Biology 12

13.2 The Structure of Ecosystems

440 MHR • Unit 5 Population Dynamics

As mentioned in section 13.1, ecosystems contain

both biotic and abiotic components. Each

ecosystem consists of all the organisms in one or

more communities, as well as the physical and

chemical factors affecting them. The boundaries of

ecosystems are not distinct — one ecosystem may

overlap or exist inside another. There are terrestrial

(land-based) and aquatic (water-based) ecosystems,

and ecosystems that contain both land and water.

An ecosystem can be small (such as the one shown

in Figure 13.14), or as large as the biosphere — a

global ecosystem.

Figure 13.14Small insects fall into the pitcher plant’s cup-

shaped leaves, drown, and decompose. This plant, which

typically lives in nitrogen-poor environments, extracts the

nutrients it requires from the bodies of insects. The pitcher

plant and its surroundings comprise a small ecosystem.

Despite the wide range in sizes and types of

ecosystems, the same basic processes take place

in all of them. Two processes are particularly

important: energy flow and chemical cycling. The

proper functioning of these processes is vital to the

survival of organisms in the ecosystem and to the

integrity of the ecosystem itself.

Trophic Structure

When ecologists refer to the trophic structureof an

ecosystem or community, they are describing the

feeding relationships among its members. Each

species is assigned to a specific trophic levelin

the structure, depending on its main source of

nutrition. Most ecosystems have several trophic

levels through which energy flows and chemicals

(matter) cycle.

The first (or lowest) trophic level consists of

autotrophic organisms. Autotrophs are organisms

that can make energy-rich organic molecules (such

as glucose) from the raw materials available in the

environment. They then break these “homemade”

organic molecules down during cellular respiration

to provide the energy that fuels the rest of their life

processes. Photosynthetic autotrophs use the

energy of the Sun to drive this manufacturing

process. Almost all plants, as well as some types

of protists (algae) and bacteria (cyanobacteria),

are photosynthetic autotrophs.

All organisms need energy to drive cellular

processes. They must, therefore, have a source of

organic molecules from which they can release this

energy during cellular respiration. Autotrophs,

which are at the first trophic level, produce organic

molecules; this makes the first level the most

important. Because the first trophic level supports

all life at the higher levels, autotrophs

are referred to as the primary producersin an

ecosystem. This first trophic level provides all the

potential energy required to drive the other levels

in the ecosystem.

All organisms in the trophic levels above this

one are heterotrophs. Heterotrophs are unable to

EXPECTATIONS

Describe what is meant by the trophic structure of a community.

Describe the ecosystem roles of producers, consumers, and decomposers.