Syllabus Edition
First teaching 2015
Last exams 2025
Spatial & Temporal Changes in Communities
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
Zonation & Succession
Zonation
Zonation in ecology refers to the gradual change in the composition of species and communities across a landscape, based on a gradient of environmental factors such as:
Altitude
Latitude
Temperature
Moisture
Light
As these factors change, the species present in an ecosystem also change, leading to distinct zones or bands of organisms that can be observed in the ecosystem
This process occurs due to the interactions between the physical environment and the biological components of an ecosystem
An example of zonation can be observed in a rocky intertidal zone, where the physical and biological characteristics of the ecosystem change gradually from the high tide mark to the low tide mark
At the highest point (sometimes referred to as the spray zone), the zone is usually dry and dominated by lichen and other hardy plants that can withstand long periods of exposure to air and sunlight
In the high tide zone, the environment becomes more hospitable for other organisms such as barnacles, mussels, chitons, limpets and sea snails that can attach themselves to the rocks and withstand waves
Further down towards the low tide zone, the environment becomes even more favourable for marine organisms such as sea stars, anemones, and sea urchins that require the constant presence of water
An example of zonation in a rocky intertidal zone
Succession
Ecosystems are dynamic, meaning that they are constantly changing
Sometimes, ecosystems change from being very simple to being relatively complex
This process is known as succession
During succession, the biotic conditions (i.e. the living factors) and the abiotic conditions (i.e. the non-living factors) change over time
Succession is the process that occurs when newly formed or newly exposed land (with no species present) is gradually colonised (inhabited) by an increasing number of species
This new uninhabited land can be created in several ways. For example:
The magma from erupting volcanoes cools and often leads to the formation of new rock surfaces or even new rocky islands in the sea
Another way new land can be exposed is by sea-level dropping or the drying up of a lake, leaving areas of bare rock
Succession does not only occur on bare rock. Any barren terrain that is slowly being colonised by living species is undergoing primary succession. For example:
Sand dunes in coastal areas
Marram grasses are the pioneer species in these environments as they have deep roots to access water that other plants can't reach and they are also able to tolerate the salty environment i.e. the high concentrations of sodium and calcium ions caused by sea spray
Succession Occurs in a Series of Stages
Firstly, seeds and spores that are carried by the wind land on the exposed rock and begin to grow
These first species to colonise the new land (often moss and lichens) are known as pioneer species
As these pioneer species die and decompose, the dead organic matter (humus) forms a basic soil
Seeds of small plants and grasses, sometimes also carried in the wind or sometimes transported other ways (e.g. in bird faeces) land on this basic soil and begin to grow (these smaller plants are adapted to survive in shallow, relatively nutrient-poor soils)
As these small plants and shrubs die and decompose, the new soil becomes deeper and more nutrient-rich
The roots of these small plants and shrubs also form a network that helps to hold the soil in place and prevent it from being washed away
Larger plants and shrubs, as well as small trees, that require deeper, more nutrient-rich soil, can now begin to grow
These larger plants and small trees also require more water, which can be stored in deeper soils
Finally, the soil is sufficiently deep, contains enough nutrients and can hold enough water to support the growth of large trees
These final species to colonise the new land become the dominant species of the now relatively complex ecosystem
The final community formed, containing all the different plant and animal species that have now colonised the new land, is known as the climax community
Succession is the process of ecosystem change over time, beginning with newly formed or newly exposed land
Changes Occurring During Succession
As the the structure and species composition of an ecosystem changes during succession, so do the patterns of energy flow, productivity, diversity, and mineral cycling that ecosystem
Energy Flow:
During the early stages of succession, the energy flow in the ecosystem is relatively low
This is because there are only a few species present, and most of the energy is used to build biomass
As the ecosystem becomes more complex, energy flow increases
Gross and Net Productivity:
During the early stages of succession, gross productivity and net productivity is low because there are only a few species present so the ecosystem’s overall gain in energy and biomass per unit area per unit time is relatively small
As the ecosystem becomes more complex, gross productivity and net productivity increase
Diversity:
Diversity refers to the number of species present in an ecosystem
During the early stages of succession, diversity is low because there are only a few species present
As the ecosystem becomes more complex, diversity increases because there are more niches available, and more species are able to coexist within the same habitats in the ecosystem
Mineral Cycling:
Mineral cycling refers to the movement of nutrients through an ecosystem
During the early stages of succession, mineral cycling is relatively simple as there are only a few species present and nutrient cycling is largely driven by abiotic processes
As the ecosystem becomes more complex, nutrient cycling becomes more complex because there are more species present and each species has unique nutrient requirements and cycling processes
Examiner Tips and Tricks
You could be presented with an example of succession other than the one provided here e.g. succession on a sand dune. As long as you understand the principles of the stages of succession you should be able to apply your knowledge to any example that an exam question might throw at you.
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