Interdependence Among Organisms in an Ecosystem
Senior 3 · Biology
Interdependence Among Organisms in an Ecosystem
Lesson 4 of 5
25 min
In Akagera, a herd of impalas grazes with heads down, but there is always one head up, watching. At one alarm snort the whole herd bounds away. A few kilometres outside the park, a farmer's cow grazes the same kind of grass, while ticks feed quietly on the cow's blood. The lion, the impala and the tick all feed on other living organisms, but in very different ways, with very different effects on the organism that is eaten.
Definition
Predation: a relationship in which one animal, the predator, kills and eats another animal, the prey. Effects: predator +, prey −. Example: a lion killing an impala.
Grazing: feeding by a herbivore on plants, usually grass, eating part of the plant without killing it. Grazer +, plant −. Animals that feed on the leaves of trees and bushes are called browsers.
Parasitism: a close relationship in which one organism, the parasite, lives on or in another organism, the host, and feeds on it, harming it but usually not killing it quickly. Parasite +, host −.
Ectoparasite: a parasite that lives on the outside of its host (a tick). Endoparasite: a parasite that lives inside its host (a tapeworm).
Predator and prey populations often rise and fall in cycles. Figure 1 shows the typical pattern, drawn from illustrative data.
Figure 1: A predator–prey cycle (illustrative data). Prey: solid blue line, left axis. Predators: dashed red line, right axis.
Worked example 1: reading a predator–prey graph
Use Figure 1. (a) In which year does the prey population first reach its peak, and when do the predators peak? (b) Calculate the lag. (c) Explain why the predator peak comes after the prey peak.
Also notice: the prey population is much larger than the predator population, as the pyramids of Lesson 2 predict.
| Predator adaptations (lion) | Prey adaptations (impala) |
|---|---|
| Eyes face forward: both eyes see the same scene, to judge the distance to the prey. | Eyes on the sides of the head: a wide field of view to spot danger from almost any direction. |
| Long, sharp canine teeth and claws to seize and kill. | Long legs: a fast runner that can leap high and change direction suddenly. |
| Sandy coat that blends with dry grass while stalking. | Lives in herds: many eyes watch for predators, and alarm calls warn the others. |
| Hunts in groups (prides), so it can catch large or fast prey. | Feeds and drinks in open ground where a stalking predator is easier to see. |
Grazing looks like predation of plants, but there is an important difference: a grazer eats part of the plant, and the plant usually survives. Grasses are well suited to being grazed, because they grow from near the base of the leaf, close to the ground. When the tips are bitten off, the leaf keeps growing from below. A tree whose leaves and shoot tips are browsed recovers much more slowly.
Worked example 2: how many cows can a pasture feed?
A 5-hectare pasture produces 4,000 kg of dry grass per hectare in a year. A cow eats about 10 kg of dry grass a day. To avoid overgrazing, no more than half of the grass should be eaten. How many cows can the pasture support for a year? (Illustrative figures.)
Answer: 2 cows. Round down: a third cow would push the pasture into overgrazing. If the farmer kept 5 cows, they would need kg, over 90% of all the grass.
| Parasite (type) | Host | How it harms the host |
|---|---|---|
| Tick (ectoparasite) | cattle, goats, dogs | sucks blood; some ticks carry disease-causing microorganisms, such as the one that causes East Coast fever in cattle |
| Louse, flea (ectoparasites) | mammals and birds | feeds on blood or skin; causes itching and sores |
| Tapeworm (endoparasite) | humans, cattle, pigs | lives in the small intestine and absorbs digested food |
| Plasmodium (endoparasite) | humans (carried by mosquitoes) | destroys liver and red blood cells: malaria |
| Dodder, Cuscuta (plant parasite) | flowering plants | a leafless climbing plant whose suckers (haustoria) grow into the host's stem and take its water and food |
Adaptations of parasites. A tapeworm has hooks and suckers on its head to hold on to the gut wall, a thick outer covering that resists the host's digestive enzymes, and no gut of its own: it absorbs food already digested by the host. It produces huge numbers of eggs, because very few reach a new host. A tick has piercing mouthparts and saliva that stops the host's blood from clotting.
Why a parasite rarely kills its host quickly: the host is its food supply and its home. A parasite that kills its host at once dies with it, unless it has already reached a new host.
Warning
Common mistakes. (1) Drawing the predator peak before the prey peak: predators respond to prey, so they peak later. (2) "Parasites kill their hosts": most weaken the host but keep it alive. (3) Calling grazing "predation": the grazer eats only part of the plant, which regrows.
GCSE Biology: Food Chains and Predator-Prey Cycles (Cognito). Watch for the explanation of why the predator population rises and falls after the prey population.
Tip
Memory hook. Predator: kills at once. Grazer: nibbles part, and the plant regrows. Parasite: lives with its host and takes a little at a time. In exams, name the type of parasite (ecto or endo) and one adaptation linked to its way of life.
Practice: predation, grazing and parasitism
Key points
Sources and credits
Written by Ganzaa: Figure 1 (illustrative data), the tables, the pasture calculation (illustrative figures), the worked examples and the practice questions.
