Mark D. Hunter

Effects of resource distribution on animal-plant interactions

M.D. Hunter and P.W. Price, Introduction: Plants as a Variable Resource Base for Animals. M.C. Rossiter, The Impact of Resource Variation on Population Quality in Herbivorous Insects: A Critical Aspect of Population Dynamics. R.S. Ostfeld, Small Mammal Herbivores in a Patchy Environment: Individual Strategies and Population Responses. A.E. Weis and D.R. Campbell, Plant Genotype: A Variable Factor in Insect-Plant Interaction. B.J. Rathcke, Nectar Distributions, Pollinator Behavior, and Plant Reproductive Success. P.W. Price, Plant Resources as the Mechanistic Basis for Insect Herbivore Population Dynamics. J.C. Schultz, Factoring Natural Enemies into Plant Tissue Availability to Herbivores. T. Ohgushi, Resource Limitation on Insect Herbivore Populations. J.R. Karr, M. Dionne, and I. Schlosser, Bottom-Up versus Top-Down Regulation of Vertebrate Populations: Lessons from Birds and Fish. M.D. Hunter, Interactions Within Herbivore Communities Mediated by the Host Plant: The Keystone Herbivore Concept. D.W. Roubik, Loose Niches in Tropical Communities: Why Are There So Few Bees and So Many Trees? T.H. Fleming, How Do Fruit-and-Nectar Feeding Birds and Mammals Track Their Food Resources? T. Inoue and M. Kato, Inter-and Intraspecific Morphological Variation in Bumblebee Species, and Competition in Flower Utilization. J.M. Scriber and R.C. Lederhouse, The Thermal Environment as a Resource Dictating Patterns of Feeding Specialization of Insect Herbivores. Each chapter includes references. Index.

What goes up must come down? Nutrient addition and predation pressure on oak herbivores

Current research suggests that the strength of top-down forces in communities increases with resource availability to primary producers. We examined the relative impacts of top-down and bottom-up forces on oak–herbivore communities by factorial manipulation of predation pressure and plant growth and quality. Plant growth and quality (phenolics, nitrogen) and avian predation intensity were altered through the addition of fertilizer to and the exclusion of birds from Quercus prinus and Q. rubra saplings. We censused the densities of insect herbivore guilds and predaceous arthropods on experimental and control trees in the summers of 1996 and 1997. We assessed changes in oak foliage quality using chemical assays for nitrogen, gallotannins, proanthocyanidins, and foliar astringency (protein-binding capacity). In general, leaf chewers, phloem feeders, and leaf miners were more frequent on fertilized than on unfertilized trees. Predaceous arthropods were also more frequent on fertilized trees and were positivel...

PHENOTYPIC DIVERSITY INFLUENCES ECOSYSTEM FUNCTIONING IN AN OAK SANDHILLS COMMUNITY

Given the drastic decline in biodiversity at all levels, it is imperative that we consider the potential effects of diversity within single species on ecosystem functioning. However, empirical data describing the relationship between intraspecific diversity and ecosystem functioning are lacking. We present field data demonstrating that the litter phenotype of individual trees affects carbon and nitrogen fluxes during decomposition, and that single-phenotype treatments differ in ecosystem processes from a phenotypic mix. Since nutrient dynamics are related to the chemistry of the litter, we then used the strength of the relationship between genetic distance and litter chemistry to infer the existence of genotypic effects on ecosystem functioning. In combination, our results provide the first evidence that losses in intraspecific diversity can affect the ecosystem processes of carbon and nitrogen cycling.

Effects of elevated atmospheric carbon dioxide on insect–plant interactions

The rare®ed atmosphere of US politics has been heating up recently. During March, President George W. Bush contributed signi®cantly to the accumulation of hot air over Washington and elsewhere by announcing two major policy shifts related to the emission of CO2 into the atmosphere. First, President Bush reneged on a campaign pledge to regulate carbon dioxide emissions from U.S. power plants. Soon afterwards, his administration explicitly opposed the Kyoto Protocol, the international agreement setting country-by-country limits on emissions of greenhouse gases. These two policy positions ensure that little will change in the near future to reduce the amounts of CO2 entering the atmosphere from US sources. In short, it is a great time to be in the CO2 business. Entomologists have recognized for some time that elevated concentrations of atmospheric CO2 may in ̄uence the distribution, abundance and performance of insects that feed on plants (Lincoln et al., 1984, 1986; Fajer et al., 1989). Major questions remain, however, on the relative importance of changes in weather, changes in plant quality and changes in predation pressure on the dynamics of insect herbivore populations under conditions of elevated CO2. If you grow crops or trees for a living, these questions boil down to one simple concern; will yields increase or decrease as CO2 levels continue to rise? The problem is that we do not really know yet, and it is going to cost signi®cant sums of money to ®nd out. There are too many interacting variables to make simple predictions about changes in pest damage to forestry and agricultural commodities. CO2± mediated changes in temperature or precipitation may affect insects directly and may in ̄uence the geographical ranges of agricultural and natural plant communities (Cannon, 1998). The predators, parasites and pathogens that maintain some level of control over insect populations may also be affected by global climate change or changes in plant phenotype (Stiling et al., 1999). Changes in the nutritional and defensive characteristics of host plants may drive changes in levels of insect damage to plants (Bezemer & Jones, 1998) and all of these ecological effects may interact with other sources of environmental variation including drought, nutrient availability and light (Arnone et al., 1995; Roth et al., 1997; Haettenschwiler & Schafellner, 1999; McDonald et al., 1999). In the longer term, elevated CO2 may in ̄uence the fundamental ecosystem properties upon which all plant productivity depends (Ball & Drake, 1997; Jones et al., 1998; Kampichler et al., 1998; Kandeler et al., 1998; Hungate et al., 1999; Strand et al., 1999). While presidents and policy makers play Russian roulette with the climate, ecologists and entomologists are exploring with ever-increasing accuracy and complexity the potential rami®cations of elevated CO2 for plant±herbivore interactions. Without long-term studies of the crucial variables, we will simply be unprepared for the enriched CO2 atmosphere that is developing (Coviella & Trumble, 1999). Atmospheric CO2 concentrations have already risen by about 25% since the industrial revolution and are expected to increase from current ambient levels of 350± 360 p.p.m. (or mL/L) to around 600 p.p.m. by the end of the century (Houghton et al., 1995). All of the potential consequences of elevated CO2 concentrations are too great to cover in detail here. In this paper, I focus upon what we know about changes in plant quality under elevated CO2 and how changing food quality might interact with other ecological variables to alter the performance and abundance of insects on plants.

INSECT CANOPY HERBIVORY AND FRASS DEPOSITION AFFECT SOIL NUTRIENT DYNAMICS AND EXPORT IN OAK MESOCOSMS

Increased nitrogen (N) mobilization and export from terrestrial forest ecosystems following canopy herbivory have been well documented, though the mechanism behind the loss is not clear. Because carbon (C) and N dynamics are closely linked, herbivore activity may also affect C distribution. We initiated a replicated mini-ecosystem experiment to test the hypothesis that insect frass (feces) influences soil C and N dynamics following insect defoliation. One hundred and sixty red oak (Quercus rubra) saplings were transplanted to seven-gallon (26.5-L) pots with soil and litter from the Coweeta Hydrologic Laboratory (CWT) (Otto, North Carolina, USA) and overwintered in experimental pot stands. During the 2002 growing season, trees were subjected to a 3 × 2 factorial experimental design with three damage groups (herbivore, mechanical, “undamaged”) and two frass depositions (frass, no frass). Frass deposition increased soil total C, total N, and the soil NH4+ pool. Leachate NO3− export also increased following f...

Self-Medication in Animals

Animal self-medication against parasites is more widespread than previously thought, with profound implications for host-parasite biology. The concept of antiparasite self-medication in animals typically evokes images of chimpanzees seeking out medicinal herbs to treat their diseases (1, 2). These images stem partly from the belief that animals can medicate themselves only when they have high cognitive abilities that allow them to observe, learn, and make conscious decisions (3). However, any concept of self-medication based solely on learning is inadequate. Many animals can use medication through innate rather than learned responses. The growing list of animal pharmacists includes moths (4), ants (5), and fruit flies (6). The fact that these animals self-medicate has profound implications for the ecology and evolution of animal hosts and their parasites.

Nonadditive effects of leaf litter species diversity on breakdown dynamics in a detritus-based stream.

Since species loss is predicted to be nonrandom, it is important to understand the manner in which those species that we anticipate losing interact with other species to affect ecosystem function. We tested whether litter species diversity, measured as richness and composition, affects breakdown dynamics in a detritus-based stream. Using full-factorial analyses of single- and mixed-species leaf packs (15 possible combinations of four dominant litter species; red maple [Acer rubrum], tulip poplar [Liriodendron tulipifera], chestnut oak [Quercus prinus], and rhododendron [Rhododendron maximum]), we tested for single-species presence/absence (additive) or species interaction (nonadditive) effects on leaf pack breakdown rates, changes in litter chemistry, and microbial and macroinvertebrate biomass. Overall, we found significant nonadditive effects of litter species diversity on leaf pack breakdown rates, which were explained both by richness and composition. Leaf packs containing higher litter species richness had faster breakdown rates, and antagonistic effects of litter species composition were observed when any two or three of the four litter species were mixed. Less-consistent results were obtained with respect to changes in litter chemistry and microbial and macroinvertebrate biomass. Our results suggest that loss of litter species diversity will decrease species interactions involved in regulating ecosystem function. To that end, loss of species such as eastern hemlock (Tsuga canadensis) accompanied by predicted changes in riparian tree species composition in the southeastern United States could have nonadditive effects on litter breakdown at the landscape scale.

Fertilization Mitigates Chemical Induction and Herbivore Responses Within Damaged Oak Trees

Previous work has shown that fertilization can mitigate across-year induced resistance among individual Alaska paper birch, supporting the hypothesis that induction results from nutrient deficiency rather than active defense. The present study suggests that fertilization can prevent induction responses in oak within one season, and among leaves within individual trees. Saplings of two oak species, divided among fertilizer treatments within a plantation, were defoliated to varying degrees by a gypsy moth outbreak during spring. We monitored foliage chemistry and insect herbivore distributions during summer and fall of the same year. Damaged leaves within unfertilized Quercus prints saplings showed increases in foliar astringency and proanthocyanidins. Within fertilized trees, dam- age did not induce increases in either astringency or proanthocyanidins. Defoliation also induced increases in proanthocyanidins and astringency within Q. rubra saplings, but in- duction was unaffected by fertilization. On both tree species, the distributions of certain insect guilds were skewed away from damaged leaves on unfertilized trees, but independent of damage on fertilized trees. We suggest that the effects of fertilization on induction can vary among tree species, and can occur at more than one spatial and temporal scale. Our data support the view that nutrient availability may affect phytochemical induction and herbivore responses.

Differential susceptibility to variable plant phenology and its role in competition between two insect herbivores on oak.

Abstract. 1. The population dynamics of Tortix viridana L. (Lepidoptera: Tortricidae) and Operophtera brumata L. (Lepidoptera: Geometridae) on the pedunculate oak Quercus robur L. are driven by the degree to which larval hatch coincides with oak budburst in spring.

Opposing effects of spring defoliation on late season oak caterpillars

ABSTRACT. 1. The pedunculate oak, Quercus rohur L., suffers high annual levels of spring defoliation in Wytham Woods. near Oxford.