Catherine E. Bach

EFFECTS OF INTERSPECIFIC COMPETITION ON FITNESS OF THE HERMIT CRAB CLIBANARIUS TRICOLOR

Clibanarius tricolor overlaps strongly in shell utilization with the other common species of intertidal hermit crabs found in the Florida Keys. Laboratory observations indicated C. tricolor can dominate Clibanarius antillenisis in shell fights, while Calcinus tibicen dominates C. tricolor. In micro-areas of sympatry with C. antillensis, C. tricolors shell fit is better and egg production parameters not affected when compared to adjacent areas where only C. tricolor occurs. Sympatry with Calcinus tibicen results in a poorer shell fit, a smaller shell size for a given size crab, a smaller mean size of crab, and a disruption of the relationship between clutch size and crab size:shell weight ratio. A strong effect of shell species on the probability of Y Y being ovigerous was noted. It is suggested that the ecological separation which characterizes these species over most of their ranges was an evolutionary response, in part, to the effects of interspecific shell competition.

Effects of Host Plant Patch Size on Herbivore Density: Patterns

The influence of host plant patch size on the population densities of three herbivorous insect species was investigated in seven sizes of experimental patches of squash plants (from 1 to 144 plants). Responses to patch size differed significantly for the three herbivore species, which are closely related but differ in the degree to which they require cucurbits as hosts. The number of individuals of Acalymma vittatum (the striped cucumber beetle) per plant was significantly greater in small patches (1-4 plants) than in larger patches (16-144 plants). The spotted cucumber beetle or southern corn rootworm (Diabrotica undecimpunctata howardi) was most abundant in intermediate-sized patches (64 or 100 plants). Population densities of Diabrotica virgifera (the western corn rootworm) were more variable, but tended to increase with increasing patch size. Relationships between herbivore density and patch size were nonlinear for all three species. The effects of patch size on herbivore population densities also differed between gen- erations of the same species. Second generation Acalymma vittatum were most abundant in large patches, which was opposite to the pattern shown by first generation individuals. Greater numbers of second generation adults emerged in large patches than in small patches, indicating an effect of patch size on larval survivorship and/or oviposition rates. Densities of the spotted cucumber beetle were also differentially affected by patch size at different times throughout the season. Patch size significantly influenced plant growth, as measured by number of leaves and biomass, but did not influence parameters of plant reproduction (numbers of flowers or yield). Plant longevity was significantly greater in larger patches (16 plants or greater) than in small patches (1 or 4 plants). To examine the role of differences in plant size associated with patch size in influencing beetle densities, analyses of the number of beetles per leaf were carried out. Even when differences in plant size were taken into account, patch size still significantly affected densities of all three herbivore species. Thus, beetles responded to patch size per se, not just to associated changes in plant size. Relationships between plant parameters and herbivore parameters were analyzed in detail for the most specialized of the herbivore species, Acalymma vittatum. Beetle densities were positively correlated with numbers of flowers early in the season, but more strongly correlated with numbers of leaves late in the season. Plant longevity was negatively cor- related with numbers of early colonizing beetles, indicating a potential effect of herbivory on plant survivorship. Variable herbivore responses to patch size are discussed with emphasis on mechanisms, including plant size/quality and insect movement.

Effects of Host Plant Patch Size on Herbivore Density: Underlying Mechanisms

This study was designed to examine the mechanisms underlying the response of herbivores to variation in the size of host plant patches. In order to explain why identical changes in patch size affect closely related herbivores in different ways, I separated the effects of patch size alone from the confounded effects of the surrounding plant community. Further tests determined whether herbivore densities were directly affected by surrounding nonhost plants (via changes in movement patterns) and/or indirectly affected by changes in host plant size/quality caused by the surrounding nonhost plants. Population densities of three closely related insect herbivores, Acalymma vittatum (SCB), Diabrotica undecimpunctata howardi (SCR), and Diabrotica virgifera (WCR), were studied in experimental patches of squash host plants (Cucurbita maxima). Patches varied in size (4, 16, or 64 plants per patch) and in the nature of their edges (no tomato plant edge, an edge of tomato plants growing in the ground (=interacting nonhost plants), and an edge of tomato plants growing in pots (=non-interacting nonhost plants)). This exper- imental design enabled a comparison of herbivore densities in patches with no nonhosts surrounding patches, patches with interacting nonhost neighbors, and patches with non- interacting nonhost neighbors. The three herbivore species differed in their response to both patch size and edge treatment. Densities of first generation SCB were significantly affected by both patch size and edge treatment, being greatest in the smallest patches. For a given patch size, densities were greatest in the patches with non-interacting tomato neighbors (tomatoes in pots). More second generation beetles emerged per plant in the treatment with non-interacting plants, but the total number of second generation individuals per plant was not affected by either patch size or edge treatment. SCR densities were significantly affected by edge treatment only, with individuals being most abundant in patches with non-interacting tomato neighbors. For both species, differences in densities as a function of edge treatment were of much greater magnitude in small patches than in large patches. In contrast, pop- ulation densities of WCR were not influenced by either patch size or edge treatment. The growth (number of leaves) and reproduction (number of flowers and yield) of squash plants were significantly affected by edge treatment, with the largest plants occurring in patches with non-interacting tomato plants and the smallest plants in patches with inter- acting tomato plants. Patch size influenced both number of leaves and yield. Plant longevity was not significantly influenced by either patch size or edge treatment. The role of these differences in plant parameters in influencing beetle population dynamics was investigated for SCB. For a given average plant size per patch, beetle densities were significantly greater in the treatment with non-interacting nonhosts than in the treatment with no tomato plant edge. A mark-recapture study with SCB and SCR revealed that rates of recapture were not affected by either patch size or edge treatment, for either herbivore species. However, a greater proportion of both SCB and SCR individuals colonized neighboring small patches than neighboring large patches on a per plant basis. In order to test the mechanisms underlying these herbivore responses to patch size, experiments were conducted with SCB to examine effects of neighboring nonhost vegetation on herbivore feeding preferences and movement patterns. When given a choice of potted plants inside cages, beetles preferred a squash plant alone compared to a squash plant growing next to a tomato plant in a separate pot. When given a choice of leaves (but no direct contact with plants), beetles did not distinguish between leaves from plants grown with nonspecific vs. tomato neighbors, indicating beetles did not perceive differences in leaf quality as a function of neighbor type. Observations of movement patterns of SCB beetles in small patches with and without surrounding tomato plants showed that a significantly greater proportion of beetles left patches without surrounding tomato plants than patches with an edge of tomato plants.

Genetic variation in resistance and tolerance to insect herbivory in Salix cordata

1. This study investigated whether sand‐dune willow Salix cordata, exhibits genetic variation in resistance and tolerance to herbivory.

LONG-TERM EFFECTS OF INSECT HERBIVORY AND SAND ACCRETION ON PLANT SUCCESSION ON SAND DUNES

This study examined how long-term insect herbivory, sand accretion, and the interaction of these factors affect patterns of plant succession on sand dunes. Individuals of a common dune species, Salix cordata, were either exposed to insect herbivory (by a specialist flea beetle, Altica subplicata) or protected from insect herbivory with cages from 1990 to 1992. Plant community composition and sand levels were then assessed from 1993 to 1996, during which time natural sand accretion occurred and flea beetles were rare. The goal was to examine how plant species composition and successional patterns were affected by past herbivory on one plant species, sand accretion, and an interaction between past herbivory and sand accretion. Past herbivory on Salix cordata caused a significant decrease in the proportional representation of herbaceous monocots in the plant community, as well as changes in the abundance of four of the 10 most common plant species. Other grasses and Aster/Solidago had greater increases in abundance in plots with past herbivory, Populus tremuloides had lesser decreases in abundance in plots with past herbivory, and Potentilla anserina had greater increases in abundance in plots without past herbivory. Past herbivory did not significantly affect changes in species richness over the study. Over the entire study, herbaceous monocots, herbaceous dicots, and horsetails increased in absolute abundance, but woody plant abundance decreased. Sand accretion significantly decreased the changes in abundance of three categories of plants: herbaceous monocots, herbaceous dicots, and woody plants. Only for horsetails -was there was a positive rela- tionship between change in abundance and sand accretion. As sand levels increased, the percentage of herbaceous dicots increased, whereas the percentage of herbaceous monocots decreased. There was a greater decrease in species richness with increasing amounts of sand deposition. For eight of the 10 most common species, there was a negative relationship between sand level and change in abundance; only beach grasses responded positively to sand accretion. Plant height was a significant factor in explaining which species increased vs. decreased in abundance; species that increased during the study were significantly taller than species that decreased. Clonal and nonclonal species responded similarly to sand accretion. There were very few interactive effects of past herbivory and sand level. Thus, past herbivory on Salix cordata had direct effects on the future composition of the plant community, via changes in plant abundance, but did not appear to modify the response of the plant community to sand accretion.

Plant successional stage and insect herbivory: flea beetles on sand-dune willow

The effects of early plant successional stage on the abundance and amounts of damage caused by a specialist herbivore were investigated for sand—dune willow, Salix cordata (Salicaceae), growing on sand dunes of varying ages. Both adults and larvae of Altica subplicata (Coleoptera: Chrysomelidae) were very abundant on host plants growing on the youngest and intermediate—aged dunes, but were absent from most plants growing on the oldest dunes. Amounts of beetle damage were greatest on the youngest dune, with nearly half of the plants having no leaves at the end of the season, and damage decreased with increasing successional stage. To explain these patterns of herbivory, laboratory and field studies we used to examine beetle survivorship, feeding preferences, and movement patterns. There was no difference in larval survivorship or growth rate as a function of the age of the community in which host plants were growing. Plant successional stage also did not influence larval feeding preferences, however, adult...

Effects of herbivory and genotype on growth and survivorship of sand‐dune willow (Salix cordata)

Abstract. 1 The response of different clones of sand‐dune willow, Salix cordata, to herbivory by a specialist herbivore, Altica subplicata, was studied in three glasshouse experiments. Plants were caged and exposed to three herbivory treatments: no beetles, low number of beetles, and high number of beetles. 2 Plants consistently had significantly higher growth rates in the absence of herbivory than under conditions of low or high herbivory (1.5–6 times higher). 3 Herbivore treatment influenced mortality from drought stress; more plants from the low and high herbivory treatments (40% and 80%) died from drought stress than did control plants (0%). 4 Clone genotype significantly influenced growth rates and the susceptibility of plants to drought stress. However, clones showed similar growth responses to herbivory, suggesting a lack of genetic variation in tolerance or resistance to herbivory.

Interactive effects of herbivory and sand burial on growth of a tropical dune plant, Ipomoea pes-caprae

1. This study examined the effects of insect herbivory, sand burial, and the interactive effects of these factors, on the growth of beach morning glory, Ipomoea pes‐caprae, a common tropical dune plant. Levels of herbivory and sand burial were manipulated on individual shoots, and effects on stem growth, leaf production, and production of adventitious roots and axillary branches by nodes were examined.

LONG-TERM EFFECTS OF INSECT HERBIVORY ON RESPONSES BY SALIX CORDATA TO SAND ACCRETION

This study examined the effects of long-term insect herbivory on the response of sand dune willow, Salix cordata, to sand accretion. Plants were either exposed to insect herbivory (by a specialist flea beetle, Altica subplicata) or protected from insect herbivory with cages for three years. Plant growth and mortality, as well as sand levels, were then assessed for the next four years, during which time natural sand accretion occurred. The goal was to examine how growth and mortality were affected by past herbivory and sand accretion, and most importantly, whether these two factors interacted to affect plant per- formance. There was a highly significant positive correlation between mortality rate and amount of sand accretion. Plants with past herbivory had significantly greater mortality rates than plants with no past herbivory, but this difference resulted from a plant size effect. Shorter plants were more susceptible to mortality from sand burial, and plants with past herbivory were significantly shorter than plants without past herbivory. Past herbivory stimulated both stem diameter and height growth over the entire study. The stem diameter growth response to sand accretion varied strongly for plants with and without past herbivory, and the nature of the relationships also varied among dunes. On the west dune, where sand accumulated at a much faster rate, there was a negative rela- tionship between sand accumulation and stem diameter growth, but this relationship was only significant for plants with past herbivory. On the east dune, with a slower sand accumulation rate, stem diameter growth was positively related to sand level for plants with past herbivory, but negatively related to sand level for plants with no past herbivory. Past herbivory and sand accretion also affected future susceptibility to herbivory. Beetle densities were significantly greater on plants with no past herbivory than on plants expe- riencing herbivory three years earlier. For plants with no past herbivory, plants experiencing high amounts of sand had greater beetle densities than plants experiencing low amounts of sand. Thus, this study demonstrated long-term effects of herbivory on increasing plant mortality (indirectly via a decrease in plant size), increasing both stem diameter and height growth of surviving plants, and decreasing future susceptibility to herbivory. Past herbivory also strongly altered the stem diameter growth response of plants to another stress, sand accretion.

Individuality in the predator defense behaviour of the crab Heterozius rotundifrons

When individuals of the crab Heterozius rotundifrons are stimulated by tactile input they assume a rigid appendage-spread posture for a number of seconds. This posture is effective in reducing predation. When tested repeatedly, individuals both differed from one another in the duration of maintaining this posture and were consistent in their responses. This result held if crabs were tested five times in one day with the same stimuli (with either tactile input alone or tactile input + alarm odour), stimulated with three different sensory input combinations (tactile, tactile + alarm odour, tactile + alarm odour + shadows), or tested once a day with tactile input for five days. Crab sex and size were not important in explaining the variation in duration of this predator defense behaviour.