Exploitation: predation, herbivory and parasitism

What exploitation is

• Exploitation is a relationship in which one species benefits at the expense of another, which is harmed. It is a +/– relationship.
• The standard names three forms:

Form	Exploiter	Effect on the exploited organism
Predation	Predator	Prey is killed and eaten
Herbivory	Herbivore	Plant tissue eaten; usually damaged, not killed
Parasitism	Parasite	Host harmed over time, usually not killed quickly

• The distinctions matter because they produce different selection pressures.
  • A predator kills, so selection on prey is intense and immediate — failure means death.
  • A herbivore usually removes part of a plant, so selection favours tolerance and regrowth as well as defence.
  • A parasite depends on its host staying alive, so selection on the parasite favours restraint — a parasite that kills its host quickly destroys its own habitat.

Predation

• Predators and prey exert reciprocal selection pressure on each other, producing an evolutionary arms race.

• Predator adaptations: acute senses, speed, stealth, camouflage, claws and teeth, and hunting strategies such as ambush or pursuit.

• Prey adaptations:

  • Avoiding detection — camouflage, remaining still, nocturnal activity.
  • Escaping once detected — speed, agility, sudden startling movement.
  • Defence — spines, toxins, armour, group defence.
  • Warning colouration — bright patterning advertising that the animal is toxic or dangerous.

• Predator–prey population cycles.

  • Prey numbers rise → more food for predators → predator numbers rise.
  • Increased predation → prey numbers fall → predators starve → predator numbers fall.
  • Reduced predation → prey numbers rise again.
  • The predator peak always lags behind the prey peak, because predators can only increase after prey have become abundant. Identifying which curve lags is how you tell predator from prey in an unlabelled graph.

• New Zealand's problem is naive prey. Native birds evolved with no mammalian predators, so their anti-predator adaptations are matched to avian predators that hunt by sight from above.

  • Freezing, camouflage and remaining still are effective against a hawk but useless against a stoat or rat hunting by scent.
  • Ground nesting, flightlessness and low reproductive rates were viable without mammals but are severely disadvantageous with them.
  • There has been no time for appropriate adaptations to evolve, which is why introduced mammalian predators have been so destructive.

Herbivory

• Herbivores usually damage rather than kill, so the relationship is prolonged.

• Plant defences:

  • Physical — thorns, spines, tough or hairy leaves, high silica content that wears down teeth.
  • Chemical — toxins, bitter compounds, and digestibility reducers such as tannins that bind proteins.
  • Tolerance — meristems positioned close to the ground so the growing point survives grazing, plus rapid regrowth from stored reserves.

• Herbivore counter-adaptations: detoxifying enzymes, specialised teeth, gut microorganisms that digest cellulose, and feeding selectively on younger or less defended tissue.

• Divaricating shrubs are a distinctive New Zealand form — densely interlaced, wiry, small-leaved stems with leaves held inside the tangle. The widely supported explanation is that this evolved as a defence against moa browsing: the tough outer stems are hard to grip and strip, and the leaves are protected inside.

  • Supporting evidence: many divaricating species change to a normal branching form above about 3 m, roughly the browsing height of the largest moa.

• Possums now browse New Zealand canopy trees such as rātā and kāmahi that evolved without mammalian browsers, so their defences are poorly matched and whole canopies can be defoliated.

Parasitism

• A parasite lives on or in a host, obtaining nutrients from it and harming it, usually without killing it quickly.

  • Ectoparasites live on the surface — fleas, ticks, lice.
  • Endoparasites live inside — tapeworms, flukes, many protozoa.

• Parasite adaptations: attachment structures, resistance to host digestive enzymes and immune attack, huge reproductive output to offset the low chance any one offspring finds a host, and often complex life cycles using more than one host species.

• Host defences: immune responses, grooming, and behavioural avoidance of infected individuals or contaminated areas.

• Why parasites usually avoid killing the host. The host is the parasite's habitat and food supply. A parasite that killed its host quickly would destroy its own resource, often before it had reproduced or transmitted to a new host. Selection therefore favours intermediate virulence — enough exploitation to reproduce well, not so much that transmission fails.

Selective advantage

• Anti-predator responses. Camouflage, vigilance, fleeing and grouping all reduce the probability of being killed. An individual that survives an encounter lives to reproduce, while one that does not leaves no offspring — so selection on these traits is exceptionally strong and direct.
  • Grouping works through several mechanisms: more eyes detecting the predator, dilution of individual risk, and confusion of the predator by many moving targets.
• Predator responses. Better detection and capture mean more energy gained per unit of hunting effort, so more energy is available for reproduction and the predator is more likely to survive lean periods.
• Plant defences. Less tissue lost means more photosynthetic area retained and fewer resources spent on repair, so more is available for seed production. Chemical defence is costly to produce, so it is favoured only where herbivore pressure is high enough to justify it.
• Parasite restraint. A parasite that harms its host less has a host that lives longer, so the parasite has more time to reproduce and transmit — more offspring in total than a more damaging strain would achieve.