14.1 Essential ideas

14.1.2 Communities and ecosystems

food webFigure 14.1.2a – A food web from a marine ecosystem

Food webs are more realistic representations of feeding relationships than food chains. From the example shown here, it is clear that most species occupy different trophic levels in multiple food chains. 

Productivity and respiration

You should recall from 4.1.2 Energy flow that ecosystems are closed systems in which energy, but not matter, is exchanged with the surroundings.

  • The amount of energy converted into biomass at each trophic level is small compared with the amount of energy available. This is because all consumers create heat energy through respiration. Heat is energy that cannot be converted to biomass and is lost to the surroundings.
  • Productivity is the term given to biomass accumulation in a community, measured in units of energy, kjm-2yr-1.
  • Plants produce biomass through photosynthesis. The amount of biomass accumulated by autotrophs is called primary productivity.
  • Animals produce biomass by absorbing food through digestion. The amount of biomass accumulated by heterotrophs is called secondary productivity.
  • Organisms convert some of the energy acquired to heat during respiration. So we make a distinction between gross and net productivity as follows: 

Net productivity = gross productivity – energy lost in respiration

As a community develops, the rate of net secondary productivity affects total productivity of the system, due to increases in respiration. A community comprising mostly plants has high net productivity and low biomass. As more heterotrophs are added to the community, the net productivity of the system decreases as more biomass is added and respiration rates increase. Eventually, a community becomes stable when the productivity to respiration ratio (P/R) approaches 1. This is illustrated in Figure 14.1.2b.

relationship biomass accumulation, respiration, gross productionFigure 14.1.2b – Development of a stable community
The relationship between biomass accumulation, respiration and gross productivity. In a stable community, the ratio of productivity to respiration is close to 1.

Succession and disturbance

Ecological succession is the process through which a community develops from a simple one containing a few pioneer species, into a biodiverse and mature climax community.

  • Primary succession occurs when a new habitat is colonised for the first time (e.g. bare rock exposed by retreating ice is populated with lichens and mosses). It may take thousands of years for a pioneer community to develop into a climax community.
  • Secondary succession occurs when a climax community is disturbed, either naturally (e.g. by strong winds or a forest fire) or through human interference (e.g. abandoning agricultural land). The resulting pioneer community may develop into a climax community over a span of 150 years.

succession timelineFigure 14.1.2c – A secondary succession timeline

Secondary succession occurs after a disturbance in a climax community. Climax communities emerge rapidly because ground soil is already well developed.

Climax communities have the following structure:

  • Rates of productivity and respiration fluctuate, but are relatively stable (P/R = 1).
  • Total amount of biomass is high.
  • Species are highly adapted – they occupy narrow niches.
  • Food webs are complex.
  • Biodiversity is high.
  • Soil is deep and nutrient-rich, containing layers of detritus, and efficient decomposers.
  • Nutrients are cycled within the system. 

Disturbances alter the structure of ecosystems by exposing new habitats or interfering with existing climax communities. After a disturbance, the rate of change in productivity and respiration is likely to increase until a new climax community is established.

Climate influences ecosystem development

The type of stable ecosystem that emerges in an area can be predicted based on climate patterns in the area.

A climograph shows the climactic conditions of temperature and precipitation in each of the world’s major biomes. 

climographFigure 14.1.2d – Climograph

Activity 1

Activity 2

Define the following terms:

  • gross primary productivity (GPP)
  • net primary productivity (NPP)
  • gross secondary productivity (GSP)
  • net secondary productivity (NSP).

Tip

The P in the P/R ratio refers to gross productivity. This means that when (P/R)>1 biomass accumulates, and when (P/R)<1 biomass is depleted. The diagram below shows how gross productivity PG, and respiration R, are related in the secondary succession of a forest. The area between the two lines represents net productivity PN.

net productivity

Figure 14.1.2e – Net productivity

Did you know?

Certain animals are more efficient at generating biomass than others. Homeotherms, like mammals, are animals that use cellular respiration to maintain a constant internal body temperature. Poikilotherms (see Figure 14.1.2f), like reptiles and amphibians, are animals that live with varying body temperatures – they do not lose much heat during respiration and are able to convert food to biomass more efficiently.

poikilothermFigure 14.1.2f – Poikilotherm

Nature of Science

Models are simplified representations of the real world. A pyramid of energy is a model of how energy flows through an ecosystem Food webs attempt to represent all the trophic relationships – but many organisms occupy more than one trophic level. Do trophic levels really exist, or are they simply useful inventions for explaining phenomena?

Activity 3

Determine the range of conditions for which each of the biomes will form.