Describe the types of interspecific relationships and explain their effects on the organisms involved.

What this dot point is asking

Predation

Predation is an interaction in which one organism, the predator, kills and eats another, the prey. It directly transfers energy up the food chain and is a powerful selection pressure: predators tend to catch the slowest or least wary prey, while prey evolve defences such as camouflage, speed, or toxins. Predator and prey numbers are linked, so a rise in prey can be followed by a rise in predators, which then drives prey numbers down again, producing cyclic patterns.

Herbivory (animals eating plants) is a form of predation, and so is the relationship between a parasite and its host in the broad sense, though parasitism is usually treated separately.

Competition

Competition occurs when organisms need the same limited resource, such as food, water, light, space, or mates. It comes in two forms:

• Intraspecific competition: between members of the same species. It is often intense because individuals need exactly the same resources, and it is a key factor in limiting population size.
• Interspecific competition: between members of different species that overlap in their requirements.

Competition harms both competitors to some degree because resources are shared rather than used fully by one party. Over time, interspecific competition can drive species to use slightly different resources, a process called resource partitioning, which reduces overlap and allows coexistence.

Mutualism

Mutualism is a symbiosis in which both species benefit. Examples include:

• Pollination, where a bee gains nectar and the plant gains pollen transfer.
• The relationship between nitrogen-fixing bacteria in root nodules and legume plants, where bacteria gain sugars and the plant gains usable nitrogen.
• Coral and the algae living in their tissues, where the algae gain shelter and the coral gains sugars from photosynthesis.

Commensalism

Commensalism is a symbiosis in which one species benefits and the other is neither helped nor harmed. An example is a bird nesting in a tree: the bird gains shelter while the tree is unaffected. True commensalism is hard to confirm, because careful study often reveals a small benefit or cost to the second partner, but it remains a useful category.

Parasitism

Parasitism is a symbiosis in which one species, the parasite, benefits while the other, the host, is harmed. Parasites usually live in or on the host and take nutrients from it. Examples include tapeworms in the gut, ticks on the skin, and many disease-causing organisms. Successful parasites usually do not kill the host quickly, because the host is their food and habitat; weakening the host while keeping it alive maximises the parasite's success.

Why interactions matter for communities

These interactions are not isolated. The size and behaviour of one population affects many others through chains of predation, competition, and symbiosis. A keystone species is one whose interactions have an effect on the community far larger than its abundance suggests, such as a top predator that controls prey numbers and so allows many other species to coexist. Removing such a species can collapse the wider community.