White-tailed deer browse on an invasive shrub with extended leaf phenology meets assumptions of an apparent competition hypothesis.

Kylie L Martinod,David L Gorchov

doi:10.1093/aobpla/plx006

Abstract

It has been hypothesized that invasive plant species with extended leaf phenology (ELP) elevate generalist herbivore populations, increasing herbivory on native plants (apparent competition). This hypothesis assumes that consumption of the invasive is associated with periods of ELP, the invasive is a major component of the herbivore's diet, and that it is more nutritious than native plants during periods of ELP. We tested these assumptions by estimating the proportion of the white-tailed deer diet comprised of Lonicera maackii, an invasive shrub with ELP, quantifying the seasonal pattern of deer browse on this invasive shrub, and comparing its nutritional quality to leafless woody stems. In the Miami University Natural Areas in southwest Ohio we quantified the frequency of leafy twigs of woody species 0.3-2.1 m high in three habitats (deciduous forest, Juniperus virginiana forest, and forest-field edge). Monthly we quantified deer browse on marked L. maackii twigs, and estimated the mass of leaf and stem tissue consumed with allometric relationships using diameter and length of unbrowsed twig portions. We estimated the percent of the annual deer diet comprised of L. maackii by dividing the sum of these estimates by the product of deer abundance (estimated by pellet-based distance sampling) and consumption estimates from the literature. Crude protein of L. maackii stems and leaves was determined by C:N analyser. In each habitat the frequency of L. maackii was greater than all other woody species combined. We estimated L. maackii comprised 14-47 % of the annual deer diet. Deer browsed L. maackii each month, but consumption was high in early spring and late summer. Crude protein of leafy twigs of L. maackii in early spring was 12.9 %, much higher than leafless twigs of native species on-site. These findings support the assumptions of the hypothesis that invasive plants with ELP impact native plants via deer-mediated apparent competition.

Highlights

Impacts of invasive plants on native plants have been demonstrated in many systems, and generally attributed to resource competition or changes in ecosystem processes (Vila et al 2011; Gioria and Osborne 2014, Jauni and Ramula 2015)
We found that deer browsed on L. maackii throughout the year, but in spring and summer, and estimated this invasive shrub comprised a large fraction of the annual food consumed by deer in the study area
Though we documented deer browsed on L. maackii throughout the year, we estimate the mass consumed was high in early spring (April, May) and summer (August)

Summary

Introduction

Impacts of invasive plants on native plants have been demonstrated in many systems, and generally attributed to resource competition or changes in ecosystem processes (Vila et al 2011; Gioria and Osborne 2014, Jauni and Ramula 2015). Less well studied is the potential impact of invasive plants on native plants via apparent competition, where the negative interaction is indirect, a consequence of both species interacting with a shared enemy, e.g. a predator (Holt 1977, Connell 1990). For plant invasions in temperate forests, several studies report support for non-trophic apparent competition, greater predation on seeds of native species by rodents in areas with cover of non-native shrubs (Meiners 2007, Dutra et al 2011 Bartowitz and Orrock 2016, but see Mattos et al 2013). The only evidence for food-mediated apparent competition comes from Orrock et al.’s (2015) finding that the fruits of the invasive shrub, Lonicera maackii, increased the negative effect of rodents on native plant species richness. The potential for large-bodied, generalist grazers and browsers to affect food-mediated apparent competition between invasive and native plants due to mediate apparent competition has been hypothesized (Smith and Hall 2015), but not tested

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