Species’ geographic distributions have already shifted during the Anthropocene. However, we often do not know what aspects of the environment drive range dynamics, much less which traits mediate organisms’ response to these environmental gradients. Most studies focus on possible climatic limits to species’ distributions and have ignored the role of biotic interactions, despite theoretical support for their importance in setting distributional limits. We used field experiments and simulations to estimate contributions of mammal herbivory to a range boundary in the California annual plant Clarkia xantiana ssp. xantiana. A steep gradient of increasing probability of herbivory occurred across the boundary, and a reanalysis of prior transplant experiments revealed that herbivory drove several-fold declines in lifetime fitness at and beyond the boundary. Simulations showed that populations could potentially persist beyond the range margin in the absence of herbivory. Using data
from a narrowly sympatric subspecies, C. x. parviflora, we also showed that delayed phenology is strongly associated with C. xantiana ssp. xantiana’s susceptibility to herbivory and low fitness beyond its border. Overall, our results provide some of the most comprehensive evidence to date of how the interplay of demography, traits, and spatial gradients in species interactions can produce a geographic range limit, and lend empirical support to recent developments in range limits theory.
Phenotypic data for C. xantiana transplant experiment
These are phenotypic data from a two year transplant experiment planting two subspecies of Clarkia xantiana (C. x. xantiana and C. x. parviflora) into three sites in Southern California. Data were collected in the field during the 1997-1998 and 1998-1999 growing seasons. Column definitions: YR (Year of the transplant (first or second)); ST (Site); year (Calendar year of the transplant); block (planting block); pol (whether plant was supplemented with pollen (1) or not (0)); population (population from which seed was sourced); ssp (subspecies (C. x. xantiana or C. x. parviflora); dtfl (Julian date of flowering); herb (whether plant experienced herbivory (1) or not (0)); germPerc (percentage of 8 seeds that germinated); fitness1 (seed set for focal plant); cell_fitness (fitness for cell, equal to product of number of germinants and focal plant seed set); Jan_size (size in January (product of number of leaves and mean leaf size)); Feb_size (size in February); March_size (size in March); April_size (size in April); May_size (size in May); June_size (size in June); size (largest size recorded for the plant)
PhenHerb_Dryad_Transplant.csv
Herbivory on translocated stems of Clarkia xantiana ssp. xantiana
Results from a stem translocation experiment exploring mammalian herbivory on C. x. xantiana at multiple sites across two years in Southern California. Stems of adult plants were harvested and placed in transects, maintained in florist tubes filled with water, and left for five days before censusing for herbivory. Column definitions: Round (Census round (1 early growing season, 4 late growing season); Date (Date of herbivory census); Year (Year of experiment); Site; Easting (Easting location of site); Transect (Transect within site); ID (Position along transect); Source (In 2015, source genotype (Center vs Edge of range)); StemHerb (Herbivory on stem (Yes or No). "discard" is due to either plant dying or experimental caging treatment imposed (not analyzed)); Comments
PhenHerb_Dryad_Translocations.csv
Herbivory in natural populations of Clarkia xantiana ssp. xantiana
Herbivory in natural populations of C. xantiana ssp. xantiana, recorded during experimental translocation of C. x. xantiana stems to explore geographic variation in mammal herbivory. Column headings: Site; Transect (transect within site); Easting (location of site); Round (census round); Date (date of census); Plant (individual plant ID); Snips (herbivory by mammal); DeadlySnip (fatal herbivory by mammal); Comments
PhenHerb_Dryad_InSituHerb.csv
R Analyses and Simulations for Benning et al. 2019
R code (in Markdown) for analyses and simulations used in Benning et al. 2019. Main sections are: analysis of stem translocation experiments to explore geographic variation in mammal herbivory; fitness analyses for simulations of 'no' and 'reduced' herbivory; analysis of relationship between phenology and herbivory. Includes supplementary analyses included in Appendix.
Analysis_PhenHerb_Dryad.Rmd