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#connectance

3 public questions tagged with this topic.

Food Chain and Web

Connectance in food web is calculated by:

Food-web connectance is the fraction of possible feeding links that are actually realized, calculated as actual links divided by potential links. If directed links among S species are allowed, potential links may be S², S(S−1), or another denominator depending on whether cannibalism and basal species are included. Because conventions differ, comparisons require the same definition and sampling effort. Connectance influences possible pathways for energy flow and indirect effects, but observed values can decline artificially as more poorly resolved species are added. Energy budgets must distinguish stocks from rates: standing biomass can remain high or low even when production and transfer through that compartment are rapid. Matter can cycle repeatedly through producers, consumers, and decomposers, whereas usable energy requires continuous external input because respiration degrades it to heat. The amount reaching a consumer level depends jointly on resource production, the fraction consumed, assimilation efficiency, and conversion of assimilates into new biomass.

Ref: Fundamentals of Ecology, Odum & Barrett, 5th Ed., Ch. 3

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Ecosystem Ecology

Connectance indicates:

“Lower complexity with richness” for connectance indicates. This relationship follows from the ecological mechanism represented by the terms in the item, not merely from an association between their names. Rates depend on temperature, moisture, substrate quality, consumer physiology, and the elemental balance between organisms and their food. These controls explain why the same process can differ among terrestrial, freshwater, and marine systems without changing its definition. The remaining alternatives—“Higher complexity with higher richness”, “No relation”, “Random variation”—refer to different states, processes, or scales and therefore do not express the same causal relationship. Ecosystem processes are constrained by energy conservation and by the cycling of matter. Energy enters mainly through primary production, is lost as metabolic heat at every transfer, and therefore cannot be recycled in the way that carbon, nitrogen, phosphorus, or water can. Linking the wording to measurable consequences for fitness, abundance, or flux gives the conclusion its scientific meaning and prevents a purely mnemonic interpretation.

Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 17-19

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Ecosystem Ecology

Connectance tends to:

Food-web connectance is the proportion of realised feeding links among all possible links. As species richness increases, the number of possible pairwise interactions rises approximately with the square of species number, whereas each consumer usually interacts with only a limited subset. Consequently connectance often decreases with richness even if the absolute number of links increases. The pattern depends on sampling and web definition, so it is an empirical tendency rather than an inviolable law. Mechanistic interpretation connects individual physiology and species interactions to population change, community composition, and ecosystem-level fluxes. Reliable inference requires the complete experimental design, definitions, units, and statistical evidence; missing labels cannot be reconstructed from an answer key alone. Net primary production equals gross primary production minus plant respiration and represents biomass or energy made available for growth and consumers. Rates must be compared on the same area, biomass, leaf-area, and time basis because changing the denominator can reverse an apparent ecosystem ranking.

Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 17-19

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