Insect Ecology - An Ecosystem Approach 2nd ed - Chapter 12 pot

12 Herbivory I Types and Patterns of Herbivory A Herbivore Functional Groups B Measurement of Herbivory C Spatial and Temporal Patterns of Herbivory II Effects of Herbivory A Plant Productivity, Survival and Growth Form B Community Dynamics C Water and Nutrient Fluxes D Effects on Climate and Disturbance Regime III Summary HERBIVORY IS THE RATE OF CONSUMPTION BY ANIMALS OF ANY PLANT parts, including foliage, stems, roots, flowers, fruits, or seeds Direct effects of insects on plant reproductive parts are addressed in Chapter 13 Herbivory is a key ecosystem process that reduces density of plants or plant materials, transfers mass and nutrients to the soil or water column, and affects habitat and resource conditions for other organisms Insects are the primary herbivores in many ecosystems, and their effect on primary production can equal or exceed that of more conspicuous vertebrate grazers in grasslands (e.g., A Andersen and Lonsdale 1990, Gandar 1982, Sinclair 1975, Weisser and Siemann 2004, Wiegert and Evans 1967) Loss of plant material through herbivory generally is negligible, or at least inconspicuous, but periodic outbreaks of herbivores have a well-known capacity to reduce growth and survival of host species by as much as 100% and to alter vegetation structure over large areas A key aspect of herbivory is its variation in intensity among plant species, reflecting biochemical interactions between the herbivore and the various host and nonhost species that comprise the vegetation (see Chapter 3) Effects of herbivory on ecosystem processes depend on the type of herbivore and pattern of consumption, as well as its intensity Measurement and comparison of herbivory and its effects among ecosystems and environmental conditions remain problematic as a result of lack of standardized techniques for measuring or manipulating intensity Few studies have assessed the effects of herbivory on ecosystem processes other than primary production Nevertheless, accumulating evidence indicates that effects of herbivory on ecosystem processes, including primary production, are complex Ecosystem management practices that exacerbate or suppress herbivory may be counterproductive 347 348 12 HERBIVORY I TYPES AND PATTERNS OF HERBIVORY A Herbivore Functional Groups Herbivorous insects that have similar means of exploiting plant parts for food can be classified into feeding guilds or functional groups Groups of plant-feeders include chewers that consume foliage, stems, flowers, pollen, seeds, and roots; miners and borers that feed between plant surfaces; gall-formers that reside and feed within the plant and induce the production of abnormal growth reactions by plant tissues; sap-suckers that siphon plant fluids; and seed predators and frugivores that consume the reproductive parts of plants (Romoser and Stoffalano 1998) Some species, such as seed predators, seedling-eaters, and tree-killing bark beetles, are true plant predators, but most herbivores function as plant parasites because they normally not kill their hosts, but instead feed on the living plant without causing death (Price 1980) These different modes of consumption affect plants in different ways For example, folivores (species that chew foliage) directly reduce the area of photosynthetic tissue, whereas sap-sucking insects affect the flow of fluids and nutrients within the plant and root-feeders reduce plant capacity to acquire nutrients or remain upright Folivory is the best-studied aspect of herbivory In fact, the term herbivory often is used even when folivory alone is measured because loss of foliage is the most obvious and easily quantified aspect of herbivory The loss of leaf area can be used to indicate the effect of herbivory In contrast, other herbivores such as sap-suckers or root-borers cause less conspicuous losses that are more difficult to measure Nonetheless, Schowalter et al (1981c) reported that calculated loss of photosynthates to sap-suckers greatly exceeded measured foliage loss to folivores in an early successional deciduous forest Sap-suckers and root-feeders also may have long-term effects (e.g., through disease transmission or altered rates of nutrient acquisition or growth) (J.P Smith and Schowalter 2001) B Measurement of Herbivory Effects of herbivory on ecosystem processes are determined by temporal and spatial variability in the magnitude of consumption Clearly, evaluation of the effects of herbivory requires robust methods for measuring herbivory as well as primary productivity and other ecosystem processes Measurement of herbivory can be difficult, especially for underground plant parts and forest canopies, and has not been standardized Several methods commonly used to measure herbivory have been compared by Filip et al (1995), Landsberg (1989), and Lowman (1984) The simplest and most widely used technique is the measurement of feeding rate by individual herbivores and extrapolation to feeding rate by a population This technique provides relatively accurate rates of consumption and can be used to estimate per capita feeding rate for sap-suckers as well as folivores (e.g., Gandar 1982, Schowalter et al 1981c, B Stadler and Müller 1996) Insect folivores usually consume 50–150% of their dry body mass per day (Blumer and I TYPES AND PATTERNS OF HERBIVORY Diemer 1996, Reichle and Crossley 1967, Reichle et al 1973, Schowalter et al 1981c) Rates of sap and root consumption are difficult to measure, but a few studies have provided limited information For example, honeydew production by individual sap-sucking insects can be used as an estimate of their consumption rates Stadler and Müller (1996) and Stadler et al (1998) reported that individual spruce aphids, Cinara spp., produced from 0.1 mg honeydew day-1 for first instars to mg day-1 for adults, depending on aphid species, season, and nutritional status of the host Schowalter et al (1981c) compiled consumption data from studies of eight herb- and tree-feeding aphids (Auclair 1958, 1959, 1965, Banks and Macaulay 1964, Banks and Nixon 1959, M Day and Irzykiewicz 1953, Llewellyn 1972, Mittler 1958, 1970, Mittler and Sylvester 1961, Van Hook et al 1980, M Watson and Nixon 1953), a leafhopper (M Day and McKinnon 1951), and a spittlebug (Wiegert 1964) that yielded an average consumption rate of 2.5 mg dry sap mg-1 dry insect day-1 Several factors affect the rate of sap consumption P Andersen et al (1992) found that leafhopper feeding rate was related to xylem chemistry and fluid tension Feeding rates generally increased with amino-acid concentrations and decreased with xylem tension, ceasing above tensions of 2.1 Mpa when plants were water stressed Stadler and Müller (1996) reported that aphids feeding on poor-quality hosts with yellowing needles produced twice the amount of honeydew as did aphids feeding on high-quality hosts during shoot expansion, but this difference disappeared by the end of shoot expansion Banks and Nixon (1958) reported that aphids tended by ants approximately doubled their rates of ingestion and egestion Measurement of individual consumption rate has limited utility for extrapolation to effects on plant growth because more plant material may be lost, or not produced, than actually consumed as a consequence of wasteful feeding or mortality to meristems (e.g., Blumer and Diemer 1996, Gandar 1982) For example, Schowalter (1989) reported that feeding on Douglas-fir, Pseudotsuga menziesii, buds by a bud moth, Zeiraphera hesperiana, caused an overall loss of 50% of photosynthetic biomass removed daily Rates were greatest in some phytoplankton communities where herbivores consumed all production daily and least in some forests where herbivores removed