Life history and predation

Every species tries to reproduce as successfully as possible, its Darwinian fitness, and different species have evolved different life histories to do this. Some breed only once in their life, such as Pacific salmon and bamboo; others breed many times, as most birds and mammals do. Some produce many small offspring, such as oysters and fish; others produce a few large ones, such as birds and mammals.

No species lives alone. Species in a habitat interact, and each interaction can be described by its effect on each partner: + (benefit), − (harm) or 0 (no effect):

• Mutualism (+, +): both benefit.
• Competition (−, −): both are harmed.
• Predation and parasitism (+, −): one benefits, the other is harmed.
• Commensalism (+, 0): one benefits, the other is unaffected.
• Amensalism (−, 0): one is harmed, the other is unaffected.

Predation is nature's way of moving the energy stored in plants up to higher levels: a tiger eats a deer, a sparrow eats a seed. Predators are important:

• They control prey numbers. When the prickly pear cactus was brought into Australia in the 1920s, it had no natural enemies and spread over millions of hectares. It was finally controlled by bringing in a moth that feeds on it in its native land: biological control.
• They help keep species diversity by reducing competition among prey. When the starfish Pisaster, a predator, was removed from a rocky shore in America, more than 10 species of invertebrates disappeared within a year, outcompeted by others.

Prey defend themselves:

• Camouflage: some insects and frogs are coloured to blend with their surroundings.
• Poison: the Monarch butterfly tastes horrible to birds, because as a caterpillar it fed on a poisonous weed and stored the chemical in its body.

Plants defend themselves too, since they cannot run away:

• Thorns, as in Acacia and cactus.
• Chemicals: the weed Calotropis makes highly poisonous cardiac glycosides, so cattle and goats never browse on it. Many chemicals we use, such as nicotine, caffeine, quinine and strychnine, are made by plants as defences against grazers.