E Regulation of Net Primary Productivity by Insects
During the 1960s, a number of studies, including Crossley and Howden 1961 , Crossley and Witkamp 1964 , C. Edwards and Heath 1963 , and Zlotin and Khodashova 1980 , indicated that arthropods potentially control energy and nutrient fluxes in ecosystems. Clearly, phytophages could affect, without regulating, ecosystem properties. However, phytophages respond to changes in vegetation density or physiological condition in ways that provide both positive and negative feedback, depending on the...
A Properties of Cybernetic Systems
Cybernetic systems generally are characterized by 1 information systems that integrate system components, 2 low-energy feedback regulators that have high-energy effects, and 3 goal-directed stabilization of high-energy processes. Mechanisms that sense deviation perturbation in system condition communicate with mechanisms that function to reduce the amplitude and period of deviation. Negative feedback is the most commonly recognized method for stabilizing outputs. A thermostat represents a...
Attraction
Chemical, visual, and acoustic signaling are used to attract potential mates. Attraction of mates can be accomplished by either sex in Coleoptera, but only females of Lepidoptera release sex pheromones and only males of Orthoptera stridulate. Sex pheromones greatly improve the efficiency with which insects find potential mates over long distances in heterogeneous environments Card 1996, Law and Regnier 1971, Mustaparta 1984 . The particular blend of compounds and their enantiomers, as well as...
E Mechanism of Dispersal
The probability that suitable resources can be found and colonized depends on the mode of dispersal. Three general mechanisms can be identified random, phoretic, and directed. Random dispersal direction and path is typical of most small insects with little capacity to detect or orient toward environmental cues. Such insects are at the mercy of physical barriers or wind or water currents, and their direction and path of movement are determined by obstacles and patterns of air or water movement....
Ecosystem Structure and Function
C. Factors Influencing Cycling Processes V. Ecosystem Modeling VI. Summary TANSLEY 1935 COINED THE TERM ECOSYSTEM TO RECOGNIZE THE integration of the biotic community and its physical environment as a fundamental unit of ecology within a hierarchy of physical systems that span the range from atom to universe. Shortly thereafter, Lindeman's 1942 study of energy flow through an aquatic ecosystem introduced the modern concept of an ecosystem as a feedback system capable of redirecting and...
C Dispersal
Dispersal is the movement of individuals away from their source and includes spread, the local movement of individuals, and migration, the cyclic mass movement of individuals among areas L. Clark et al. 1967, Nathan et al. 2003 . As discussed in Chapter 2, long-distance dispersal maximizes the probability that habitat or food resources created by environmental changes or disturbances are colonized before the source population depletes its resources or is destroyed by disturbance. However,...
B DensityDependent Factors
Primary density-dependent factors include intraspecific and interspecific competition, for limited resources, and predation. The relative importance of these factors has been the topic of much debate. Malthus 1789 wrote the first theoretical treatise describing the increasing struggle for limited resources by growing populations. Effects of intraspecific competition on natality, mortality, and dispersal have been demonstrated widely see Chapter 5 . As competition for finite resources becomes...
B Water Balance
Maintenance of homeostatic water balance also is a challenge for organisms with high ratios of surface area to volume Edney 1977, N. Hadley 1994 . The arthropod exoskeleton is an important mechanism for control of water loss. Larger, more heavily sclerotized arthropods are less susceptible to desiccation than are smaller, more delicate species Alstad et al. 1982, Kharboutli and Mack 1993 . Arthropods in xeric environments usually are larger, have a thicker cuticle, and secrete more waxes to...
D Effects on Climate and Disturbance Regime
Herbivore-induced changes in vegetation structure likely affect soil temperature, relative humidity, erosion, soil moisture, and soil fertility see Chapter 11 . Changes in litter accumulation can affect ecosystem vulnerability or sensitivity to some disturbances, especially fire. However, relatively few studies have demonstrated herbivore effects on these variables. Herbivory increases vegetation porosity Fig. 12.17 and penetration of light, precipitation, and wind to the understory and soil...
C Disturbances
Within biomes, characteristic abiotic and biotic factors interact to influence the pattern of disturbances, relatively discrete events that alter ecosystem conditions, and create a finer-scale landscape mosaic of patches with different disturbance and recovery histories Harding et al. 1998, Schowalter et al. 2003, Willig and Walker 1999 . Disturbances, such as fire, storms, drought, flooding, anthropogenic conversion Fig. 2.8 , alter vertical and horizontal gradients in temperature, moisture,...
Ii Effects Of Pollination
Pollination contributes to genetic recombination and survival of plant species in heterogeneous environments. Many plants can reproduce vegetatively or by self-fertilization, but these mechanisms are not conducive to long-distance colonization or genetic recombination. Species survival and adaptation to changing environmental conditions requires outcrossing and environmental selection among diverse genotypes. Some long-lived perennials may endure adverse conditions and persist by vegetative...
B Measurement of Seed Predation and Dispersal
A number of factors influence rates of seed predation and dispersal. The extent of seed mortality, mechanism of seed transport, distance moved from the parent plant, attraction of particular dispersal agents, and thermodynamic constraints determine the probability that seeds will survive and be moved to suitable or distant locations. Pollinators and seed predators can have opposing effects on seed production. Steffen-Dewenter et al. 2001 reported that pollinator activity decreased, but seed...
B Factors Affecting Succession
Succession generally progresses toward the community type characteristic of the biome within which it occurs e.g., toward deciduous forest within the deciduous forest biome or toward chaparral within the chaparral biome e.g.,Whittaker 1953, 1970 . However, succession can progress along various alternative pathways and reach alternative endpoints such as stands dominated by beech, Fagus, maple, Acer, or hemlock, Tsuga, within the eastern deciduous forest in North America , depending on a variety...
C Energy Budgets
Energy budgets can be developed from measurements of available solar energy, primary productivity, secondary productivity, decomposition, and respiration. Comparison of budgets and conversion efficiencies among ecosystems can indicate factors affecting energy flow and contributions to global energy budget. Development of energy budgets for agricultural ecosystems can be used to evaluate the efficiency of human resource production. Lindeman 1942 was the first to demonstrate that ecosystem...
I THE PHYSICAL TEMPLATE A Biomes
Global patterns of temperature and precipitation, reflecting the interaction among latitude, global atmospheric and oceanic circulation patterns, and topography, establish a regional template of physical conditions that support characteristic communities, called biomes Fig. 2.1 Finch and Trewartha 1949 . Latitudinal gradients in temperature from Earth's equator to its poles define the tropical, subtropical, temperate, and arctic zones. Precipitation patterns overlay these temperature gradients....
B Secondary Productivity
Net primary production provides the energy for all heterotrophic activity. Consumers capture the energy stored within the organic molecules of their food sources. Therefore, each trophic level acquires the energy represented by the biomass consumed from the lower trophic level. The rate of conversion of NPP into heterotroph tissues is secondary productivity. As with primary productivity, we can distinguish the total rate of energy consumption by secondary producers from the energy incorporated...
A Plant Productivity Survival and Growth Form
Traditionally, herbivory has been viewed solely as a process that reduces primary production. As described in the preceding text, herbivory can remove several times the standing crop of foliage, alter plant growth form, or kill all plants of selected species over large areas during severe outbreaks. However, several studies indicate more complex effects of herbivory. The degree to which her-bivory affects plant survival, productivity, and growth form depends on the plant parts affected plant...
A Exponential and Geometric Models
The simplest model of population growth describes change in numbers as the initial population size times the per capita rate of increase see Fig. 6.7 Berryman 1997, Price 1997 . This model integrates per capita natality, mortality, immi- Exponential and logistic models of population growth. The exponential model describes an indefinitely increasing population, whereas the logistic model describes a population reaching an asymptote at the carrying capacity of the environment K . Exponential and...
D Species Interactions
Species interactions can enhance or preclude persistence of some species, as discussed in Chapter 8. As noted in the preceding text, species populations cannot persist where their host species are absent. However, the presence of competitors, predators, and mutualists also affects persistence of associated species, both directly and indirectly. In the past, species interactions often were viewed as evidence of co-evolution. However, species colonizing new areas can occupy niches through...
B Community Dynamics
Differential herbivory among plants and plant species in an ecosystem affects both the distribution of individuals of a particular plant species and the opportunities for growth of plant species resistant to or tolerant of herbivory.The intensity of herbivory determines its effects on plant communities. Low to moderate intensities that prevail most of the time generally ensure a slow turnover of plant parts or individual plants. High intensities during outbreaks or as a result of management can...
Courtship Behavior
Courtship often involves an elaborate, highly ritualized sequence of stimulus and response actions that must be completed before copulation occurs Fig. 4.6 . This provides an important mechanism that identifies species and sex, thereby enhancing reproductive isolation. Color patterns, odors, and tactile stimuli are important aspects of courtship. For many species, ultraviolet patterns are revealed, close-range pheromones are emitted, or legs or mouthparts stroke the mate as necessary stimuli L....
Trophic Loops and Intraguild Predation
Loops, or reciprocal predation, in which two species feed on each other or a third species feeds on one and is eaten by the other, should be rare or absent because the size range of prey is constrained by physical limits and because loops potentially reduce population recovery following disturbance Pimm 1982, Pimm and Rice 1987 . Intraguild predation involves predation among members of the predator guild on each other. Cannibalism is considered a self-loop see Fox 1975a . Polis 1991b and Reagan...
E Factors Affecting Expression of Defenses
Some plant groups are characterized by particular defenses. For example, ferns and gymnosperms rely primarily on phenolics, terpenoids, and insect hormone analogues, whereas angiosperms more commonly produce alkaloids, phenolics, and many other types of compounds. However, most plants apparently produce compounds representing a variety of chemical classes Harborne 1994, Newman 1990 . Each plant species can be characterized by a unique chemical fingerprint conferred by these chemicals....
Iii Paleoecology
Paleoecology provides a context for understanding extant interactions and community structure. Although most paleoecological study has focused on biogeo-graphic patterns e.g., Price 1997 , fossils also reveal much about prehistoric species interactions and community structure Labandeira 1998, Labandeira and Sepkoski 1993, Poinar and Poinar 1999 and even the consequences of prehistoric changes in climate Wilf and Labandeira 1999, Wilf et al. 2001 or other disturbances Labandeira et al. 2002 ....
D Arthropod Defenses
1. Antipredator Defenses Arthropods also use various defenses against predators and parasites. Physical defenses include hardened exoskeleton, spines, claws, and mandibles. Chemical defenses are nearly as varied as plant defenses. Hence, predaceous species also must be capable of evaluating and exploiting defended prey resources. The compounds used by arthropods, including predaceous species, generally belong to the same categories of compounds described previously for plants. Many insect...
Ii Ecosystems As Cybernetic Systems
The cybernetic nature of ecosystems, from patch to global scales, has been a central theme of ecosystem ecology. J. Lovelock 1988 suggested that autotroph-heterotroph interactions have been responsible for the development and regulation of atmospheric composition and climate that are suitable for the persistence of life. The ability of ecosystems to minimize variability in climate and rates of energy and nutrient fluxes would affect responses to anthropogenic changes in global conditions.
Vi Summary
An ecosystem represents the integration of the biotic community and the abiotic environment. The capacity of the community to modify its environment depends on its structure and the degree to which it controls energy flow, biogeochemical cycling, and climatic conditions. Ecosystem structure reflects the organization of various abiotic and biotic pools that exchange energy and matter. Abiotic pools are the atmosphere, oceans, and sediments that represent the sources of energy and matter for...
A Pollinator Functional Groups
Functional groups of pollinators may be more or less restricted to groups of plants based on floral or habitat characteristics Bawa 1990 . A large number of pollinators are generalists with respect to plant species. This functional group includes many beetles, flies, thrips, etc. that forage on any floral resources available. Specialist pollinators often exploit particular floral characteristics that may exclude other pollinators. For example, nocturnally flowering plants with large flowers...
A Decomposition and Mineralization
An extensive literature has addressed the effects of detritivores on decomposition and mineralization rates Coleman et al. 2004 . Generally, the effect of arthropods on the decay rate of litter can be calculated by subtracting the decay rate when arthropods are excluded from the decay rate when arthropods are present see Table 14.1 . Detritivores affect decomposition and mineralization processes, including fluxes of carbon as CO2 or CH4, by fragmenting litter and by affecting rates of microbial...
Iii Anthropogenic Effects On Spatial Dynamics
The disappearance of M. spretus indicates the vulnerability to extinction of even cyclically abundant species when populations decline to near or below their extinction thresholds see Chapter 6 . Populations always have been vulnerable to local extinctions as a result of disturbances or habitat loss during environmental changes. Species persist to the extent that dispersal capabilities are adapted to the frequency and scale of these changes. Species adapted to relatively unstable habitats...
A Patterns of Succession
Two types of succession can be recognized. Primary succession occurs on newly exposed substrates e.g., lava flows, uplifted marine deposits, dunes, newly deposited beaches, etc. . Primary succession usually involves a long period of soil formation and colonization by species requiring little substrate modification. Secondary succession occurs on sites where the previous community was disturbed and is influenced by remnant substrate and surviving individuals. Although most studies of succession...
Ii Successional Change In Community Structure
Relatively predictable changes in community structure occur over periods of decades to centuries as a result of succession on newly exposed or disturbed sites. New habitats become available for colonization as a result of tectonic activity, glacial movement, sea level change, and sediment deposition or erosion. Species colonizing newly exposed surfaces usually are small in stature, tolerant of exposure or able to exploit small shelters, and able to exploit nonorganic or exogenous resources....
B Mortality
Mortality is the population death rate i.e., the per capita number of individuals dying per unit time . As with natality, we can distinguish a potential longevity or lifespan, resulting only from physiological senescence, from the realized longevity, resulting from the action of mortality factors. Hence, mortality can be viewed both as reducing the number of individuals in the population and as reducing survival. Both have importance consequences for population dynamics. Organisms are...
I Development Of The Concept
The intellectual roots of ecosystem self-regulation lie in Darwin's 1859 recognition that some adaptations apparently benefit a group of organisms more than the individual, leading to selection for population stability. The concept of altruism and selection for homeostasis at supraorganismal levels has remained an important issue, despite recurring challenges and alternative models e.g., Axelrod and Hamilton 1981, Schowalter 1981, E.Wilson 1973,1997 . Behavioral ecologists have been challenged...
B Logistic Model
A mathematic model to account for density-dependent regulation of population growth was developed by Verhulst in 1838 and again, independently, by Pearl and Reed 1920 . This logistic model see Fig. 6.7 often is called the Pearl-Verhulst equation Berryman 1981, Price 1997 . The logistic equation is as follows where K is the carrying capacity of the environment. This model describes a sigmoid S-shaped curve see Fig. 6.7 that reaches equilibrium at K. If N lt K, then the population will increase...
C Models of Succession
Clements 1916 noted that comparison of successional stages is less useful than is understanding of processes affecting the transitions from one sere to another. Nevertheless, few studies have continued over sufficient periods to evaluate the mechanism s producing successional transitions. Rather, a number of nonmutually-exclusive models, all of which may affect particular transitions to varying degrees, have been proposed and debated widely e.g., Connell and Slatyer 1977, H. Horn 1981, Mcintosh...
D Other Abiotic Factors
Many aquatic insects are sensitive to water level and flow rate Ward 1992 .These factors can fluctuate dramatically, especially in seasonal habitats, such as desert playas, intermittent streams, wetlands, and perched pools in treeholes and bromeliads phytotelmata . Water level affects both temperature and water quality, temperature because smaller volumes absorb or lose heat more quickly than do larger volumes, water quality because various solutes become more concentrated as water evaporates....
Mutualism
Mutualistic interactions benefit both partners positive effects on each and therefore represent cooperative or mutually exploitative relationships. One member of a mutualism provides a resource that is exploited by the other the symbiont . The symbiont, in turn, unintentionally provides a service to its host. Commensalism an unidentified mite in an ambrosia beetle, Trypodendron lineatum, mine in Douglas-fir. A variety of predaceous and detritivorous mites exploit resources in bark and ambrosia...
Parasitism
Parasitism affects the host prey population in ways that are similar to predation and can be described using predation models. However, whereas predation involves multiple prey killed and consumed during a predator's lifetime, parasites feed on living prey. Parasitoidism is unique to insects, especially flies and wasps, and combines attributes of both predation and parasitism. The adult par-asitoid usually deposits eggs or larvae on, in, or near multiple hosts, and the larvae subsequently feed...
B Dispersion
Dispersion is the spatial pattern of distribution of individuals. Dispersion is an important characteristic of populations that affects spatial patterns of resource use and population effect on community and ecosystem attributes. Dispersion pattern can be regular, random, or aggregated. A regular uniform dispersion pattern is seen when individuals space themselves at regular intervals within the habitat. This dispersion pattern is typical of species that contest resource use, especially...
B Ecosystem Homeostasis
E. Odum 1969 presented a number of testable hypotheses concerning ecosystem capacity to develop and maintain homeostasis, in terms of energy flow and biogeochemical cycling, during succession. Although subsequent research has shown that many of the predicted trends are not observed, at least in some ecosystems, Odum's hypotheses focused debate on ecosystems as cybernetic systems. Engelberg and Boyarsky 1979 argued that ecosystems do not possess the critical goal-directed communication and...
A Factors Affecting Efficiency
Efficiency is affected by a number of constraints on energy and resource allocation. Clearly, selection should favor physiological and behavioral adaptations that improve overall efficiency. However, adaptive strategies reflect the net current result of many factors that have variable and interactive effects on survival and reproduction. Hence, individual responses to current conditions vary in efficiency. Whereas physiological, and many behavioral, responses are innate genetically based, hence...
B Environmental Variation
Physical conditions vary seasonally in most biomes see Fig. 2.5 . Temperate ecosystems are characterized by obvious seasonality in temperature, with cooler winters and warmer summers, and also may show distinct seasonality in precipitation patterns, resulting from seasonal changes in the orientation of Earth's axis relative to the sun. Although tropical ecosystems experience relatively consistent temperatures, precipitation often shows pronounced seasonal variation see Fig. 2.5 . Aquatic...
AVHRR 12 x 103 pixels Landsat TM 13 x 106 pixels 6 polygons33170 polygons
Examples of geostatistical representation. The Seeley-Swan watershed in Montana, United States, is depicted with 1-km2 raster cells on the left, by 30 x 30 m raster cells on the right, and with vector polygons defined from topographic analysis in the inset. The more heterogeneneous the landscape, the greater the number of cells required to provide an accurate assessment of spatial variation. From R. Waring and Running 1998 . Please see extended permission list pg 570. Examples of...
Commensalism
Commensalism benefits the symbiont without significantly affecting the host. This is a relatively rare type of interaction because few hosts can be considered to be completely unaffected by their symbionts. Epiphytes, plants that benefit by using their hosts for aerial support but gain their resources from the atmosphere, and cattle egrets, which eat insects flushed by grazing cattle, are well-known examples of commensalism. However, epiphytes may capture and provide nutrients to the host a...
A Primary Productivity
Primary productivity is the rate of conversion of solar energy into plant matter. The total rate of solar energy conversion into carbohydrates total photosynthesis is gross primary productivity GPP . However, a portion of GPP must be expended by the plant through metabolic processes necessary for maintenance, growth, and reproduction and is lost as heat through respiration. The net rate at which energy is stored as plant matter is net primary productivity. The energy stored in net primary...
A DensityIndependent Factors
Insect populations are highly sensitive to changes in abiotic conditions, such as temperature, water availability, etc., which affect insect growth and survival see Chapter 2 . Changes in population size of some insects have been related directly to changes in climate or to disturbances e.g., Greenbank 1963, Kozar 1991, Porter and Redak 1996, Reice 1985 . In some cases, climate fluctuation or disturbance affects resource values for insects. For example, loss of riparian habitat as a result of...
A Management of Crop Forest and Urban Pests
Management of crop, forest, and urban pests has been a major application of insect ecology. Insect roles in ecosystems may conflict with crop and livestock production and human health and habitation when conditions favor insect population growth. For example, densely planted monocultures of crop species, often bred to reduce bitter defensive flavors, provide ideal conditions for population growth of herbivorous species see Chapter 6 . Similarly, buildings provide protected habitats for ants,...
A Thermoregulation
Insects, as well as other invertebrates, are generally heterothermic, meaning that their body temperatures are determined primarily by ambient temperature. Rates of metabolic activity hence, energy and carbon flux generally increase with temperature. Developmental rate and processes also are temperature dependent. However, at least some species regulate body temperature to some degree through physiological or behavioral responses to extreme temperatures. Insect species show characteristic...
Iii Summary 1
Decomposition and pedogenesis are major ecosystem processes that affect bio-geochemical cycling, trace gas fluxes, soil fertility, and primary production. Decomposition of organic matter involves four component processes photooxi-dation, leaching, comminution, and mineralization. Arthropods are key factors influencing comminution and mineralization. Functional groups involved in decomposition include coarse comminuters that fragment large materials and fine comminuters that fragment smaller...