DOI: 10.5061/dryad.jd423
title: Data from: The tortoise and the hare: reducing resource availability shifts competitive balance between plant species

description: 1.Determining how changes in abiotic conditions influence community
 interactions is a fundamental challenge in ecology. Meeting this challenge
 is increasingly imperative in the Anthropocene where climate change and
 exotic species introductions alter abiotic context and biotic composition
 to reshuffle natural systems. 2.We created plant assemblages consisting of
 monocultures or equal abundance of the native community dominant bluebunch
 wheatgrass (Pseudoroegneria spicata) and the exotic spotted knapweed
 (Centaurea stoebe), a co-occurring invasive forb that has overtaken
 grasslands across the western United States. We subjected these
 composition treatments to drought (20% of average precipitation vs
 average) and herbivory on C. stoebe by its biocontrol agent Cyphocleonus
 achates to explore how reduced precipitation might influence the effects
 of competition and biocontrol herbivory on C. stoebe's abundance.
 3.At the end of seven years, C. stoebe dominated mixed-species plots under
 normal precipitation conditions, with biomass 50% greater than that of the
 native P. spicata. However, under drought stress, P. spicata's
 biomass was &gt;200% greater than C. stoebe's. Interestingly,
 both species were impervious to drought in monoculture, indicating the
 importance of the drought by competition interaction. The biocontrol
 herbivore reduced C. stoebe abundance and indirectly increased P. spicata
 biomass in mixed-species drought plots, but these effects were only
 marginally significant and relatively weak. Overall, C. stoebe abundance
 was primarily driven by the drought by competition interaction, with
 negatively additive but weak effects of the drought by herbivory
 interaction. 4.The response of the exotic to the treatments was driven by
 rapid changes in population density linked to its fast life history
 strategy, while the native's response was driven by changes in per
 capita plant biomass linked to its slower life history strategy.
 Individual plant performance metrics did not predict overall population
 responses for the invader, indicating the importance of longer-term
 population measures. 5.Synthesis. These results demonstrate that reduced
 precipitation inputs linked to climate change can dramatically shift the
 balance of plant competition, even toggling the advantage from exotic to
 native dominance. They also illustrate the importance of biotic
 interactions in predicting species responses to abiotic change.
description: DROUGHT STUDY POPULATION
 DATA 2013Data were collected in
 plots representing assemblages of varying species composition, which were
 exposed to drought and herbivory treatments. Included are population data
 (population size, canopy cover, and biomass per plant species) for the
 final year of the seven-year experiment, 2013. Headings for columns are as
 follows: BLOCK: 2 x 5 arrangement of plots with the experimental garden,
 numbered 1-15; PLOT: plot within block, numbered 1-10; DROUGHT:
 experimental factor, with D=DROUGHT, N=NORMAL; COMPOSITION: experimental
 factor, with M=MIXED SPECIES PLOT, C=CENTAUREA STOEBE MONOCULTURE,
 P=PSEUDOROEGNARIA SPICATA MONOCULTURE; HERBIVORY: experimental factor,
 with Y=HERBIVORE ADDED, N=HERBIVORE EXCLUDED; NADULTS: number of adult
 plants in plot counted per species, where suffix indicates species (C=C.
 STOEBE and P=P. SPICATA); COVER: percent canopy cover per species, where
 suffix indicates species (C=C. STOEBE and P=P. SPICATA); BIOMASS: total
 biomass in plot per species (as estimated from cover-biomass relationship
 for 2014), where suffix indicates species (C=C. STOEBE and P=P.
 SPICATA).