Ecology is the study of living things and their interactions with individuals of the same species, different species and their environment.   Ecologists are interested the interactions within communities that live in the same ecosystem.  Communities contain many different species and often have several separate populations of the same species.  The area in which each population lives is known as its habitat and the number of individuals living within each habitat is the population size.   

The population size of mobile organisms can be estimated using the capture-mark-release-recapture technique.  This technique provides values for the Lincoln Index, which is a mathematical means of estimating population size. 

Ecologists and conservationists are interested in how diverse communities are.  To assess diversity, species richness and species evenness are measured.  With non-mobile species, randomly sampling using quadrats provides values for the Simpson’s Diversity Index, which is a mathematical means of comparing the diversity of two or more habitats.   

Interactions between organisms are known as biotic factors.  Biotic factors are often density-dependent.  Scarcity of density-dependent factors leads to intraspecific competition, which in turn drives survival of the fittest.  While intraspecific competition is relatively common, interspecific competition is rare because different species occupy different ecological niches. 

Both abiotic factors, which include the climate, topography and edaphic factors, and biotic factors affect the carrying capacity.   Once a population has reached its carrying capacity, there are small fluctuations but the overall numbers of individuals remain relatively stable.

Ecosystems change and develop over time; this gradual process is known as ecological succession.  The typical seral stages are coloniser; pioneer; herbaceous plants; shrubs, with mixed deciduous woodland being the climax community in the UK.  Species diversity and biomass, along with soil composition, gradually change as succession takes place.  Systematic sampling techniques using transects are often used to measure succession.  

Conservation is the maintenance of biodiversity including genetic, species and habitat diversity.  International Conventions, European Union Directives and UK?laws have led to the establishment of protected areas (Ramsar sites; SPAs; SACs; MPAs; National Parks; NNRs; LNRs; SSSIs: MNRs), which?are managed for conservation by statutory bodies. 

Managing grasslands and heathlands for conservation prevents succession to the climax community from taking place and so results in a plagioclimax.  Both grasslands and heathlands are grazed, and unwanted species are removed.  Grasslands can also be managed by mowing and cutting; heathlands are often subjected to controlled burning. 

Sustainable management of woodland aims to balance the needs of humans for timber, recreation and wellbeing against their importance for wildlife, air quality and flood mitigation.  Traditional woodland management practices include coppicing and pollarding; sustainable practices in commercial forestry include selective cutting, long rotational time and selection of appropriate species.