Effects of a Specialist Herbivore (Altica Subplicata) on Salix Cordata and Sand Dune Succession

Abstract

Although effects of insect herbivory on host plant growth and reproduction are well documented, few studies have examined how decreases in host plant fitness might influence community—level patterns, such as plant succession. The purpose of this study was to examine how a specialist flea beetle, Altica subplicata, influences (1) the growth and survivorship of its host plant, sand—dune willow (Salix cordata), and consequently, (2) patterns of plant succession on sand dunes. Growth and mortality of S. cordata were compared over a 3—yr period in experimental plots in which host plants were either protected from beetle feeding (with mesh cages) or exposed to beetle feeding (without cages). The changes in the abundance and biomass of other plant species in these experimental plots were also monitored for the 3—yr period. Growth rates of host plants were significantly influenced by the herbivore exclusion treatment. Plants protected from beetle feeding added 2.2 times as much height and 2.0 times as much diameter as did plants exposed to beetle feeding. Differences between growth rates of plants with and without beetles were greater on the west dune, which had significantly greater amounts of beetle damage, than on the east dune. Furthermore, within plots exposed to beetle feeding, areas with greater amounts of beetle damage had significantly lower height and diameter growth rates than did areas with lesser amounts of beetle damage. A separate experiment showed that mesh cages per se did not influence height growth, diameter growth, or number of shoots added over a 2—yr period. Plant mortality was also strongly affected by the herbivore exclusion treatment. Eighty—four percent of the plants that died over the 3—yr period of the study were in plots with beetles present. Mortality was 3x and 6x higher for plants with beetles than for plants without beetles on the east and west dune, respectively. On the west dune, which had higher beetle damage and plant mortality, the amount of beetle damage incurred by plants explained 88% of the variation in plant mortality. Herbivory on S. cordata also had significant, indirect effects on the changes in abundance of neighboring, non—host plants. For both monocot and dicot herbs, there were greater increases in numbers of plants in plots with beetle feeding than in plots with beetles absent, but only on the west dune with its greater beetle damage. Numbers of herbaceous monocots and herbaceous dicots increased 2.5 times and 1.5 times more, respectively, in plots with beetles than in plots without beetles. The changes in abundance of other Salix plants displayed the same trends, but differences were not statistically significant. In contrast, other woody plants showed significantly greater decreases in abundance in plots with beetles than in plots without beetles on the west dune. Individual plant species displayed both positive and negative changes in abundance in response to exclusion of herbivores from S. cordata. Biomass of two common herbaceous monocots (Juncus spp.) increased more in plots with beetles present, but for the most common herbaceous dicots (Aster and Solidago spp.), increases were greater in plots with beetles absent. Biomass of Salix myricoides, the most common other willow, on the east dune increased more in plots with beetles than in plots without beetles. In general, species that did not show differential increases in abundance also did not show differential growth to already—present plants. After 3 yr of herbivore exclusion, there were no significant differences in species richness, species diversity, or species evenness in plots with and without beetles. Thus, herbivory on S. cordata caused increases in the abundance of herbaceous plants and decreases in the abundance of other woody plants, but over 3 yr did not influence overall species diversity. It appears that selective herbivory by Altica subplicata directly decreases growth and increases mortality of S. cordata and that damage of this single plant species also exerts an indirect impact on the patterns of plant succession on sand dunes.