Distribution describes where a species is found within a habitat and how its individuals are spread out. Abundance tells us how many; distribution tells us where.Distribution is determined by abiotic factors (such as light intensity, soil pH, soil moisture, temperature, mineral ions and exposure to wind or waves) and biotic factors (such as competition, predation, grazing and the availability of food). A species can only live where conditions fall within its range of tolerance.Individuals in a population may be spread in three patterns:
- Random: the position of one individual is unrelated to others, often where resources are evenly spread (e.g. dandelions on a lawn).
- Clumped: individuals are grouped, around a resource or because offspring stay near their parents (e.g. rushes in damp hollows).
- Uniform (regular): individuals are evenly spaced, often because of competition or territories (e.g. nesting seabirds).
Distribution often changes gradually along an environmental gradient. On a rocky shore, different seaweeds form bands from low to high tide level, because species differ in how well they tolerate drying out. This banding is called zonation. Moving from open grassland into woodland, shade-tolerant plants replace light-demanding ones as light intensity falls.To study distribution, ecologists sample systematically along a line across the gradient and measure abiotic factors at each point. They then look for a correlation between a factor and the species present.
- Random: the position of one individual is unrelated to others, often where resources are evenly spread (e.g. dandelions on a lawn).
- Clumped: individuals are grouped, around a resource or because offspring stay near their parents (e.g. rushes in damp hollows).
- Uniform (regular): individuals are evenly spaced, often because of competition or territories (e.g. nesting seabirds).
Distribution often changes gradually along an environmental gradient. On a rocky shore, different seaweeds form bands from low to high tide level, because species differ in how well they tolerate drying out. This banding is called zonation. Moving from open grassland into woodland, shade-tolerant plants replace light-demanding ones as light intensity falls.To study distribution, ecologists sample systematically along a line across the gradient and measure abiotic factors at each point. They then look for a correlation between a factor and the species present.