Ecosystems as open systems

Overview

Inputs to a small-scale ecosystem, outputs from a small-scale ecosystem and main stores in an ecosystem.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Why is an ecosystem described as an open system?

   Model answer

   Because both energy and matter cross its boundary. Solar energy enters and is eventually lost as heat, and water, gases, dissolved nutrients, seeds and mobile animals all move in and out. Nothing about an ecosystem is sealed off from its surroundings, and this is why events outside its boundary - pollution carried in by a river, a change in grazing pressure on adjoining land, or a change in climate - can alter it.

2. 2

   What are the inputs to a small-scale ecosystem?

   Model answer

   The main energy input is solar radiation, which is the only significant source of energy the system has and which producers convert into chemical energy by photosynthesis. Material inputs are precipitation, together with the dissolved nutrients it carries from the atmosphere; carbon dioxide and other atmospheric gases, including nitrogen fixed by bacteria; nutrients released from the parent rock by weathering; and the arrival of organisms - seeds, spores and animals moving in from surrounding areas.

3. 3

   What are the outputs from a small-scale ecosystem?

   Model answer

   Energy leaves the system as heat, lost at every transfer through respiration and never returning. Water is lost by evapotranspiration and by runoff and throughflow leaving the site. Nutrients are lost in solution by leaching down beyond the reach of roots and by runoff carrying them out of the system, and soil and organic matter are lost by erosion. Gases such as oxygen and carbon dioxide are exchanged with the atmosphere. Organic matter also leaves when animals move out or migrate, and where people are involved, when timber, crops or livestock are harvested and removed.

4. 4

   What are the main stores in an ecosystem?

   Model answer

   The three principal nutrient stores are biomass, meaning the nutrients held in living plants and animals; litter, meaning the nutrients in dead organic matter lying on or in the surface of the ground; and soil, meaning the nutrients held in the soil in a form available to plant roots. Energy is stored in the chemical bonds of living and dead organic matter, and water is stored in the soil, in the vegetation and on the surface. The relative size of the three nutrient stores differs sharply between ecosystems and is a good indicator of how the system works.

5. 5

   Name the main transfers within an ecosystem.

   Model answer

   Photosynthesis transfers solar energy into chemical energy held in plant tissue. Feeding transfers energy and matter along the food chain from producers to consumers. Litterfall transfers dead leaves, branches, remains and waste from the biomass store to the litter store. Decomposition transfers nutrients from litter into the soil in soluble form. Uptake by roots transfers nutrients from the soil back into the biomass. Weathering transfers nutrients from the parent rock into the soil, and precipitation transfers nutrients from the atmosphere into the system. Respiration transfers energy out as heat, and leaching and runoff transfer nutrients out in solution.

6. 6

   What is the essential difference between the way energy and the way nutrients move through an ecosystem?

   Model answer

   Energy flows through the system in one direction and is lost; nutrients cycle within it and are reused. Solar energy enters, is fixed by producers and is passed along the food chain, but at every step the great majority is given off as heat during respiration and that heat cannot be recaptured, so the system depends on a continuous fresh supply of sunlight. Nutrients, by contrast, are chemical elements that are not destroyed: they pass from soil to plant to animal to litter and back to the soil through decomposition, going round the same loop indefinitely, with only relatively small inputs from weathering and rainfall and small losses to leaching and runoff.

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