Ecology: The Biology of Interactions. 3.11. Trophic Links and Levels
Autotrophs obtain biogenic elements and the necessary energy from the environment and create organic matter. The organic matter produced by autotrophs is consumed by some heterotrophs; those heterotrophs are consumed by others, and so on until the organic matter synthesized by autotrophs is almost completely decomposed. These re...
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3.10. The Nature and Characteristics of Communities
D. Shabanov, M. Kravchenko. Ecology: The Biology of Interactions Chapter 3. Biogeocoenology and Community Ecology
3.12. Ecological Efficiencies
3.11. Trophic Links and Levels The transfer of matter and transformation of energy in ecosystems occur through the nutrition of organisms. Global processes that sustain the biosphere and make human existence possible are connected with the feeding of innumerable individual living beings. Autotrophs obtain biogenic elements and the energy they need from the environment and produce organic matter. The organic matter of autotrophs is consumed by some heterotrophs, those heterotrophs by others, and so on until the organic matter synthesized by autotrophs is decomposed almost completely. These feeding-based relationships are called trophic (food) links. Their sequences form trophic chains. A trophic chain is the path by which organic matter and the energy it contains are transferred from its primary recipients (autotrophs) through a series of organisms that eat one another. Two types of trophic chains are distinguished. Grazing chains run from green plants to herbivores and then to predators. Detrital chains run from dead organic matter (detritus) to microorganisms, detritivores, and their predators (Fig. 3.11.1). [IMG_1] Fig. 3.11.1. Grazing and detrital trophic chains are interconnected If we examine where and how the elements of grazing and detrital chains are arranged, we see that most biogeocoenoses are divided into two tiers: an autotrophic tier, well illuminated and dominated by production, and a heterotrophic tier, without light and dominated by respiration. In terrestrial biogeocoenoses, the autotrophic tier is above the soil, and the heterotrophic tier is below its surface. In aquatic ecosystems, the autotrophic tier is the water column illuminated by sunlight, while the heterotrophic tier is the dark depths and bottom sediments. Grazing chains run in the autotrophic tier of ecosystems, and detrital chains in the heterotrophic one. However, these sequences are not independent. Some animals can obtain energy from different chains. A toad that comes out in the evening may eat a leaf beetle that has just fed on a garden plant (and belongs to a grazing chain), or it may catch a ground beetle, a predatory beetle that fed on subterranean invertebrates from detrital food chains. Therefore, all trophic chains within a given ecosystem can be said to form its trophic web. We still need one more concept related to the trophic structure of communities, and this is the one we will use most often later. A trophic level is a set of community organisms that receive solar energy after the same number of transformations. Naturally, the first trophic level is the producer level. Producers are eaten by first-level consumers, those by second-level consumers, and so on. Some species can occupy different levels in different manifestations; therefore, the concept of trophic level characterizes not a species as such, but features of its lifestyle in a specific ecological situation. Humans can eat potatoes, pork, or even a delicacy frog. In these situations, humans act as first-level consumers, second-level consumers, or even third- or fourth-level consumers. Nevertheless, if we disregard exotic and delicacy foods, we can conclude that humans belong to levels I-II of consumers, i.e., they obtain solar energy transformed once (by plants) or twice (first by plants, then by herbivores). An energy flow passes through each trophic level, and the output of one level is the input of another. Energy flows through trophic-web levels at different rates. A useful characteristic of this flow is transfer (turnover) time = biomass / net productivity. For algae, transfer time is about several days; for steppe ecosystems, 3 years; for forests, 25 years. In forest litter, transfer time ranges from 3 months in humid tropical forest to 100 years in mountain coniferous forest. Experiments introducing radioactive tracers into ecosystems, whose transfer through trophic webs can be tracked, make it possible to observe the movement of matter through ecosystems over time. The cycling of matter in an individual ecosystem is linked to matter cycling in the biosphere as a whole. People sometimes speak of small and large cycles of matter exchange. 3.10. The Nature and Characteristics of Communities
D. Shabanov, M. Kravchenko. Ecology: The Biology of Interactions Chapter 3. Biogeocoenology and Community Ecology
D. Shabanov, M. Kravchenko. Ecology: Biology of Interactions Section 3. Biogeocenology and Ecology of Communities
3.12. Ecological Efficiencies