Biotic and abiotic components

Overview

Biotic components of an ecosystem and vertical structure of a temperate deciduous woodland ecosystem.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is an ecosystem?

   Model answer

   An ecosystem is a community of living organisms together with the non-living physical environment they occupy, and the interactions and flows of energy and matter between them. It is a system, so it has inputs, outputs, stores and transfers, and it functions as a whole rather than as a collection of separate parts. Ecosystems exist at any scale, from a rotting log or a pond up to a whole biome.

2. 2

   What are the biotic components of an ecosystem?

   Model answer

   The biotic components are all the living parts and the dead organic matter derived from them. They comprise the producers, which are the green plants that make their own food, the consumers, which are the animals that feed on plants or on other animals, and the decomposers, which are the bacteria, fungi and detritivores that break down dead material. Together they form the food webs of the ecosystem, and their remains form the litter that supplies the soil with organic matter.

3. 3

   What are the abiotic components of an ecosystem?

   Model answer

   The abiotic components are the non-living physical and chemical conditions. They include climate - temperature, precipitation, sunlight, wind and humidity; soil - its depth, texture, structure, pH, nutrient content, moisture and aeration; relief - altitude, slope angle and aspect; the underlying geology and parent rock; and water and drainage conditions. These set the physical limits within which the biotic community must operate.

4. 4

   How do biotic and abiotic components interact in an ecosystem?

   Model answer

   The relationship runs both ways. Abiotic conditions determine which species can survive: temperature and light govern the rate of photosynthesis, soil depth and moisture govern rooting and water supply, pH governs nutrient availability, and aspect governs the warmth and light a slope receives. In turn the biotic components modify the abiotic environment: a tree canopy reduces light, temperature range and wind speed and raises humidity beneath it, leaf litter builds soil depth, organic matter and moisture retention and alters pH, roots bind and aerate the soil and draw nutrients up from depth, and burrowing animals mix it. This two-way interaction is why the environment inside a wood is measurably different from that of the open field beside it.

5. 5

   Describe the vertical structure of a temperate deciduous woodland ecosystem.

   Model answer

   There are four broad layers. The canopy, generally between about fifteen and thirty metres up, is formed by the crowns of the dominant trees such as oak, ash and beech, and it intercepts most of the light and rainfall. Beneath it is the sub-canopy or understorey of smaller and younger trees such as holly, hazel, rowan and hawthorn, tolerant of partial shade. Below that is the shrub layer of woody bushes, brambles and saplings. At the bottom is the herb or field layer of grasses, ferns and flowering plants, and a ground layer of mosses, fungi and leaf litter on the soil surface. The layering exists because light is progressively absorbed with depth, so each stratum is occupied by species adapted to the light level available there.

6. 6

   Why do woodland herbs such as bluebells flower in early spring?

   Model answer

   Because the abiotic conditions on the woodland floor change dramatically with the season. Before the deciduous canopy comes into leaf, light reaches the ground almost unimpeded, and the soil is moist and beginning to warm. Once the canopy closes in late spring, it intercepts the great majority of the light and the trees also draw heavily on soil moisture, so conditions at ground level become too shaded for these species to photosynthesise effectively. They therefore complete their growth, flowering and seed setting in the few weeks of the spring window and survive the rest of the year as bulbs or underground storage organs.

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