Interdependence Among Organisms Within Their Environment
Senior 5 · Biology
Interdependence Among Organisms Within Their Environment
Lesson 1 of 4
25 min
In a maize field in Musanze, a farmer drops two or three seeds into each planting hole. A few weeks later she pulls out all but one or two seedlings in each hole and throws away plants that look perfectly healthy. She is not wasting them. Seedlings that stay crowded share the same light, water and nitrate, and each one ends up small with a poor cob. This is competition, and it is only one of the ways in which organisms affect each other. In Senior 3 you met food webs and named these relationships. In this unit you analyse them as an ecologist does: what each partner gains or loses, what evidence shows it, and how it controls the size and distribution of populations.
Definition
Interspecific relationship: an interaction between individuals of different species, such as a lion and a buffalo. Intraspecific relationship: an interaction between individuals of the same species, such as two maize plants in one hole.
Symbiosis (in the broad sense): a close, usually long-lasting association between two different species. It includes mutualism, commensalism and parasitism.
Competition: an interaction in which two or more organisms need the same resource, and that resource is in short supply, so each one gets less than it would alone. Effect: harmful (−) to both.
Ecologists describe each interaction by its effect on the growth, survival or reproduction of each partner: + (benefits), − (is harmed) or 0 (is not significantly affected).
| Interaction | Species A | Species B | Example |
|---|---|---|---|
| Competition | − | − | Lions and spotted hyenas taking the same prey in Akagera |
| Predation | + (predator) | − (prey) | A leopard killing a bushbuck |
| Parasitism | + (parasite) | − (host) | A tick feeding on the blood of a cow |
| Mutualism | + | + | Rhizobium bacteria in the root nodules of a bean plant |
| Commensalism | + (commensal) | 0 (host) | An orchid growing on the branch of a forest tree |
| Amensalism | 0 | − | Penicillium mould releasing penicillin that kills nearby bacteria |
Saprophytism is the odd one out: a saprotroph such as a mushroom feeds on dead organic matter, so there is no living partner to be harmed. It is still a form of dependence, because the saprotroph needs the bodies and wastes of other organisms, and they in turn need the minerals it releases.
These interactions do more than decide who eats whom. They:
Definition
Intraspecific competition: competition between members of the same species for a limited resource (food, light, water, space, mates).
Interspecific competition: competition between members of different species for a limited resource that both need.
Scramble competition: every competitor gets some of the resource, but less than it needs (crowded maize seedlings sharing nitrate). Contest competition: some individuals win the whole resource and others get none (a male defending a territory).
Members of one species need exactly the same resources, so intraspecific competition is usually the most intense competition of all. As density rises, each individual's share falls. The results are:
Because its strength rises with density, intraspecific competition is the main reason a population levels off at the carrying capacity.
Worked example 1: crowding maize
An agronomy trial (illustrative data) sowed maize at four densities and weighed the dry grain from each square metre at harvest.
| Plants per m² | Grain per m² / g |
|---|---|
| 2 | 600 |
| 4 | 1 000 |
| 8 | 1 200 |
| 16 | 1 200 |
(a) Calculate the mean grain mass per plant at each density. (b) Explain the pattern.
Use: trials like this tell farmers the spacing that gives the most grain for the least seed.
Two species compete only if they share a resource that is in short supply. How strongly they compete depends on how much their ways of life overlap, which ecologists describe with the idea of a niche.
Definition
Ecological niche: the role of a species in its ecosystem: the full range of conditions it can tolerate (temperature, pH, moisture) and the resources it uses (food, space, time of activity), together with its interactions with other species.
Fundamental niche: the whole niche a species could occupy if no competitors or predators were present. Realised niche: the part of the fundamental niche it actually occupies when other species are present. The realised niche is never larger than the fundamental niche.
Competitive exclusion principle (Gause's principle): two species with identical niches cannot coexist indefinitely in the same place when a resource is limiting; one will out-compete and eliminate the other.
In the early 1930s the Russian ecologist G. F. Gause grew two ciliates, Paramecium aurelia and Paramecium caudatum, in tubes of nutrient liquid with bacteria as food. Grown in separate tubes, each species increased and then levelled off. Grown together in one tube, P. aurelia persisted but P. caudatum died out within about 16 days: P. aurelia took the bacteria faster. When Gause instead paired P. caudatum with P. bursaria, both survived, because the two species fed in different parts of the tube. Their niches did not overlap completely.
On rocky shores in Scotland, J. H. Connell (published 1961) studied two barnacles. Small Chthamalus lives high on the shore; larger Balanus lives lower down. When Connell scraped Balanus off some rocks, young Chthamalus survived in the lower zone too. Where Balanus was left, it grew over, crushed or undercut the Chthamalus. So the lower limit of Chthamalus is set by competition, and its upper limit by drying out at low tide, which Balanus tolerates less well.
Many similar species do live together. They avoid exclusion by resource partitioning: dividing a resource so that their realised niches differ. In a classic study, R. H. MacArthur (1958) found five species of warbler feeding in the same spruce trees in North America, each searching mainly in a different part of the tree. On the savanna, zebras crop the tall, coarse grass, while wildebeest and Thomson's gazelles graze the shorter, leafier grass beneath it, a pattern well studied in the Serengeti.
Definition
Resource partitioning: the division of a limited resource among species with similar needs, by feeding in different places, at different times or on different sizes of food, so that their niches overlap less and they can coexist.
Worked example 2: reading Gause's result as data
Illustrative counts of P. caudatum per 0.5 cm³ of culture:
| Day | 0 | 4 | 8 | 12 | 16 |
|---|---|---|---|---|---|
| Alone | 2 | 40 | 120 | 130 | 130 |
| With P. aurelia | 2 | 35 | 60 | 25 | 0 |
(a) Calculate the percentage by which the mixed-culture population on day 8 is below the population alone. (b) Explain the difference between the two cultures after day 8.
Warning
Common mistakes
CrashCourse: "Community Ecology: Interspecies Interactions" (Crash Course Biology #6). Watch for competition between species, the niche, and how species divide resources. The video also previews predation and symbiosis, which the next lessons study in detail.
Tip
Exam technique: when a question shows two population curves, first say what each species does alone (this shows intraspecific competition and the carrying capacity), then say what changes together. Name the resource and the principle: "competitive exclusion, because the niches overlap completely".
Practice: interrelationships and competition
Key points
Sources and credits
Figures 1 and 2, the tables, the worked examples and the practice questions are written by Ganzaa. The shapes in Figure 1 follow Gause's published curves; the numbers in both worked examples are illustrative.
