DOI: 10.5061/dryad.s1v35
title: Data from: Ecological correlates of the distribution limits of two poeciliid species across a salinity gradient

description: 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.
description: Torres-Dowdall etal.
 BJLS 2555.R1. Statistical niche model dataData 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 dataData 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.