Identifying the environmental factors responsible for the formation of a species' distribution limit is challenging because organisms interact in complex ways with their environments. However, the use of statistical niche models in combination with the analysis of phenotypic variation along environmental gradients can help to reduce such complexity and identify a subset of candidate factors. In the present study, we used such approaches to describe and identify factors responsible for the parapatric distribution of two closely-related livebearer fish species along a salinity gradient in the lowlands of Trinidad, West Indies. The downstream distribution limits of Poecilia reticulata were strongly correlated with the brackish–freshwater interface. We did not observe significant phenotypic variation in life-history traits for this species when comparing marginal with more central populations, suggesting that abrupt changes in conditions at the brackish–freshwater interface limit its distribution. By contrast, Poecilia picta was present across a wide range of salinities, although it gradually disappeared from upstream freshwater localities. In addition, P. picta populations exhibited an increase in offspring size in localities where they coexist with P. reticulata, suggesting a role for interspecific competition. The parapatric distribution of these two species, suggests that P. reticulata distributions are limited by an abiotic factor (salinity), whereas P. picta is limited by a biotic factor (interspecific competition). Similar parapatric patterns have been previously described in other systems, suggesting they might be a common pattern in nature.
Torres-Dowdall etal. BJLS 2555.R1. Statistical niche model data
Data used in the construction and testing of the Statistical niche model of Poecilia reticulata and Poecilia picta. The column labeled ‘Data’ specifies if that location was used for training the model or testing it. The ‘Drainage/Area’ column indicates either the drainage or the geographic region where the river is located. The ‘Locality’ column gives more information on the location of sampling or the name of the river sampled. The ‘P.picta’ and ‘P.reticulata’ columns specify if that particular species was present or absent at that sampling locality. Then information about the sites is presented in the following order: latitude, longitude, elevation (measured in meters above sea level), salinity (measured in part per thousands), dissolved oxygen (measured in milligrams per liter), and pH. Finally, the date of sampling is indicated.
Torres-Dowdall etal. BJLS 2555.R1. Phenotypic data
Data used in the analysis of phenotypic variation observed in wild-caught fish of Poecilia reticulata and Poecilia picta (‘Species’ column). Phenotypes measured in males include size and condition index (Kn). Phenotypes measured in females include reproductive allocation, fecundity, mean size of developing embryos, and condition index (Kn). ‘ID’ indicates internal collection number. ‘Sex’ specifies if data was collected from a male or a female. ‘Drainage’ refers to the drainage from where wild fish were collected. ‘Locality’ refers to the place along each drainage from where fish were collected (‘Upstream’ correspond to a freshwater-upstream locality where only P. reticulata occurred, ‘Midstream’ to a freshwater-midstream locality where both species were present, and ‘Downstream’ to a brackish water-downstream locality where only P. picta occurred). Size was measured as standard length of the fish (‘Length’ column, units in millimeters). Adult mass is presented as eviscerated dry mass (‘Mass’ column, units milligrams). ‘Kn’ refers to condition index, which was determined using this formula (Kn= Mi/aLib); where (a) is the intercept and (b) the slope of the least squares regression of log-transformed individual dry mass (Mi) on log-transformed individual standard length (Li). ‘#_Embryos’ was used to determine fecundity, and is a measure the number of developing embryos found in a female at the time of euthanasia. ‘Mean_offspring_mass’ is a measure of the size of each developing embryo at the time of the female’s euthanasia. ‘Reproductive_allocation’ is a measure of the percentage of the mass of a female dedicated to reproduction. ‘Mean_Stage’ is a measure of the developmental stage in which the embryos were at the time of the female’s euthanasia. Missing data or not corresponding data are showed as a dash.
