Plant-Animal Interactions

Coevolution and the Web of Life

by Rodrigo Medel

Galler-induced reduction of shoot growth and fruit production in the shrub Colliguaja integerrima (Euphorbiaceae)

by Rodrigo Medel

Gonzales, W.L., P.P. Caballero & R. Medel. 2005. Galler-induced reduction of shoot growth and fruit production in the shrub Colliguaja integerrima (Euphorbiaceae). Revista Chilena de Historia Natural 78: 393-399.

We evaluated experimentally the effect of a gall-maker insect on vegetative and reproductive traits of the shrub... more We evaluated experimentally the effect of a gall-maker insect on vegetative and reproductive traits of the shrub Colliguaja integerrima (Euphorbiaceae). We performed two experiments: (1) a chemical (insecticide) exclusion to prevent gall formation, and (2) a mechanical removal of new galls at the early stage to prevent gall growth. In the study area, galled shrubs were common (77 %). Because the pattern of insect attack may
influence plant fitness, the distribution of egg clusters deposited by the galler and the number of galls among shrubs were also evaluated. Fruit production was inversely associated with the number of galls, but did not correlate with shrub height, shrub cover, and number of shoots. Ungalled shoots were longer than galled shoots after 12 months initiated the experiment. Chemical exclusion produced a delayed positive effect on plant reproduction. Fruit production was higher in experimental than control branches after 24 months. Mechanical removal of galls increased fruit production in comparison to control branches in the next reproductive season. These results indicate that the galler reduces shoot growth, and has a delayed detrimental impact on fruit production of C. integerrima. Because the chance of finding new galls was higher on the previously infected shrubs, it is possible that reinfection processes account for the cumulative negative effects of the bud-galling insect on plant fitness.

The impact of waterfowl herbivory on plant standing crop: a meta-analysis

by Kevin Wood

Wood, K.A., Stillman, R.A., Clarke, R.T., Daunt, F. & O'Hare, M.T. (2012). The impact of waterfowl herbivory on plant standing crop: a meta-analysis. Hydrobiologia, 686: 157-167.

Waterfowl can cause substantial reductions in plant standing crop, which may have ecological and economic... more Waterfowl can cause substantial reductions in plant standing crop, which may have ecological and economic consequences. However, what determines the magnitude of these reductions is not well understood. Using data from published studies, we derived the relationship between waterfowl density and reduction in plant standing crop. When waterfowl density was estimated as individuals ha−1 no significant relationship with reduction in plant standing crop was detected. However, when waterfowl density was estimated as kg ha−1 a significant, positive, linear relationship with reduction in plant standing crop was found. Whilst many previous studies have considered waterfowl species as homologous, despite large differences in body mass, our results suggest that species body mass is a key determinant of waterfowl impact on plant standing crop. To examine relative impacts of waterfowl groups based on species body mass, a measure of plant biomass reduction (R s) per bird per hectare was calculated for each group. Comparison of R s values indicated some differences in impact between different waterfowl groups, with swans having a greater per capita impact than smaller-bodied waterfowl groups. We present evidence that this difference is linked to disparities in individual body size and associated differences in intake rates, diet composition and energy requirements. Future research priorities are proposed, particularly the need for experiments that quantify the importance of factors that determine the magnitude of waterfowl impacts on plant standing crop.

An individual-based model of swan-macrophyte conflicts on a chalk river

by Kevin Wood

Wood, K.A., Stillman, R.A., Daunt, F. & O’Hare, M.T. (2012). An individual-based model of swan-macrophyte conflicts on a chalk river. In: (Eds. Boon, P.J. & P.J. Raven) River Conservation and Management. Wiley-Blackwell, Chichester (UK): pp.339-343.

Foraging mute swans Cygnus olor can cause a substantial reduction of aquatic macrophyte biomass in chalk rivers in... more Foraging mute swans Cygnus olor can cause a substantial reduction of aquatic macrophyte biomass in chalk rivers in southern England. To reduce the adverse effects of this on ecology, hydrology and fisheries interests a river management strategy needs to be able to predict where and when grazing pressure will be greatest. To assess the suitability of mathematical modelling for this purpose, an individual-based model (IBM) was constructed and tested for swans grazing a 1.1km length of the River Frome and adjacent pasture land in Dorset, England. Comparison of predicted and observed data indicates that the model was good at estimating the number of swan days, macrophyte biomass depletion, swan feeding time and feeding patch use. The results indicate that IBMs used at the appropriate scale could help to reconcile river management conflicts involving swans.

Reliance on prey derived nitrogen by the carnivorous plant Drosera rotundifolia decreases with increasing nitrogen deposition.

by Jonathan Millett

Millett J, Svensson BM, Newton J, Rydin H. 2012. New Phytologist. doi: 10.1111/j.1469-8137.2012.04139.x

• Carnivory in plants is presumed to be an adaptation to a low nutrient environment. Nitrogen (N) from carnivory is... more • Carnivory in plants is presumed to be an adaptation to a low nutrient environment. Nitrogen (N) from carnivory is expected to become a less important component of their N budget as root N availability increases.
• We investigated the uptake of N via roots versus prey of the carnivorous plant Drosera rotundifolia growing in ombrotrophic bogs along a latitudinal nitrogen deposition gradient through Sweden, using a natural abundance stable isotope mass balance technique.
• D. rotundifolia plants receiving the lowest level of N deposition obtained a greater proportion of N from prey (57%) than plants on bogs with higher N deposition (22% at intermediate and 33% at the highest deposition). When adjusted for differences in plant mass this pattern was also present when considering total prey N uptake (66, 26 and 26 µg prey N plant-1 at the low, intermediate and high N deposition sites respectively). The pattern of mass adjusted root N uptake was opposite to this (47, 75 and 86 µg N plant-1).
• D. rotundifolia plants in this study switched from reliance on prey N to reliance on root derived N as a result of increasing N availability due to atmospheric N deposition.

Effects of sampling completeness on the structure of plant - pollinator networks

by Rodrigo Medel

Accepted for publication in Ecology
Co-authored with A. Rivera-Hutinel, R.O. Bustamante & V.H. Marin

Plant-animal interaction networks provide important information on community organization. One of the most critical... more Plant-animal interaction networks provide important information on community organization. One of the most critical assumptions of network analysis is that the observed interaction patterns constitute an adequate sample of the set of interactions present in plant-animal communities. In spite of its importance, few studies have evaluated this assumption and in consequence there is no consensus on the sensitivity of network metrics to sampling methodological shortcomings. In this study we examine how variation in sampling completeness influences the estimation of six network metrics frequently used in the literature (connectance, nestedness, modularity, robustness to species loss, path length, and centralization). We analyze data of 186 flowering plants and 336 pollinator species in ten networks from a forest fragmented system in central Chile. Using species-based accumulation curves we estimated the deviation of network metrics in undersampled communities with respect to exhaustively sampled communities and the effect of network size and sampling evenness on network metrics. Our results indicate that: 1) most metrics were affected by sampling completeness, but they differ in their sensitivity to sampling effort, 2) nestedness, modularity, and robustness to species loss were less influenced by insufficient sampling than connectance, path length, and centralization, 3) robustness was mildly influenced by sampling evenness. These results caution studies that summarize information from databases with high, or unknown, heterogeneity in sampling effort per species, and stimulate researchers to report sampling intensity to standardize its effects in the search for broad patterns in plant-pollinator networks.

Baikalophyllum lobatum and Rehezamites anisolobus: two seed plants with “cycadophyte” foliage from the Early Cretaceous of eastern Asia

by Stephen McLoughlin

Reference:
Pott, C., McLoughlin, S., Lindström, A., Wu Shunqing, & Friis, E.M., 2012. Baikalophyllum lobatum and Rehezamites anisolobus: two seed plants with “cycadophyte” foliage from the Early Cretaceous of eastern Asia. International Journal of Plant Sciences 173, 192–208.

Two fossil seed plants with ‘‘cycadophyte’’ foliage, Baikalophyllum lobatum and Rehezamites anisolobus, are... more Two fossil seed plants with ‘‘cycadophyte’’ foliage, Baikalophyllum lobatum and Rehezamites anisolobus, are reinvestigated and described on the basis of new material from Lower Cretaceous strata of Liaoning and Inner Mongolia, China, and previously published material from China and Transbaikalia, Russia. The new fossils demonstrate that the Baikalophyllum plants were slender-stemmed, loosely branched, and shrublike. Yixianophyllum jinjiagouensis, described from the Zhuanchengzi beds of the Yixian Formation, Liaoning, is determined to be a junior synonym of B. lobatum. Rehezamites anisolobus, informally described from the Lower Cretaceous Yixian Formation in previous studies, is validated here, and its foliage is compared with that of Baikalophyllum. Baikalophyllum and Rehezamites have leaf morphologies and venation patterns that, in terms of general architecture, resemble those of leaves traditionally assigned to extinct members of Cycadales, Bennettitales, and Pentoxylales, but because of ambiguous circumscription and the need for thorough revisions of these groups, leaves of Baikalophyllum and Rehezamites are retained unassigned.

Biased oviposition and biased survival together help resolve a fig-wasp conflict

by Douglas Yu

Hui Wang, Jo Ridley, Derek W. Dunn, Ruiwu Wang, James M. Cook, Douglas W. Yu. Manuscript

We report evidence that helps to resolve competing explanations for stability in the classic fig-wasp mutualism. Fig... more We report evidence that helps to resolve competing explanations for stability in the classic fig-wasp mutualism. Fig trees are pollinated by wasps that lay their eggs in the tree’s ovules. Each wasp larva then develops at the expense of a fig seed. Upon maturity, the female wasps collect pollen and disperse to a new tree, continuing the cycle. Fig fitness is increased by producing both seeds and wasps, whereas short-term wasp fitness increases only with more wasps, thereby leading to a conflict of interests. We show experimentally that wasps exploit the inner layers of ovules first (biased oviposition), which is consistent with optimal-foraging theory. As oviposition increases, seeds in the middle layer are replaced on a one-to-one basis by pollinator offspring. Finally, in the outer layer of ovules, seeds disappear but are only partially replaced by pollinator offspring, which suggests high wasp mortality, perhaps due to plant defences (biased survival). We propose that selection to increase growth rate in inner ovules trades off against survivorship in outer ovules, and since there are more wasp offspring in inner ovules, biased survival is maintained. Our results suggest that both biased oviposition and biased survivorship ensure seed production in figs, thereby stabilising the mutualism. Finally, we report evidence suggesting that fig trees direct more resources to figs with more seeds, and this may help balance seed and wasp production at the level of the tree. Because an equal allocation would appear to favour the fig’s interests, the fig appears to be in control of the mutualism.

Arms Race Coevolution: The Local and Geographical Structure of a Host–Parasite Interaction

by Rodrigo Medel

Co-authored with M.A. Mendez, C.G. Ossa & C. Botto-Mahan
Evolution Education & Outreach 2910.

Consideration of complex geographic patterns of mreciprocal adaptation has provided insight into new features of the... more Consideration of complex geographic patterns of mreciprocal adaptation has provided insight into new features of the coevolutionary process. In this paper, we provide mecological, historical, and geographical evidence for coevolution under complex temporal and spatial scenarios that include intermittent selection, species turnover across localities, and a range of trait match/mismatch across populations. Our study focuses on a plant host–parasitic plant interaction endemic to arid and semiarid regions of Chile. The long spines of Chilean cacti have been suggested to evolve under parasite-mediated selection as a first line of defense against the mistletoe Tristerix aphyllus. The mistletoe, in turn, has evolved an extremely long morphological structure that emerges from the seed endosperm (radicle) to reach the host cuticle. When spine length was traced along cactus phylogenies, a significant association between spine length and parasitism was detected, indicating that defensive traits evolved in high correspondence with the presence or absence of parasitism in two cactus lineages. Assessment of spine-radicle matching across populations revealed a potential for coevolution in 50% of interaction pairs. Interestingly, hot spots for coevolution did not distribute at random across sites. On the contrary, interaction pairs showing high matching values occur mostly in the northern distribution of the interaction, suggesting a geographical structure for coevolution in this system. Only three sampled interaction pairs were so mismatched that reciprocal selection could not occur given current trait distributions. Overall, different lines of evidence indicate that arms-race coevolution is an ongoing phenomenon that occurs in the global system of interconnected populations.

Ecology and evolution of negative and positive interactions in Cactaceae: lessons and pending tasks

by Rodrigo Medel

Plant Ecology & Diversity 4: 1-11. 2011
Co-authored with Pablo C. Guerrero, Gastón O. Carvallo, Jafet M. Nassar, Julissa Rojas-Sandoval & Virginia Sanz

The Cactaceae is a diverse and conspicuous Neotropical family that has evolved a wide variety of adaptations during... more The Cactaceae is a diverse and conspicuous Neotropical family that has evolved a wide variety of adaptations during co-evolution with their interacting species. Recent research has indicated complex ecological and evolutionary interactions involving cacti and other organisms. In the framework provided by ecological interactions theory we show four case studies involving cacti: (1) the evolutionary ecology of a mistletoe–cactus parasitism in central Chile; (2) the effect of an exotic grass on the demography of a threatened cactus in Puerto Rico; (3) the herpetochory in the genus Melocactus in Venezuela; and (4) the
role of abiotic and biotic factors on the floral morphology in eight cacti in northern Chile. In addition, we highlight some key aspects that might be useful as guidelines and recommendations for structuring future research and collaborative efforts in cactus biology research; these are related to the development of a cactus interaction database that integrates published information; in invasion biology, aspects that account for eventual detrimental effects of invasive species to cactus diversity; and the role of multi-species interactions such as pollination, seed dispersal and nurse plant associations for the maintenance of cacti and the ecosystem services that they provide.

Trichomes on the leaves of Anomozamites villosus sp. nov. (Bennettitales) from the Daohugou beds (Middle Jurassic), Inner Mongolia, China: Mechanical defence against herbivorous arthropods

by Stephen McLoughlin

Reference:
Pott, C., McLoughlin, S., Wu Shunqing, Friis, E.E., 2012. Trichomes on the leaves of Anomozamites villosus sp. nov. (Bennettitales) from the Daohugou beds (Middle Jurassic), Inner Mongolia, China: Mechanical defence against herbivorous arthropods. Review of Palaeobotany and Palynology 169, 48–60.

The abaxial leaf surfaces of Anomozamites villosus sp. nov., a new species of bennettitalean foliage from the Middle... more The abaxial leaf surfaces of Anomozamites villosus sp. nov., a new species of bennettitalean foliage from the Middle Jurassic Daohugou beds of Inner Mongolia, China, are covered with soft, shaggy hairs that are loosely, but regularly distributed across the lamina. Selectively arranged, long, stiff or acerate hairs directed or bent towards the leaf apex, project on the abaxial side of the leaf from the rachis margins. Both types of hairs are interpreted predominantly as adaptations for passive defence against damage by arthropods (sap-sucking, leaf-chewing or oviposition) rather than being morphological responses to physical environmental parameters, e.g. xeromorphic features. As several of the leaves show evidence for high levels of leaf-margin feeding by phytophagous arthropods, these mechanisms have not been entirely effective and may have evolved to defend against only selected arthropods or behavioural strategies. These findings contribute to the reconstruction of ecological interactions in the exceptionally preserved Jurassic Daohugou biota of northern China, and provide a rare insight into the plant–arthropod co-evolutionary ‘arms race’.

Effects of vector behavior and host resistance on mistletoe aggregation

by Rodrigo Medel

Co-authored with E. Vergara, A. Silva & M. Kalin
Ecology 2004

Understanding the factors affecting parasite aggregation in natural host populations is one of the central questions... more Understanding the factors affecting parasite aggregation in natural host populations is one of the central questions in parasite ecology. While different biological mechanisms giving rise to aggregation have been documented in the literature, the role of established parasites in vector attraction, and its importance in determining clumped parasite distributions has received less attention. In a two-year field study, we evaluated the importance of a bird vector, Mimus thenca (Mimidae), on the aggregation dynamics of the holoparasitic mistletoe, Tristerix aphyllus, on its cactus host, Echinopsis chilensis. Removal
of T. aphyllus from cacti decreased the number of visits and the time spent by the bird vector, which resulted in a 3.5-fold lower seed deposition of the mistletoe on experimental hosts than on control hosts. Vector preference, however, was not the only factor affecting aggregation in this system. Spine length of the cactus acted as a first line of defense against parasitism, by discouraging bird perching on top of host columns. While heavily parasitized hosts received more seeds than unparasitized hosts, spines counteracted this effect. These results provide field evidence that parasite aggregation results from the balance between vector behavior and host resistance traits.

Convergence and historical effects in harvester ant assemblages of Australia, North America, and South America

by Rodrigo Medel

Biological Journal of the Linnean Society. 1995.

In this paper I examine the extent to which contemporary ecological patterns in 42 ant assemblages of three continents... more In this paper I examine the extent to which contemporary ecological patterns in 42 ant assemblages of three continents can be explained as a result of present-day environments or from differences in the history of each ant biota. The contribution of each factor to the overall variation of six community characters was evaluated through Schluter`s ANOVA procedure.

Notes on the floral biology and pollination syndrome of Echinopsis chiloensis (Cactaceae) in a population of semiarid Chile

by Rodrigo Medel

Co-authored with C.G. Ossa
Gayana Botanica (Chile). 2011.

We report some aspects of the floral biology of Echinopsis chiloensis ssp. chiloensis in a population near to the... more We report some aspects of the floral biology of Echinopsis chiloensis ssp. chiloensis in a population near to the northern limit of its distribution. Anthesis is strictly diurnal and flowers remain open during 492 min on the average. Echinopsis chiloensis is self-incompatible. Flower longevity, nectar production along day, and floral visitors indicate that this E. chiloensis population fits well to a diurnal insect pollination syndrome. These results contrast with a previous report indicating nocturnal anthesis and mixed mellitophylous-sphingophylous pollination syndrome in populations near to the
southern limit of the distribution range. We suggest that the pollination syndrome of E. chiloensis is a labile rather than fixed
condition that may depend on the abiotic characteristics and geographical location of the population under assessment.

Shifting species interactions in terrestrial dryland ecosystems under altered water availability and climate change

by Kevin McCluney

Species interactions play key roles in linking the responses of populations, communities, and ecosystems to... more Species interactions play key roles in linking the responses of populations, communities, and ecosystems to environmental
change. For instance, species interactions are an important determinant of the complexity of changes in trophic biomass
with variation in resources. Water resources are a major driver of terrestrial ecology and climate change is expected to
greatly alter the distribution of this critical resource. While previous studies have documented strong effects of global
environmental change on species interactions in general, responses can vary from region to region. Dryland ecosystems
occupy more than one-third of the Earth’s land mass, are greatly affected by changes in water availability, and are
predicted to be hotspots of climate change. Thus, it is imperative to understand the effects of environmental change on
these globally significant ecosystems.
Here, we review studies of the responses of population-level plant-plant, plant-herbivore, and predator-prey
interactions to changes in water availability in dryland environments in order to develop new hypotheses and
predictions to guide future research. To help explain patterns of interaction outcomes, we developed a conceptual
model that views interaction outcomes as shifting between (1) competition and facilitation (plant-plant), (2) herbivory,
neutralism, or mutualism (plant-herbivore), or (3) neutralism and predation (predator-prey), as water availability crosses
physiological, behavioural, or population-density thresholds. We link our conceptual model to hypothetical scenarios
of current and future water availability to make testable predictions about the influence of changes in water availability
on species interactions. We also examine potential implications of our conceptual model for the relative importance of
top-down effects and the linearity of patterns of change in trophic biomass with changes in water availability. Finally,
we highlight key research needs and some possible broader impacts of our findings. Overall, we hope to stimulate and
guide future research that links changes in water availability to patterns of species interactions and the dynamics of
populations and communities in dryland ecosystems.

Seed dispersers as disease vectors: bird transmission of mistletoes seeds to plant hosts

by Rodrigo Medel

Co-authored with C. Martinez del Rio, A. Silva and M. Hourdequin
Ecology. 1996.

The relationship between mistletoes and birds has been studied from the perspectives of mutualism and seed dispersal.... more The relationship between mistletoes and birds has been studied from the perspectives of mutualism and seed dispersal. Here, we emphasize the role that avian dispersers play as agents of mistletoe seed transmission to plant hosts. We describe the patterns of transmission of the seeds of Tristerix aphyllus, an endophytic Chilean mistletoe, on two of its columnar cacti hosts (Eulychnia acida and Echinopsis skottsbergii) by the Chilean Mockingbird Mimus thenca. In north-central Chile, these cacti grow in relatively discrete subpopulations on north-facing slopes. We measured variation in seed transmission within 10 subpopulations varying in species composition, host density, parasite density, parasite prevalence (defined as the percentage of hosts infested in a given population), and disperser abundance. Seed transmission was independent of species, but was strongly de- pendent on prior parasitism. Parasitized individuals received seeds much more frequently than expected from their relative abundance. We found no correlation between the density of hosts and seed transmission. We found strong positive correlations, however, between parasite prevalence and seed transmission to both parasitized and nonparasitized hosts. Seed transmission of T. aphyllus seeds by M. thenca appeared to be frequency- rather than density- dependent. Seed transmission was also tightly and positively correlated with the abundance of seed-dispersing birds at each site. Because bird abundance and parasite prevalence were correlated, we conducted path analysis to disentangle their relative effect on seed trans- mission. A model including only the direct effect of bird abundance and the indirect effect of parasite prevalence through bird abundance explained roughly the same variance as a full model including both the direct and indirect effects of bird abundance and prevalence on seed transmission. Apparently, variation in bird abundance was the main determinant of variation in transmission. We suggest that mistletoes, host plants, and the birds that disperse mistletoe seeds are systems well suited for studies of the ecological and evolu- tionary dynamics of disease transmission.

Pollinator-mediated selection on the nectar guide phenotype in the Andean monkey flower, Mimulus luteus

by Rodrigo Medel

Co-authored with C. Botto-Mahan and M.T. Kalin-Arroyo
Ecology. 2003.

Mimulus luteus (Scrophulariaceae) is a perennial herb occurring in the South American Andes that shows a wide... more Mimulus luteus (Scrophulariaceae) is a perennial herb occurring in the South American Andes that shows a wide variation in the size and shape of a red spot on the lower lobe of the yellow flower. We describe the preference of four insects (three bees and one butterfly) and one hummingbird species for floral characters, and estimated the strength, direction, and form of pollinator-mediated selection through female fitness. We applied geometric morphometrics to describe the preference of pollinator species for different guide shapes. Our results revealed striking differences in the floral phenotypes preferred by insects and hummingbirds. Insects visited flowers with corollas 1.25-fold larger and guides 1.72-fold larger than the hummingbird species did. While insects preferred flowers with nectar guides pointing toward the corolla tube, the hummingbird preferred flowers with heartshaped nectar guides. Most of the floral preferences shown by pollinators translated into significant linear and nonlinear selection coefficients. When selection was analyzed on a per-flower basis and for female fitness, corolla size was under positive directional selection, and nectar guide size and shape were under disruptive selection. Because the insect and hummingbird pollinators showed a strong segregation in their daily activity time, we suggest that current disruptive selection on the nectar guide phenotype can result from the differential availability of the rewarding floral variants over a day. Our findings suggest that pollinator-mediated selection favoring extreme phenotypes in M. luteus may not only contribute to high nectar guide variation found in this species, but also can promote divergence of corolla and nectar guide traits.

Pollinator-mediated selection in a specialized pollination system: matches and mismatches across populations

by Rodrigo Medel

Co-authored with J. Nattero and A.A. Cocucci
Journal of Evolutionary Biology

Most studies on pollinator-mediated selection have been performed in
generalized rather than specialized... more Most studies on pollinator-mediated selection have been performed in
generalized rather than specialized pollination systems. This situation has
impeded evaluation of the extent to which selection acts on attraction or
specialized key floral traits involved in the plant-pollinator phenotypic
interphase. We studied pollinator-mediated selection in four populations of
Nierembergia linariifolia, a self-incompatible and oil-secreting plant pollinated
exclusively by oil-collecting bees. We evaluated whether floral traits experience
variable selection among populations and whether attraction and fit traits
are heterogeneously selected across populations. Populations differed in every
flower trait and selection was consistently observed for corolla size and flower
shape, two traits involved in the first steps of the pollination process. However,
we found no selection acting on mechanical-fit traits. The observation that
selection occurred upon attraction rather than mechanical-fit traits, suggests
that plants are not currently evolving fine-tuned morphological adaptations to
local pollinators and that phenotypic matching is not necessarily an expected
outcome in this specialized pollination system.

Pollinator‐mediated selection and experimental manipulation of the flower phenotype in Chloraea bletioides

by Rodrigo Medel

Co-authored with M. Cuartas-Dominguez
Functional Ecology. 2010.

1. While many studies of flower evolution have relied on the assessment of selection coefficients, these estimates... more 1. While many studies of flower evolution have relied on the assessment of selection coefficients, these estimates represent correlative evidence of selection. Few studies have combined correlative with experimental evidence, and hence the mechanisms underlying selection processes are not well understood.

2. In this study we quantified the magnitude, direction and form of pollinator-mediated selection acting upon morphological traits in the Chilean endemic orchid Chloraea bletioides. Results revealed strong selection differentials for plant height and flower number, and absence of total selection for morphological flower characters. Multivariate analysis of selection revealed a strong and positive directional selection gradient for flower number.

3. The labellar lobe and flower tube were under negative correlational selection, indicating that selection promoted alternative combinations of traits. Plants with long floral tubes and small labellar lobes, and short floral tubes with large labellar lobes were selected for. Interestingly, selection on flower number increased at a higher rate through the female function than through the male function, which suggests that size-dependent resource limitation may be more important than pollinator-mediated selection for fruit set.

4. In addition to estimates of phenotypic selection, we evaluated through phenotype modification the impact of pollinators on the correlation and potential integration of flower traits coming from different whorls. We removed the tip of the lateral sepals and the labellar lobe in a 2 x 2 factorial design.

5. Results revealed that phenotypic modification had no effect on male and female fitness. The absence of effect was consistent with the low and non-significant selection coefficients. Nevertheless, the observation of significant correlational selection for labellar lobe and flower tube suggests that experimental studies performed on these characters may be of interest to evaluate the functional and adaptive integration of flower design in orchids.

Ecología y evolución de interacciones planta-animal

by Rodrigo Medel

Edited by R. Medel,M. Aizen & R. Zamora. Editorial Universitaria. 399 pp. Spanish