CLIMEX is a dynamic simulation model which manipulates available meteorological data to estimate an animal or plant's geographic distribution and relative abundance as determined by climate.

CLIMEX is applied to different species by selecting the values of a series of parameters which describe the species' response to temperature and moisture. An annual population 'Growth Index' (GI_A) describes the potential for growth of a population during the favourable season and four stress indices (Cold, Hot, Wet and Dry) describe the probability of the population surviving through the unfavourable season. The GI_A and Stress Indices are combined into an 'Ecoclimatic Index' (EI) to give an overall measure of favourableness of the location or year for permanent occupation by the target species. The results are presented as tables, graphs or maps.

A species' climatic requirements are inferred from its known geographical distribution (its native range or in another location where it is has been introduced), relative abundance and seasonal phenology. Some life cycle data, such as developmental threshold temperatures, can be used to fine tune or interpret the CLIMEX parameter values. Initial estimates of parameter values are refined by comparing predictions with the known presence or absence, or preferably relative abundance, of the species in each location. Once parameter values have been estimated, CLIMEX can be used to make predictions for other, independent locations from a database of meteorological data from nearly 3000 locations worldwide.

CLIMEX assumes that the distribution of a species is solely determined by climate. This 'potential' distribution is often modified by physical and biological factors such as soil type, microclimatic factors, topography, food quality and availability, parasites, predators and pathogens. The impact of these modifiers must be considered when making assessments of CLIMEX predictions. Past experience indicates that effects are marginal compared to climate in the majority of cases.

CLIMEX assumes that animal and plant populations experience a season which is favourable for population growth and one that is unfavourable and may jeopardise its persistence in a given area. In practice exceptions do occur, with annual plants for example, frequently not being limited by the conditions during the period that is unfavourable for growth because they are in the seed stage. There are also small areas of the world, in East Africa for example, where there are two seasons of favourable and unfavourable conditions each year associated with bimodal rainfall distributions. The former case can be handled with CLIMEX, but the latter is not easily included at present.

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