Species Interactions: Mutualism, Parasitism and Predation

The correct answer is C, Mutualism. A lichen is a close partnership between an alga and a fungus in which both partners gain. The alga prepares carbohydrates by photosynthesis and shares them with the fungus, while the fungus absorbs and holds water and minerals and anchors the pair to bare rock or tree bark. Neither partner can colonise bare rock alone, so the association is described as obligate mutualism, and lichens are therefore called pioneer species of a bare rock succession. Option A is wrong because in parasitism one partner is harmed, whereas both the alga and the fungus benefit here. Option B is wrong because commensalism requires one partner to be unaffected, which is not the case. Option D is wrong because amensalism means one species is harmed while the other is unaffected, as when Penicillium kills bacteria.

The correct answer is A, Total stem parasite. Cuscuta, known as dodder or amarbel, is a flowering plant that has lost its chlorophyll and its leaves and cannot make its own food. Its twining stem clings to the host and sends sucker-like haustoria into the host's vascular tissue, from which it draws both water and prepared food. Because it depends on the host for organic food as well as water, it is a total or holoparasite, and because the contact is through the stem it is a stem parasite. Option B is wrong because partial root parasites such as sandalwood keep their chlorophyll and attach at the root. Option C is wrong because an epiphyte, such as an orchid on a mango branch, only uses the host for support. Option D is wrong because saprophytes feed on dead organic matter, not on a living host.

The correct answer is B, Commensalism. The egret walks beside grazing cattle and feeds on the grasshoppers and other insects flushed out of the grass by the cattle's hooves. The bird gains an easy meal, while the cattle are neither helped nor harmed by its presence, which is exactly the plus and zero pattern of commensalism. Barnacles on a whale's back and an orchid on a mango branch belong to the same category. Option A is wrong because mutualism needs both species to gain, and the cattle gain nothing. Option C is wrong because the interaction between the egret and the cattle is not predation, even though the egret preys on insects. Option D is wrong because competition requires both species to seek the same limited resource and both to suffer, which does not happen here.

The correct answer is A, the moth Cactoblastis cactorum. Prickly pear cactus was taken to Australia in the nineteenth century and spread uncontrollably over grazing land because no local animal fed on it. The cactus-feeding moth Cactoblastis cactorum was introduced, and its larvae bored into the cactus pads and destroyed them, bringing the weed under control. This is the standard textbook case of biological control, the use of a natural predator or herbivore in place of chemicals. Option B is wrong because Pisaster is the starfish used by Robert Paine in the keystone species experiment on the American Pacific coast. Option C is wrong because Balanus is the barnacle that excluded Chathamalus in Connell's competition study on the Scottish coast. Option D is wrong because Penicillium is the fungus quoted as an example of amensalism, not of weed control.

The correct answer is B, G. F. Gause. The Russian ecologist G. F. Gause worked with cultures of Paramecium and concluded that when two species need exactly the same limited resource in the same space, one of them is always eliminated in the long run; this is the competitive exclusion principle, also called Gause's principle. Species that are seen living together have usually divided the resource by time, depth or size, which is called resource partitioning. Option A is wrong because Robert Paine demonstrated the idea of a keystone species by removing the starfish Pisaster. Option C is wrong because Joseph Connell showed competitive exclusion in the field with barnacles but did not state the principle. Option D is wrong because Charles Elton is remembered for the ecological niche and the pyramid of numbers.

The correct answer is B, the starfish Pisaster. Robert Paine removed the predatory starfish Pisaster from rocky intertidal pools and found that within a short time the mussels it had been eating took over the rock surface and crowded out other animals, so that more than ten invertebrate species were lost from the pools. The experiment gave ecology the idea of a keystone species, a species whose effect on community structure is far greater than its numbers suggest. Option A is wrong because the sea otter is a keystone species of kelp forests, but it was not the animal of Paine's intertidal experiment. Option C is wrong because the sea anemone is quoted with the clownfish as an example of commensalism. Option D is wrong because the remora is the sucker fish that rides on sharks and feeds on their leftovers.

The correct answer is C, Mutualism. Rhizobium lives inside the nodules on the roots of legumes such as gram, pea, groundnut and arhar. The bacterium converts atmospheric nitrogen into ammonia, a form the plant can absorb and build into proteins, while the plant supplies the bacterium with shelter and with sugars made in photosynthesis. Both partners gain, so the association is mutualism, and it is the reason legumes are grown in rotation to restore soil fertility. Option A is wrong because the plant is not harmed; its growth actually improves. Option B is wrong because amensalism means one species is harmed and the other unaffected. Option D is wrong because predation involves one organism killing and eating another animal or plant, which does not describe a bacterium settled peacefully inside a root nodule.

The correct answer is A, Amensalism. In amensalism one species is harmed while the other neither gains nor loses, a pattern written as minus and zero. Penicillium releases penicillin into its surroundings and the antibiotic kills or stops the growth of nearby bacteria, yet the fungus itself gets no measurable benefit from their death. The release of a growth-inhibiting chemical by one organism against another is also called allelopathy, as when the leaf litter of some trees prevents other seedlings from growing beneath them. Option B is wrong because commensalism benefits one partner and leaves the other unaffected, with no harm involved. Option C is wrong because a parasite feeds on a living host and gains from it. Option D is wrong because mutualism needs both species to gain.

The correct answer is A, Crow. Brood parasitism is the habit of laying eggs in the nest of another species and letting the host bird incubate them and feed the young. The Asian koel, a cuckoo, lays its eggs in a crow's nest, and natural selection has made the koel's eggs resemble the crow's eggs in size and colour closely enough that the crow does not detect them. The host loses because it spends its energy rearing another bird's chick. Option B is wrong because the house sparrow is not the regular host of the koel. Option C is wrong because the pigeon builds a flimsy nest and is not parasitised in this way. Option D is wrong because the myna nests in holes and cavities, which the koel does not use; the crow remains the textbook host.

The correct answer is B, a fungus and the roots of a higher plant. In a mycorrhiza the threads of a fungus grow around or inside the fine roots of a plant and greatly extend the surface from which minerals, especially phosphorus, can be absorbed from the soil. The fungus passes these minerals and extra water to the plant and receives sugars produced by photosynthesis in return, so both partners gain and the relationship is mutualism. Pine seedlings, for instance, grow poorly in soil without their mycorrhizal fungi. Option A is wrong because an alga with a fungus is a lichen. Option C is wrong because no such standard association is described under this name. Option D is wrong because fungus-growing termites and ants are a separate example and are not called mycorrhiza.

The correct answer is A, Interspecific competition. Connell showed in the field that on the intertidal rocks of the Scottish coast the larger and faster-growing barnacle Balanus physically crowds out the smaller Chathamalus wherever the two settle together, so that Chathamalus survives only higher up the shore where Balanus cannot tolerate the exposure. Both species seek the same limited resource, space on the rock, and both are harmed by the other's presence, which is the minus and minus pattern of competition. The study is also quoted as field proof of Gause's competitive exclusion principle. Option B is wrong because neither barnacle gains from the other. Option C is wrong because brood parasitism concerns birds laying eggs in another nest. Option D is wrong because in commensalism one species would have to be unaffected.