Explore the vast range of titles available.

Reproductive efficiency reflects the gap between an organism's potential success and its actual achievement. Exploring how theoretical sex allocation models deal with reproductive efficiency reveals shortcomings in two fundamental elements of the theory, fitness gain curves and the Shaw‐Mohler equation. Gain curves depict success but make no reference to the production of the entities that accrue fitness, so they imply nothing about efficiency in its common sense, the fraction of successful reproductive entities in relation to the number produced. Because the number of successful entities cannot exceed the number produced, a gain curve implies a minimum allowable production, but the implication itself exposes new problems. Gain curves are used to replace terms in the Shaw‐Mohler equation that were meant to represent an individual's fair competitive share of total population fitness. If gain curves themselves represent individual fitness outcomes, the theoretical solutions can imply unequal aggregate success of male and female mating agents, a biological impossibility in a sexually reproducing population. If gain curves represent inputs to an arena of mating interactions, the theoretical solutions can imply misleading or biologically impossible patterns of reproductive efficiency. The relationship between pollination efficiency and pollen‐ovule ratios in flowering plants is used as a window into the shortcomings of gain curves and the Shaw‐Mohler equation. There are new modeling approaches to sex allocation that are more explicit about the processes that lead from production of male and female reproductive entities to their eventual fitness outcomes. Exploring the concept of reproductive efficiency reveals several shortcomings in fitness gain curves and the Shaw‐Mohler equation for sex ratio evolution. Gain curves that refer to the gamete inputs to mating interactions make no statement about the production of gametes. Gain curves that refer to the fitness outcomes of mating often violate the required equality of total male and female fitness in a population. The example of pollination efficiency in flowering plants is used as a window onto these and other conceptual difficulties of sex allocation models based on the Shaw‐Mohler equation.

Plant–pollinator interactions provide a natural experiment in signal evolution. Flowers are known to have evolved colour signals that maximise their ease of detection by the visual systems of important pollinators such as bees. Whilst most angiosperms are bee pollinated, our understanding on how the second largest group of pollinating insects, flies, may influence flower colour evolution is limited to the use of categorical models of colour discrimination that do not reflect the small colour differences commonly observed between and within flower species. Here we show by comparing flower signals that occur in different environments including total absence of bees, a mixture of bee and fly pollination within one plant family (Orchidaceae) from a single community, and typical flowers from a broad taxonomic sampling of the same geographic region, that perceptually different colours, empirically measured, do evolve in response to different types of insect pollinators. We show evidence of both convergence among fly-pollinated floral colours but also of divergence and displacement of colour signals in the absence of bee pollinators. Our findings give an insight into how both ecological and agricultural systems may be affected by changes in pollinator distributions around the world.

Land use change, by disrupting the co-evolved interactions between plants and their pollinators, could be causing plant reproduction to be limited by pollen supply. Using a phylogenetically controlled meta-analysis on over 2200 experimental studies and more than 1200 wild plants, we ask if land use intensification is causing plant reproduction to be pollen limited at global scales. Here we report that plants reliant on pollinators in urban settings are more pollen limited than similarly pollinator-reliant plants in other landscapes. Plants functionally specialized on bee pollinators are more pollen limited in natural than managed vegetation, but the reverse is true for plants pollinated exclusively by a non-bee functional group or those pollinated by multiple functional groups. Plants ecologically specialized on a single pollinator taxon were extremely pollen limited across land use types. These results suggest that while urbanization intensifies pollen limitation, ecologically and functionally specialized plants are at risk of pollen limitation across land use categories. An insufficient amount of pollen transfer by pollinators (pollen limitation) could reduce plant reproduction in human-impacted landscapes. Here the authors conduct a global meta-analysis and find that pollen limitation is high in urban environments and depends of plant traits such as pollinator dependency.

Colour is an important signal that flowering plants use to attract insect pollinators like bees. Previous research in Germany has shown that nectar volume is higher for flower colours that are innately preferred by European bees, suggesting an important link between colour signals, bee preferences and floral rewards. In Australia, flower colour signals have evolved in parallel to the Northern hemisphere to enable easy discrimination and detection by the phylogenetically ancient trichromatic visual system of bees, and native Australian bees also possess similar innate colour preferences to European bees. We measured 59 spectral signatures from flowers present at two preserved native habitats in South Eastern Australia and tested whether there were any significant differences in the frequency of flowers presenting higher nectar rewards depending upon the colour category of the flower signals, as perceived by bees. We also tested if there was a significant correlation between chromatic contrast and the frequency of flowers presenting higher nectar rewards. For the entire sample, and for subsets excluding species in the Asteraceae and Orchidaceae, we found no significant difference among colour categories in the frequency of high nectar reward. This suggests that whilst such relationships between flower colour signals and nectar volume rewards have been observed at a field site in Germany, the effect is likely to be specific at a community level rather than a broad general principle that has resulted in the common signalling of bee flower colours around the world.

Baker's law refers to the tendency for species that establish on islands by long-distance dispersal to show an increased capacity for self-fertilization because of the advantage of self-compatibility when colonizing new habitat. Despite its intuitive appeal and broad empirical support, it has received substantial criticism over the years since it was proclaimed in the 1950s, not least because it seemed to be contradicted by the high frequency of dioecy on islands. Recent theoretical work has again questioned the generality and scope of Baker's law. Here, we attempt to discern where the idea is useful to apply and where it is not. We conclude that several of the perceived problems with Baker's law fall away when a narrower perspective is adopted on how it should be circumscribed. We emphasize that Baker's law should be read in terms of an enrichment of a capacity for uniparental reproduction in colonizing situations, rather than of high selfing rates. We suggest that Baker's law might be tested in four different contexts, which set the breadth of its scope: the colonization of oceanic islands, metapopulation dynamics with recurrent colonization, range expansions with recurrent colonization, and colonization through species invasions.

Few ant species construct cleared trails. Among those that do, leaf-cutting Atta ants build the most prominent networks, with single colonies clearing debris and obstructions from hundreds of meters of trails annually. Workers on cleared paths move at higher speed than they do over uncleared litter, and one measurement of the time and energetic costs of trail clearance suggests that benefits of trail usage far outweigh the investment costs of trail clearing. The ecological basis of trail clearing remains uncertain, however, because no full account has been made of benefits and costs in common units that allow comparison. We make such an account using a scalable, integrative model of trail investment and foraging energetics. Contrary to assumptions in previous work, we find that trail clearing needs not always be energetically profitable for leafcutting ants. Profitability depends on the workforce composition, specifically, on how many ants in a traffic stream act as maintenance workforce to respond to sudden and unpredictable obstructions, such as leaf fall. Such maintenance patrols have not previously been recognized as a cost of trail building. If the patrolling workforce is not too large, the energetic savings from foraging over cleared trails offset the investment and maintenance costs within a few days. Under some conditions, however, amortization can take weeks or months, or trail clearing can become unprofitable altogether. This suggests that Atta colonies must have a mechanism to regulate the intensity of their trail clearing behavior. We explore possible mechanisms and make testable predictions for future research.

Colour signals are a major cue in putative pollination syndromes. There is evidence that the reflectance spectra of many flowers target the distinctive visual discrimination abilities of hymenopteran insects, but far less is known about bird-pollinated flowers. Birds are hypothesized to exert different selective pressures on floral colour compared with hymenopterans because of differences in their visual systems. We measured the floral reflectance spectra of 206 Australian angiosperm species whose floral visitors are known from direct observation rather than inferred from floral characteristics. We quantified the match between these spectra and the hue discrimination abilities of hymenopteran and avian vision, and analysed these metrics in a phylogenetically informed comparison of flowers in different pollination groups. We show that bird-visited flowers and insect-visited flowers differ significantly from each other in the chromatic cues they provide, and that the differences are concentrated near wavelengths of optimal colour discrimination by whichever class of pollinator visits the flowers. Our results indicate that angiosperms have evolved the spectral signals most likely to reinforce their pollinators' floral constancy (the tendency of individual pollinators to visit flowers of the same species) in communities of similarly coloured floral competitors.

Because establishing a new population often depends critically on finding mates, individuals capable of uniparental reproduction may have a colonization advantage. Accordingly, there should be an over-representation of colonizing species in which individuals can reproduce without a mate, particularly in isolated locales such as oceanic islands. Despite the intuitive appeal of this colonization filter hypothesis (known as Baker’s law), more than six decades of analyses have yielded mixed findings. We assembled a dataset of island and mainland plant breeding systems, focusing on the presence or absence of self-incompatibility. Because this trait enforces outcrossing and is unlikely to re-evolve on short timescales if it is lost, breeding system is especially likely to reflect the colonization filter. We found significantly more self-compatible species on islands than mainlands across a sample of > 1500 species from three widely distributed flowering plant families (Asteraceae, Brassicaceae and Solanaceae). Overall, 66% of island species were self-compatible, compared with 41% of mainland species. Our results demonstrate that the presence or absence of self-incompatibility has strong explanatory power for plant geographical patterns. Island floras around the world thus reflect the role of a key reproductive trait in filtering potential colonizing species in these three plant families.

Geographical variation in the interacting traits of plant–pollinator mutualism can lead to local adaptive differentiation. We tested Darwin’s hypothesis of reciprocal selection as a key driving force for the evolution of floral traits of an alpine ginger (Roscoea purpurea) and proboscis length of a tabanid fly (Philoliche longirostris). We documented the pattern of trait variation in R. purpurea and P. longirostris across five populations. At each site, we quantified pollinator-mediated selection on floral display area, inflorescence height and corolla length of R. purpurea by comparing selection gradients for flowers exposed to natural pollination and to supplemental hand pollination. Reciprocal selection between plant and fly was examined at two sites via the relationship between proboscis length and nectar consumption (fly benefit) and corolla length and pollen deposition (plant benefit). Local corolla tube length was correlated with local fly proboscis length among the five sites. We found strong linear selection imposed by pollinators on corolla tube length at all sites, but there was no consistent relationship of fitness to inflorescence height or floral display area. Selection between corolla length and proboscis length was reciprocal at the two experimental sites examined. The geographical pattern of trait variation and the evidence of selection is consistent with a mosaic of local, species-specific reciprocal selection acting as the major driving force for the evolution of corolla length of R. purpurea and proboscis length of P. longirostris.

Blue is a favored color of many humans. While blue skies and oceans are a common visual experience, this color is less frequently observed in flowers. We first review how blue has been important in human culture, and thus how our perception of blue has likely influenced the way of scientifically evaluating signals produced in nature, including approaches as disparate as Goethe’s Farbenlehre, Linneaus’ plant taxonomy, and current studies of plant-pollinator networks. We discuss the fact that most animals, however, have different vision to humans; for example, bee pollinators have trichromatic vision based on UV-, Blue-, and Green-sensitive photoreceptors with innate preferences for predominantly short-wavelength reflecting colors, including what we perceive as blue. The subsequent evolution of blue flowers may be driven by increased competition for pollinators, both because of a harsher environment (as at high altitude) or from high diversity and density of flowering plants (as in nutrient-rich meadows). The adaptive value of blue flowers should also be reinforced by nutrient richness or other factors, abiotic and biotic, that may reduce extra costs of blue-pigments synthesis. We thus provide new perspectives emphasizing that, while humans view blue as a less frequently evolved color in nature, to understand signaling, it is essential to employ models of biologically relevant observers. By doing so, we conclude that short wavelength reflecting blue flowers are indeed frequent in nature when considering the color vision and preferences of bees.