Mar Sobral

Do seed-dispersing birds exert selection on optimal plant trait combinations? Correlated phenotypic selection on the fruit and seed size of hawthorn ( Crataegus monogyna )

Seed-dispersing birds can be expected to optimize their energy intake through fruit selection and hence exert a positive selection on fruit pulp content, that is to say, favoring big fruits with small seeds. On the other hand they may select both, average fruit size and its variation. We addressed this issue by analyzing the phenotypic selection exerted by the dispersers of Crataegus monogyna on the fruit and seed size of this species. Fruit and seed size were analyzed at two ontogenic plant stages: the initial size of seeds (and their fruits) as future individuals and the mean size of fruits and seeds produced by adult plants. Fruit diameter and its within-individual variation are the actual targets of selection for maternal fruit traits, negatively affecting relative fitness, although total selection acts only on mean fruit size. For individual seeds, size selection is positive and directed at fruit diameter and seed length, while the remaining traits are subject to indirect selection. Birds exerted a correlational selection favoring big fruits with small seeds. Nevertheless the evolutionary consequences of this selection are expected to be limited by several factors. For example, the positive correlation between fruit and seed sizes, the existence of counteracting selective pressures exerted by other mutualistic or antagonist interactions and temporal or spatial changes in the environmental conditions that may alter the selective forces and even the optimum phenotype in each situation.

Selective Pressures Explain Differences in Flower Color among Gentiana lutea Populations.

Flower color variation among plant populations might reflect adaptation to local conditions such as the interacting animal community. In the northwest Iberian Peninsula, flower color of Gentiana lutea varies longitudinally among populations, ranging from orange to yellow. We explored whether flower color is locally adapted and the role of pollinators and seed predators as agents of selection by analyzing the influence of flower color on (i) pollinator visitation rate and (ii) escape from seed predation and (iii) by testing whether differences in pollinator communities correlate with flower color variation across populations. Finally, (iv) we investigated whether variation in selective pressures explains flower color variation among 12 G. lutea populations. Flower color influenced pollinator visits and differences in flower color among populations were related to variation in pollinator communities. Selective pressures on flower color vary among populations and explain part of flower color differences among populations of G. lutea. We conclude that flower color in G. lutea is locally adapted and that pollinators play a role in this adaptation.

Are pollinators and seed predators selective agents on flower color in Gentiana lutea

Animals which interact with plants often cause selective pressures on plant traits. Flower color variation within a species might be shaped by the action of animals feeding on the plant species. Pollinators might exert natural selection on color if flower color is related to their foraging efficiency. For example, some pollinator species might require more time to detect particular colors. If that is the case, flower color might have evolved as a pollination exploitation barrier—ensuring that flowers are more visited by the most efficient pollinators. In addition, non-pollinator agents such as predispersal seed predators may select on flower color, if color indicates food resources (seeds) or if color is related to deterrent compounds. We address selection on flower color in a population of Gentiana lutea where color varies among individuals from yellow to orange. We hypothesize that opposed selection from mutualists (pollinators) and antagonists (predispersal seed predators) maintains flower color variation in this population. By means of path analysis we addressed the role of both interactors in flower color selection. We found that selection acts on flower color, mediated by both pollinators and seed predators. Both agents favored yellow-flowered individuals, thus selection by pollinators and seed predators does not maintain flower color variation in this population.

Selective Pressure along a Latitudinal Gradient Affects Subindividual Variation in Plants

Individual plants produce repeated structures such as leaves, flowers or fruits, which, although belonging to the same genotype, are not phenotypically identical. Such subindividual variation reflects the potential of individual genotypes to vary with micro-environmental conditions. Furthermore, variation in organ traits imposes costs to foraging animals such as time, energy and increased predation risk. Therefore, animals that interact with plants may respond to this variation and affect plant fitness. Thus, phenotypic variation within an individual plant could be, in part, an adaptive trait. Here we investigated this idea and we found that subindividual variation of fruit size of Crataegus monogyna, in different populations throughout the latitudinal gradient in Europe, was explained at some extent by the selective pressures exerted by seed-dispersing birds. These findings support the hypothesis that within-individual variation in plants is an adaptive trait selected by interacting animals which may have important implications for plant evolution.

FRUIT-SIZE PREFERENCES IN WILD AND NAIVE EURASIAN BLACKBIRDS (TURDUS MERULA) FEEDING ON ONESEED HAWTHORN (CRATAEGUS MONOGYNA)

ABSTRACT. Frugivorous birds are expected to maximize pulp ingestion and, hence, prefer large fruits. Experience and learning may play a role in fruit-size choices, and species may also have innate preferences. We investigated preference for fruit size of Oneseed Hawthorn (Crataegus monogyna) in wild and naive Eurasian Blackbirds (Turdus merula). The preference patterns of wild and naive birds were similar, although greater differences were found among fruit-size classes in wild birds. When presented with a similar number of fruits per size class, both naive and wild birds preferred large fruits. However, when the same fruit mass per size class was offered, they reduced overall fruit consumption, and preferences for large fruits became weaker. The results suggest that Eurasian Blackbirds behave as energy-intake maximizers when feeding on Oneseed Hawthorn fruit and that they are driven, to some extent, by innate preferences for fruit size. However, birds in aviaries may not always behave in the same way as their wild counterparts, and further work is needed to verify that wild birds behave similarly under natural conditions.

Seed predators exert selection on the subindividual variation of seed size

Subindividual variation among repeated organs in plants constitutes an overlooked level of variation in phenotypic selection studies, despite being a major component of phenotypic variation. Animals that interact with plants could be selective agents on subindividual variation. This study examines selective pressures exerted during post-dispersal seed predation and germination on the subindividual variation of seed size in hawthorn (Crataegus monogyna). With a seed offering experiment and a germination test, we estimated phenotypic selection differentials for average and subindividual variation of seed size due to seed predation and germination. Seed size affects germination, growth rate and the probability of an individual seed of escaping predation. Longer seeds showed higher germination rates, but this did not result in significant selection on phenotypes of the maternal trees. On the other hand, seed predators avoided wider seeds, and by doing so exerted phenotypic selection on adult average and subindividual variation of seed size. The detected selection on subindividual variation suggests that the levels of phenotypic variation within individual plants may be, at least partly, the adaptive consequence of animal-mediated selection.

Mammal diversity influences the carbon cycle through trophic interactions in the Amazon

Biodiversity affects many ecosystem functions and services, including carbon cycling and retention. While it is known that the efficiency of carbon capture and biomass production by ecological communities increases with species diversity, the role of vertebrate animals in the carbon cycle remains undocumented. Here, we use an extensive dataset collected in a high-diversity Amazonian system to parse out the relationship between animal and plant species richness, feeding interactions, tree biomass and carbon concentrations in soil. Mammal and tree species richness is positively related to tree biomass and carbon concentration in soil—and the relationship is mediated by organic remains produced by vertebrate feeding events. Our research advances knowledge of the links between biodiversity and carbon cycling and storage, supporting the view that whole community complexity—including vertebrate richness and trophic interactions—drives ecosystem function in tropical systems. Securing animal and plant diversity while protecting landscape integrity will contribute to soil nutrient content and carbon retention in the biosphere.A high-diversity Amazonian system reveals the influence of mammalian diversity on the carbon cycle, mediated through vertebrate feeding events.

Is there a hybridization barrier between Gentiana lutea color morphs

In Gentiana lutea two varieties are described: G. lutea var. aurantiaca with orange corolla colors and G. lutea var. lutea with yellow corolla colors. Both color varieties co-occur in NW Spain, and pollinators select flower color in this species. It is not known whether a hybridization barrier exists between these G. lutea color varieties. We aim to test the compatibility between flower color varieties in G. lutea and its dependence on pollen vectors. Within a sympatric population containing both flower color morphs, we analyzed differences in reproductive success (number, weight, viability and germinability of seeds) depending on fertilization treatments (autogamy and xenogamy within variety and among varieties). We found a 93% reduction in number of seeds and a 37% reduction in seed weight respectively of autogamy treatments compared to xenogamy crossings. Additionally, reproductive success is higher within color varieties than among varieties, due to a 45% seed viability reduction on hybrids from different varieties. Our results show that G. lutea reproductive success is strongly dependent on pollinators and that a partial hybridization barrier exists between G. lutea varieties.

Flower colour variation in the montane plant Gentiana lutea L. (Gentianaceae) is unrelated to abiotic factors

Background: Flower colour variation among populations may result from the spatial variation of selective agents. The structure of phenotypic variation informs on the ecological processes related to this variation. Variation in floral traits is mainly attributed to variation in the pollinator fauna, while variation in vegetative traits is usually linked to abiotic factors or herbivores. Aims: We investigated the geographical variation of flower colour (and correlated traits) in Gentiana lutea and the relationship with the variation in abiotic factors. Methods: Phenotypic variation (flower colour, petal length, petal width, stalk length, leaf length, flower number, petal number and number of basal leaves) was assessed in 429 plants of 12 populations located at north-west Iberia. Additionally, we obtained data on the geographical coordinates, elevation, temperature, rainfall and radiation for each population. Results: Populations mostly differed in flower colour, from orange to yellow from west to the east. Abiotic factors were unrelated to variation in either floral or vegetative traits. Conclusions: Phenotypic variation among G. lutea populations does not result from adaptation to environmental factors. Other factors, such as historical events or selective pressure exerted by biotic interactions, might explain the flower colour variation in G. lutea along the Cantabrian Range.

Author Correction: Mammal diversity influences the carbon cycle through trophic interactions in the Amazon

In the version of this Article originally published, the surname of Ted K. Raab was misspelt. This error has now been corrected in all versions of the Article.