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The ecology and evolution of seed predation by Darwin’s finches on Tribulus cistoides on the Gal�apagos Islands

SOF�IA CARVAJAL-ENDARA ,1,10 ANDREW P. HENDRY ,1,2 NANCY C. EMERY ,3 COREY P. NEU ,4

DIEGO CARMONA ,5 KIYOKO M. GOTANDA ,6 T. JONATHAN DAVIES ,1,7 JAIME A. CHAVES ,8 AND MARC T. J. JOHNSON 9

1Department of Biology, McGill University, 1205 Avenue Docteur Penfield, Montr�eal, Quebec H3A 1B1 Canada 2Redpath Museum, McGill University, 859 Sherbrooke Street West, Montr�eal, Quebec H3A 0C4 Canada

3Department of Ecology and Evolutionary Biology, University of Colorado Boulder, Boulder, Colorado 80309-0334 USA 4Department of Mechanical Engineering, University of Colorado Boulder, Boulder, Colorado 80309-0427 USA

5Departamento de Ecolog�ıa Tropical, Campus de Ciencias Biol�ogicas y Agropecuarias, Universidad Aut�onoma de Yucat�an, M�erida, Yucat�an M�exico

6Department of Zoology, University of Cambridge, Cambridge, CB2 3EJ United Kingdom 7Biodiversity Research Centre, Departments of Botany, Forest and Conservation Sciences, University of British Columbia, 2212 Main

Mall, Vancouver, British Columbia V6T 1Z4 Canada 8Colegio de Ciencias Biol�ogicas y Ambientales – Extensi�on Gal�apagos, Universidad San Francisco de Quito, Campus Cumbay�a, Casilla

Postal 17-1200-841, Quito, Ecuador 9Department of Biology, University of Toronto Mississauga, Mississauga, Ontario L5L 1C6 Canada

Citation: Carvajal-Endara, S., A. P. Hendry, N. C. Emery, C. P. Neu, D. Carmona, K. M. Gotanda, T. J. Davies, J. A. Chaves, and M. T. J. Johnson. 2020. The ecology and evolution of seed predation by Darwin’s finches on Tribulus cistoides on the Gal�apagos Islands. Ecological Monographs 90(1):e01392. 10. 1002/ecm.1392

Abstract. 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 evolu- tion 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 is a potent agent of selection that drives contemporary evolutionary change in finch beak morphology. Although the effects of these interactions 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 inten- sity of seed predation and the strength of natural selection by finches on fruit defense traits vary 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 examin- ing seed predation by finches in 30 populations of T. cistoides over 3 yr. 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 interannual 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.

Key words: adaptive divergence; coevolutionary arms race; geographic mosaic; phenotypic selection; plant defense; trophic interactions.

INTRODUCTION

Antagonistic interactions play a major role in the evo- lutionary diversification of traits that mediate species interactions (Thompson 1999, Vamosi 2005, Paterson

Manuscript received 20 December 2018; revised 8 May 2019; accepted 9 July 2019. Corresponding Editor: Todd M. Palmer

10 E-mail: [email protected]

Article e01392; page 1

Ecological Monographs, 90(1), 2020, e01392 © 2019 by the Ecological Society of America

et al. 2010). Plant–herbivore interactions have long been used as a model to understand the evolution and ecology of antagonistic interactions (Ehrlich and Raven 1964, Fritz and Simms 1992, Agrawal 2011). Plants employ a wide diversity of mechanical and chemical defense strategies to avoid the negative effects of herbivores, including seed predators (Crawley 1983, Carmona et al. 2011). In turn, herbivores and predators use a variety of strategies to counteract plant defenses, including behav- ioral, morphological, and physiological offensive traits (Karban and Agrawal 2002). Selection that favors traits that better protect plants against herbivores and preda- tors can lead to contemporary evolutionary changes in plant defense traits (Agrawal et al. 2012, Z€ust et al. 2012, Didiano et al. 2014). Here, we study the effect of seed predation by Darwin’s finches on plant ecology, and its potential role in the evolution of seed defense traits by natural selection. The interaction between Darwin’s finches and their food

plants on the Gal�apagos Islands is a famous andwell-studied example of contemporary evolution (Grant and Grant 2014). Previous studies in agroup of Darwin’s finches known as ground finches show that evolutionary changes in beak size and shape are driven by the availability and distribution of seeds (Lack 1947, Grant 1986, Grant and Grant 1995). Ground finches are primarily seed predators and poor seed dispersers; they usually crush the seeds before ingesting them, and their feces and gut samples rarely contain viable seeds (Buddenhagen and Jewell 2006, Guerrero and Tye 2009). In general, ground finches are opportunistic feeders that eat a large variety of seed species, but when resources are limited following droughts, finches become dependent on the seeds of a smaller number of plant species that are often harder and more difficult to open (Grant and Grant 1995, De Le�on et al. 2014). The ability to exploit those seeds is largely influ- enced by beak size and shape (Lack 1947, Grant and Grant 1995, De Le�on et al. 2011). Because seeds are a major part of their diet, and because ground finches exhibit preferences for certain seeds, it is anticipated that finches have an important effect on the ecology and evolution of plants on the Gal�apagos Islands. However, despite the well-developed liter- ature on the interactions between Darwin’s finches and plants (Boag and Grant 1981, Schluter and Grant 1984, Price 1987, Grant and Grant 1999, De Le�on et al. 2014), the eco- logical and evolutionary consequences of seed predation by finches on plants remains largely unexplored. The effects of seed predation by finches on plants on

the Gal�apagos Islands are expected to be mediated by both climate and the strength of species interactions. Pre- dation pressure by finches on seeds during periods with high precipitation might be negligible owing to the high production of seeds, and the increased availability of other food resources such as insects (Grant and Boag 1980, Boag and Grant 1984, Price 1985, Gibbs and Grant 1987). However, during extended droughts, when seed production is reduced, selective seed predation by finches (Grant 1986, De Le�on et al. 2014, Grant and Grant 2014) could greatly influence seed survival, plant

distributions, and the evolution of seed defense traits. Selection imposed by finches on seed defense traits is expected to play the most important role for plant species that are commonly exploited by finches. Caltrop (Tribulus cistoides) is one of the main food sources for some species of ground finches during dry periods, and it is credited with driving the evolution of beak morphology in the Medium Ground Finch (Geospiza fortis) during periods of drought (Grant and Grant 2006, 2014). The fruits of T. cistoides possess morphological features thought to provide defenses against predation, including multiple long spines and a hard protective tissue (Grant 1981; Fig. 1). Grant (1981) showed that, within a T. cistoides population on Daphne Major island, fruits with two spines were eaten more frequently than fruits with four spines, suggesting that finches impose selection on T. cis- toides fruit morphology. However, selection on T. cis- toides fruits has not been assessed across years or in populations on other islands, and the association between fruit morphology and seed survival in response to finch predation across the archipelago remains unclear. An additional factor that might influence the effects of

seed predation by finches on plants on the Gal�apagos Islands is variation in the composition of finch communi- ties. Ground finches are broadly distributed within the archipelago and most of the islands harbor several species that differ in beak size and shape. Among ground finches, only the Large Ground Finch (G. magnirostris), the Large Cactus Finch (G. conirostris), and the Medium Ground Finch (G. fortis) are able to exploit T. cistoides seeds (Grant 1981, Grant and Grant 1982). These species, however, are not uniformly distributed across the islands. The contempo- rary faunas of some major islands have one of the large- beaked G. magnirostris and G. conirostris species and the small-beaked G. fortis, such as Santa Cruz and Isabela (Fig. 2), whereas others lack the large-beaked species, such as Floreana and San Crist�obal. This spatial variation in the finch community could have large ecological and evolution- ary consequences because G. magnirostris are superior at feeding on T. cistoides seeds relative to G. fortis (Grant 1981), which could lead to divergent patterns of predation and selection imposed on fruit morphology across the Gal�apagos Islands. Our study focuses on understanding the effects of seed

predation by Darwin’s finches on the ecology and evolu- tion of T. cistoides. We asked the following three ques- tions: (1) Does seed predation by finches vary among populations, islands, finch community composition, and years? We expected seed predation to vary among years; due to variation in annual precipitation, and also in asso- ciation with finch community composition (small-beaked finches are expected to eat fewer seeds of T. cistoides dur- ing wetter conditions). (2) Do finches impose selection on T. cistoides fruit morphology, and does selection vary among populations, islands, years, and with finch com- munity composition? We expected the strength of selec- tion on fruit morphology to vary over time in correspondence with precipitation, and spatially among

Article e01392; page 2 SOF�IA CARVAJAL-ENDARA ET AL. Ecological Monographs Vol. 90, No. 1

islands in association with finch community composition: large-beaked finch species eat seeds more readily and likely impose differing selection on fruit morphology compared to communities with only small-beaked

finches. (3) Does T. cistoides fruit morphology differ among islands with contrasting finch community compo- sition (i.e., the presence/absence of large-beaked finches)? We expected spatial variation in fruit morphology to

FIG. 1. (a) Tribulus cistoides fruits (schizocarps), from left to right: a green immature fruit, a mature dry fruit, and a fruit attached to a maternal plant. (b) Two sets of dry mericarps, corresponding to two fruits of different plants, showing variation in size and number of spines. Mericarps in the upper set are larger and have four spines while mericarps in the lower set are smaller and have only two spines. (c) Opened mericarp to expose seed compartments, one empty compartment and three compartments with seeds inside. (d) Geospiza fortis (Medium Ground Finch) holding a T. cistoides mericarp. Mericarps showing marks observed (e) when seeds are eaten by finches, (f) when seeds are eaten by insects, and (g) when seeds germinate. Photo credits: Marc T. J. John- son (a [left and middle], c, and f), Andrew P. Hendry (b), Kiyoko M. Gotanda (d and e), and Sof�ıa Carvajal-Endara (a [right] and g).

February 2020 DARWIN’S FINCHES AS AGENTS OF SELECTION Article e01392; page 3

reflect spatial variation in finch community composition, which would be consistent with adaptive responses to divergent selective pressure. To address these questions, we examined variation in T. cistoides fruit morphology and patterns of seed predation in 30 natural populations across seven islands of the Gal�apagos archipelago over 3 yr, and performed a seed predation experiment in a popu- lation on one of the islands. Our study is one of the first to address the potential effect of seed predation by Dar- win’s finches on the evolution of Gal�apagos plants. We consider the importance of these results for understand- ing the potential coevolutionary interactions between Darwin’s finches and the plants whose seeds they con- sume.

METHODS

Study site and system

The Gal�apagos archipelago is located in the Pacific Ocean approximately 1,000 km west of the Ecuadorian coast in South America, and it comprises 14 major

islands and many small islets (Geist 1996). We restricted our study to seven islands that vary in finch community composition (Fig. 2), and that harbor at least one of the three finch species that consume T. cistoides seeds: G. fortis, G. conirostris, and G. magnirostris. The diet of these three finch species varies according to the size and shape of their beaks, as well as the spatial and temporal availability of seeds (Schluter and Grant 1984; Grant and Grant 1999, De Le�on et al. 2014). During dry peri- ods, especially the droughts that accompany La Ni~na events, preferred foods are limited and, hence, T. cis- toides seeds become a main food source for these finch species (Grant and Grant 2014). Tribulus cistoides (Zygophyllaceae) is a perennial pros-

trate herb native to subtropical and tropical Africa and now is widespread in tropical and subtropical arid coastal habitats around the world (Porter 1972). Broadly distributed across the Gal�apagos archipelago, it is usu- ally found in arid lowlands and coastal regions, where it grows in discrete patches close to roads, trails, and shorelines (Porter 1971). Tribulus cistoides plants can flower at any time of year on the Gal�apagos Islands, but most of its vegetative growth occurs during the wet sea- son (from January to May), they produce fruits called schizocarps (Fig. 1a), which contain five individual seg- ments referred to as mericarps that typically separate from one another as the fruit dries (Fig. 1b) (Wiggins and Porter 1971). Each T. cistoides mericarp is a hard fibrous structure that includes from one to seven seeds contained within individual compartments (Fig. 1c). Mericarps typically have four spines (two upper and two lower sharp protuberances), but the size and position of spines varies greatly among individual plants, and some mericarps completely lack some or all spines (Fig. 1b). The spiny mericarps are also a means of seed dispersal (Porter 1972); fruits adhere easily to animals, such as the feet of seabirds (Wiggins and Porter 1971). Ocean cur- rents and humans are considered important vectors of long-distance dispersal, whereby fruits travel long dis- tances by getting attached to shoes and rubber tires (Holm et al. 1977). To extract the seeds, finches pick up mericarps from

the ground after they have dropped from the plant. The finches often hold the mericarp laterally between their mandibles, and apply pressure by closing their beak, moving the upper and lower mandibles sideways to each other, to crack the mericarp wall, sometimes stabilizing the mericarp against a rock or the ground (Fig. 1d, see Video S1). The mericarps are very durable and long lived and this, combined with the very distinct damage left by finch predation, makes it possible to determine which mericarps have been depredated even months after a pre- dation event. Specifically, finches remove the ventral sur- face of the hard mericarp tissue protecting the seeds, exposing the empty seed compartments from which seeds are removed (Fig. 1c), often one compartment at a time (Video S1) (Grant 1981). Mericarps depredated by finches (Fig. 1e) are easily distinguished from mericarps

FIG. 2. Map showing the seven islands of the Gal�apagos archipelago where Tribulus cistoides fruits were sampled. Black and blue identify the islands where large-beaked ground finches are present: the Large Ground Finch (Geospiza magnirostris) is present on Isabela and Santa Cruz and the Large Cactus Finch (G. conirostris) is found on Espa~nola. Orange identifies the islands where these large-beaked finches are absent. The Med- ium Ground Finch (G. fortis) is present in all visited islands except in Espa~nola.

Article e01392; page 4 SOF�IA CARVAJAL-ENDARA ET AL. Ecological Monographs Vol. 90, No. 1

consumed by insects, which make smaller circular “drill” holes (Fig. 1f), and from mericarps from which seeds have germinated, which are apparent as empty seed com- partments are still partially enclosed by the mericarp wall (Fig. 1g), without the rough damage characteristic of seed predation by finches (Fig. 1e). Other than finches and insects, no other common predators of T. cistoides seeds are found on the Gal�apagos Islands. Unopened mericarps of T. cistoides were found in the gizzard contents of a Gal�apagos dove (Zenaida galapa- goensis); however, T. cistoides fruits are not a typical part of the diet of this species (Grant and Grant 1979).

Population sampling and experimental design

To explore impacts of seed predation by finches, we sampled nearly 7,000 mericarps from 30 T. cistoides populations across seven islands of the archipelago over 3 yr (2015–2017). Considering only ground finch species that consume T. cistoides seeds, finch seed-predator communities on three of the selected islands (Santa Cruz, Isabela, and Espa~nola) include large-beaked finch species (G. magnirostris or G. conirostris), whereas finch communities on the other four islands (San Crist�obal, Floreana, Baltra, and Seymour Norte) lack large-beaked finch species (Fig. 2). The medium-beaked species, G. fortis, is present on all sampled islands except Espa~nola (Fig. 2). Sampling was performed between the months of February and March, corresponding to the end of the dry season and beginning of the wet season (Fig. 3a), which is when the finches’ preferred food is expected to be most scarce and their consumption of T. cistoides seeds becomes highest. On four of the islands (Santa Cruz, Isabela, San Crist�obal, and Floreana), we repeated sampling annually from 2015 to 2017. During this period, the archipelago experienced strong climatic variation, including an El Ni~no event that occurred in 2015 (Stramma et al. 2016) and resulted in higher pre- cipitation relative to the preceding and subsequent years (Fig. 3b). The number of T. cistoides populations sampled var-

ied among islands (one to eight populations) due to spa- tial variation in the abundance of plants, with a “population” considered to be a discrete patch of T. cis- toides plants separated by at least 500 m from any other patch. Information about the populations sampled each year (island, geographic coordinates) is provided in Appendix S1: Table S1. From each population, we col- lected approximately 100 mericarps chosen haphazardly across the area; we made every effort to select mericarps “blindly” to avoid biases, so that mericarps represented a random subset of the morphological traits present in the population as much as possible. Most mericarps are expected to be from the previous season, but it is possi- ble that some mericarps were >1 yr old. A total of 6,391 mericarps were collected across all islands, populations, and years. For each mericarp, we used digital calipers to measure mericarp length (mm), width (mm), and the

distance between the tips of the upper spines (upper spine size, mm) located toward the distal end of the mericarp, and we noted the presence or absence of lower spines and the number of seeds removed by finches (Fig. 4a). To estimate the total number of seeds origi- nally produced in each mericarp we opened and counted the number of seeds in 752 mericarps, collected from five populations on Santa Cruz island in 2015. We evaluated the relationship between the number of seeds per meri- carp and mericarp morphology by fitting the following allometric equation: number of seeds = log(length) + log(width) + log(length) 9 log(width). We then used this model to predict the total number of seeds per mericarp (R2 = 0.48). To test whether there was variation in fruit morphol-

ogy among individual plants for selection to act upon, we sampled mericarps from two T. cistoides populations (AB and EG) on Santa Cruz island during February 2015 (see geographic information in Appendix S1: Table S1). From each population, we sampled 15 indi- vidual plants, from each of which we collected four com- plete (i.e., uneaten) and mature fruits (schizocarps), with each schizocarp having four to five mericarps. In total,

FIG. 3. Variation in (a) monthly and (b) annual precipita- tion (mm) from 2014 to 2017 on Santa Cruz island. Precipita- tion data were obtained from a meteorological station at the Charles Darwin Research Station (CDRS).

February 2020 DARWIN’S FINCHES AS AGENTS OF SELECTION Article e01392; page 5

we sampled 583 mericarps for measurement of morpho- logical traits including length, width, upper spine size, presence/absence of lower spines, and mericarp mass (to the nearest milligram using a digital balance GEM20; Smart Weigh, Jintan, China). To experimentally test whether finches impose selec-

tion on mericarp morphology, we performed a seed pre- dation experiment during March 2016. First, we collected 600 mature and intact mericarps from a T. cis- toides population (EG) located on Santa Cruz island (see geographic information in Appendix S1: Table S1). We measured four traits from each mericarp (length, width, upper spine size, and presence/absence of lower spines), and gave each mericarp a unique mark with indelible ink so mericarps could be individually identi- fied. We also applied an experimental removal of spines from a haphazard subset of the 400 mericarps by clipping either one or both of the upper spines, which allowed us to experimentally test the functional role of spines in defense. The marked mericarps were then exposed to natural finch predation on 40 circular plastic trays (~15 cm in diameter). The trays were placed across the area where the mericarps were collected, at least 30 cm apart from each other, and were monitored every three days. The mericarps were recovered after 30 d. Finally, to evaluate the relationship between mericarp

morphology and hardness, we used 102 mericarps col- lected in 2017 from three populations on Isabela island and seven populations on Santa Cruz island (Appendix S1: Table S1). For each mericarp, we mea- sured hardness (0–100 value on a Shore D scale; Pam- push et al. 2011) using a handheld durometer (Asker, Super Ex, Type D, Kyoto, Japan). As the structure of the mericarp wall varies over its surface (Fig. 4b), we mea- sured hardness at six locations on each mericarp (see detailed information in Appendix S2: Fig. S1). In addi- tion, on each mericarp, we measured six morphological traits (length, width, depth, upper spine size, longest spine length, and spine position; Fig. 4a).

Statistical analyses

All statistical analyses were performed using R v. 3.4.2 (R Development Core Team 2008).

Does seed predation by finches vary among populations, islands, finch community composition, or years?—We used logistic linear mixed-effects models with the func- tion glmer in lme4 v. 1.1-14 package (Bates et al. 2015) to model the proportion of seed predation per popula- tion (proportion of mericarps with one or more seeds removed by finches). This model was fit as follows: pre- dation per population = year + finch community com- position + year 9 finch community composition + island + error. Year, finch community composition, and their interaction were treated as fixed effects, whereas island was included as a random effect. Finch commu- nity composition was categorized as 0 on islands where large-beaked finch species (G. magnirostris and G. conirostris) were absent (Floreana, San Crist�obal, Baltra, and Seymour Norte), and 1 on islands where large-beaked finch species were present (Isabela, Santa Cruz, and Espa~nola). To examine the association of pre- cipitation with seed predation during our study, we fit a similar model in which we replaced the fixed factor year with the total annual precipitation (mm) registered dur- ing the year that preceded each sampling. Precipitation measurements, obtained from a meteorological station placed on Santa Cruz island at the Charles Darwin Research Station (0°44037.600 S, 90°50021.900 W), were log10-transformed. We also fit the following model where the response variable was the proportion of seeds removed per mericarp, and mericarp was the unit of replication: proportion of seeds removed = year + finch community composition + year 9 finch community composition + island + popu- lation(island) + error. In this analysis, the proportion of seeds consumed per mericarp was calculated as the ratio between the number of seeds removed from the mericarp

FIG. 4. (a) Mericarp traits and morphological measurements. (b) Micro-computed tomography (lCT) image showing mericarp wall variation over its surface.

Article e01392; page 6 SOF�IA CARVAJAL-ENDARA ET AL. Ecological Monographs Vol. 90, No. 1

and the number of seeds predicted based on the traits of the mericarp. We included year and finch community composition as fixed effects, whereas island and popula- tion were included as nested random effects, with the parentheses denoting nested factors. Significance of fixed effects was assessed using a type II Wald’s chi-squared test, and the significance of random effects was assessed with likelihood-ratio tests. P values were divided by two because tests of the significance of random effects are one-tailed given that variance > 0 (Littell et al. 1996). Finally, to evaluate more directly the effect of the finch community on seed predation per year (at the level of population and mericarps), we fit the logistic mixed- effects models separately for each year. We performed the analyses described above including all islands and exclud- ing data from the three islands that were sampled only in 2016 (Espa~nola, Baltra, and Seymour Norte).

Do finches impose selection on T. cistoides fruit morphol- ogy and does selection vary among populations, islands, years, or with finch community composition?—We first confirmed that most mericarp traits examined (length, upper spine size, presence/absence of lower spines, and mass) exhibit substantial variation among individual plants, with the exception of mericarp width (Appendix S3: Table S1). Next, we measured phenotypic selection (sensu Lande and Arnold 1983) on mericarps sampled from natural populations using logistic mixed- effects models in the R package lme4 v. 1.1-14 (Bates et al. 2015) to examine the relationship between T. cis- toides fitness (seed survival) and fruit morphology (Jan- zen and Stern 1998). Estimates of T. cistoides seed survival included two variables: (1) a binary response, where 0 corresponded to a mericarp that had at least one seed removed and 1 to a mericarp that had no seeds removed, and (2) the prop

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