The competitive exclusion principle, which states that two species competing for the same limited resource cannot coexist indefinitely, was given by

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, 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.