EFFECTS OF A SPECIALIST HERBIVORE (ALTICA-SUBPLICATA) ON SALIX-CORDATA AND SAND DUNE SUCCESSION

Although effects of insect herbivory on host plant growth and reproduc tion are well documented, few studies have examined how decreases in h ost plant fitness might influence community-level patterns, such as pl ant succession. The purpose of this study was to examine how a special ist flea beetle, Altica subplicata, influences (1) the growth and surv ivorship of its host plant, sand-dune willow (Salix cordata), and cons equently, (2) patterns of plant succession on sand dunes. Growth and m ortality of S. cordata were compared over a 3-yr period in experimenta l plots in which host plants were either protected from beetle feeding (with mesh cages) or exposed to beetle feeding (without cages). The c hanges in the abundance and biomass of other plant species in these ex perimental plots were also monitored for the 3-yr period. Growth rates of host plants were significantly influenced by the herbivore exclusi on 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 be etle feeding. Differences between growth rates of plants with and with out beetles were greater on the west dune, which had significantly gre ater amounts of beetle damage, than on the east dune. Furthermore, wit hin plots exposed to beetle feeding, areas with greater amounts of bee tle damage had significantly lower height and diameter growth rates th an did areas with lesser amounts of beetle damage. A separate experime nt showed that mesh cages per se did not influence height growth, diam eter growth, or number of shoots added over a 2-yr period. Plant morta lity was also strongly affected by the herbivore exclusion treatment. Eighty-four percent of the plants that died over the 3-yr period of th e study were in plots with beetles present. Mortality was 3x and 6x hi gher for plants with beetles than for plants without beetles on the ea st and west dune, respectively. On the west dune, which had higher bee etle 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 i n abundance of neighboring, non-host plants. For both monocot and dico t herbs, there were greater increases in numbers of plants in plots wi th beetle feeding than in plots with beetles absent, but only on the w est dune with its greater beetle damage. Numbers of herbaceous monocot s and herbaceous dicots increased 2.5 times and 1.5 times more, respec tively, in plots with beetles than in plots without beetles. The chang es in abundance of other Salix plants displayed the same trends, but d ifferences were not statistically significant. In contrast, other wood y plants showed significantly greater decreases in abundance in plots with beetles than in plots without beetles on the west dune. Individua l plant species displayed both positive and negative changes in abunda nce 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 abse nt. Biomass of Salix myricoides, the most common other willow, on the east dune increased more in plots with beetles than in plots without b eetles. In general, species that did not show differential increases i n abundance also did not show differential growth of already-present p lants. After 3 yr of herbivore exclusion, there were no significant di fferences in species richness, species diversity, or species evenness in plots with and without beetles. Thus, herbivory on S. cordata cause d increases in the abundance of herbaceous plants and decreases in the abundance of other woody plants, but over 3 yr did not influence over all species diversity. It appears that selective herbivory by Altica s ubplicata directly decreases growth and increases mortality of S. cord ata and that damage to this single plant species also exerts an indire ct impact on the patterns of plant succession on sand dunes.