DOI: 10.5061/dryad.qq4715f
title: Data from: The ecology and evolution of seed predation by Darwin's finches on Tribulus cistoides on the Galápagos Islands

description: Predator-prey interactions play a key role in the evolution of species
 traits through antagonistic coevolutionary arms-races. The evolution of
 beak morphology in the Darwin’s finches in response to competition for
 seed resources is a classic example of evolution by natural selection. The
 seeds of Tribulus cistoides are an important food source for the largest
 ground finch species (Geospiza fortis, G. magnirostris, and G.
 conirostris) in dry months, and the hard spiny morphology of the fruits
 are a potent agent of selection that drives contemporary evolutionary
 change in finch beak morphology. Although the effects of these interaction
 on finches are well known, how seed predation affects the ecology and
 evolution of the plants is poorly understood. Here we examine whether seed
 predation by Darwin’s finches affects the ecology and evolution of T.
 cistoides. We ask whether the intensity of seed predation and the strength
 of natural selection by finches on fruit defense traits varies among
 populations, islands, years, or with varying finch community composition
 (i.e., the presence/absence of the largest beaked species, which feed on
 T. cistoides most easily). We then further test whether T. cistoides fruit
 defenses have diverged among islands in response to spatial variation in
 finch communities. We addressed these questions by examining seed
 predation by finches in 30 populations of T. cistoides over three years.
 Our study reveals three key results. First, Darwin’s finches strongly
 influence T. cistoides seed survival, whereby seed predation varies with
 differences in finch community composition among islands and in response
 to inter-annual fluctuations in precipitation. Second, finches impose
 phenotypic selection on T. cistoides fruit morphology, whereby smaller and
 harder fruits with longer or more spines exhibited higher seed survival.
 Variation in finch community composition and precipitation also explains
 variation in phenotypic selection on fruit defense traits. Third,
 variation in the number of spines on fruits among islands is consistent
 with divergent phenotypic selection imposed by variation in finch
 community composition among islands. These results suggest that Darwin’s
 finches and T. cistoides are experiencing an ongoing coevolutionary
 arms-race, and that the strength of this coevolution varies in space and
 time.
description: Seed predation of
 Tribulus cistoides seeds by Darwin's finches on natural
 populationsMericarps were sampled
 from 30 T. cistoides populations across seven islands of the archipelago
 over three years (2015-2017). For each mericarp, we measured length (mm),
 width (mm), and the distance between the tips of the upper spines (upper
 spine size, mm) located towards the distal end of the mericarp, noted the
 presence or absence of lower spines, and the number of seeds removed by
 finches. Geographic coordinates of each population is provided in Appendix
 S1: Table S1.Data_1.txtVariation in fruit morphology among individual plantsMorphological measurements of Tribulus cistoides fruits from two natural populations (AB and EG) on Santa Cruz island. We sampled 15 individual plants on each site, from each of which we collected four complete (i.e., uneaten) and mature fruits (schizocarps), with each schizocarp having 4 to 5 mericarps. From each mericarp we registered length (mm), width (mm), upper spine size, presence of lower spines, and mericarp mass (mm).Data_2.txt