Steven D. Johnson

Bio: Steven D. Johnson is an academic researcher from University of KwaZulu-Natal. The author has contributed to research in topics: Pollination & Pollinator. The author has an hindex of 70, co-authored 360 publications receiving 16127 citations. Previous affiliations of Steven D. Johnson include University of Natal & University of Cape Town.

Generalization versus specialization in plant pollination systems

Abstract: The long-standing notion that most angiosperm flowers are specialized for pollination by particular animal types, such as birds or bees, has been challenged recently on the basis of apparent widespread generalization in pollination systems. At the same time, biologists working mainly in the tropics and the species-rich temperate floras of the Southern hemisphere are documenting pollination systems that are remarkably specialized, often involving a single pollinator species. Current studies are aimed at understanding: (1) the ecological forces that have favoured either generalization or specialization in particular lineages and regions; (2) the implications for selection on floral traits and divergence of populations; and (3) the risk of collapse in plant-pollinator mutualisms of varying specificity.

Mechanisms and evolution of deceptive pollination in orchids

Abstract: The orchid family is renowned for its enormous diversity of pollination mechanisms and unusually high occurrence of non-rewarding flowers compared to other plant families. The mechanisms of deception in orchids include generalized food deception, food-deceptive floral mimicry, brood-site imitation, shelter imitation, pseudoantagonism, rendezvous attraction and sexual deception. Generalized food deception is the most common mechanism (reported in 38 genera) followed by sexual deception (18 genera). Floral deception in orchids has been intensively studied since Darwin, but the evolution of non-rewarding flowers still presents a major puzzle for evolutionary biology. The two principal hypotheses as to how deception could increase fitness in plants are (i) reallocation of resources associated with reward production to flowering and seed production, and (ii) higher levels of cross-pollination due to pollinators visiting fewer flowers on non-rewarding plants, resulting in more outcrossed progeny and more efficient pollen export. Biologists have also tried to explain why deception is overrepresented in the orchid family. These explanations include: (i) efficient removal and deposition of pollinaria from orchid flowers in a single pollinator visit, thus obviating the need for rewards to entice multiple visits from pollinators; (ii) efficient transport of orchid pollen, thus requiring less reward-induced pollinator constancy; (iii) low-density populations in many orchids, thus limiting the learning of associations of floral phenotypes and rewards by pollinators; (iv) packaging of pollen in pollinaria with limited carry-over from flower to flower, thus increasing the risks of geitonogamous self-pollination when pollinators visit many flowers on rewarding plants. All of these general and orchid-specific hypotheses are difficult to reconcile with the well-established pattern for rewardlessness to result in low pollinator visitation rates and consequently low levels of fruit production. Arguments that deception evolves because rewards are costly are particularly problematic in that small amounts of nectar are unlikely to have a significant effect on the energy budget of orchids, and because reproduction in orchids is often severely pollen-, rather than resource-limited. Several recent experimental studies have shown that deception promotes cross-pollination, but it remains unknown whether actual outcrossing rates are generally higher in deceptive orchids. Our review of the literature shows that there is currently no evidence that deceptive orchids carry higher levels of genetic load (an indirect measure of outcrossing rate) than their rewarding counterparts. Cross-pollination does, however, result in dramatic increases in seed quality in almost all orchids and has the potential to increase pollen export (by reducing pollen discounting). We suggest that floral deception is particularly beneficial, because of its promotion of outcrossing, when pollinators are abundant, but that when pollinators are consistently rare, selection may favour a nectar reward or a shift to autopollination. Given that nectar-rewardlessness is likely to have been the ancestral condition in orchids and yet is evolutionarily labile, more attention will need to be given to explanations as to why deception constitutes an 'evolutionarily stable strategy'.

Pollinator-mediated evolution of floral signals

Abstract: Because most plants rely on animals for pollination, insights from animal sensory ecology and behavior are essential for understanding the evolution of flowers. In this review, we compare and contrast three main types of pollinator responses to floral signals--receiver bias, 'adaptive' innate preferences, and associative learning--and discuss how they can shape selection on floral signals. We show that pollinator-mediated selection on floral signals can be strong and that the molecular bases of floral signal variation are often surprisingly simple. These new empirical and conceptual insights into pollinator-mediated evolution provide a framework for understanding patterns of both convergent (pollination syndromes) and advergent (floral mimicry) floral signal evolution.

Darwin's beautiful contrivances: evolutionary and functional evidence for floral adaptation

Abstract: Contents Summary 530 I. Introduction 530 II. The process of floral and inflorescence adaptation 532 III. Experimental studies of flowers as adaptations 538 IV. Floral diversification: microevolution writ large? 539 V. Concluding comments 541 Acknowledgements 542 References 542 Summary Although not ‘a professed botanist’, Charles Darwin made seminal contributions to understanding of floral and inflorescence function while seeking evidence of adaptation by natural selection. This review considers the legacy of Darwin's ideas from three perspectives. First, we examine the process of floral and inflorescence adaptation by surveying studies of phenotypic selection, heritability and selection responses. Despite widespread phenotypic and genetic capacity for natural selection, only one-third of estimates indicate phenotypic selection. Second, we evaluate experimental studies of floral and inflorescence function and find that they usually demonstrate that reproductive traits represent adaptations. Finally, we consider the role of adaptation in floral diversification. Despite different diversification modes (coevolution, divergent use of the same pollen vector, pollinator shifts), evidence of pollination ecotypes and phylogenetic patterns suggests that adaptation commonly contributes to floral diversity. Thus, this review reveals a contrast between the inconsistent occurrence of phenotypic selection and convincing experimental and comparative evidence that floral traits are adaptations. Rather than rejecting Darwin's hypotheses about floral evolution, this contrast suggests that the tempo of creative selection varies, with strong, consistent selection during episodes of diversification, but relatively weak and inconsistent selection during longer, ‘normal’ periods of relative phenotypic stasis.

Long-tongued fly pollination and evolution of floral spur length in the Disa draconis complex (Orchidaceae)

Abstract: Field studies in South Africa showed that floral spur length in the Disa draconis complex (Orchidaceae) varies enormously between populations in the southern mountains (means = 32-38 mm), lowland sandplain (mean = 48 mm), and northern mountains (means = 57-72 mm) We tested the hypothesis that divergence in spur length has resulted from selection exerted through pollinator proboscis length Short-spurred plants in several southern mountain populations, as well as long-spurred plants in one northern mountain population, were pollinated by a horsefly, Philoliche rostrata (Tabanidae), with a proboscis length that varied from 22 to 35 mm among sites Long-spurred plants on the sandplain were pollinated by the tanglewing fly, Moegistorynchus longirostris (Nemestrinidae), which has a very long proboscis (mean = 57 mm) Selection apparently favors long spurs in sandplain plants, as artificial shortening of spurs resulted in a significant decline in pollen receipt and fruit set, although pollinaria removal was not significantly affected Fruit set in the study populations was limited by pollen availability, which further suggests that selection on spur length occurs mainly through the female component of reproductive success

The Theory of Island Biogeography

Abstract: Preface to the Princeton Landmarks in Biology Edition vii Preface xi Symbols Used xiii 1. The Importance of Islands 3 2. Area and Number of Speicies 8 3. Further Explanations of the Area-Diversity Pattern 19 4. The Strategy of Colonization 68 5. Invasibility and the Variable Niche 94 6. Stepping Stones and Biotic Exchange 123 7. Evolutionary Changes Following Colonization 145 8. Prospect 181 Glossary 185 References 193 Index 201

Citation classic - optimal foraging - a selective review of theory and tests

Abstract: Beginning with Emlen (1966) and MacArthur and Pianka (1966) and extending through the last ten years, several authors have sought to predict the foraging behavior of animals by means of mathematical models. These models are very similar,in that they all assume that the fitness of a foraging animal is a function of the efficiency of foraging measured in terms of some "currency" (Schoener, 1971) -usually energy- and that natural selection has resulted in animals that forage so as to maximize this fitness. As a result of these similarities, the models have become known as "optimal foraging models"; and the theory that embodies them, "optimal foraging theory." The situations to which optimal foraging theory has been applied, with the exception of a few recent studies, can be divided into the following four categories: (1) choice by an animal of which food types to eat (i.e., optimal diet); (2) choice of which patch type to feed in (i.e., optimal patch choice); (3) optimal allocation of time to different patches; and (4) optimal patterns and speed of movements. In this review we discuss each of these categories separately, dealing with both the theoretical developments and the data that permit tests of the predictions. The review is selective in the sense that we emphasize studies that either develop testable predictions or that attempt to test predictions in a precise quantitative manner. We also discuss what we see to be some of the future developments in the area of optimal foraging theory and how this theory can be related to other areas of biology. Our general conclusion is that the simple models so far formulated are supported are supported reasonably well by available data and that we are optimistic about the value both now and in the future of optimal foraging theory. We argue, however, that these simple models will requre much modification, espicially to deal with situations that either cannot easily be put into one or another of the above four categories or entail currencies more complicated that just energy.

How many flowering plants are pollinated by animals

Abstract: It is clear that the majority of fl owering plants are pollinated by insects and other animals, with a minority utilising abiotic pollen vectors, mainly wind. However there is no accurate published calculation of the proportion of the ca 352 000 species of angiosperms that interact with pollinators. Widely cited fi gures range from 67% to 96% but these have not been based on fi rm data. We estimated the number and proportion of fl owering plants that are pollinated by animals using published and unpublished community-level surveys of plant pollination systems that recorded whether each species present was pollinated by animals or wind. Th e proportion of animal-pollinated species rises from a mean of 78% in temperate-zone communities to 94% in tropical communities. By correcting for the latitudinal diversity trend in fl owering plants, we estimate the global number and proportion of animal pollinated angiosperms as 308 006, which is 87.5% of the estimated species-level diversity of fl owering plants. Given current concerns about the decline in pollinators and the possible resulting impacts on both natural communities and agricultural crops, such estimates are vital to both ecologists and policy makers. Further research is required to assess in detail the absolute dependency of these plants on their pollinators, and how this varies with latitude and community type, but there is no doubt that plant – pollinator interactions play a signifi cant role in maintaining the functional integrity of most terrestrial ecosystems. Plant – pollinator relationships may be one of the most ecologically important classes of animal – plant interaction: without pollinators, many plants could not set seed and reproduce; and without plants to provide pollen, nectar and other rewards, many animal populations would decline, with consequent knock-on eff ects for other species (Kearns et al.

The nested assembly of plant-animal mutualistic networks

Abstract: Most studies of plant–animal mutualisms involve a small number of species. There is almost no information on the structural organization of species-rich mutualistic networks despite its potential importance for the maintenance of diversity. Here we analyze 52 mutualistic networks and show that they are highly nested; that is, the more specialist species interact only with proper subsets of those species interacting with the more generalists. This assembly pattern generates highly asymmetrical interactions and organizes the community cohesively around a central core of interactions. Thus, mutualistic networks are neither randomly assembled nor organized in compartments arising from tight, parallel specialization. Furthermore, nestedness increases with the complexity (number of interactions) of the network: for a given number of species, communities with more interactions are significantly more nested. Our results indicate a nonrandom pattern of community organization that may be relevant for our understanding of the organization and persistence of biodiversity.

Pollination Syndromes and Floral Specialization

Abstract: ▪ Abstract Floral evolution has often been associated with differences in pollination syndromes. Recently, this conceptual structure has been criticized on the grounds that flowers attract a broader spectrum of visitors than one might expect based on their syndromes and that flowers often diverge without excluding one type of pollinator in favor of another. Despite these criticisms, we show that pollination syndromes provide great utility in understanding the mechanisms of floral diversification. Our conclusions are based on the importance of organizing pollinators into functional groups according to presumed similarities in the selection pressures they exert. Furthermore, functional groups vary widely in their effectiveness as pollinators for particular plant species. Thus, although a plant may be visited by several functional groups, the relative selective pressures they exert will likely be very different. We discuss various methods of documenting selection on floral traits. Our review of the literatur...