How a Cuckoo Tricks Crows Into Raising Its Chicks

No species lives alone. Plants, animals and microbes eat, compete with, live on and cooperate with one another, and these interactions shape which species survive where.

This part covers the types of interaction between species, predation and prey defences, competition, and parasitism and mutualism.

Interactions between species are classified by whether each partner benefits (+), is harmed (–) or is unaffected (0): mutualism is (+, +), competition is (–, –), predation and parasitism are (+, –), commensalism is (+, 0) and amensalism is (–, 0).

The six interactions:

- Mutualism (+, +) — both species benefit, as in lichens
- Competition (–, –) — both are harmed as they compete for a limited resource
- Predation (+, –) — the predator benefits and the prey is harmed
- Parasitism (+, –) — the parasite benefits and the host is harmed
- Commensalism (+, 0) — one benefits while the other is unaffected, as with an orchid growing on a mango branch or a barnacle on a whale
- Amensalism (–, 0) — one is harmed while the other is unaffected

An everyday example. Cattle egrets walking beside grazing buffaloes in Indian fields catch insects the animals stir up — the egrets benefit while the buffaloes are unaffected.

The substance. Predation and parasitism share the same signs (+, –), but a predator usually kills its prey quickly while a parasite often lives on its host for a long time.

Predation passes energy from plants to herbivores and from prey to predators and keeps prey populations in check; prey defend themselves with camouflage and chemical defences, and plants defend against herbivores with thorns and poisons such as cardiac glycosides.

Roles of predation:

- A route for energy transfer across trophic levels
- Controls prey populations — when the prickly pear cactus was introduced into Australia without natural enemies, it spread over huge areas until a cactus-feeding moth was brought in to control it
- Helps maintain species diversity by reducing competition among prey

Prey defences:

- Camouflage — insects and frogs coloured to blend with their surroundings
- Chemical defence — the Monarch butterfly is distasteful to predators because of a chemical it takes in as a caterpillar by feeding on a poisonous weed

Plant defences against herbivores:

- Thorns — in Acacia and the cactus Opuntia
- Poisons — Calotropis makes highly poisonous cardiac glycosides, so cattle and goats never browse it

An everyday example. Goats grazing along a roadside leave the aak bushes (Calotropis) untouched, avoiding their poisonous glycosides.

The substance. Predators are not simply harmful — by keeping prey numbers in check they prevent over-grazing and ecosystem collapse.

Competition occurs when species use the same limited resources, lowering the fitness of each; Gause's competitive exclusion principle states that two closely related species competing for the same resource cannot coexist indefinitely because the weaker one is eliminated, but species can coexist by partitioning resources, such as feeding in different places or at different times.

Features of competition:

- It can occur between unrelated species, such as flamingos and fish competing for zooplankton in shallow lakes
- Resources need not be scarce — in interference competition, one species lowers another's feeding efficiency simply by its presence

Connell's barnacle experiment:

- On the rocky coasts of Scotland, the larger barnacle Balanus dominates the intertidal area
- It excludes the smaller barnacle Chthamalus from that zone

Gause's principle:

- Two closely related species competing for the same resource cannot coexist indefinitely
- The competitively inferior species is eventually eliminated, particularly when resources are limited

Resource partitioning:

- Competing species avoid direct competition by using different feeding times or different foraging patterns
- MacArthur found that five closely related warbler species living on the same tree coexisted by foraging in different parts of the tree

An everyday example. Street vendors who sell at different hours on the same pavement share the space without competing head-on — a human version of partitioning.

The substance. Competition is strongest between closely related species, because they use the most similar resources.

Parasites evolve adaptations such as loss of unnecessary sense organs, suckers for attachment, loss of the digestive system and high reproductive capacity, and brood parasites like the cuckoo trick other birds into raising their young; in mutualism both partners gain, as in lichens, mycorrhizae, and the pollination partnerships of figs with wasps and Ophrys orchids with bees.

Parasitism:

- Many parasites are host-specific and evolve along with their hosts
- Adaptations — loss of unnecessary sense organs, adhesive organs or suckers, loss of the digestive system and high reproductive capacity
- Ectoparasites live on the host's surface, such as lice on humans and ticks on dogs; endoparasites live inside, such as liver flukes
- Brood parasitism — the koel, a cuckoo, lays its eggs in a crow's nest; the eggs resemble the crow's eggs, so the crow incubates them

Mutualism:

- Lichens — a partnership between a fungus and photosynthesising algae or cyanobacteria
- Mycorrhizae — fungi help plant roots absorb nutrients, and the plant supplies carbohydrates
- Fig and wasp — each fig species is pollinated by its own wasp species; the female wasp pollinates the flowers while laying eggs, and the larvae feed on some developing seeds
- ***Ophrys* orchid and bee — a petal resembles a female bee, so the male bee attempts to mate with the flower and carries away pollen

An everyday example. Koels calling in Indian towns in summer are cuckoos that often leave their eggs in crows' nests.

The substance. A parasite that kills its host quickly harms itself** — successful parasites usually weaken hosts rather than kill them.