Concept of an Ecosystem
Senior 6 · Biology
Concept of an Ecosystem
Lesson 1 of 5
25 min
Drive east from Kigali to Akagera National Park and you reach open grassland dotted with acacia trees, where zebra, impala and giraffe graze, and where lakes and papyrus swamps line the Akagera River. Drive south-west instead, to Nyungwe National Park, and you climb into cool, misty montane rainforest, with tree ferns, orchids and troops of chimpanzees and other primates. Rwanda is small, yet these two parks hold completely different communities of living things.
Why? In Senior 2 you met ecosystems, food chains and the difference between biotic and abiotic factors. This unit asks the A-level questions: which ecological factors decide what can live where, how energy and nutrients move through an ecosystem, and how people change it. This first lesson classifies ecosystems and analyses the factors that shape them.
Definition
Ecosystem: a community of organisms together with the non-living environment they interact with, linked by the flow of energy and the cycling of nutrients. Example: Lake Kivu, with its algae, fish and the water, light and dissolved minerals they depend on.
Community: all the populations of different species living in the same place at the same time.
Habitat: the place where an organism normally lives. Example: the forest floor and undergrowth of Nyungwe for the L'Hoest's monkey.
Ecological niche: the role of a species in its ecosystem: what it eats, what eats it, where and when it is active, and how it uses the abiotic conditions around it. Two species cannot share exactly the same niche for long in one place.
Biome: a large region of the Earth with a characteristic climate and vegetation, such as tropical rainforest or savanna.
Ecosystems are first divided by where they are: on land (terrestrial) or in water (aquatic). Aquatic ecosystems are then divided by salt content (freshwater or marine) and by whether the water is still or flowing. Wetlands sit between the two: the ground is waterlogged for all or part of the year.
Table 1: Types of ecosystems, with Rwandan examples
| Type | Main features | Example in or near Rwanda |
|---|---|---|
| Tropical montane rainforest (terrestrial) | High rainfall, cool because of altitude, several layers of trees, very high species diversity | Nyungwe National Park |
| Savanna (terrestrial) | Grassland with scattered trees; a long dry season; fire and grazing keep trees few | The plains of Akagera National Park |
| Agro-ecosystem (terrestrial) | A farmed ecosystem: people choose the producers and add water and nutrients | Tea estates, rice schemes in the valleys |
| Lentic freshwater (aquatic) | Still water: lakes and ponds, layered by temperature and light | Lake Kivu, Lake Ihema in Akagera |
| Lotic freshwater (aquatic) | Flowing water: rivers and streams, well oxygenated, organisms that cling or swim strongly | The Nyabarongo and Akagera rivers |
| Wetland | Soil waterlogged most of the year; plants adapted to low soil oxygen | The Rugezi wetland; the papyrus swamps of Akagera |
| Marine (aquatic) | Salt water, about 35 g of salt per litre; oceans, coral reefs, estuaries | The Indian Ocean coast of Tanzania and Kenya |
Every organism is affected by two groups of factors. Table 2 sorts them.
Table 2: Ecological factors
| Group | Factor | How it acts on organisms |
|---|---|---|
| Abiotic: climatic | Light | Intensity and day length control photosynthesis; in a forest, only a few per cent of the light reaches the floor |
| Temperature | Sets the rate of enzyme-controlled reactions; falls as altitude rises | |
| Water (rainfall, humidity) | Needed for photosynthesis and transport; low humidity increases transpiration | |
| Abiotic: edaphic (soil) | pH, texture, mineral ions, water and air content | Decide which plants can take up enough water and minerals; waterlogged soil has little oxygen for roots |
| Abiotic: topographic | Altitude, slope, aspect (the direction a slope faces) | Change temperature, drainage and soil depth over short distances |
| Biotic | Competition, predation, herbivory | Limit the size of populations and where they can live |
| Parasitism, mutualism, commensalism, human activity | Harm or help the organisms involved; people change every other factor |
Biotic interactions are named by who gains (+), who loses (−) and who is unaffected (0):
Definition
Abiotic factor: a non-living physical or chemical feature of the environment that affects organisms. Example: soil pH.
Biotic factor: the effect of other living organisms on an organism. Example: competition for grass.
Limiting factor: the factor in shortest supply relative to need, which therefore controls the rate of a process or the size of a population. Adding more of any other factor has no effect until the limiting factor is increased.
Range of tolerance: the range of values of a factor, between a lower and an upper limit, within which a species can survive.
If you count one species at many sites that differ in only one abiotic factor, such as temperature, the counts form a tolerance curve (Figure 1). This is the law of tolerance (V. E. Shelford, 1913): each species survives only between a lower and an upper limit of each factor, and does best within an optimum range.
Figure 1: A tolerance curve for temperature (illustrative). Green: optimum range. Cream: zones of physiological stress. Grey: zones of intolerance, where the species is absent.
Three points to read from Figure 1:
Species with a wide range of tolerance for many factors are widespread; species with narrow ranges are restricted to a few places. The species of Nyungwe that live only in the mountains of the Albertine Rift, such as the L'Hoest's monkey, are examples of narrow tolerance.
Liebig's law of the minimum (Justus von Liebig, 1840) states the same idea for resources: growth is controlled not by the total of resources available but by the scarcest one. A field with plenty of water and nitrogen still yields poorly if phosphate is scarce.
Warning
Three common mistakes
Worked example 1: altitude as an abiotic factor
In the lower atmosphere, air temperature falls with altitude by about for every climbed on average (the standard environmental lapse rate). The mean temperature at a valley site at is (illustrative). Estimate the mean temperature on a ridge at , and explain one effect on the vegetation.
Effect: enzyme-controlled reactions, including photosynthesis, run more slowly at , so plants grow more slowly, and only species whose range of tolerance extends this low can live on the ridge. This is one reason why the vegetation changes as you climb a mountain.
Condition: the lapse rate is an average; on a real day it varies with humidity and weather, so the answer is an estimate.
Worked example 2: finding the limiting factor
A student grew beans on four plots of an acidic field and measured the dry-bean yield (illustrative data).
| Treatment | Yield / tonnes per hectare |
|---|---|
| A: no fertiliser (control) | 1.2 |
| B: nitrogen added | 1.3 |
| C: phosphate added | 2.0 |
| D: nitrogen and phosphate added | 2.6 |
(a) Calculate the percentage increase in yield for B and C compared with A. (b) Identify the limiting factor on the unfertilised field and explain the results.
Table 3: Three Rwandan ecosystems and the factors that shape them
| Akagera (east) | Nyungwe (south-west) | Rugezi (north) | |
|---|---|---|---|
| Ecosystem type | Savanna with lakes and papyrus swamps | Montane rainforest, with bamboo and bogs | Highland wetland (a marsh on peat) |
| Key abiotic factors | Low altitude for Rwanda, warm, a long dry season | High altitude, cool, high rainfall and humidity | Waterlogged, acidic peat soil with little oxygen |
| Key biotic factors | Grazing, browsing and predation by large mammals | Shade from a closed canopy; seed dispersal by primates and birds | Dense sedges and grasses; birds that nest in the marsh |
| Examples of organisms | Acacia, grasses, papyrus, zebra, impala, giraffe, lion | Tree ferns, mahogany, orchids, chimpanzee, L'Hoest's monkey | Sedges, grasses, grey crowned crane |
Notice that fire and grazing, not rainfall alone, keep Akagera's grassland open: where large grazers and fire are removed, bushes and trees spread. You will meet this idea again as succession in Lesson 2.
CrashCourse: "Community Ecology: Feel the Love" (Crash Course Ecology #4). Watch for the competitive exclusion principle and resource partitioning, and link them to the definition of a niche above.
Tip
Exam technique. When a question asks you to "explain the distribution" of a species, name the factor, say which way it acts (too little or too much), and link it to a process in the organism: "At high altitude the temperature is lower, so enzyme activity and photosynthesis are slower, so growth is slower." Three linked steps earn the marks; naming the factor alone does not.
Practice: ecosystems and ecological factors
Key points
Sources and credits
Figure 1, Tables 1 to 3, the worked examples and the practice questions are written by Ganzaa. Figure 1 and the data in the worked examples are illustrative.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
