The role of biotic interactions in shaping plant flowering phenology has long been controversial; plastic responses to the abiotic environment, limited precision of biological clocks and inconsistency of selection pressures have generally been emphasized to explain phenological variation. However, part of this variation is heritable and selection analyses show that biotic interactions can modulate selection on flowering phenology. Our review of the literature indicates that pollinators tend to favour peak or earlier flowering, whereas pre-dispersal seed predators tend to favour off-peak or later flowering. However, effects strongly vary among study systems. To understand such variation, future studies should address the impact of mutualist and antagonist dispersal ability, ecological specialization, and habitat and plant population characteristics. Here, we outline future directions to study how such interactions shape flowering phenology. IntroductionFor plant reproduction, timing is everything. An individual plant that flowers too early, before it has had time to accumulate sufficient material resources, will have a limited capacity for seed production. One that delays flowering might gain higher capacity, but might also run out of time to use it before the end of the season. Flowering phenology is affected by many environmental factors, among which temperature and photoperiod, which are reliable signals of seasons, are probably the best studied. Accurate detection of such environmental cues and the resulting plastic response of plants enable flowering to occur when climatic conditions are most suitable for reproduction. Thus, resources and conditions impose bottom-up selective forces on phenology.By contrast, top-down forces act on reproductive timing, particularly those imposed by mutualists (pollinators and seed dispersers) and antagonists (floral pathogens and predispersal seed predators). Here, we review recent progress in understanding some of the top-down selective forces that act on reproductive timing. We highlight what is known,

The orchid Dactylorhiza sambucina shows a stable and dramatic flower-color polymorphism, with both yellow-and purple-flowered individuals present in natural populations throughout the range of the species in Europe. The evolutionary significance of flower-color polymorphisms found in many rewardless orchid species has been discussed at length, but the mechanisms responsible for their maintenance remain unclear. Laboratory experiments have suggested that behavioral responses by pollinators to lack of reward availability might result in a reproductive advantage for rare-color morphs. Consequently, we performed an experiment varying the relative frequency of the two color morphs of D. sambucina to test whether rare morph advantage acted in the natural habitat of the species. We show here clear evidence from this manipulative experiment that rare-color morphs have reproductive advantage through male and female components. This is the first demonstration, to our knowledge, that negative frequency-dependent selection through pollinator preference for rare morphs can cause the maintenance of a flower-color polymorphism.

More than one-third of orchid species do not provide their pollinators with either pollen or nectar rewards. Floral mimicry could explain the maintenance of these rewardless orchid species, but most rewardless orchids do not appear to have a rewarding plant that they mimic specifically. We tested the hypothesis that floral mimicry can occur through similarity based on corolla colour alone, using naive bumble-bees foraging on arrays of plants with one rewarding model species, and one rewardless putative mimic species (Dactylorhiza sambucina) which had two colour morphs. We found that when bees were inexperienced, they visited both rewardless morphs randomly. However, after bees had gained experience with the rewarding model, and it was removed from the experiment, bees resampled preferentially the rewardless morph most similar to it in corolla colour. This is the first clear evidence, to our knowledge, that pollinators could select for floral mimicry. We suggest that floral mimicry can be a selective force acting on rewardless orchids, but only under some ecological conditions. In particular, we argue that selection on early-flowering rewardless orchids that receive visits from a large pool of naive pollinators will be weakly influenced by mimicry.

The Orchidaceae characteristically contain a very large number of species that attract pollinators but do not offer them any form of reward in return for visitation. Such a strategy is highly unusual in the plant kingdom. We conducted experiments in order to manipulate the reward strategy of the rewardless bumble-bee-pollinated orchid Barlia robertiana by adding sucrose solution to inflorescences. We found that supplementation decreased the probability of a pollinator removing pollinia by approximately ten times. Despite pollinators visiting many more flowers per inflorescence on supplemented plants, eight times fewer pollinia were removed from supplemented inflorescences during each visit. Pollinia deposition patterns were not significantly affected by supplementation and no geitonogamous deposition was recorded. In populations where inflorescences were supplemented for 20 days, pollinia removal was reduced by over half for supplemented inflorescences, whereas fruit set was unmodified by supplementation. We conclude that rewardlessness would increase total seed paternity, but not change either total seed maternity or the probability that offspring were outcrossed in this species. To the authors' knowledge this is the first time that there has been an unequivocal experimental demonstration of an evolutionary advantage for rewardlessness in the Orchidaceae.

The evolution of plants that provide no form of reward for their pollinators is puzzling because they receive low numbers of pollinator visits and so have low reproductive success. To predict the evolutionary dynamics of empty morphs within a plant population, we modeled different foraging strategies that pollinators could use to avoid them. We predicted that the optimal strategy was to visit empty inflorescences randomly when these were infrequent but to use strategies such as visiting fewer flowers per inflorescence to avoid wasting time on them. As the frequencies of empty inflorescences increased, discriminating directly against empty morphs was more likely to be an optimal strategy than was avoiding the species altogether and switching to an alternative one. An experimental test of this model using artificial inflorescences showed that bumblebees used a variety of strategies to minimize time wasted on empty inflorescences. They showed weak discrimination against empty inflorescences but switched to an alternative type of inflorescence as the frequency of empty inflorescences increased. We predicted that empty morphs would be at a visitation rate disadvantage even when at low frequencies in a plant population. Differences in outcrossing rates, or male function, may explain how rewardlessness spreads in a plant population.

Summary1. While many plant species offer rewards (e.g. nectar) to pollinators, some species, particularly in orchids, do not provide rewards. Ecological factors, such as interactions with rewarding co-flowering species may affect pollinator visitation rates to such deceptive species by influencing pollinator ability to learn to avoid deceptive plants (avoidance learning). 2. We tested the effect of flower colour similarity (similar vs dissimilar) and fine-scale spatial mingling (monospecific vs heterospecific patches) of rewarding and deceptive artificial plants on pollinator visitation in a fully crossed design. We also examined the effect of these factors on learning of initially naïve bumblebees. 3. Over time, bumblebees increasingly avoided the deceptive plants, but at a significantly faster rate when deceptive and rewarding plants had dissimilar flower colours than when they were similar. 4. Deceptive plants received more visits when mingled in heterospecific patches with rewarding plants of similar flower colour than when mingled with dissimilar ones. This difference was not significant when rewarding and deceptive plants were spatially separated in monospecific patches. 5. In conclusion, both spatial mingling and flower colour similarity affected pollinator visitation to and avoidance learning of deceptive plants. This proves the validity of artificial experimental systems to study the isolated and joint effect of plant traits, and ecological factors that are crucial for the maintenance of deceptive species in natural populations.

Several hypotheses that attempt to explain invasive processes are based on the fact that plants have been introduced without their natural enemies. Among them, the EICA (Evolution of Increased Competitive Ability) hypothesis is the most influential. It states that, due to enemy release, exotic plants evolve a shift in resource allocation from defence to reproduction or growth. In the native range of the invasive species Ulex europaeus, traits involved in reproduction and growth have been shown to be highly variable and genetically correlated. Thus, in order to explore the joint evolution of life history traits and susceptibility to seed predation in this species, we investigated changes in both trait means and trait correlations. To do so, we compared plants from native and invaded regions grown in a common garden. According to the expectations of the EICA hypothesis, we observed an increase in seedling height. However, there was little change in other trait means. By contrast, correlations exhibited a clear pattern: the correlations between life history traits and infestation rate by seed predators were always weaker in the invaded range than in the native range. In U. europaeus, the role of enemy release in shaping life history traits thus appeared to imply trait correlations rather than trait means. In the invaded regions studied, the correlations involving infestation rates and key life history traits such as flowering phenology, growth and pod density were reduced, enabling more independent evolution of these key traits and potentially facilitating local adaptation to a wide range of environments. These results led us to hypothesise that a relaxation of genetic correlations may be implied in the expansion of invasive species.

European food-deceptive orchids generally flower early in spring and rely on naı¨ve pollinators for their reproduction. Some species however, flower later in the summer, when many other rewarding plants species are also in bloom. In dense flowering communities, deceptive orchids may suffer from competition for pollinator resources, or might alternatively benefit from higher community attractiveness. We investigated the pollination strategy of the deceptive species Traunsteinera globosa, and more specifically whether it benefited from the presence of coflowering rewarding species. We carried out a population survey to quantify the density and reproductive success of the orchid as well as the density of all coflowering species. Our results suggest that the deceptive orchid not only benefited from the presence of coflowering species, but that interestingly the density of the species Trifolium pratense was significantly positively correlated with the orchid's reproductive success. This species might simply act as a magnet species attracting pollinators near T. globosa, or could influence the orchid reproductive fitness through a more species-specific interaction. We propose that morphological or colour similarities between the two species should be investigated in more detail to decipher this pollination facilitation effect.