Different things may determine the population sizes within an ecosystem.
Non-living factors such as light intensity, temperature and humidity determine the number of organisms that a habitat can sustain. All species have a varying degree of ability to withstand harsh or fluctuating conditions, called resilience. If an abiotic factor changes dramatically in favour of a population – for example, plenty more light in a field – then the population will increase provided no other factors are limiting. The opposite is true if an abiotic factor changes against the resilience limit of a population – it will decrease.
“Living factors” refer to all interactions between organisms, be it a bunny rabbit being predated, or two shrubs competing for sunlight. All individual actions between organisms form a web which impacts on all populations in an ecosystem, therefore determining their sizes.
Interspecific competition refers to competition between members of different species for the same resources (food, light, water, etc.). Often when a new species is introduced in a habitat, say the American ladybird to the UK, if the invader species is better adapted, then the host population decreases in size. This may lead to…