The coexistence of ecologically similar species might be counteracted by ecological drift and demographic stochasticity, both of which erode local diversity. With niche differentiation, species can be maintained through performance trade-offs between environments, but trade-offs are difficult to invoke for species with similar ecological niches. Such similar species might then go locally extinct due to stochastic ecological drift but there is little empirical evidence for such processes. Previous studies have relied on biogeographical surveys and inferred process from pattern, while experimental field investigation of ecological drift are rare. Mechanisms preserving local species diversity, such as frequency-dependence (e. g. rare-species advantages), can oppose local ecological drift, but the combined effects of ecological drift and such counteracting forces have seldom been investigated. Here, we investigate mechanisms between coexistence of ecologically similar but strongly sexually differentiated damselfly species (Calopteryx virgo and C. splendens). Combining field surveys, behavioral observations, experimental manipulations of species frequencies and densities, and simulation modelling, we demonstrate that species coexistence is shaped by the opposing forces of ecological drift and negative frequency-dependence (rare species advantage), generated by interference competition. Stochastic and deterministic processes therefore jointly shape coexistence. The role of negative frequency-dependence in delaying the loss of ecologically similar species, such as those formed by sexual selection, should therefore be considered in community assembly, macroecology, macroevolution and biogeography.
R-script (code) to Fig. 1A-B and Table A1
This file contains the R-script and code that can be used to generate Fig. 1A-B and Table A1 in the manuscript. These figures show geographic variation in species frequencies (Fig. 1A) and density-differences between population categories (Fig. 1B), as well as the statistical tests (ANOVA and GLM) for comparison of these categories. Data-file associated with this script is provided separately.
DryadRScriptFig1TableA1.txt
Geographic variation in species frequencies and densities
This file contains the data behind Fig. 1A-B and Table A1 in the manuscript. These figures show geographic variation in species frequencies (Fig. 1A) and density-differences between population categories (Fig. 1B). The script-file (R-code) associated with this datafile is provided separately, and can be used to reproduce the statistical test associated with Table A1.
DryadCalopteryxSpeciesFrequenciesData.txt
Male aggression dataset (Fig. 3A-B, Tables A6-A7)
This file contains the C. splendens male aggression data in the intruder experiments that were performed in two allopatric and two sympatric populations (Fig. 3A-B, Tables A6-A7).
MaleAggressionDataSetDryad.txt
Script (R-code) of longevities of C. splendens and C. virgo in the mesocosm experiments
This file contains the script (R-code) needed to generate Fig. 2 and the statistical tests in Tables A2-A5, as well as the supporting Fig. D1. Three datafiles (in CSV-format) are uploaded separately, and all of these data come from the mesocosm experiments where species densities- and frequencies were experimentally manipulated in outdoor cages.
DryadRScriptFig2TablesA2ToA5.txt
Datafile "Sympatric" from the mesocosm experiments (Fig. 2A)
This datafile contains longevities of the two different species (C. splendens and C. virgo) under three different sympatric frequency-treatments and two density-treatments which are shown in Fig. 2A and Table A2. Together with the script with R-code (provided separately), readers should be able replicate the statistical tests in Table A2.
Sympatric.txt
Datafile "Splendens" in the mesocosm experiments in Fig. 2B and Table A3
This datafile contains longevity information from the mesocosm experiments for the C. splendens males, shown in Fig. 2B and Table A3. Script with R-code is provided separately and should be used with this datafile to re-generate the statistical results in Table A3.
Splendens.txt
Datafile "Virgo" for the mesocosm experiments on male longevity (Fig. 2C, Tables A4-5)
This datafile contains longevity information from the mesocosm experiments for the C. splendens males, shown in Fig. 2C and Table A4-A5. Script with R-code is provided separately and should be used with this datafile to re-generate the statistical results in Table A4-A5.
Virgo.txt
Script (R-code) for simulating negative frequency-dependence and ecological drift (Fig. 4)
This file contains the R-code needed to run the simulations in Fig. 4A-F and associated Supplemental figures. The code can be modified to vary community size (J), the strength of negative frequency-dependence (NF) and fitness ratio (FR). Each simulation run follows only 10 populations to speed up computing power, but the code can easily be modified to follow more populations, as well as the number of generations to follow. For further details, see Material and Methods in our paper and Vellend (2016; see the reference list in the paper).
The original code that we have modified in this study can be found here:
https://github.com/aammd/ecotheory/blob/master/vignettes/Box2.Rmd [MacDonald & Vellend. R code accompanying Vellend (2016) The Theory of Ecological Communities. Princeton University Press].
DryadRCodeFig4DriftSimulations.txt
