Unit 1 Interdependence between organisms within their environment
Key unit competence: Explain complex relationships among organisms within their environment.
Key ideas
- Organisms interact in intraspecific (same species) and interspecific (different species) relationships. The main types are competition, predation, parasitism, saprophytism and symbiosis (mutualism and commensalism). These interactions control population sizes, energy flow and nutrient cycling.
- Competition occurs when organisms need the same limited resource (food, water, light, space, mates). Intraspecific competition is usually more intense (identical needs) and is density-dependent. Interspecific competition: by Gause's competitive exclusion principle, two species with identical niches cannot coexist indefinitely; one is eliminated or they undergo niche (resource) partitioning (e.g. different feeding heights of giraffes and zebras). Types: scramble (resource shared until exhausted) and contest (some individuals win the resource by fighting or territory).
- Economic value of competition: guides crop spacing and weeding, choice of intercrops with different root depths, and biological control (competitor organisms suppress pests or weeds).
- Parasitism: the parasite lives on (ectoparasite: ticks, lice, fleas, Cuscuta/dodder) or in (endoparasite: tapeworm, Ascaris, Plasmodium, schistosomes) a host and harms it. Obligate parasites cannot live without a host; facultative parasites can also live freely. Adaptations: hooks and suckers, resistant cuticle, reduced organs, hermaphroditism, huge numbers of eggs, resistant stages, intermediate hosts and vectors (e.g. snail for Schistosoma, pig/cow for Taenia), anticoagulants in blood-feeders.
- Economic importance of parasites: crop losses (Striga on maize and sorghum, nematodes), livestock diseases (ticks spread East Coast fever, trypanosomes), human disease (malaria, bilharzia, worms) and treatment costs; parasitoids (wasps) are used in biological control of pests.
- Predation: a predator kills and eats prey. Forms: carnivory, herbivory (grazing, browsing; plant "predation"), insectivorous plants (sundew, pitcher plant), parasitoids. Predator-prey cycles: prey numbers rise, predators increase after a time lag, prey fall, predators fall (e.g. lynx and snowshoe hare). Predators remove weak and sick prey, control herbivores and maintain biodiversity; prey evolve defences (camouflage, mimicry, warning colours, toxins, speed, herds).
- Saprophytism: saprophytes (many fungi and bacteria) secrete enzymes onto dead organic matter and absorb the products (extracellular digestion). They are decomposers that recycle C, N and P; economically they make compost, biogas, fermented foods, antibiotics, and also cause food spoilage and wood rot.
- Mutualism: both partners benefit: Rhizobium in legume root nodules (fixes N₂; receives carbohydrates), mycorrhizae (fungi increase mineral and water uptake by roots; receive sugars), lichens (alga/cyanobacterium and fungus), cellulose-digesting microbes in ruminants and termites, pollinators and flowers, cleaner fish, oxpeckers (partly). Commensalism: one benefits, the other is unaffected: epiphytes (orchids, ferns on trees), cattle egrets, remoras on sharks, commensal gut bacteria.
- Overall significance: interactions regulate populations, keep ecosystem balance, drive coevolution and natural selection, and can be exploited in agriculture (legume rotation, biological control, mycorrhizal inoculants).
Common misconceptions
- "Competition always ends with one species dying out." No: species often partition resources (feeding at different times, places or on different foods) and coexist.
- "Herbivores are not predators." No: grazing and browsing are forms of plant predation (herbivory).
- "Saprophytes are parasites of dead bodies." No: parasites feed on living hosts; saprophytes feed on dead matter.
- "Mycorrhizae are fungal diseases of roots." No: they are mutualistic fungi that improve mineral and water uptake.
Worked examples
Example 1. Explain why predator numbers peak after prey numbers in a predator-prey cycle.
Solution
When prey become plentiful, predators have more food; their survival and reproduction increase, but it takes time (a time lag) for new predators to be born and grow. By the time predator numbers peak, prey are already being eaten faster than they reproduce, so prey numbers fall first, followed by predators.
Example 2. Explain how Striga (witchweed) is an economic problem in maize and sorghum farming, and suggest two control methods.
Solution
Striga is a parasitic plant whose haustoria attach to crop roots and absorb water, minerals and sugars, stunting the crop and reducing yield greatly. Control: crop rotation with legumes (trap crops make Striga seeds germinate without a host), hand-pulling before seeding, resistant varieties, manure to improve soil fertility, push-pull with Desmodium.
Practice questions
Distinguish between scramble and contest competition, with an example of each.
Show answer
Scramble: all individuals share a limited resource until it is used up, e.g. maize plants planted too densely for soil nutrients. Contest: some individuals win the resource by fighting or holding territory, e.g. male antelopes fighting for females, birds defending territories.
Common mistake: Confusing contest competition with predation.
Explain the competitive exclusion principle and how species avoid exclusion.
Show answer
Two species with identical niches cannot coexist indefinitely; the better competitor eliminates the other. Species avoid this by niche partitioning: using different foods, feeding at different times, places or heights.
Common mistake: Saying all competing species become extinct.
State five adaptations of endoparasites.
Show answer
Hooks and suckers; enzyme-resistant cuticle; reduced digestive and sense organs; absorb food over body surface; hermaphroditism; huge egg production; resistant cysts/eggs; intermediate hosts; anaerobic respiration.
Common mistake: Listing adaptations of predators.
State the economic importance of parasites.
Show answer
Harmful: crop losses (Striga, nematodes), livestock diseases (ticks, trypanosomes), human diseases (malaria, bilharzia), treatment costs and lost work. Useful: parasitoids for biological control of pests.
Common mistake: Giving only harmful effects.
Describe a predator-prey cycle and explain its significance.
Show answer
Prey increase, then predators increase after a time lag; predators reduce prey; predators then decline from lack of food; prey recover. Significance: controls population sizes, removes weak prey, maintains balance and drives coevolution.
Common mistake: Drawing predator peaks before prey peaks.
Explain the roles of saprophytes in nutrient cycling.
Show answer
They secrete enzymes onto dead organisms and wastes, digest them and absorb products; this releases CO₂, ammonium (later nitrates), phosphates and other minerals back to the soil and air for producers.
Common mistake: Saying saprophytes make their own food.
Give four examples of mutualism and state the benefit to each partner.
Show answer
Rhizobium and legumes (N₂ fixed / sugars); mycorrhizae and roots (minerals, water / sugars); lichen alga and fungus (food / shelter, water); rumen microbes and cows (food, shelter / cellulose digestion); bees and flowers (nectar / pollination).
Common mistake: Giving commensal examples.
Distinguish between mutualism and commensalism with examples.
Show answer
Mutualism: both benefit (bees and flowers). Commensalism: one benefits, the other is unaffected (orchid on a tree, remora on a shark).
Common mistake: Saying in commensalism the host is harmed.
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. Insects that lay eggs in a host insect, whose larvae then kill it, are: a. niche (resource) partitioning 2. The delay between the peaks of prey and predator numbers is the: b. parasitoids 3. The snail is the intermediate host of: c. Schistosoma (bilharzia) 4. Different species dividing a resource to reduce competition is: d. time lag Show answer
1 → b (parasitoids) · 2 → d (time lag) · 3 → c (Schistosoma (bilharzia)) · 4 → a (niche (resource) partitioning)
- Used in biological control.
- Predators take time to reproduce.
- Infective larvae leave the snail in water.
- Example: feeding at different heights.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
saprophytes · compost and biogas production · intraspecific · parasites- Saprophytes are economically useful in .
- Competition between members of the same species is .
- Organisms that secrete enzymes onto dead matter and absorb the products are .
Show answer
1: compost and biogas production · 2: intraspecific · 3: saprophytes
- They decompose organic matter.
- It is usually the most intense.
- This is extracellular digestion.
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. Competition in which a resource is shared until it runs out is: a. scramble competition 2. A parasitic plant that attacks the roots of maize and sorghum is: b. commensalism 3. Grazing of grass by cattle is a form of: c. herbivory (plant predation) 4. An orchid growing on a tree branch without harming it is an example of: d. Striga (witchweed) Show answer
1 → a (scramble competition) · 2 → d (Striga (witchweed)) · 3 → c (herbivory (plant predation)) · 4 → b (commensalism)
- In contest competition some win the resource.
- It causes large yield losses.
- The animal feeds on living plants.
- It is an epiphyte.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
protection from all insects · better uptake of phosphate and water · mycorrhiza · a thick cuticle resisting enzymes- A farmer inoculates pine seedlings with mycorrhizal fungi. The main benefit is .
- The fungus-root association that increases mineral uptake is .
- An adaptation of tapeworms to life in the gut is .
Show answer
1: better uptake of phosphate and water · 2: mycorrhiza · 3: a thick cuticle resisting enzymes
- Fungal hyphae extend the root surface.
- The fungus receives sugars.
- They absorb digested food directly.