We should rethink and reform traditional approaches to managing deer so that we can better integrate land vegetation with wildlife management to achieve broad public objectives. Interacting effects of high ungulate populations and invasive plants deserve further study to determine whether similar recommendations apply to other regions.