Structure in plant–animal interaction assemblages

We present a comprehensive approach to detect pattern in assemblages of plant and animal species linked by interactions such as pollination, frugivory or herbivory. Simple structural models produce gradient, compartmented or nested patterns of interaction; intermediate patterns between a gradient and compartments are possible, and nesting within compartments produces a combined model. Interaction patterns can be visualized and analyzed either as matrices, as bipartite networks or as multivariate sets through correspondence analysis. We argue that differences among patterns represent outcomes of distinct evolutionary and ecological processes in these highly diversified assemblages. Instead of choosing one model a priori, assemblages should be probed for a suite of patterns. A plant–pollinator assemblage exemplifies a simple nested pattern, whereas a plant–herbivore assemblage illustrates a compound pattern with nested structures within compartments. Compartmentation should reflect coevolutionary histories and constraints, whereas differences in species abundance or dispersal may generate nestedness.     

Integrating optimal foraging and optimal oviposition theory in plant–insect research

The current approach for studying host selection by phytophagous insects is mainly based on optimal oviposition theory, i.e. the preference–performance hypothesis. Almost no attention has been given to optimal foraging theory. However, recent papers and additional evidence given in this work illustrate that also optimal foraging may shape host preference patterns of phytophagous insects. Therefore and because optimal foraging and optimal oviposition may oppose conflicting needs to phytophagous insects, we plea for an integration of optimal foraging and optimal oviposition in plant–insect research. We argue how this may improve our understanding of plant–insect interactions.     

Invariant properties in coevolutionary networks of plant–animal interactions

Plant–animal mutualistic networks are interaction webs consisting of two sets of entities, plant and animal species, whose evolutionary dynamics are deeply influenced by the outcomes of the interactions, yielding a diverse array of coevolutionary processes. These networks are two‐mode networks sharing many common properties with others such as food webs, social, and abiotic networks. Here we describe generalized patterns in the topology of 29 plant–pollinator and 24 plant–frugivore networks in natural communities. Scale‐free properties have been described for a number of biological, social, and abiotic networks; in contrast, most of the plant–animal mutualistic networks (65.6%) show species connectivity distributions (number of links per species) with a power‐law regime but decaying as a marked cut‐off, i.e. truncated power‐law or broad‐scale networks and few (22.2%) show scale‐invariance. We hypothesize that plant–animal mutualistic networks follow a build‐up process similar to complex abiotic nets, based on the preferential attachment of species. However, constraints in the addition of links such as morphological mismatching or phenological uncoupling between mutualistic partners, restrict the number of interactions established, causing deviations from scale‐invariance. This reveals generalized topological patterns characteristic of self‐organized complex systems. Relative to scale‐invariant networks, such constraints may confer higher robustness to the loss of keystone species that are the backbone of these webs.     

The function of animal ‘eyespots’:Conspicuousness but not eye mimicry is key

Many animals are marked with conspicuous circular features often called 'eyespots',which intimidate predators,preventing or halting an attack.It has long been assumed that eyespots work by mimicking the eyes of larger animals,but recent experiments have indicated that conspicuousness and contrast is important in eyespot function,and not eye mimicry.We undertake two further experiments to distinguish between the conspicuousness and mimicry hypotheses,by using artificial prey presented to wild avian predators...     

Butterflies show flower colour preferences but not constancy in foraging at four plant species

1. The extent to which flower colour and other visual cues influence butterfly flower choice in the field is poorly understood, especially in comparison with choices by Hymenoptera. 2. Using a novel approach to studies of visitation behaviour by butterflies, flower colour of four Asteraceae species was phenotypically manipulated to decouple the influence of that trait from others (including morphology and nectar rewards) on visitation by Lycaena heteronea, Speyeria mormonia, Cercyonis oetus, and Phyciodes campestris. 3. Flower visits were recorded to experimental flower arrays in subalpine meadows to measure (i) spontaneous preference by butterflies for particular colours and other traits and (ii) flower constancy (longer than expected strings of visits made to flowers of the same species), a behaviour that can reduce interspecific gene flow in plants. 4. Over three field seasons, 3558 individual flower visits in 1386 foraging bouts were observed for free‐flying butterflies. All four butterfly species responded to the phenotypic manipulations of flower colour, although in different ways. Speyeria mormonia and L. heteronea also exhibited preferences based on other flower traits. Lycaena heteronea responded to combinations of traits such that the other traits it preferred depended upon the context of flower colour. 5. None of the butterfly species exhibited flower constancy in any of the arrays employed. 6. The observed preferences show that butterflies, like some other pollinators, are potentially capable of exerting selection on colour and other floral traits. Moreover, these flower preferences can depend on the context of other flower traits. The absence of constancy contrasts with reports of high constancy in many bees.     

Plant stem density as a cue in patch choice by foraging juvenile bluegill sunfish

Synopsis Juvenile bluegill sunfish,Lepomis macrochira, are restricted to vegetated habitats by predators. Variation in plant stem density has a significant effect on bluegill foraging success. Given the mosaic nature of this habitat, plant stem density may provide a cue for selecting among patches in which to forage. In this study, juvenile bluegills were offered patches of artificial vegetation differing only in plant stem density as potential foraging sites. Three densities, 100, 250, and 500 stems m^–2 were tested. Fish were presented with a choice between patches (100:250, 250:500, or 100:500). Bluegill foraging rate in, and the number of fish choosing each patch was recorded. Juvenile bluegills showed a preference for those patches which maximized their foraging rate.     

Changes in the colour of light cue circadian activity

The discovery of melanopsin, the non-visual opsin present in intrinsically photosensitive retinal ganglion cells (ipRGCs), has created great excitement in the field of circadian biology. Now, researchers have emphasized melanopsin as the main photopigment governing circadian activity in vertebrates. Circadian biologists have tested this idea under standard laboratory, 12h Light: 12h Dark, lighting conditions that lack the dramatic daily colour changes of natural skylight. Here we used a stimulus paradigm in which the colour of the illumination changed throughout the day, thus mimicking natural skylight, but luminance, sensed intrinsically by melanopsin containing ganglion cells, was kept constant. We show in two species of cichlid, Aequidens pulcher and Labeotropheus fuelleborni, that changes in light colour, not intensity, are the primary determinants of natural circadian activity. Moreover, opponent-cone photoreceptor inputs to ipRGCs mediate the sensation of wavelength change, and not the intrinsic photopigment, melanopsin. These results have implications for understanding the evolutionary biology of non-visual photosensory pathways and answer long-standing questions about the nature and distribution of photopigments in organisms, including providing a solution to the mystery of why nocturnal animals routinely have mutations that interrupt the function of their short wavelength sensitive photopigment gene.     

The effect of space in plant–animal mutualistic networks: insights from a simulation study

The topology of plant–animal mutualistic networks has the potential to determine the ecological and evolutionary dynamics of interacting species. Many mechanisms have been proposed as explanations of observed network patterns; however, the fact that plant–animal interactions are inherently spatial has so far been ignored. Using a simulation model of frugivorous birds foraging in spatially explicit landscapes we evaluated how plant distribution and the scale of bird movement decisions influenced species interaction probabilities and the resulting network properties. Spatial aggregation and limited animal mobility restricted encounter probabilities, so that the distribution of animal visits per plant deviated strongly from the binomial distribution expected for a well-mixed system. Lack of mixing in turn resulted in a strong decrease in network connectance, a weak decrease in nestedness, stronger interactions, greater strength asymmetry and the unexpected presence/absence of some interactions. Our results suggest that spatial processes may contribute substantially to structure plant–animal mutualistic networks.     

Host-selective toxins as agents of cell death in plant–fungus interactions

Host-selective toxins are known determinants of compatibility in plant–fungus interactions and provide a powerful model for understanding the specificity of these associations. The identification of genes required for toxin biosynthesis has shown that the genes are unique to the toxin producing species and are clustered in complex loci. These loci may have been acquired by horizontal gene transfer. Many, if not all, host-selective toxins act by disrupting biochemical processes and in several cases the resulting cell death has the characteristics of programmed cell death. This ability to make dead tissue from living has enabled these facultative saprophytic fungi to become plant pathogens.     

The influence of the light environment and photosynthesis on oxidative signalling responses in plant–biotrophic pathogen interactions

Plants grow in a constantly fluctuating environment, which has driven the evolution of a highly flexible metabolism and development necessary for their sessile lifestyle. In contrast to the situation in the natural world, the detailed dissection of the regulatory networks that govern plants' responses to abiotic insults and their interaction with pathogens have been studied almost exclusively in controlled environments where a single challenge has been applied. However, the question arises of how such pathways operate when the plant is subjected to multiple stresses, especially where the expression of overlapping gene sets and common signalling molecules, such as reactive oxygen species (ROS), are implicated. This review will focus on the responsiveness of leaves to their light environment and how this might influence both basal and induced resistance to infection by biotrophic pathogens. While several signalling pathways operate in a complex network of defence responses, the functioning of the salicylic acid (SA) signalling pathway will receive specific consideration. This is because information is becoming available of its role in abiotic stress responses and it dependency on light. This article covers several topics, some of which formerly have received scant attention. These include the effects of infection on photosynthetic performance and carbohydrate metabolism, the parallels between the induction of acclimation to high light and immunity to pathogens, the role of light in the functioning of the SA signalling pathway and the light sensitivity of lesion formation and the use of lesion mimic mutants and transgenic plants. Finally, a model is proposed that attempts to extrapolate these controlled environment-based studies to the functioning of defences against pathogens in a field-grown crop.     

Evidence for spatial niche partitioning in predaceous aphidophaga: Use of plant colour as a cue

A field experiment involving aphid-free control and nutrient-stressed plants of 5 maize (Zea mays L.) genotypes was conducted to determine if predaceous aphidophaga use plant cues, such as colour, to select plants on which to forage. Nutrient stress resulted in plants lighter in colour (Yellow) than control plants in all the maize genotypes. Coccinellids were significantly more abundant on yellow plants than on greener control plants whereas chrysopids were significantly more numerous on controls in 3 out of 5 maize genotypes. These two groups of predators may use plant colour to partition habitat spatially and exploit their aphid prey while minimizing intraguild interactions.     

The repellent scent-mark of the honeybeeApis mellifera tigustica and its role as communication cue during foraging

Summary Experimental evidence for flower-marking in honeybees (Apis mellifera ligustica), using pairs of workers from the same colony foraging on an artificial patch of flowers, is reported. Workers marked artificial flowers with scent and strongly rejected all flowers they had recently visited. The same rejection behavior, in a lower although significant proportion, was observed when bees visited flowers just abandoned by the other individual of the pair. The repellent nature of this scent-mark was demonstrated with the use of an air extractor connected to the patch of artificial flowers. When the apparatus was turned on, the rejection behavior disappeared and bees accepted both flowers just abandoned by themselves and flowers just abandoned by the other bee. Differences in the response level of bees to their own marks or to the partner's marks suggest that the repellent scent-mark applied by a bee during foraging would basically be a self-use signal, although it certainly has value in communicating with other workers.     

Pollen–plant–climate relationships in sub-Saharan Africa

Aim  To demonstrate that incorporating the bioclimatic range of possible contributor plants leads to improved accuracy in interpreting the palaeoclimatic record of taxonomically complex pollen types.
Location  North Tropical Africa.
Methods  The geographical ranges of selected African plants were extracted from the literature and geo-referenced. These plant ranges were compared with the pollen percentages obtained from a network of surface sediments. Climate-response surfaces were graphed for each pollen taxon and each corresponding plant species.
Results  Several patterns can be identified, including taxa for which the pollen and plant distributions coincide, and others where the range limits diverge. Some pollen types display a reduced climate range compared with that of the corresponding plant species, due to low pollen production and/or dispersal. For other taxa, corresponding to high pollen producers such as pioneer taxa, pollen types display a larger climatic envelope than that of the corresponding plants. The number of species contained in a pollen taxon is an important factor, as the botanical species included in a taxon may have different geographical and climate distributions.
Main conclusions  The comparison between pollen and plant distributions is an essential step towards more precise vegetation and climate reconstructions in Africa, as it identifies taxa that have a high correspondence between pollen and plant distribution patterns. Our method is a useful tool to reassess biome reconstructions in Africa and to characterize accurately the vegetation and climate conditions at a regional scale, from pollen data.     

Plant scents modify innate colour preference in foraging swallowtail butterflies

Flower-visiting insects exhibit innate preferences for particular colours. A previous study demonstrated that naive Papilio xuthus females prefer yellow and red, whereas males are more attracted to blue. Here, we demonstrate that the innate colour preference can be modified by olfactory stimuli in a sexually dimorphic manner. Naive P. xuthus were presented with four coloured discs: blue, green, yellow and red. The innate colour preference (i.e. the colour first landed on) of the majority of individuals was blue. When scent from essential oils of either orange flower or lily was introduced to the room, females’ tendency to select the red disc increased. Scents of lavender and flowering potted Hibiscus rosa-sinensis, however, were less effective. Interestingly, the odour of the non-flowering larval host plant, Citrus unshiu, shifted the preference to green in females. In males, however, all plant scents were less effective than in females, such that blue was always the most favoured colour. These observations indicate that interactions between visual and olfactory cues play a more prominent role in females.     

Refuge-mediated apparent competition in plant–consumer interactions

At the intersection of consumer behaviour and plant competition is the concept of refuge-mediated apparent competition: an indirect interaction whereby plants provide a refuge for a shared consumer, subsequently increasing consumer pressure on another plant species. Here, we use a simple model and empirical examples to develop and illustrate the concept of refuge-mediated apparent competition. We find that the likelihood that an inferior competitor will succeed via refuge-mediated apparent competition is greater when competitors have similar resource requirements and when consumers exhibit a strong response to the refuge and high attack rates on the superior competitor. Refuge-mediated apparent competition may create an emergent Allee effect, such that a species invades only if it is sufficiently abundant to alter consumer impact on resident species. This indirect interaction may help explain unresolved patterns observed in biological invasion, such as the different physical structure of invasive exotic plants, the lag phase, and the failure of restoration efforts. Given the ubiquity of refuge-seeking behaviour by consumers and the ability of consumers to alter the outcome of direct competition among plants, refuge-mediated apparent competition may be an underappreciated mechanism affecting the composition and diversity of plant communities.
Ecology Letters (2010) 13: 11–20     

Correlation between octopaminergic signalling and foraging task specialisation in honeybees

Regulation of pollen and nectar foraging in honeybees is linked to differences in the sensitivity to the reward. Octopamine (OA) participates in the processing of reward-related information in the bee brain, being a candidate to mediate and modulate the division of labour among pollen and nectar foragers. Here we tested the hypothesis that OA affects the resource preferences of foragers. We first investigated whether oral administration of OA is involved in the transition from nectar to pollen foraging. We quantified the percentage of OA-treated bees that switched from a sucrose solution to a pollen feeder when the sugar concentration was decreased experimentally. We also evaluated if feeding the colonies sucrose solution containing OA increases the rate of bees collecting pollen. Finally, we quantified OA and tyramine (TYR) receptor genes expression of pollen and nectar foragers in different parts of the brain, as a putative mechanism that affects the decision-making process regarding the resource type collected. Adding OA in the food modified the probability that foragers switch from nectar to pollen collection. The proportion of pollen foragers also increased after feeding colonies with OA-containing food. Furthermore, the expression level of the AmoctαR1 was upregulated in foragers arriving at pollen sources compared with those arriving at sugar-water feeders. Using age-matched pollen and nectar foragers that returned to the hive, we detected an upregulated expression of a TYR receptor gene in the suboesophageal ganglia. These findings support our prediction that OA signalling affects the decision in honeybee foragers to collect pollen or nectar.     

Symbiotic soil microorganisms as players in aboveground plant–herbivore interactions – the role of rhizobia

Rhizobia play a key role for performance of leguminous plants and ecosystem productivity. However, no studies to date have investigated the importance of the rhizobial symbiosis for legume–herbivore interactions. The additional nitrogen provided by the rhizobia improves the nutritional quality of plants, but may also be used for the synthesis of defence compounds. We performed greenhouse experiments with nodulating and non-nodulating, as well as cyanogenic and acyanogenic strains of Trifolium repen s to study the effects of rhizobia Rhizobium leguminosarum on plant growth and the performance of the chewing herbivore Spodoptera littoralis and the phloem-sucking aphid Myzus persicae . We demonstrate that for nodulating strains of T. repens rhizobia increased plant growth and the performance of Spodoptera littoralis . However, this positive effect of rhizobia on the caterpillars did not occur in a cyanogenic clover strain. Reproduction of the phloem-sucking aphid Myzus persicae was inconsistently affected by rhizobia. Our study provides evidence that the additional nitrogen provided by the rhizobia may be used for the production of nitrogen-based defence compounds, thereby counteracting positive effects on the performance of chewing herbivores. The symbiosis with rhizobia is therefore an important driver of legume–herbivore interactions.