Diverse pollinator communities enhance plant reproductive success.

Understanding the functional consequences of biodiversity loss is a major goal of ecology. Animal-mediated pollination is an essential ecosystem function and service provided to mankind. However, little is known how pollinator diversity could affect pollination services. Using a substitutive design, we experimentally manipulated functional group (FG) and species richness of pollinator communities to investigate their consequences on the reproductive success of an obligate out-crossing model plant species, Raphanus sativus. Both fruit and seed set increased with pollinator FG richness. Furthermore, seed set increased with species richness in pollinator communities composed of a single FG. However, in multiple-FG communities, highest species richness resulted in slightly reduced pollination services compared with intermediate species richness. Our analysis indicates that the presence of social bees, which showed roughly four times higher visitation rates than solitary bees or hoverflies, was an important factor contributing to the positive pollinator diversity-pollination service relationship, in particular, for fruit set. Visitation rate at different daytimes, and less so among flower heights, varied among social bees, solitary bees and hoverflies, indicating a niche complementarity among these pollinator groups. Our study demonstrates enhanced pollination services of diverse pollinator communities at the plant population level and suggests that both the niche complementarity and the presence of specific taxa in a pollinator community drive this positive relationship.

Plant species loss affects life-history traits of aphids and their parasitoids.

The consequences of plant species loss are rarely assessed in a multi-trophic context and especially effects on life-history traits of organisms at higher trophic levels have remained largely unstudied. We used a grassland biodiversity experiment and measured the effects of two components of plant diversity, plant species richness and the presence of nitrogen-fixing legumes, on several life-history traits of naturally colonizing aphids and their primary and secondary parasitoids in the field. We found that, irrespective of aphid species identity, the proportion of winged aphid morphs decreased with increasing plant species richness, which was correlated with decreasing host plant biomass. Similarly, emergence proportions of parasitoids decreased with increasing plant species richness. Both, emergence proportions and proportions of female parasitoids were lower in plots with legumes, where host plants had increased nitrogen concentrations. This effect of legume presence could indicate that aphids were better defended against parasitoids in high-nitrogen environments. Body mass of emerged individuals of the two most abundant primary parasitoid species was, however, higher in plots with legumes, suggesting that once parasitoids could overcome aphid defenses, they could profit from larger or more nutritious hosts. Our study demonstrates that cascading effects of plant species loss on higher trophic levels such as aphids, parasitoids and secondary parasitoids begin with changed life-history traits of these insects. Thus, life-history traits of organisms at higher trophic levels may be useful indicators of bottom-up effects of plant diversity on the biodiversity of consumers.

Effect of plant species loss on aphid-parasitoid communities.

1. The consequences of species loss on ecosystem functioning within a single trophic level have been extensively studied. However, the loss of basal species is likely to have profound impacts on the abundance, richness and ecosystem functioning of species at higher trophic levels. 2. Here, we used experimentally established plant communities with a species richness gradient to study the effects of plant species loss on a multi-trophic insect community in the field. We measured densities and species richness of aphids and parasitic wasps (primary, secondary and facultative tertiary parasitoids of aphids) that naturally colonized the grassland plots. 3. Furthermore, we calculated two ecosystem functions: aphid load (the number of aphid individuals per host plant biomass used as a proxy for herbivory) and parasitism rate. We used structural equation models to explore pathways of direct and indirect effects of plant species richness on higher trophic levels. 4. We found that the densities and richness of species at all trophic levels were influenced by changes in plant species richness. The effects were rarely direct, but instead mediated by the abundance and species richness of aphid host plants and subsequent trophic levels. 5. The herbivore and primary parasitoid levels were most directly affected by changes in plant species richness, with highest insect densities and species richness occurring at intermediate plant species richness. The densities and species richness of secondary parasitoids declined linearly with plant species richness owing to sparser resources, resulting in shorter food chains in communities with the highest plant species richness. 6. Aphid load was highest at intermediate plant species richness and negatively affected by both host plant biomass and host plant species richness. Parasitism rate was mostly affected indirectly via aphid density and overall only weakly negatively related to plant species richness. 7. Our results demonstrate that plant species richness can have strong cascading effects up to high trophic levels. However, their direction may differ at the lower and higher ends of the plant species richness spectrum, cautioning against simplifying consequences of plant species loss for specialist food webs that may become limited by sparse resources at high plant richness.

The robustness of pollination networks to the loss of species and interactions: a quantitative approach incorporating pollinator behaviour.

Species extinctions pose serious threats to the functioning of ecological communities worldwide. We used two qualitative and quantitative pollination networks to simulate extinction patterns following three removal scenarios: random removal and systematic removal of the strongest and weakest interactors. We accounted for pollinator behaviour by including potential links into temporal snapshots (12 consecutive 2-week networks) to reflect mutualists' ability to 'switch' interaction partners (re-wiring). Qualitative data suggested a linear or slower than linear secondary extinction while quantitative data showed sigmoidal decline of plant interaction strength upon removal of the strongest interactor. Temporal snapshots indicated greater stability of re-wired networks over static systems. Tolerance of generalized networks to species extinctions was high in the random removal scenario, with an increase in network stability if species formed new interactions. Anthropogenic disturbance, however, that promote the extinction of the strongest interactors might induce a sudden collapse of pollination networks.

Indirect interactions between invasive and native plants via pollinators.

In generalised pollination systems, the presence of alien plant species may change the foraging behaviour of pollinators on native plant species, which could result in reduced reproductive success of native plant species. We tested this idea of indirect interactions on a small spatial and temporal scale in a field study in Mauritius, where the invasive strawberry guava, Psidium cattleianum, provides additional floral resources for insect pollinators. We predicted that the presence of flowering guava would indirectly and negatively affect the reproductive success of the endemic plant Bertiera zaluzania, which has similar flowers, by diverting shared pollinators. We removed P. cattleianum flowers within a 5-m radius from around half the B. zaluzania target plants (treatment) and left P. cattleianum flowers intact around the other half (control). By far, the most abundant and shared pollinator was the introduced honey bee, Apis mellifera, but its visitation rates to treatment and control plants were similar. Likewise, fruit and seed set and fruit size and weight of B. zaluzania were not influenced by the presence of P. cattleianum flowers. Although other studies have shown small-scale effects of alien plant species on neighbouring natives, we found no evidence for such negative indirect interactions in our system. The dominance of introduced, established A. mellifera indicates their replacement of native insect flower visitors and their function as pollinators of native plant species. However, the pollination effectiveness of A. mellifera in comparison to native pollinators is unknown.

Fungal endosymbionts of plants reduce lifespan of an aphid secondary parasitoid and influence host selection.

Complex biotic interactions shape ecological communities of plants, herbivores and their natural enemies. In studies of multi-trophic interactions, the presence of small, invisible micro-organisms associated with plants and those of a fourth above-ground trophic level have often been neglected. Incorporating these neglected factors improves our understanding of the processes within a multi-trophic network. Here, we ask whether the presence of a fungal endosymbiont, which alters plant quality by producing herbivore-toxic substances, trickles up the food chain and affects the performance and host-selection behaviour of aphid secondary parasitoids. We simultaneously offered hosts from endophyte-free and endophyte-infected environments to secondary parasitoids. Older and more experienced parasitoid females discriminated against hosts from the endophyte-infected environment. Developing in lower quality hosts from the endophyte-infected environment reduced the lifespan of secondary parasitoids. This indicates that aphid secondary parasitoids can perceive the disadvantage for their developing offspring in parasitoids from the endophyte environment and can learn to discriminate against them. In the field, this discrimination ability may shift the success of primary parasitoids to endophyte-infected plants, which co-occur with endophyte-free plants. Ultimately, the control of aphids depends on complex interactions between primary and secondary parasitoids and their relative sensitivity to endophytic fungi.

Trophic cascades initiated by fungal plant endosymbionts impair reproductive performance of parasitoids in the second generation.

Variation in plant quality can transmit up the food chain and may affect herbivores and their antagonists in the same direction. Fungal endosymbionts of grasses change the resource quality by producing toxins. We used an aphid-parasitoid model system to explore how endophyte effects cascade up the food chain and influence individual parasitoid performance. We show that the presence of an endophyte in the grass Lolium perenne has a much stronger negative impact on the performance of the parasitoid Aphidius ervi than on its aphid host Metopolophium festucae. Although the presence of endophytes did not influence the parasitism rate of endophyte-naïve parasitoids or their offspring's survival to adulthood, most parasitoids developing within aphids from endophyte-infected plants did not reproduce at all. This indicates a delayed but very strong effect of endophytes on parasitoid performance, which should ultimately affect plant performance negatively by releasing endophyte-tolerant herbivores from top-down limitations.

Seed dispersal and establishment of endangered plants on Oceanic Islands: the Janzen-Connell model, and the use of ecological analogues.

BACKGROUND: The Janzen-Connell model states that plant-specific natural enemies may have a disproportionately large negative effect on progeny close to maternal trees. The majority of experimental and theoretical studies addressing the Janzen-Connell model have explored how it can explain existing patterns of species diversity in tropical mainland areas. Very few studies have investigated how the model's predictions apply to isolated oceanic islands, or to the conservation management of endangered plants. Here, we provide the first experimental investigation of the predictions of the Janzen-Connell model on an oceanic island, in a conservation context. In addition, we experimentally evaluate the use of ecological analogue animals to resurrect the functional component of extinct frugivores that could have dispersed seeds away from maternal trees. METHODOLOGY/PRINCIPAL FINDINGS: In Mauritius, we investigated seed germination and seedling survival patterns of the critically endangered endemic plant Syzygium mamillatum (Myrtaceae) in relation to proximity to maternal trees. We found strong negative effects of proximity to maternal trees on growth and survival of seedlings. We successfully used giant Aldabran tortoises as ecological analogues for extinct Mauritian frugivores. Effects of gut-passage were negative at the seed germination stage, but seedlings from gut-passed seeds grew taller, had more leaves, and suffered less damage from natural enemies than any of the other seedlings. CONCLUSIONS/SIGNIFICANCE: We provide the first experimental evidence of a distance-dependent Janzen-Connell effect on an oceanic island. Our results potentially have serious implications for the conservation management of rare plant species on oceanic islands, which harbour a disproportionately large fraction of the world's endemic and endangered plants. Furthermore, in contrast to recent controversy about the use of non-indigenous extant megafauna for re-wilding projects in North America and elsewhere, we argue that Mauritius and other oceanic islands are ideal study systems in which to empirically explore the use of ecological analogue species in restoration ecology.

Natural enemies act faster than endophytic fungi in population control of cereal aphids.

1. Fast-growing populations of phytophagous insects can be limited by the presence of natural enemies and by alkaloids that are produced by symbiotic associations of many temperate grass species with endophytic fungi. It is unclear if and how acquired plant defences derived from endophytic fungi interact with natural enemies to affect phytophagous insect populations. 2. To assess the relative importance of endophytic fungi compared to that of natural enemies on the population dynamics of phytophagous insects, we carried out a fully factorial field experiment, in which the presence of natural enemies and the presence of endophytic fungi were manipulated simultaneously. Target colonies of aphids were monitored for 8 weeks starting from their natural appearance in the field to the end of the aphid season. 3. We show that on Lolium perenne increased natural enemy densities reduced the individual numbers of two common cereal aphids, Rhopalosiphum padi and Metopolophium festucae. 4. The presence of the endophytic fungi Neotyphodium lolii reduced the number of M. festucae but did not affect the number of R. padi. The reduction in R. padi numbers by predators and parasitoids was not influenced by the presence of endophytes. For adult M. festucae, however, the negative effects of natural enemies were significant only in the absence of endophytes. 5. Over the duration of the experiment, the effect of natural enemies on aphid colony growth was much stronger than the effect of the endophytic fungi N. lolii, presumably because predator and parasitoid action on aphid colonies is much faster than any effects of endophytes. 6. Our results demonstrate that with simultaneous action of acquired endosymbionts and natural enemies, both factors can control aphid colony growth but they generally act independently of each other.

Genetic variation and covariation of susceptibility to parasitoids in the aphid Myzus persicae: no evidence for trade-offs.

Parasitoids are an important mortality factor for insects. Susceptibility to parasitoids should thus be under strong negative selection. Nevertheless, ample genetic variation for susceptibility to parasitoids is commonly observed in natural populations, suggesting that trade-offs may constrain the evolution of reduced susceptibility. This can be studied by assessing genetic variation for susceptibility and its covariation with other components of fitness. In a set of 17 clones of the peach potato aphid, Myzus persicae, for which good estimates of heritable variation for life-history traits were available, we found significant clonal variation for susceptibility to two of their common parasitoids: Aphidius colemani and Diaeretiella rapae. One clone, the only one harbouring a facultative endosymbiotic bacterium, Regiella insecticola, was entirely resistant to both parasitoids. Susceptibilities to the two parasitoids exhibited a positive genetic correlation close to unity, implying a general mechanism of defence. However, the susceptibility to parasitoids was uncorrelated to the clones' fecundity or rate of increase, providing no evidence for costs of the ability to resist parasitoids.

Strong parasitoid-mediated selection in experimental populations of aphids.

Clonal diversity in asexual populations may be maintained if different clones are favoured under different environmental conditions. For aphids, parasitoids are an important variable of the biotic environment. To test whether parasitoids can mediate selection among host clones, we used experimental populations consisting of 10 clones of the peach-potato aphid, Myzus persicae, and allowed them to evolve for several generations either without parasitoids or in the presence of two species of parasitoid wasps. In the absence of parasitoids, strong shifts in clonal frequencies occurred, mostly in favour of clones with high rates of increase. The parasitoid Diaeretiella rapae hardly affected aphid densities but changed the outcome of competition by favouring one entirely resistant clone and disfavouring a highly susceptible clone. Aphidius colemani, the more infective parasitoid, strongly reduced aphid densities and dramatically changed host clonal frequencies. The most resistant clone, not a successful clone without parasitoids, became totally dominant. These results highlight the potential of temporal or spatial variation in parasitoid densities to maintain clonal diversity in their aphid hosts.

Interaction diversity within quantified insect food webs in restored and adjacent intensively managed meadows.

1. We studied the community and food-web structure of trap-nesting insects in restored meadows and at increasing distances within intensively managed grassland at 13 sites in Switzerland to test if declining species diversity correlates with declining interaction diversity and changes in food-web structure. 2. We analysed 49 quantitative food webs consisting of a total of 1382 trophic interactions involving 39 host/prey insect species and 14 parasitoid/predator insect species. Species richness and abundance of three functional groups, bees and wasps as the lower trophic level and natural enemies as the higher trophic level, were significantly higher in restored than in adjacent intensively managed meadows. Diversity and abundance of specific trophic interactions also declined from restored to intensively managed meadows. 3. The proportion of attacked brood cells and the mortality of bees and wasps due to natural enemies were significantly higher in restored than in intensively managed meadows. Bee abundance and the rate of attacked brood cells of bees declined with increasing distance from restored meadows. These findings indicate that interaction diversity declines more rapidly than species diversity in our study system. 4. Quantitative measures of food-web structure (linkage density, interaction diversity, interaction evenness and compartment diversity) were higher in restored than in intensively managed meadows. This was reflected in a higher mean number of host/prey species per consumer species (degree of generalism) in restored than in intensively managed meadows. 5. The higher insect species and interaction diversity was related to higher plant species richness in restored than in intensively managed meadows. In particular, bees and natural enemies reacted positively to increased plant diversity. 6. Our findings provide empirical evidence for the theoretical prediction that decreasing species richness at lower trophic levels should reduce species richness at higher trophic levels, and in addition lead to even stronger reductions in interaction diversity at these higher levels. Species at higher trophic levels may thus benefit relatively more than species at lower trophic levels from habitat restoration in the grassland ecosystems studied. We also demonstrate enhanced compartment diversity and lower interaction evenness in restored than in intensively managed meadows, both of which are theoretically positively associated with increased ecosystem stability in restored meadows.

Coloured nectar: distribution, ecology, and evolution of an enigmatic floral trait.

While coloured nectar has been known to science at least since 1785, it has only recently received focused scientific attention. However, information about this rare floral trait is scattered and hard to find. Here, we document coloured nectar in 67 taxa worldwide, with a wide taxonomical and geographical distribution. We summarise what is currently known about coloured nectar in each of the lineages where it occurs. The most common nectar colours are in the spectrum from yellow to red, but also brown, black, green, and blue colours are found. Colour intensity of the nectar varies, sometimes even within one taxa, as does the level of contrast between flower petals and nectar. Coloured nectar has evolved independently throughout the angiosperms at least 15 times at the level of family, and is in many cases correlated with one or more of three parameters: (1) vertebrate pollination, known or hypothesised, (2) insularity -- many species are from islands or insular mainland habitats, and (3) altitude -- many species are found at relatively high altitudes. We discuss the evolution and speculate on possible ecological functions of coloured nectar. Apart from being a non-functional, perhaps pleiotropic, trait, we present several hypotheses on possible ecological functions of coloured nectar. Firstly, for some plant species it can be interpreted as an honest signal, leading to high pollination efficiency. Secondly, it can function as a deterrent against nectar-thieves or inefficient pollinators, thus acting as a floral filter. Thirdly, nectar colour-pigments can have anti-microbial qualities that may protect the nectar in long-lived flowers. Neither of these possibilities are mutually exclusive. Recent studies have provided experimental evidence for the first two hypotheses, and we suggest promising avenues for future research into this little-known floral trait.

Positive indirect interactions between neighboring plant species via a lizard pollinator.

In natural communities, species are embedded in networks of direct and indirect interactions. Most studies on indirect interactions have focused on how they affect predator-prey or competitive relationships. However, it is equally likely that indirect interactions play an important structuring role in mutualistic relationships in a natural community. We demonstrate experimentally that on a small spatial scale, dense thickets of endemic Pandanus plants have a strong positive trait-mediated indirect effect on the reproduction of the declining endemic Mauritian plant Trochetia blackburniana. This effect is mediated by the endemic gecko Phelsuma cepediana moving between Pandanus thickets, a preferred microhabitat, and nearby T. blackburniana plants, where it feeds on nectar and pollinates the plants. Our findings emphasize the importance of considering plant-animal interactions such as pollination at relatively small spatial scales in both basic ecological studies and applied conservation management.

Mauritian coloured nectar no longer a mystery: a visual signal for lizard pollinators.

Most floral nectars are clear as water, and the enigmatic coloured nectar in three endemic plant species in Mauritius has puzzled scientists studying it. One hypothesis about the possible ecological function of coloured nectar is that it serves as a visual signal for pollinators. Recent studies have shown that at least two of the three Mauritian plant species with coloured nectar are visited and pollinated by endemic Phelsuma geckos. We here provide experimental evidence for the visual signal hypothesis by showing that Phelsuma ornata geckos prefer coloured over clear nectar in artificial flowers. In flowering plants, coloured nectar could additionally function as an honest signal that allows pollinators to assert the presence and judge the size of a reward prior to flower visitation, and to adjust their behaviour accordingly, leading to increased pollinator efficiency. Our study provides a first step in understanding this rare and intriguing floral trait.

Fungal plant endosymbionts alter life history and reproductive success of aphid predators.

Endosymbionts occur in most plant species and may affect interactions among herbivores and their predators through the production of toxic alkaloids. Here, we ask whether effects of mycotoxins produced by the symbiosis of the fungal endophyte Neotyphodium lolii and the grass Lolium perenne are transmitted to the aphidophagous ladybird Coccinella septempunctata when feeding on cereal aphids Rhopalosiphum padi on infected plants. The larval development of coccinellids was extended, while their survival was reduced when feeding exclusively on aphids from infected plants. Ladybirds developing on aphids from infected plants showed reduced fecundity and impaired reproductive performance. Body size and symmetries of ladybird adults were not affected by the endophytes. Consistently strong, negative effects of endophytes on the lifetime performance of ladybirds indicates that mycotoxins are transmitted along food chains, causing significant damage for top predators. Such cascading effects will influence the population dynamics of aphid predators in the long term and could feedback to the primary plant producers.

Symbiosis between grasses and asexual fungal endophytes.

The symbiosis between vertically transmitted asexual endophytic fungi and grasses is common and generally considered to be mutualistic. Recent studies have accumulated evidence of negative effects of endophytes on plant fitness, prompting a debate on the true nature of the symbiosis. Genetic factors in each of the two partners show high variability and have a range of effects (from positive to negative) on plant fitness. In addition, interacting environmental factors might modify the nature of the symbiosis. Finally, competition and multitrophic interactions among grass consumers are influenced by endophytes, and the effects of plant neighbours or consumers could feedback to affect plant fitness. We propose a mutualism-parasitism continuum for the symbiosis between asexual endophytes and grasses, which is similar to the associations between plants and mycorrhizal fungi.

Relative importance of fertiliser addition to plants and exclusion of predators for aphid growth in the field.

Herbivore dynamics and community structure are influenced both by plant quality and the actions of natural enemies. A factorial experiment manipulating both higher and lower trophic levels was designed to explore the determinants of colony growth of the aphid Aphis jacobaeae, a specialist herbivore on ragwort Senecio jacobaea. Potential plant quality was manipulated by regular addition of NPK-fertiliser and predator pressure was reduced by interception traps; the experiment was carried out at two sites. The size and persistence of aphid colonies were measured. Fertiliser addition affected plant growth in only one site, but never had a measurable effect on aphid colony growth. In both habitats the action of insect predators dominated, imposing strong and negative effects on aphid colony performance. Ants were left unmanipulated in both sites and their performance on the aphid colonies did not significantly differ between sites or between treatments. Our results suggest that, at least for aphid herbivores on S. jacobaea, the action of generalist insect predators appears to be the dominant factor affecting colony performance and can under certain conditions even improve plant productivity.

Body sizes of hosts and parasitoids in individual feeding relationships.

In a natural community of 49 species (12 species of aphids and 37 species of their parasitoids), body lengths of 2,151 parasitoid individuals were, to an excellent approximation, related to the body lengths of their individual aphid hosts by a power law with an exponent close to 3/4. Two alternative models predict this exponent. One is based on surface area to volume relationships. The other is based on recent developments in metabolic ecology. Both models require a changing ratio (in both host and parasitoid) of length to diameter with increasing body length. These changing ratios are manifested differently in the two models and result in testably different predictions for the scaling of body form with increasing size. The estimated exponent of 3/4 for the relationship between individual host body size and individual parasitoid body size degrades to an exponent of nearly 1/2, and the scatter in the relationship between aphid and parasitoid body length is substantially increased, if the average length of a parasitoid species is examined as a function of the average length of its aphid host species instead of using measurements of individuals.