How do organisms depend on one another and on the flow of energy and nutrients?
Interdependence: ecological niche, competition, predation and herbivory, energy flow through food chains and webs, the recycling of nutrients, and ecological succession.
An SQA Higher Environmental Science answer on interdependence, covering the ecological niche, competition, predation and herbivory, energy flow and trophic levels, nutrient cycling, and primary and secondary succession towards a climax community.
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What this dot point is asking
The SQA wants you to explain how organisms in an ecosystem depend on one another through the ecological niche, competition, predation and herbivory, how energy flows through food chains and webs and is lost at each level, how nutrients are recycled, and how communities change over time through succession. This is the conceptual core of the Living Environment.
The ecological niche
When two species have overlapping niches, they compete for the same limited resources. Interspecific competition is between different species; intraspecific competition is within one species and is usually more intense because the individuals need exactly the same resources. Strong competition can exclude one species or force species to use slightly different resources, which reduces overlap.
Predation and herbivory
Predation (a predator eating prey) and herbivory (an animal eating plants) are interactions that link organisms in a food web and regulate population sizes. Predator and prey numbers often follow a cyclical pattern: prey increase, predators then increase as food is plentiful, the rising predator numbers reduce the prey, and the predators then fall, allowing the prey to recover. These interactions are biotic factors that help control abundance.
Energy flow through food chains and webs
Energy enters an ecosystem when producers (green plants and algae) capture light energy in photosynthesis. It then flows along a food chain through trophic levels: producer, primary consumer (herbivore), secondary consumer, and so on. A food web shows the many interconnected chains in a real community.
The energy is lost because organisms respire (releasing heat), and because not all of an organism is eaten or absorbed (faeces, bones and roots carry energy away). Only the energy stored as new body tissue can be eaten by the next level. A pyramid of energy shows this loss, narrowing towards the top.
Recycling of nutrients
Energy flows through and is lost, but nutrients are recycled within the ecosystem and reused.
- In the carbon cycle, photosynthesis fixes carbon dioxide into organic compounds; respiration, decomposition and combustion return it to the atmosphere.
- In the nitrogen cycle, nitrogen-fixing bacteria convert nitrogen gas into usable forms, plants take up nitrates, and decomposers break down dead matter and waste, returning nitrogen to the soil; denitrifying bacteria return nitrogen gas to the air.
Decomposers (bacteria and fungi) are central: by breaking down dead organisms and waste, they release the locked-up nutrients back into the soil and air so that producers can reuse them.
Ecological succession
- Primary succession starts on bare ground with no soil, such as bare rock or a new sand dune. Pioneer species (lichens, mosses) tolerate the harsh conditions and, as they die, build up soil, changing the habitat so later species can colonise.
- Secondary succession occurs where a community has been disturbed (for example after a fire) but soil remains, so it proceeds faster.
At each stage the existing species modify the environment, making it suitable for the next group, until a climax community is reached that is in balance with the prevailing conditions.
Examples in context
Example 1. The lynx and snowshoe hare cycle. Long-term Canadian trapping records show snowshoe hare and lynx numbers rising and falling in linked, repeating cycles roughly a decade apart. The hares rise first, the lynx follow as food is plentiful, then heavy predation and food shortage cut the hares, and the lynx fall in turn, a textbook predator-prey relationship.
Example 2. Succession on Surtsey. Surtsey, a volcanic island that emerged off Iceland in 1963, has been colonised step by step: first microbes and mosses on bare lava, then vascular plants as soil formed, then nesting birds whose droppings enriched the soil further. It is a living record of primary succession from pioneer species towards a developing community.
Try this
Q1. Explain why decomposers are essential to an ecosystem. [2 marks]
- Cue. They break down dead organisms and waste, releasing nutrients back to the soil and air for producers to reuse.
Q2. A food chain has five links. Suggest why chains rarely have more links than this. [2 marks]
- Cue. About 90 percent of energy is lost at each level, so too little remains to support a further level.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher specimen4 marksExplain why the amount of energy available decreases at each successive trophic level in a food chain.Show worked answer →
A 4-mark explain answer needs several reasons energy is lost between levels.
Only a fraction of the energy in one trophic level is passed to the next. Energy is lost because not all of an organism is eaten or digested (bones, roots and faeces are not absorbed), and some material is lost as undigested waste.
Living organisms also use energy in respiration to power movement, growth and other life processes, and this energy is released as heat to the surroundings rather than being stored as new tissue.
Because only the energy stored as new body tissue can be passed on, roughly 10 percent transfers between levels. This is why food chains rarely have more than four or five links and why top predators are few.
Markers reward losses through uneaten and undigested material, losses as heat from respiration, the idea that only stored tissue is passed on, and the consequence for chain length.
SQA Higher specimen3 marksDescribe what is meant by ecological succession, and explain the role of pioneer species in primary succession.Show worked answer →
This is a 3-mark describe-and-explain answer about succession.
Succession is the gradual, directional change in the community of species occupying an area over time, leading towards a stable climax community.
In primary succession, which begins on bare ground with no soil (such as bare rock or a new sand dune), pioneer species (for example lichens and mosses) are the first colonisers. They tolerate the harsh conditions and, as they grow and die, they break down the surface and add organic matter, beginning to form soil.
This change in conditions allows other species to colonise, so the pioneers modify the habitat and make it suitable for the next stage.
Markers reward a correct definition of succession, what pioneer species are, and the idea that they modify the environment to allow later species.
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Sources & how we know this
- Higher Environmental Science Course Specification (C826 76) — SQA (2021)
- Higher Environmental Science course overview and resources — Qualifications Scotland (2026)