Whilst plants and some micro-organisms produce their own energy by photo- or chemosynthesis, many species are reliant on eating someone else to get the energy and nutrients they need. The relationships between predators and their prey or parasites and their hosts can have a great influence on species distributions.
Predators
We tend to think of predators as fierce hunters such as wolves and eagles. But for many ecologists, predation has a wider meaning that includes all interactions where one organism kills and eats many others. So, as well as owls being predators of mice, doves are predators of seeds, and limpets are predators of the single-celled algae they scrape off the rocks on the sea shore.
Parasites
Parasites affect virtually every living organism and it has been estimated that around 50% of all species are parasitic. Parasites, such as head-lice and tapeworms, do not always kill their host organism, but they do have negative effects, and will often have serious consequences for the survival and reproductive success of the host. Parasites tend to be smaller than their host organism and often spend their lives on or inside the host.
 This female mosquito is feeding on human blood.A spectrum of interactions
Predators and parasites represent two extremes in a range of interactions where one organism gains nutrients from another. Many species fall somewhere between these definitions of predator and parasite. Parasitoids, usually specialised wasps or flies, have larvae that eat a single insect host, often from the inside, with fatal results. The host acts as a living larder and often remains alive until the parasitoid is ready to become an adult. Herbivores include the grazers and browsers such as caterpillars and deer that eat but do not necessarily kill plants. Hemiparasites are partly parasitic plants, such as mistletoe, which lives on trees and taps into them to obtain water and minerals, but produces its own food by photosynthesis. All these interactions can be important in regulating both the numbers of those eating and those being eaten.
Attack and defence
Co-evolution is the adaptation of species as a result of their interaction with others. Over time the prey with the best adaptations to deter or avoid predators or parasites are most likely to survive and reproduce. This in turn may encourage the survival of predators with the best adaptations for catching them.
Over long periods of time co-evolution between predators, parasites, and their victims has resulted in an enormous range of armour, escape tactics, camouflage and weaponry. Hedgehogs and some woodlice roll into protective balls. Many caterpillars rely on camouflage to avoid predation and sea cucumbers exude long sticky threads if disturbed and in extreme cases will eject their entire digestive system.
For plants, which cannot run away, thorns, barbs, and host of toxic or distasteful chemicals can help make them more resistant to attack. Every time you are stung by stinging nettles you are being stabbed by its needle-like hairs and injected with a mild toxin, an incentive not to take a bite.
 Sharp spines help protect thistles from some grazers.Next: Competing
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