This is commensalism — the interaction in which one species benefits and the other is neither harmed nor benefited (+ / 0).

What the orchid gains. It grows as an epiphyte, using the mango branch for support and for a position high in the light. An orchid on the forest floor would be shaded out; on a branch it reaches the light without having to build a trunk of its own.

Why the mango is unaffected. The orchid is an epiphyte, not a parasite: it does not tap the mango's vascular tissue and takes no nutrition from it. Compare this with Cuscuta, a parasitic plant on hedge plants, which has lost its chlorophyll and leaves and derives its nutrition from the host it parasitises. The orchid keeps its own chlorophyll and makes its own food. So the mango tree derives no apparent benefit, and suffers no apparent cost.

What would overturn the classification. The chapter's wording is careful: the mango derives no apparent benefit. If measurement showed that a heavily colonised branch received significantly less light, or bore the extra weight and water load at a measurable cost to the tree's growth, the interaction would be reclassified as parasitism or at least amensalism. Conversely, if the orchid's presence were shown to attract pollinators that also visited the mango's flowers, it would edge towards mutualism. Classification here depends on measured effect, not on appearance.

(a) Commensalism. The interaction in which one species benefits and the other is neither harmed nor benefited (+ / 0). Example: the cattle egret and grazing cattle. The egrets always forage close to where the cattle are grazing, because the cattle as they move stir up and flush out insects from the vegetation that might otherwise be difficult for the egrets to find and catch; the cattle are unaffected. (Other examples: an orchid as an epiphyte on a mango branch; barnacles on the back of a whale; the clown fish among a sea anemone's stinging tentacles.)

(b) Parasitism. An interaction in which the parasite benefits at the expense of the host, which is harmed (+ / −). Parasites may reduce the survival, growth and reproduction of the host, reduce its population density, and render it more vulnerable to predation by making it physically weak. Example: lice on humans, an ectoparasite; or the human liver fluke, an endoparasite which depends on two intermediate hosts, a snail and a fish, to complete its life cycle.

(c) Camouflage. A prey defence in which the animal's colouration prevents it from being detected easily by a predator; such species are said to be cryptically coloured. Example: some species of insects and frogs.

(d) Mutualism. An interaction that confers benefits on both the interacting species (+ / +). Example: lichens, an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria. (Also mycorrhizae, where the fungus aids nutrient absorption and the plant supplies carbohydrates; and the fig and its one partner wasp species.)

(e) Interspecific competition. A process in which the fitness of one species, measured as its r or intrinsic rate of increase, is significantly lower in the presence of another species (− / −). Example: in some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, the zooplankton in the lake.

The simplification. In accordance with their lifestyles, parasites have evolved special adaptations: loss of unnecessary sense organs, adhesive organs or suckers to cling to the host, loss of the digestive system, and high reproductive capacity. Three of the four are losses, and the reason is that the host supplies what the lost organ used to provide. A parasite bathed in the host's digested food has no use for a gut; one that never has to search for food or a mate has no use for elaborate sense organs. Every structure retained has to be paid for, so structures that earn nothing are lost.

Where the saved resource goes. Into reproduction. The endoparasite's morphological and anatomical features are greatly simplified while emphasising their reproductive potential — the two facts are one fact.

Why reproduction has to be prodigious. Because reaching the next host is extremely improbable. Life cycles are often complex, involving one or two intermediate hosts or vectors: the human liver fluke depends on two intermediate hosts, a snail and a fish, and the malarial parasite needs a mosquito vector. At each transfer the great majority of offspring die without finding the next host, so only enormous numbers make the cycle viable.

Ectoparasite versus endoparasite. An ectoparasite such as a tick on a dog lives on the external surface: it still needs to attach, so adhesive organs are prominent, and it retains more of its structure because it is exposed to the outside world. An endoparasite living in the liver, kidney, lungs or red blood cells faces no such demands, so its simplification goes much further and its life cycle is more complex because of its extreme specialisation.

The arrangement. In many species of fig trees there is a tight one-to-one relationship with the pollinator species of wasp, meaning that a given fig species can be pollinated only by its partner wasp species and no other.

What the fig gains and gives. It gains pollination: the wasp pollinates the fig inflorescence while searching for suitable egg-laying sites. In return for that favour, the fig offers the wasp some of its developing seeds as food for the developing wasp larvae. So the fig pays for pollination in seeds — it sacrifices part of its own reproductive output to secure the rest.

What the wasp gains and gives. The female wasp uses the fruit as an oviposition site and uses the developing seeds within the fruit to nourish its larvae. What it gives is the pollination service, performed incidentally while it searches for egg-laying sites. Note that neither partner is being altruistic: each is pursuing its own reproduction, and the benefit to the other is a by-product.

The risk of tight specialisation. Absolute mutual dependence. If the wasp species is lost — to disease, to a pesticide, to a change in season — that fig species cannot be pollinated at all by any other insect, and its reproduction ends. Equally, the wasp has no alternative egg-laying site. Specialisation buys efficiency and reliability, and pays for them with fragility.

And the system must also be defended. Mutually beneficial systems have to be safeguarded against ‘cheaters’ — for instance animals that try to steal nectar without aiding pollination. A one-to-one relationship is one solution to that problem: a reward that only the partner species can reach cannot be stolen.

What happens. Not all orchids offer rewards. Ophrys employs ‘sexual deceit’: one petal of its flower bears an uncanny resemblance to the female of a bee species in size, colour and markings. The male bee is attracted to what it perceives as a female, pseudocopulates with the flower, and is dusted with pollen in the process; when it pseudocopulates with another flower it transfers the pollen and pollinates it.

The case for calling it parasitism. The orchid gains pollination; the bee gains nothing. Worse, it loses time and energy that it would otherwise have spent finding a real mate, which is a direct cost to its reproduction. On the sign convention that would be (+ / −) — and the chapter's definition of parasitism, an interaction detrimental to the other species, fits. The orchid is, in effect, a cheater — the mirror image of the animals that steal nectar without aiding in pollination, against which mutualistic systems must be safeguarded.

Why the chapter places it under mutualism anyway. Because it appears in the discussion of plant-pollinator co-evolution, and it illustrates that point better than any honest flower does. The mechanism of the relationship — a flower shaped by selection to fit one pollinator species exactly — is the mechanism of mutualism, even though the payment has been withdrawn.

The best answer. Call it a deceptive or exploitative pollination relationship: parasitic in its accounting, mutualistic in its machinery. And notice what keeps it honest in the long run: if the female bee's colour patterns change even slightly during evolution, pollination success will be reduced unless the orchid co-evolves to maintain the resemblance. The cheat survives only by tracking its victim, which is co-evolution just as surely as the fig and the wasp.