Unit 1 Interdependence among organisms in an ecosystem
Key unit competence: Assess interactions among organisms within the same ecosystem.
Key ideas
- Organisms in an ecosystem are interdependent: they depend on one another for food, shelter, pollination, seed dispersal and protection. Interactions are intraspecific (between members of the same species) or interspecific (between different species).
- Intraspecific competition: members of one species compete for the same food, water, space, light or mates, e.g. maize plants planted too close together, male goats fighting for mates. It is usually strong because they need exactly the same resources; it limits population size.
- Intraspecific cooperation: members work together and share tasks, e.g. social insects (bees, ants, termites) with queens, workers and drones or soldiers; lions hunting together; elephants protecting calves. Cooperation improves survival of the group.
- Interspecific competition: different species need the same resource, e.g. zebras and wildebeest for grass, weeds and crops for light, water and minerals. The better-adapted species may push out the other; species with very similar needs cannot live together for long (competitive exclusion).
- Predation: a predator kills and eats its prey, e.g. lion and zebra, owl and rat. Predator and prey numbers rise and fall in cycles: more prey → more predators → fewer prey → fewer predators. Prey adaptations: camouflage, speed, warning colours, living in herds.
- Symbiosis means living closely together. Mutualism: both benefit, e.g. Rhizobium bacteria in root nodules of legumes (fix nitrogen; get sugars), bees and flowers, cellulose-digesting microbes in the rumen of cows, lichens (alga and fungus).
- Commensalism: one benefits and the other is not affected, e.g. cattle egrets eating insects disturbed by grazing cattle, epiphytes (orchids, ferns) growing on tree branches for light.
- Parasitism: the parasite lives on or in a host, gets food from it and harms it. Ectoparasites live outside (ticks, lice, fleas, mistletoe-like plants); endoparasites live inside (tapeworm, Plasmodium, Ascaris). Parasites have hooks or suckers, high reproduction rates and resistant eggs or cysts.
- Saprophytism: saprophytes (many fungi and bacteria) feed on dead organic matter, secreting enzymes and absorbing the products; they act as decomposers and recycle nutrients.
- Interactions keep ecosystems balanced. Human actions (removing predators, introducing species such as water hyacinth, overgrazing) can upset these relationships and reduce biodiversity.
Common misconceptions
- "Competition only happens between different species." No: competition within the same species is often stronger because they need identical resources.
- "A parasite always kills its host quickly." No: a successful parasite usually keeps its host alive, because it depends on it for food and shelter.
- "In commensalism both organisms benefit." No: that is mutualism; in commensalism one benefits and the other is unaffected.
- "Predators are bad for an ecosystem." No: predators control prey numbers and remove weak or sick animals, keeping ecosystems balanced.
Worked examples
Example 1. Explain why a farmer who plants maize seeds very close together gets a low yield.
Solution
The maize plants are the same species and need the same light, water and minerals. Close planting causes strong intraspecific competition; each plant gets less, grows thin with small cobs, so the total yield is low. Correct spacing reduces competition.
Example 2. Explain how the relationship between legumes and Rhizobium is an example of mutualism.
Solution
Rhizobium bacteria live in root nodules of beans and other legumes. They fix nitrogen from the air into compounds the plant uses to make proteins. The plant gives the bacteria sugars and a protected place to live. Both benefit, so it is mutualism.
Practice questions
Distinguish between intraspecific and interspecific relationships, with one example of each.
Show answer
Intraspecific: between members of the same species, e.g. maize plants competing for light, bees cooperating in a hive. Interspecific: between different species, e.g. lion and zebra, beans and Rhizobium.
Common mistake: Giving examples that do not match the definition.
Define mutualism, commensalism and parasitism, with an example of each.
Show answer
Mutualism: both benefit (bees and flowers, Rhizobium and legumes). Commensalism: one benefits, the other unaffected (egret and cattle, orchid on a tree). Parasitism: one benefits, the host is harmed (tick and cow, tapeworm and human).
Common mistake: Calling egrets and cattle mutualism.
State three adaptations of endoparasites such as the tapeworm.
Show answer
Hooks and suckers to hold on; thick cuticle resisting digestive enzymes; absorbs digested food through its body surface (no gut); produces very many eggs; eggs with resistant shells.
Common mistake: Giving adaptations of predators.
Explain the cycle of predator and prey numbers.
Show answer
When prey are many, predators get plenty of food and increase; more predators eat more prey, so prey decrease; with less food, predators decrease; prey then recover; the cycle repeats.
Common mistake: Saying predators eat all prey and both die out.
Give two benefits of cooperation in animals.
Show answer
Better hunting success (lions); defence of young and group (elephants, bees); sharing of tasks (social insects); warmth and protection.
Common mistake: Describing competition instead.
Explain the role of saprophytes in an ecosystem.
Show answer
They feed on dead organic matter, secreting enzymes and absorbing products; this decomposes dead bodies and waste and returns minerals such as nitrates to the soil for plants.
Common mistake: Saying saprophytes are parasites.
Explain how a farmer can reduce competition among crops.
Show answer
Correct spacing; weeding; thinning seedlings; adding manure/fertiliser; irrigation; mixed cropping of plants with different needs.
Common mistake: Only listing pesticides.
Explain how removing predators such as leopards from a forest might affect the ecosystem.
Show answer
Prey such as bushbuck and monkeys would increase, overgraze or damage plants, compete strongly, and may later suffer disease and starvation; biodiversity falls.
Common mistake: Saying nothing would change.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Two species competing for the same food show: a. recycling nutrients from dead matter 2. Interactions between members of the same species are: b. root nodules of legumes 3. Rhizobium bacteria live in: c. intraspecific 4. Saprophytes help the ecosystem by: d. interspecific competition Show answer
1 → d (interspecific competition) · 2 → c (intraspecific) · 3 → b (root nodules of legumes) · 4 → a (recycling nutrients from dead matter)
- Example: zebras and wildebeest.
- Intra means within.
- They fix nitrogen.
- They act as decomposers.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
ectoparasite · intraspecific · an alga and a fungus · interspecific- A tick on a cow is an example of an .
- A lichen is a mutualism between .
- Interactions between different species are .
Show answer
1: ectoparasite · 2: an alga and a fungus · 3: interspecific
- It lives on the outside of its host.
- The alga makes food; the fungus gives shelter and water.
- Inter means between.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Microbes in a cow's rumen digest: a. parasite 2. Plasmodium in human red blood cells is an example of: b. saprophytes 3. An organism that lives on or in a host and harms it is a: c. parasitism 4. Organisms that feed on dead organic matter are: d. cellulose Show answer
1 → d (cellulose) · 2 → c (parasitism) · 3 → a (parasite) · 4 → b (saprophytes)
- The cow cannot make cellulase itself.
- It causes malaria.
- It gets food from the host.
- Many fungi and bacteria are saprophytes.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
live in colonies and share tasks · saprophytes on dead wood · epiphytes (commensals) · symbiosis- Close living together of two different species is called .
- Orchids growing on tree branches to reach light are .
- Bees, ants and termites are called social insects because they .
Show answer
1: symbiosis · 2: epiphytes (commensals) · 3: live in colonies and share tasks
- It includes mutualism, commensalism and parasitism.
- They do not take food from the tree.
- This is intraspecific cooperation.