Energy flows and nutrient cycles in ecosystems - Fundamentals of the Physical Environment - page 660

Chapter 22
Energy flows and nutrient cycles in
ecosystems

As well as being distinctive assemblages of plants and animals, ecosystems also carry out
work. Solar energy is captured by plants and passed on to other organisms. All life
ultimately depends upon the sun's energy through the process of photosynthesis.
Together with the flow of energy, ecosystems also depend upon the circulation of
sufficient nutrients to maintain the healthy vegetative growth. Nutrients enter ecosystems
from the atmosphere and from rock weathering, and can exit the ecosystem into the
atmosphere and into drainage water. Nutrient cycles thus link the air, the rocks and the
soils of the abiotic environment with the organisms of the biotic component of the
ecosystem. Although each nutrient element has its own unique cycle, it is possible to
generalize about different types of nutrient cycle. The processes involved in energy flow
and nutrient cycling are analysed in the present chapter.

GENERAL PRINCIPLES OF ENERGY FLOW

The growth of interest during the twentieth century in the study of the world's
ecosystems brought new ideas, concepts and models into the ways that humans study
nature. Central to investigations of ecological systems is the need to understand their
structure and function at various size levels, ranging from simple, local communities to
the global biosphere as a whole. In the 1960s the US ecologist E. P. Odum suggested that
ecology could best be defined as 'the study of the relationships between structure and
function in nature'. Table 22.1 simplifies the major items which are studied under the two
fundamental headings of structure and function.
The behaviour of energy in ecosystems is referred to as 'energy flow' because energy
transformations are directional, in contrast to the cyclical behaviour of nutrients. Green
plants photosynthesize organic compounds from water and carbon dioxide, using incident
solar radiation as the energy source. Solar energy is thereafter fixed in a chemical form in
the photosynthates until released as thermal energy during the respiration of plants and
animals and the decomposition of organic matter. Total organic material fixed by
photosynthesis over a unit time is called the gross production or gross primary
productivity (GPP). The proportion which remains after respiration losses in the plant is
termed net production or net primary productivity (NPP). The organic matter

Table 22.1 The subject matter of ecology

Structure 1 Composition of the biological community Species, numbers, biomass, etc.

2 Quantity of abiotic materials Nutrients, water, etc.