Pollen carryover, pollinator movement, and spatial context impact the delivery of pollination services in apple orchards.

Assessing the relative contributions of different pollinator taxa to pollination services is a central task in both basic eco-evolutionary research and applied conservation and agriculture. To that end, many studies have quantified single-visit pollen deposition and visitation frequency, which together determine a pollinator species' rate of conspecific pollen delivery. However, for plant species that require or benefit from outcrossing, pollination service quality further depends upon the ratio of outcross to self-pollen deposited, which is determined by two additional pollinator traits: pollen carryover and movement patterns among genetically compatible plant individuals. Here, we compare the pollination capacities of managed honey bees, native bumble bees, and native mining bees in apple-a varietally self-incompatible commercial crop-when pollen carryover and pollinator movement patterns are considered. We constructed simulation models of outcross pollen deposition parameterized using empirically measured single-visit pollen deposition, visitation frequency, and probabilities of intertree movement exhibited by each pollinator type, as well as pollen carryover patterns simulated based on parameters reported in the literature. In these models, we also explicitly specified the spatial relationships among cross-compatible trees based on field-realistic orchard layout schemes. We found that estimated pollination service delivery was considerably reduced for all pollinator types when pollen carryover and pollinator movement patterns were considered, as compared to when only single-visit pollen deposition and visitation frequency were considered. We also found that the performance of different pollinator types varied greatly across simulated orchard layout schemes and pollen carryover scenarios, including one instance where bumble and mining bees reversed their relative rankings. In all simulations, native bumble and mining bees outperformed managed honey bees in terms of both outcross pollen delivery per unit time and per flower visited, with disparities being greatest under scenarios of low pollen carryover. We demonstrate the degree to which pollination studies may reach inaccurate conclusions regarding pollination service delivery when pollen carryover and pollinator movement patterns are ignored. Our finding of the strong context dependence of pollination efficiency, even within a single plant-pollinator taxon pair, cautions that future studies in both basic and applied pollination biology should explicitly consider the ecological context in which pollination interactions take place.

Progressive deterioration of pollination service detected in a 17-year study vanishes in a 26-year study.

Widespread reports of declining populations of pollinators have raised concerns that plant populations may be incurring increasing shortfalls in pollination, but few studies have measured pollination deficits over enough seasons to detect such changes. I have conducted pollen-supplementation experiments in a wild population of the glacier lily (Erythronium grandiflorum, Liliaceae) from 1993 to 2018. Pollination deficits were estimated by comparing the fruit set of hand-pollinated, single-flowered plants to that of open-pollinated controls. For a subset of years, seed set data were also available. A previous publication reported a significant deterioration of pollination in this population from 1993 to 2009, and suggested phenological dislocation as a possible cause. That deterioration is no longer evident in the longer-term data set. Very long time series may be necessary to detect temporal trends in pollination service. This population consistently experiences stronger pollination deficits before its flowering peak than after. This heterogeneity suggests caution in characterizing a population as pollination-limited or not, even within a single season.

Effects of spatial patterning of co-flowering plant species on pollination quantity and purity.

Background and Aims: If two plant species share pollinators, it has been proposed that the interaction between them may range from competitive to facilitative, depending on the way in which they intermingle. In particular, the presence of a rewarding plant species may increase the rate of pollinator visitation to a less rewarding species in its vicinity, but the beneficial increase in visitation may be counteracted by a detrimental increase in heterospecific pollen transfer. We assessed this trade-off using bumble-bees foraging over a gradual spatial transition between two plant species in an indoor cage experiment. Methods: We used two 'species' of artificial flowers - one more rewarding than the other - in arrays that varied in the degree of species intermingling. The flowers dispensed and received powdered food dyes serving as pollen analogues. Captive bumble-bees visited to collect sucrose solution. We quantified dye delivery to the adhesive-tape 'stigmas' in flowers by spectrophotometry. Key Results: Across the spatial transition between species, the less attractive species received more dye (more bee visits) when in proximity to the more attractive species than it did when alone, but the larger dye loads were less pure (more heterospecific pollen transfer). The decline in purity cancelled out the gain in acquisition, so conspecific pollen receipt by the less attractive species was neutrally affected. The more attractive species received fewer visits when surrounded by the less attractive species, so the interaction between the two species was amensalism when considering conspecific pollen reception. Conclusions: Pollinator-mediated interactions between plant species depend on pollination quantity and purity, both of which can depend on spatial intermingling.

Confounding effects of spatial variation on shifts in phenology.

Shifts in the timing of life history events have become an important source of information about how organisms are responding to climate change. Phenological data have generally been treated as purely temporal, with scant attention to the inherent spatial aspects of such data. However, phenological data are tied to a specific location, and considerations of sampling design, both over space and through time, can critically affect the patterns that emerge. Focusing on flowering phenology, we describe how purely spatial shifts, such as adding new study plots, or the colonization of a study plot by a new species, can masquerade as temporal shifts. Such shifts can look like responses to climate change but are not. Furthermore, the same aggregate phenological curves can be composed of individuals with either very different or very similar phenologies. We conclude with a set of recommendations to avoid ambiguities arising from the spatiotemporal duality of phenological data.

Site fidelity by bees drives pollination facilitation in sequentially blooming plant species.

Plant species can influence the pollination and reproductive success of coflowering neighbors that share pollinators. Because some individual pollinators habitually forage in particular areas, it is also possible that plant species could influence the pollination of neighbors that bloom later. When flowers of a preferred forage plant decline in an area, site-fidelity may cause individual flower feeders to stay in an area and switch plant species rather than search for preferred plants in a new location. A newly blooming plant species may quickly inherit a set of visitors from a prior plant species, and therefore experience higher pollination success than it would in an area where the first species never bloomed. To test this, we manipulated the placement and timing of two plant species, Delphinium barbeyi and later-blooming Gentiana parryi. We recorded the responses of individually marked bumble bee pollinators. About 63% of marked individuals returned repeatedly to the same areas to forage on Delphinium. When Delphinium was experimentally taken out of bloom, most of those site-faithful individuals (78%) stayed and switched to Gentiana. Consequently, Gentiana flowers received more visits in areas where Delphinium had previously flowered, compared to areas where Delphinium was still flowering or never occurred. Gentiana stigmas received more pollen in areas where Delphinium disappeared than where it never bloomed, indicating that Delphinium increases the pollination of Gentiana when they are separated in time. Overall, we show that individual bumble bees are often site-faithful, causing one plant species to increase the pollination of another even when separated in time, which is a novel mechanism of pollination facilitation.

Ants and ant scent reduce bumblebee pollination of artificial flowers.

Ants on flowers can disrupt pollination by consuming rewards or harassing pollinators, but it is difficult to disentangle the effects of these exploitative and interference forms of competition on pollinator behavior. Using highly rewarding and quickly replenishing artificial flowers that simulate male or female function, we allowed bumblebees (Bombus impatiens) to forage (1) on flowers with or without ants (Myrmica rubra) and (2) on flowers with or without ant scent cues. Bumblebees transferred significantly more pollen analogue both to and from ant-free flowers, demonstrating that interference competition with ants is sufficient to modify pollinator foraging behavior. Bees also removed significantly less pollen analogue from ant-scented flowers than from controls, making this the first study to show that bees can use ant scent to avoid harassment at flowers. Ant effects on pollinator behavior, possibly in addition to their effects on pollen viability, may contribute to the evolution of floral traits minimizing ant visitation.

The psychophysics of sugar concentration discrimination and contrast evaluation in bumblebees.

The capacity to discriminate between choice options is crucial for a decision-maker to avoid unprofitable options. The physical properties of rewards are presumed to be represented on context-dependent, nonlinear cognitive scales that may systematically influence reward expectation and thus choice behavior. In this study, we investigated the discrimination performance of free-flying bumblebee workers (Bombus impatiens) in a choice between sucrose solutions with different concentrations. We conducted two-alternative free choice experiments on two B. impatiens colonies containing some electronically tagged bumblebees foraging at an array of computer-automated artificial flowers that recorded individual choices. We mimicked natural foraging conditions by allowing uncertainty in the probability of reward delivery while maintaining certainty in reward concentration. We used a Bayesian approach to fit psychometric functions, relating the strength of preference for the higher concentration option to the relative intensity of the presented stimuli. Psychometric analysis was performed on visitation data from individually marked bumblebees and pooled data from unmarked individuals. Bumblebees preferred the more concentrated sugar solutions at high stimulus intensities and showed no preference at low stimulus intensities. The obtained psychometric function is consistent with reward evaluation based on perceived concentration contrast between choices. We found no evidence that bumblebees reduce reward expectations upon experiencing non-rewarded visits. We compare psychometric function parameters between the bumblebee B. impatiens and the flower bat Glossophaga commissarisi and discuss the relevance of psychophysics for pollinator-exerted selection pressures on plants.

Seasonal change in a pollinator community and the maintenance of style length variation in Mertensia fusiformis (Boraginaceae).

BACKGROUND AND AIMS: In sub-alpine habitats, patchiness in snowpack produces marked, small-scale variation in flowering phenology. Plants in early- and late-melting patches are therefore likely to experience very different conditions during their flowering periods. Mertensia fusiformis is an early-flowering perennial that varies conspicuously in style length within and among populations. The hypothesis that style length represents an adaptation to local flowering time was tested. Specifically, it was hypothesized that lower air temperatures and higher frost risk would favour short-styled plants (with stigmas more shielded by corollas) in early-flowering patches, but that the pollen-collecting behaviour of flower visitors in late-flowering patches would favour long-styled plants. METHODS: Floral morphology was measured, temperatures were monitored and pollinators were observed in several matched pairs of early and late populations. To evaluate effects of cold temperatures on plants of different style lengths, experimental pollinations were conducted during mornings (warm) and evenings (cool), and on flowers that either had or had not experienced a prior frost. The effectiveness of different pollinators was quantified as seed set following single visits to plants with relatively short or long styles. KEY RESULTS: Late-flowering populations experienced warmer temperatures than early-flowering populations and a different suite of pollinators. Nectar-foraging bumble-bee queens and male solitary bees predominated in early populations, whereas pollen-collecting female solitary bees were more numerous in later sites. Pollinators differed significantly in their abilities to transfer pollen to stigmas at different heights, in accordance with our prediction. However, temperature and frost sensitivity did not differ between long- and short-styled plants. Although plants in late-flowering patches tended to have longer styles than those in early patches, this difference was not consistent. CONCLUSIONS: Seasonal change in pollinator-mediated selection on style length may help maintain variation in this trait in M. fusiformis, but adaptation to local flowering time is not apparent. The prevalence of short styles in these populations requires further explanation.

Trade-off mitigation: a conceptual framework for understanding floral adaptation in multispecies interactions.

Explanations of floral adaptation to diverse pollinator faunas have often invoked visitor-mediated trade-offs in which no intermediate, generalized floral phenotype is optimal for pollination success, i.e. fitness valleys are created. In such cases, plant species are expected to specialize on particular groups of flower visitors. Contrary to this expectation, it is commonly observed that flowers interact with various groups of visitors, while at the same time maintaining distinct phenotypes among ecotypes, subspecies, or congeners. This apparent paradox may be due to a gap in our understanding of how visitor-mediated trade-offs could affect floral adaptation. Here we provide a conceptual framework for analysing visitor-mediated trade-offs with the hope of stimulating empirical and theoretical studies to fill this gap. We propose two types of visitor-mediated trade-offs to address negative correlations among fitness contributions of different visitors: visitor-mediated phenotypic trade-offs (phenotypic trade-offs) and visitor-mediated opportunity trade-offs (opportunity trade-offs). Phenotypic trade-offs occur when different groups of visitors impose conflicting selection pressures on a floral trait. By contrast, opportunity trade-offs emerge only when some visitors' actions (e.g. pollen collection) remove opportunities for fitness contribution by more beneficial visitors. Previous studies have observed disruptive selection due to phenotypic trade-offs less often than expected. In addition to existing explanations, we propose that some flowers have achieved 'adaptive generalization' by evolving features to avoid or eliminate the fitness valleys that phenotypic trade-offs tend to produce. The literature suggests a variety of pathways to such 'trade-off mitigation'. Trade-off mitigation may also evolve as an adaptation to opportunity trade-offs. We argue that active exclusion, or floral specialization, can be viewed as a trade-off mitigation, occurring only when flowers cannot otherwise avoid strong opportunity trade-offs. These considerations suggest that an evolutionary strategy for trade-off mitigation is achieved often by acquiring novel combinations of traits. Thus, phenotypic diversification of flowers through convergent evolution of certain trait combinations may have been enhanced not only through adaptive specialization for particular visitors, but also through adaptive generalization for particular visitor communities. Explorations of how visitor-mediated trade-offs explain the recurrent patterns of floral phenotypes may help reconcile the long-lasting controversy on the validity of pollination syndromes.

Fur versus feathers: pollen delivery by bats and hummingbirds and consequences for pollen production.

One floral characteristic associated with bat pollination (chiropterophily) is copious pollen production, a pattern we confirmed in a local comparison of hummingbird- and bat-adapted flowers from a cloud forest site in Ecuador. Previous authors have suggested that wasteful pollen transfer by bats accounted for the pattern. Here we propose and test a new hypothesis: bats select for increased pollen production because they can efficiently transfer larger amounts of pollen, which leads to a more linear male fitness gain curve for bat-pollinated plants. Flight cage experiments with artificial flowers and flowers of Aphelandra acanthus provide support for this hypothesis; in both instances, the amount of pollen delivered to stigmas by birds is not related to the amount of pollen removed from anthers on the previous visit, while the same function for bats increases linearly. Thus, increased pollen production will be linearly related to increased male reproductive success for bat flowers, while for bird flowers, increased pollen production leads to rapidly diminishing fitness returns. We speculate that fur takes up and holds more pollen than feathers, which seem to readily shed excess grains. Our gain-curve hypothesis may also explain why evolutionary shifts from bird to bat pollination seem more common than shifts in the opposite direction.

Flowering phenology in subalpine meadows: does climate variation influence community co-flowering patterns?

Climate change is expected to alter patterns of species co-occurrence, in both space and time. Species-specific shifts in reproductive phenology may alter the assemblages of plant species in flower at any given time during the growing season. Temporal overlap in the flowering periods (co-flowering) of animal-pollinated species may influence reproductive success if competitive or facilitative interactions between plant species affect pollinator services. We used a 33-year data set on flowering phenology in subalpine meadows in Colorado, USA, to determine whether interannual variation in snowmelt date, which marks the start of the growing season, affected co-flowering patterns. For two of four species considered, we found a significant relationship between snowmelt timing and composition of the assemblage of co-flowering plants. In years of early snowmelt, Lathyrus lanszwertii var. leucanthus (Fabaceae), the species we investigated in most detail, tended to overlap with earlier-flowering species and with fewer species overall. In particular, overlap with the flowering period of Lupinus polyphyllus var. prunophilus, with which Lathyrus leucanthus shares pollinators, was significantly reduced in early-snowmelt years. The observed association between timing of snowmelt and patterns of flowering overlap could not have been predicted simply by examining temporal trends in the dates of peak flowering of the dominant species in the community, as peak flowering dates have largely shifted in parallel with respect to snowmelt date. However, subtle interspecific differences in responsiveness of flowering time, duration, and intensity to interannual climate variation have likely contributed to the observed relationship. Although much of the year-to-year variation in flowering overlap remains unexplained by snowmelt date, our finding of a measurable signal of climate variation suggests that future climate change may lead to altered competitive environments for these wildflower species.

Consumption of a nectar alkaloid reduces pathogen load in bumble bees.

Diet has a significant effect on pathogen infections in animals and the consumption of secondary metabolites can either enhance or mitigate infection intensity. Secondary metabolites, which are commonly associated with herbivore defense, are also frequently found in floral nectar. One hypothesized function of this so-called toxic nectar is that it has antimicrobial properties, which may benefit insect pollinators by reducing the intensity of pathogen infections. We tested whether gelsemine, a nectar alkaloid of the bee-pollinated plant Gelsemium sempervirens, could reduce pathogen loads in bumble bees infected with the gut protozoan Crithidia bombi. In our first laboratory experiment, artificially infected bees consumed a daily diet of gelsemine post-infection to simulate continuous ingestion of alkaloid-rich nectar. In the second experiment, bees were inoculated with C. bombi cells that were pre-exposed to gelsemine, simulating the direct effects of nectar alkaloids on pathogen cells that are transmitted at flowers. Gelsemine significantly reduced the fecal intensity of C. bombi 7 days after infection when it was consumed continuously by infected bees, whereas direct exposure of the pathogen to gelsemine showed a non-significant trend toward reduced infection. Lighter pathogen loads may relieve bees from the behavioral impairments associated with the infection, thereby improving their foraging efficiency. If the collection of nectar secondary metabolites by pollinators is done as a means of self-medication, pollinators may selectively maintain secondary metabolites in the nectar of plants in natural populations.

Pollinator experience, neophobia and the evolution of flowering time.

Environmental changes, such as current climate warming, can exert directional selection on reproductive phenology. In plants, evolution of earlier flowering requires that the individuals bearing genes for early flowering successfully reproduce; for non-selfing, zoophilous species, this means that early flowering individuals must be visited by pollinators. In a laboratory experiment with artificial flowers, we presented captive bumble-bees (Bombus impatiens) with flower arrays representing stages in the phenological progression of a two-species plant community: Bees that had been foraging on flowers of one colour were confronted with increasing numbers of flowers of a second colour. Early flowering individuals of the second 'species' were significantly under-visited, because bees avoided unfamiliar flowers, particularly when these were rare. We incorporated these aspects of bee foraging behaviour (neophobia and positive frequency dependence) in a simulation model of flowering-time evolution for a plant population experiencing selection against late flowering. Unlike simple frequency dependence, a lag in pollinator visitation prevented the plant population from responding to selection and led to declines in population size. Pollinator behaviour thus has the potential to constrain evolutionary adjustments of flowering phenology.

Going to great lengths: selection for long corolla tubes in an extremely specialized bat-flower mutualism.

In a hypothesis that has remained controversial since its inception, Darwin suggested that long-tubed flowers and long-tongued pollinators evolved together in a coevolutionary race, with each selecting for increasing length in the other. Although the selective pressures that flowers impose on tongue length are relatively straightforward, in that longer tongues allow access to more nectar, selective pressures that pollinators impose on flower length are less clear. Here, we test for such selective pressures in the highly specialized mutualism between the nectar bat Anoura fistulata, which can extend its tongue twice as far as other nectar bats, and Centropogon nigricans, which has flowers of a similar length (8-9 cm). We used flight cage experiments to examine the effects of artificially manipulated flower lengths on (i) bat behaviour and (ii) pollen transfer. Increased length produced longer visits, but did not affect the force bats applied during visits. In the second experiment, flower length increased both the male and female components of flower function: long male flowers delivered more pollen grains and long female flowers received more pollen grains. However, pollen transfer was not correlated with visit duration, so the mechanism behind differences in pollen transfer remains unclear. By demonstrating that bats select for increasing flower length, these results are consistent with the hypothesis that A. fistulata evolved its remarkable tongue in a coevolutionary race with long-tubed flowers similar to that envisioned by Darwin.

Trapline foraging by pollinators: its ontogeny, economics and possible consequences for plants.

BACKGROUND: Trapline foraging (repeated sequential visits to a series of feeding locations) has been often observed in pollinators collecting nectar or pollen from flowers. Although field studies on bumble-bees and hummingbirds have clarified fundamental aspects of this behaviour, trapline foraging still poses several difficult questions from the perspectives of both animals and plants. These questions include whether and how traplining improves foraging performance, how animals develop traplines with accumulating foraging experience, and how traplining affects pollen flow or plant reproduction. SCOPE: First, we review our previous work performed by using computer simulations and indoor flight-cage experiments with bumble-bees foraging from arrays of automated feeders. Our findings include the following: (1) traplining benefits foragers that are competing for resources that replenish in a decelerating way, (2) traplining is a learned behaviour that develops over a period of hours and (3) the establishment of traplines could be hampered by spatial configuration of plants such as zigzags. Second, using a simulation model linking pollinator movement and pollen transfer, we consider how service by pollinators with different foraging patterns (searchers or trapliners) would affect pollen flow. Traplining increases mating distance and mate diversity, and reduces 'iterogamy' (self-pollination caused by return visits) at the population level. Furthermore, increased visitation rates can have opposite effects on the reproductive success of a plant, depending on whether the visitors are traplining or searching. Finally, we discuss possible consequences of traplining for plants in the light of new experimental work and modelling. CONCLUSIONS: We suggest that trapline foraging by pollinators increases variation among plant populations in genetic diversity, inbreeding depression and contributions of floral traits to plant fitness, which should in turn affect the rates and directions of floral evolution. More theoretical and empirical studies are needed to clarify possible outcomes of such a neglected side of pollination.

A generalized pollination system in the tropics: bats, birds and Aphelandra acanthus.

BACKGROUND AND AIMS: A number of different types of flower-visiting animals coexist in any given habitat. What evolutionary and ecological factors influence the subset of these that a given plant relies on for its pollination? Addressing this question requires a mechanistic understanding of the importance of different potential pollinators in terms of visitation rate (pollinator 'quantity') and effectiveness at transferring pollen (pollinator 'quality') is required. While bat-pollinated plants typically are highly specialized to bats, there are some instances of bat-pollinated plants that use other pollinators as well. These generalized exceptions tend to occur in habitats where bat 'quantity' is poor due to low or fluctuating bat densities. METHODS: Aphelandra acanthus occurs in tropical cloud forests with relatively high densities of bat visitors, yet displays a mix of floral syndrome characteristics, suggesting adaptation to multiple types of pollinators. To understand its pollination system better, aspects of its floral phenology and the 'quantity' and 'quality' components of pollination by its floral visitors are studied here. KEY RESULTS: Flowers were found to open and senesce throughout the day and night, although anther dehiscence was restricted to the late afternoon and night. Videotaping reveals that flowers are visited nocturnally by bats and moths, and diurnally by hummingbirds. Analysis of pollen deposition shows that bats regularly transfer large amounts of conspecific pollen, while hummingbirds occasionally transfer some pollen, and moths rarely do so. CONCLUSIONS: Hummingbirds and bats were comparable in terms of pollination 'quantity', while bats were the most effective in terms of 'quality'. Considering these components together, bats are responsible for approx. 70 % of A. acanthus pollination. However, bats also transferred remarkably large amounts of foreign pollen along with the conspecific grains (three of four grains were foreign). It is suggested that the negative effects of interspecific pollen transfer may decrease bat 'quality' for A. acanthus, and thus select for generalization on multiple pollinators instead of specialization on bats.

Background complexity affects colour preference in bumblebees.

Flowers adapted for hummingbird pollination are typically red. This correlation is usually explained by the assertion that nectar- or pollen-stealing bees are "blind" to red flowers. However, laboratory studies have shown that bees are capable of locating artificial red flowers and often show no innate preference for blue over red. We hypothesised that these findings might be artefacts of the simplified laboratory environment. Using bumblebees (Bombus impatiens) that had been trained to visit red and blue artificial flowers, we tested whether colour preference was influenced by complexity of the background on which they were foraging. Many bees were indifferent to flower colour when tested using a uniform green background like those commonly used in laboratory studies, but all bees showed strong colour preferences (usually for blue) when flowers were presented against a photograph of real foliage. Overall, preference for blue flowers was significantly greater on the more realistic, complex background. These results support the notion that the red of "hummingbird syndrome" flowers can function to reduce bee visits despite the ability of bees to detect red and highlight the need to consider context when drawing inferences about pollinator preferences from laboratory data.

Geographic patterns and pollination ecotypes in Claytonia virginica.

Geographical variation in pollinators visiting a plant can produce plant populations adapted to local pollinator environments. We documented two markedly different pollinator climates for the spring ephemeral wildflower Claytonia virginica: in more northern populations, the pollen-specialist bee Andrena erigeniae dominated, but in more southern populations, A. erigeniae visited rarely and the bee-fly Bombylius major dominated. Plants in the northern populations experienced faster pollen depletion than plants in southern populations. We also measured divergent pollen-related plant traits; plants in northern populations produced relatively more pollen per flower and anther dehiscence was more staggered than plants in southern populations. These plant traits might function to increase pollen dispersal via the different pollen vectors.

Ecological context influences pollinator deterrence by alkaloids in floral nectar.

Secondary compounds may benefit plants by deterring herbivores, but the presence of these defensive chemicals in floral nectar may also deter beneficial pollinators. This trade-off between sexual reproduction and defense has received minimal study. We determined whether the pollinator-deterring effects of a nectar alkaloid found in the perennial vine Gelsemium sempervirens depend on ecological context (i.e. the availability of alternative nectar sources) by monitoring the behavioural response of captive bumblebees (Bombus impatiens, an important pollinator of G. sempervirens in nature) to nectar alkaloids in several ecologically relevant scenarios. Although alkaloids in floral nectar tended to deter visitation by bumblebees, the magnitude of that effect depended greatly on the availability and nectar properties of alternative flowers. Ecological context should thus be considered when assessing ecological costs of plant defense in terms of pollination services. We consider adaptive strategies that would enable plants to minimize pollinator deterrence because of defensive compounds in flowers.