A regional assessment of white-tailed deer effects on plant invasion

Abstract

Herbivores can profoundly influence plant species assembly, including plant invasion, and resulting community composition. Population increases of native herbivores, e.g. white-tailed deer (Odocoileus virginianus), combined with burgeoning plant invasions raise concerns for native plant diversity and forest regeneration. While individual researchers typically test for the impact of deer on plant invasion at a few sites, the overarching influence of deer on plant invasion across regional scales is unclear. We tested the effects of deer on the abundance and diversity of introduced and native herbaceous and woody plants across 23 white-tailed deer research sites distributed across the east-central and north-eastern USA and representing a wide range of deer densities and invasive plant abundance and identity. Deer access/exclusion or deer population density did not affect introduced plant richness or community-level abundance. Native and total plant species richness, abundance (cover and stem density) and Shannon diversity were lower in deer-access vs. deer-exclusion plots. Among deer-access plots, native species richness, native and total cover, and Shannon diversity (cover) declined as deer density increased. Deer access increased the proportion of introduced species cover (but not of species richness or stem density). As deer density increased, the proportion of introduced species richness, cover and stem density all increased. Because absolute abundance of introduced plants was unaffected by deer, the increase in proportion of introduced plant abundance is likely an indirect effect of deer reducing native cover. Indicator species analysis revealed that deer access favoured three introduced plant species, including Alliaria petiolata and Microstegium vimineum, as well as four native plant species. In contrast, deer exclusion favoured three introduced plant species, including Lonicera japonica and Rosa multiflora, and 15 native plant species. Overall, native deer reduced community diversity, lowering native plant richness and abundance, and benefited certain invasive plants, suggesting pervasive impacts of this keystone herbivore on plant community composition and ecosystem services in native forests across broad swathes of the eastern USA.

Keywords: Biological invasions; Odocoileus virginianus; exotic plants; herbivory; introduced plants; palatability; plant invasion; regional pooled analysis.

Figure 1.

Locations of 23 deer research sites in east-central and north-eastern USA included in pooled analyses. Symbol size indicates sampling intensity across sites, which are labelled with two-letter codes (see Table 1 for additional site information).

Figure 2.

Effects of white-tailed deer access/exclusion on (A, E) introduced and native plant abundance, (B, F) total plant abundance (includes unknown species), (C, G) proportion of introduced (intr.) plants and (D, H) Shannon Diversity (H′) in east-central and north-eastern USA. Means (±SE) are presented according to the abundance metric used for data collection, stem density (A–D) and/or cover (E–H) (see Table 1 for additional site information). An asterisk between bars indicates a significant effect of deer; ns = not significant; n = sample size (number of plots). The number of plots was constrained in the proportion introduced richness and Shannon diversity analyses due to plots with zero vegetation [see Supporting Information—Text S1].

Figure 3.

Effects of white-tailed deer access/exclusion on mean (±SE) (A) introduced (white shading) and native (black shading) plant species richness, (B) total plant species richness (includes unknown species) and (C) proportion introduced plant species richness in east-central and north-eastern USA. An asterisk between bars indicates a significant effect of deer; ns = not significant; n = sample size (number of plots). The number of plots was constrained in the proportion introduced richness analysis due to plots with zero vegetation [see Supporting Information—Text S1].

Table 7.

Effects of deer on the abundance of the most frequent introduced (in bold type) and native plant species in east-central and north-eastern USA based on mixed models using floristic composition data collected from deer-access and deer-exclusion plots. Only significant effects are shown of the 13 introduced and 20 native species analysed. Abundance (+SE) in deer-access and deer-exclusion plots is presented; units for density are plants m⁻² and for cover are per cent cover. Results are arranged by deer access/exclusion and then by abundance; each species’ results are listed together.