The hierarchical organization (see Fig. 1.2 or Table 1.1) of this text emphasizes linkages and feedbacks among levels of ecological organization. Linkages and feedbacks are strongest between neighboring levels but are significant even between individual and ecosystem levels of the hierarchy. Physiological and behavioral responses to environmental variation are under genetic control and determine individual fitness, but they also affect the rate and geographic pattern of resource acquisition and allocation that control climate and energy and bio-geochemical fluxes at the ecosystem level. These feedbacks are an important and largely neglected aspect of insect ecology that affect ecosystem stability and global processes.
The geographic distribution of individual species generally reflects the environmental template established by continental history, latitude, mountain ranges, and global atmospheric and oceanic circulation patterns. The great diversity of insects reflects their rapid adaptation, conferred by small size, short life spans, and rapid reproductive rates, to environmental variation. These attributes have facilitated speciation at multiple scales: among geographic regions, habitats, and resources and at microscales on or within resources (e.g., individual leaves). However, within the potential geographic range of a species, the spatial and temporal patterns of abundance reflect disturbance dynamics, resource distribution, and interactions with other species that affect individual fitnesses and enhance or limit colonization and population growth.
Energy and resource budgets (see Fig. 4.1) are key aspects of individual fitness, population persistence, and community interactions. All organisms require energy to accumulate resources, necessary for growth and reproduction, against resource concentration gradients and thereby maintain the thermodynamic disequilibrium characteristic of life. Where resources are more concentrated, relative to individual needs, less energy is required for acquisition. Interactions among organisms often may be controlled by mass balances of multiple nutrients. Resource use requires adaptations to acquire necessary limiting nutrients, such as nitrogen, while avoiding or circumventing toxic or defensive chemicals as well as overabundant nutrients.
Much research has addressed plant defenses against feeding by insects and other herbivores. Insect herbivores have evolved a variety of mechanisms for avoiding, detoxifying, or inhibiting expression of plant defenses. All species have mobile stages adapted to find new resources before current resources are depleted or destroyed. The early evolution of flight among insects greatly facilitated foraging, escape from unsuitable environmental or resource conditions, and discovery of more optimal conditions. Individuals or populations that fail to acquire sufficient energy and nutrients to grow and reproduce do not survive.
Adaptations for detecting and acquiring resources are highly developed among insects. Many insects can detect the presence and location of resources from chemical cues carried at low concentrations on wind or water currents. The diversity of strategies among insect species for acquiring resources has perhaps drawn the most ecological attention. These strategies range from ambush to active foraging; often demonstrate considerable learning ability (especially among social insects); and involve insects in all types of interactions with other organisms, including competition (e.g., for food, shelter, and oviposition site resources), predation and parasitism (on plant, invertebrate, and vertebrate prey or hosts and as prey or hosts), and mutualism (e.g., for protection, pollination, and seed dispersal).
Spatial and temporal variation in population and community structure reflects net effects of environmental conditions. Changes in population and community structure also constrain survival and reproduction of associated species. Population density and competitive, predatory, and mutualistic interactions affect foraging behavior and energy and nutrient balances of individuals. Individuals forced to move constantly to avoid intraspecific or interspecific competitors or predators will be unable to forage sufficiently for energy and nutrient resources. However, energy and nutrient balances can be improved through mutualistic interactions that enhance the efficiency of resource acquisition. The relative contributions of intraspecific and interspecific interactions to individual survival and reproduction remain a central theme of ecology but have been poorly integrated with ecosystem conditions. Debate over the importance of bottom-up versus top-down controls of populations perhaps reflects variation in the contributions of these factors among species as well as spatial and temporal variation in their effect.
Ecosystems represent the level at which complex feedbacks among abiotic and biotic processes are integrated. Ecosystems can be viewed as dynamic energy-and nutrient-processing engines that modify global energy and nutrient fluxes. Cycling and storage processes controlled by organisms reduce variation in abiotic conditions and resource availability. Although ecosystem properties are largely determined by vegetation structure and composition, insects and other animals modify ecosystem conditions, often dramatically, through effects on primary production, decomposition and mineralization, and pedogenesis. Insect herbivore effects on vegetation structure affect albedo, evapotranspiration, and wind abatement. Changes in decomposition processes affect fluxes of carbon and trace gases as well as soil structure and fertility. Insect roles as ecosystem engineers mitigate or exacerbate environmental changes resulting from anthropogenic activities. Resolution of environmental issues requires attention to these roles of insects as well as to their responses to environmental changes.
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