The Hierarchy of Subsystems

Complex systems with feedback mechanisms can be partitioned into component subsystems, which are themselves composed of sub-subsystems. Viewing the ecosystem as a nested hierarchy of subsystems (Table 1.1), each with its particular properties and processes (Coulson and Crossley 1987, Kogan 1998, O'Neill et al 1986), facilitates understanding of complexity. Each level of the hierarchy can be studied at an appropriate level of detail, and its properties can be explained by the integration of its subsystems. For example, population responses to changing environmental conditions reflect the net physiological and behavioral

ÂĦABQeDBB Ecological hierarchy and the structural and functional properties characterizing each level.

Ecological level

Structure

Function

Global

Biome distribution, atmospheric condition, climate, sea level, total biomass

Gas, water, nutrient exchange between terrestrial and marine systems

Biome

Landscape pattern, temperature, moisture profile, integrated biomass of ecosystems

Energy and matter fluxes, disturbance regimen, migration

Landscape

Disturbance pattern, community distribution, metapopulation structure

Energy and matter fluxes, integrated NPP of ecosystems, colonization and extinction

Ecosystem

Vertical and horizontal structure, disturbance type and frequency, biomass, functional organization

Energy and matter fluxes, succession, NPP, herbivory, decomposition, pedogenesis

Community

Diversity, trophic organization

Species interactions, temporal and spatial changes

Population

Density, dispersion, age structure, genetic structure

Natality, mortality, dispersal, gene flow, temporal and spatial changes

Individual

Anatomy, genome

Physiology/learning/behavior, resource acquisition and allocation

NPP, net primary productivity.

NPP, net primary productivity.

responses of individuals that determine their survival and reproduction. Changes in community structure reflect the dynamics of component populations. Fluxes of energy and matter through the ecosystem reflect community organization and interaction. Landscape structure reflects ecosystem processes that affect movement of individuals. Hence, the integration of structure and function at each level determines properties at higher levels.

At the same time, the conditions produced at each level establish the context, or template, for responses at lower levels. Population structure resulting from individual survival, dispersal, and reproduction determines future survival, dispersal, and reproduction of individuals. Ecosystem conditions resulting from community interactions affect subsequent behavior of individual organisms, populations, and the community. Recognition of feedbacks from higher levels has led to developing concepts of inclusive fitness (fitness accruing through feedback from benefit to a group of organisms) and ecosystem self-regulation (see Chapter 15). The hypothesis that insects function as cybernetic regulators that stabilize ecosystem properties (M. Hunter 2001b, Mattson and Addy 1975, Schowalter

1981) has been one of the most important and controversial concepts to emerge from insect ecology.

Ecosystem processes represent the integration of processes at the level of component communities. Component communities are subsystems (i.e., more or less discrete assemblages of organisms based on particular resources). For example, the relatively distinct soil faunas associated with fungal, bacterial, or plant root resources represent different component communities (J. Moore and Hunt 1988). Component communities are composed of individual species populations, with varying strategies for acquiring and allocating resources. Species populations, in turn, are composed of individual organisms with variation in individual physiology and behavior. Ecosystems can be integrated at the landscape or biome levels, and biomes can be integrated at the global (biosphere) level. Spatial and temporal scales vary across this hierarchy. Whereas individual physiology and behavior operate on small scales of space and time (i.e., limited to the home range and life span of the individual), population dynamics span landscape and decadal scales, and ecosystem processes, such as patterns of resource turnover, recovery from disturbance, or contributions to atmospheric carbon, operate at scales from the patch to the biome and from decades to millenia.

Modeling approaches have greatly facilitated understanding of the complexity and consequences of interactions and linkages within and among these organizational levels of ecosystems. The most significant challenges to ecosystem modelers remain (1) the integration of appropriately detailed submodels at each level to improve prediction of causes and consequences of environmental changes and (2) the evaluation of contributions of various taxa (including particular insects) or functional groups to ecosystem structure and function. In particular, certain species or structures have effects disproportionate to their abundance or biomass. Studies focused on the most abundant or conspicuous species or structures fail to address substantial contributions of rare or inconspicuous components, such as many insects.

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