This blog post on ‘Ecosystem structure and energy flow’ is part of the BES ‘Key Concepts in Ecology’ blog series, designed to help ecologists in learning the key topics in ecology. See the full blog series for a list of key topics that you might typically find in an ecology textbook, each providing a short introduction to the topic, and a list of suggested papers for students to refer to.
All life depends on energy, and ecosystems are structured by the flow of energy through different organisms. Energy from the sun is captured by primary producers, largely through photosynthesis by plants and algae, and the captured energy flows through different trophic levels as a result of decomposition or consumption of organisms at a lower level (e.g. plants, herbivores) by those at a higher level (e.g. predators, parasites).
Primary production on land and in the oceans varies depending on the traits of the photosynthetic organisms involved and on the availability of limiting resources such as light, nutrients and water. Being able to explain variation in primary productivity by using easily measured organismal traits rather than knowing the identity of the species involved is useful as it simplifies production estimation (Jänesa et al. 2016). Nitrogen pollution in the atmosphere is caused by human activity and can provide additional nutrients than those available in the soil. Indeed, nitrogen from the atmosphere deposited on the canopy of a Sitka spruce forest, and absorbed directly by the canopy, may be enough to support the entire crop of new leaves grown every year (Ferraretto et al. 2022).
Species from different trophic levels interact with each other through consumption. Trophic chains are linear interactions between organisms where, for example, a plant is eaten by a herbivore which in turn is eaten by a predator. Trophic webs are more complex sets of interactions including intraguild predation, where two consumers within the same trophic level both compete for shared resources and engage in predator-prey interactions and apparent competition between prey species which do not compete for the same resources mediated through a shared predator. As the number and types of species within a community increases the number of potential interactions and therefore potential complexity of the food web also increases. However, Torres-Campos et al. (2019) found that the realized interactions were much simpler than predicted from all potential interactions, simplifying food web structure. Trophic structure can be fundamentally modified by humans; in urban areas top predators may be missing, accidental or intentional food subsidies may be provided which support smaller, opportunistic omnivores (El-Sabaawi 2018).
Biodiversity influences ecosystem function where local communities with more species tend to have higher productivity. It is commonly thought that resource partitioning, whereby different species take up different limiting resources in space or time, is a key mechanism governing the biodiversity-ecosystem function relationship. Jesch et al. (2018) found no evidence for below-ground resource partitioning and instead suggest that facilitation, biotic feedbacks or above ground resource partitioning may be important. The effects of biodiversity on ecosystem function also depend on the spatial and temporal scale at which this relationship is assessed (Barry et al. 2020). This means that relationships between biodiversity and ecosystem function observed in small scale experiments may not simply be extrapolated to larger scales. The types of plants in a producer community have a strong influence on energy flow through decomposition. For example, tall deciduous species in the Arctic will have quite different consequences for the cycling of carbon compared to evergreen dwarf shrubs (Vowles & Björk 2018). Therefore, predictions of ecosystem function under climate change depend on accurate predictions of vegetation change, including complex feedbacks between living and dead vegetation components and mycorrhizal partners.
Introduction written by Yvonne Buckley (Senior Editor, Journal of Ecology). Reading list curated by the BES journal Editors.
References and suggested reading
Drivers of primary production