Interdependence Among Organisms in an Ecosystem
Senior 3 · Biology
Interdependence Among Organisms in an Ecosystem
Lesson 3 of 5
25 min
A farmer who sows maize or beans far too thickly, hoping for a bigger harvest, usually gets thin, weak plants with small cobs or few pods. Each plant needs light, water and mineral salts, and a crowded field does not have enough for all of them. The plants are competing, and they are all members of the same species. In the same field, weeds compete with the crop too.
Organisms of the same species and of different species affect each other in many ways. This lesson sorts those relationships and looks closely at the first of them: competition.
Definition
Intraspecific relationship: an interaction between individuals of the same species. Example: two bean plants in the same row; worker bees in one hive.
Interspecific relationship: an interaction between individuals of different species. Example: a lion and an impala; weeds and maize.
Competition: an interaction in which organisms struggle for the same resource that is in short supply, so that each gets less than it needs. Both sides are harmed.
Resource: anything an organism uses that can run short: light, water, mineral salts and space for plants; food, water, mates, nesting sites and territory for animals.
Ecologists describe each relationship by its effect on the two partners: + the partner benefits, − it is harmed, 0 it is not affected.
| Relationship | Effect on species 1 | Effect on species 2 | Example | Lesson |
|---|---|---|---|---|
| Competition | − | − | weeds and maize | this lesson |
| Predation | + (predator) | − (prey) | lion and impala | 4 |
| Grazing | + (grazer) | − (plant) | zebra and grass | 4 |
| Parasitism | + (parasite) | − (host) | tick and cow | 4 |
| Mutualism | + | + | bean plant and nitrogen-fixing bacteria | 5 |
| Commensalism | + | 0 | cattle egret and cattle | 5 |
| Amensalism (and allelopathy) | 0 | − | a mould and the bacteria it kills | 5 |
| Neutralism | 0 | 0 | two species that do not affect each other | 5 |
Members of one species share the same needs, so they are each other's closest competitors: intraspecific competition. Seedlings crowd each other for light; male impalas fight for territories and mates. But members of a species also cooperate. In a honeybee colony, thousands of worker bees collect food, feed the young and defend the hive for the whole colony. Chimpanzees live in communities that share a territory and defend it together; Nyungwe has three semi-habituated chimpanzee communities that visitors can track.
Definition
Intraspecific competition: competition between members of the same species. It is usually intense, because they need exactly the same resources.
Interspecific competition: competition between members of different species that use some of the same resources.
Competitive exclusion principle: two species that need exactly the same resources cannot live together in the same place for long; the better competitor eventually drives the other out.
Resource partitioning: the sharing out of a resource between species that use different parts of it, which lets them live together. Example: grazers that eat different heights of the same grass.
Aim: to find out how crowding affects the growth of bean plants.
You need: three equal seed trays of the same soil, 65 bean seeds of the same variety, a ruler, water, and access to a drying oven and a balance.
Figure 1: The three trays, seen from above. Only the number of plants changes; everything else is kept the same.
| Tray | Number of plants | Total dry mass / g | Mean dry mass per plant / g |
|---|---|---|---|
| A | 5 | 15.0 | ? |
| B | 20 | 36.0 | ? |
| C | 40 | 40.0 | ? |
Illustrative results, of the kind a class obtains.
Warning
Safety. Wash your hands after handling soil. Only your teacher should load and empty the drying oven: the trays and plants are hot. Do not eat beans used in experiments.
Worked example 1: analysing the bean results
Complete the last column of the table and explain the pattern.
Figure 2: Mean dry mass per plant falls as crowding increases (illustrative data).
In the 1930s the Russian biologist Georgy Gause grew two species of Paramecium, single-celled organisms that feed on bacteria, in tubes of culture liquid. Grown in separate tubes, both species did well (Figure 3). Grown together in one tube, only one survived (Figure 4).
Figure 3: Each species grown alone: both populations grow and level off (redrawn to show the pattern; illustrative numbers).
Figure 4: The two species grown together: P. caudatum dies out (redrawn to show the pattern; illustrative numbers).
Worked example 2: interpreting Gause's results
Use Figures 3 and 4. (a) What was the largest population of P. caudatum alone, and together with P. aurelia? (b) Explain the difference. (c) What principle does this show?
Why this matters: in nature, species that seem to share a resource usually divide it. On a savanna, some grazers crop tall, coarse grass while others take the short, leafy regrowth; birds in one tree feed in different layers of its crown. This resource partitioning reduces interspecific competition.
GCSE Biology: Competition and Interdependence (Cognito). Watch for the resources that plants and animals compete for, and how a change in one population affects the others.
Tip
Exam technique. Always name the resource being competed for, and say whether the competition is intraspecific (same species: "intra" = within) or interspecific (different species: "inter" = between, as in an inter-school match).
Practice: relationships and competition
Key points
Sources and credits
Written by Ganzaa: the relationships table, the bean investigation and its illustrative results, Figures 1 to 4 (Figures 3 and 4 redrawn to show the pattern of Gause's results, with illustrative numbers), the worked examples and the practice questions.
