New method for field studies on the parapatric distribution of sibling species

Spatial segregation (parapatry) often occurs between closely related species. The distributions of the two species are sometimes defined with a small overlapping zone (called a `sympatric area') which generally shifts. Exclusion is necessary to explain the persistence and shift of such a spatial pattern. Field studies are carried out to identify the type of interaction that leads to the required exclusion. This is usually achieved by estimating competition and predation parameters to define the type of interaction strong enough to imply exclusion. But interaction parameters are estimated by quantitative methods which require prolonged observation (5–10 years). These estimations are thus difficult to obtain and are open to criticism because of the spatial and temporal variations in the biotic context. Now the study of these variations requires estimation of several other interaction parameters (for different times and sites); therefore, we are unable to discuss these criticisms from a study of realistic duration. We have, therefore, developed a faster study method using the spatial properties of the sympatric area (width and velocity). We used a between-species competition and a predation model, spatially extended with coupled map lattice formalism (CML), to generate substitution waves corresponding to the `moving sympatric area' and to study their spatial properties (asymptotic width and velocity). The relationships between the asymptotic width and the interaction level had different shapes with competition and predation parameters. We used this difference to define a rapid study method that does not use quantitative estimations of the interaction parameters. This method is more reliable than the usual method with respect to the above criticisms. These findings may have considerable consequences for field studies of parapatry.