THE INDIVIDUAL ORGANISM IS A FUNDAMENTAL unit of ecology. Organisms interact with their environment and affect ecosystem processes largely through their cumulative physiological and behavioral responses to environmental variation. Individual success in finding and using necessary habitats and resources to gain reproductive advantage determines fitness. Insects have a number of general attributes that have contributed to their ecological success (Romoser and Stoffolano 1998).
First, small size (an attribute shared with other invertebrates and microorganisms) has permitted exploitation of habitat and food resources at a microscopic scale. Insects can take shelter from adverse conditions in microsites too small for larger organisms (e.g., within individual leaves). Large numbers of insects can exploit the resources represented by a single leaf, often by partitioning leaf resources. Some species feed on cell contents, others on sap in leaf veins, some on top of the leaf, others on the underside, and some internally. At the same time, small size makes insects sensitive to changes in temperature, moisture, air or water chemistry, and other factors.
Second, the exoskeleton (shared with other arthropods) provides protection against predation and desiccation or water-logging (necessary for small organisms) and innumerable points of muscle attachment (for flexibility). However, the exoskeleton also limits the size attainable by arthropods. The increased weight of exoskeleton required to support larger body size would limit mobility. Larger arthropods occurred prehistorically, before the appearance of faster, more flexible
vertebrate predators. Larger arthropods also occur in aquatic environments, where water helps support their weight.
Third, metamorphosis is necessary for (exoskeleton-limited) growth but permits partitioning of habitats and resources among life stages. Immature and adult insects can differ dramatically in form and function and thereby live in different habitats and feed on different resources, reducing intraspecific competition. For example, dragonflies and mayflies live in aquatic ecosystems during immature stages but in terrestrial ecosystems as adults. Many Lepidoptera feed on foliage as immatures and on nectar as adults. Among holometabolous insects, the quiescent, pupal stage facilitates survival during unfavorable environmental conditions. However, insects, as well as other arthropods, are particularly vulnerable to desiccation and predation during ecdysis (molting).
Finally, flight evolved first among insects and conferred a distinct advantage over other organisms. Flight permits rapid long-distance movement that facilitates discovery of new resources, as well as escape from predators or unfavorable conditions. Flight remains a dominant feature of insect ecology.
This section focuses on aspects of physiology and behavior that affect insect interactions with environmental conditions, specifically adaptations that favor survival and reproduction in variable environments and mechanisms for finding, exploiting, and allocating resources. Physiology and behavior are closely integrated. For example, movement, including dispersal, is affected by physiological perception of chemical gradients, fat storage, rapid oxygen supply, etc. Similarly, physiological processes are affected by insect selection of thermally suitable location, choice of food resources, etc. Chemical defenses against predators are based on physiological processes but often are enhanced by behaviors that facilitate expression of chemical defenses (e.g., thrashing or regurgitation). Organisms affect ecosystem processes, such as energy and nutrient fluxes, through their spatial and temporal patterns of energy and nutrient acquisition and allocation.
Chapter 2 deals with physiological and behavioral responses to changing environmental conditions. Chapter 3 addresses physiological and behavioral mechanisms for finding and exploiting resources. Chapter 4 describes allocation of resources to various metabolic pathways and behaviors that facilitate resource acquisition, mate selection, reproduction, interaction with other organisms, etc. Physiology and behavior interact to determine the conditions under which insects can survive and the means by which they acquire and use available resources. These ecological attributes affect population ecology (such as population structure, responses to environmental change and disturbances, biogeography, etc., Section II), community attributes (such as use of, or use by, other organisms as resources, Section III), and ecosystem attributes (such as rates and directions of energy and matter flows, Section IV).
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