From bottom-up to top-down control of invertebrate herbivores in a retrogressive chronosequence.

In the long-term absence of disturbance, ecosystems often enter a decline or retrogressive phase which leads to reductions in primary productivity, plant biomass, nutrient cycling and foliar quality. However, the consequences of ecosystem retrogression for higher trophic levels such as herbivores and predators, are less clear. Using a post-fire forested island-chronosequence across which retrogression occurs, we provide evidence that nutrient availability strongly controls invertebrate herbivore biomass when predators are few, but that there is a switch from bottom-up to top-down control when predators are common. This trophic flip in herbivore control probably arises because invertebrate predators respond to alternative energy channels from the adjacent aquatic matrix, which were independent of terrestrial plant biomass. Our results suggest that effects of nutrient limitation resulting from ecosystem retrogression on trophic cascades are modified by nutrient-independent variation in predator abundance, and this calls for a more holistic approach to trophic ecology to better understand herbivore effects on plant communities.

Sick plants in grassland communities: a growth-defense trade-off is the main driver of fungal pathogen abundance.

Aboveground fungal pathogens can substantially reduce biomass production in grasslands. However, we lack a mechanistic understanding of the drivers of fungal pathogen infection and impact. Using a grassland global change and biodiversity experiment we show that the trade-off between plant growth and defense is the main determinant of infection incidence. In contrast, nitrogen addition only indirectly increased incidence via shifting plant communities towards faster growing species. Plant diversity did not decrease incidence, likely because spillover of generalist pathogens or dominance of susceptible plants counteracted negative diversity effects. A fungicide treatment increased plant biomass production and high levels of infection incidence were associated with reduced biomass. However, pathogen impact was context dependent and infection incidence reduced biomass more strongly in diverse communities. Our results show that a growth-defense trade-off is the key driver of pathogen incidence, but pathogen impact is determined by several mechanisms and may depend on pathogen community composition.

Effects of plant functional group removal on structure and function of soil communities across contrasting ecosystems.

Loss of plant diversity has an impact on ecosystems worldwide, but we lack a mechanistic understanding of how this loss may influence below-ground biota and ecosystem functions across contrasting ecosystems in the long term. We used the longest running biodiversity manipulation experiment across contrasting ecosystems in existence to explore the below-ground consequences of 19 years of plant functional group removals for each of 30 contrasting forested lake islands in northern Sweden. We found that, against expectations, the effects of plant removals on the communities of key groups of soil organisms (bacteria, fungi and nematodes), and organic matter quality and soil ecosystem functioning (decomposition and microbial activity) were relatively similar among islands that varied greatly in productivity and soil fertility. This highlights that, in contrast to what has been shown for plant productivity, plant biodiversity loss effects on below-ground functions can be relatively insensitive to environmental context or variation among widely contrasting ecosystems.

Plant soil feedback strength in relation to large-scale plant rarity and phylogenetic relatedness.

Understanding why some species are rare while others are common remains a central and fascinating question in ecology. Recently, interactions with soil organisms have been shown to affect local abundances of plant species within communities, however, it is not known whether they might also drive patterns of rarity at large scales. Further, little is known about the specificity of soil-feedback effects, and whether closely related plants share more soil pathogens than more distantly related plants. In a multi-species soil-feedback experiment (using 19 species) we tested whether regionally and locally rare species differed in their response to soil biota. Regional rarity was measured using range size or IUCN status and local rarity by typical abundance within an area. All species were grown on soils trained by a variety of regionally and locally rare and common species, which also varied in their degree of relatedness to the target. We found that, in general, regionally rare species suffered more than twice as much from soil biota than regionally common species. Soil cultured by regionally rare species also had a more negative effect on subsequent plant growth, suggesting they may have also accumulated more pathogens. Local rarity did not predict feedback strength. Further, soil trained by closely related plants had a more negative effect on growth than soil trained by distant relatives, which indicates a phylogenetic signal in the host range of soil biota. We conclude that soil biota may well contribute to plant rarity at large spatial scales, which offers a novel explanation for plant rarity and commonness. Moreover, our results show that phylogenetic relatedness between plants was a good predictor of the likelihood that two plant species interacted negatively via soil biota, which might mean that soil pathogens could prevent the coexistence of closely related plants and could drive patterns of phylogenetic overdispersion. Our results suggest that soil pathogens could restrict the ability of rare species to shift their ranges and might need to be considered by conservation biologists seeking to protect populations of rare plants.

Determinants of plant establishment success in a multispecies introduction experiment with native and alien species.

Determinants of plant establishment and invasion are a key issue in ecology and evolution. Although establishment success varies substantially among species, the importance of species traits and extrinsic factors as determinants of establishment in existing communities has remained difficult to prove in observational studies because they can be confounded and mask each other. Therefore, we conducted a large multispecies field experiment to disentangle the relative importance of extrinsic factors vs. species characteristics for the establishment success of plants in grasslands. We introduced 48 alien and 45 native plant species at different seed numbers into multiple grassland sites with or without experimental soil disturbance and related their establishment success to species traits assessed in five independent multispecies greenhouse experiments. High propagule pressure and high seed mass were the most important factors increasing establishment success in the very beginning of the experiment. However, after 3 y, propagule pressure became less important, and species traits related to biotic interactions (including herbivore resistance and responses to shading and competition) became the most important drivers of success or failure. The relative importance of different traits was environment-dependent and changed over time. Our approach of combining a multispecies introduction experiment in the field with trait data from independent multispecies experiments in the greenhouse allowed us to detect the relative importance of species traits for early establishment and provided evidence that species traits--fine-tuned by environmental factors--determine success or failure of alien and native plants in temperate grasslands.

Herbivore preference drives plant community composition.

Herbivores are important drivers of plant species coexistence and community assembly. However, detailed mechanistic information on how herbivores affect dominance hierarchies between plant species is scarce. Here, we used data of a multi-site herbivore exclusion experiment in grasslands to assess changes in the cover of 28 plant species in response to aboveground pesticide. application. Moreover, we assessed species-specific values of plant defense of these 28 species measured as the performance of a generalist caterpillar, and the preference of the caterpillar and a slug species in no-choice and choice feeding experiments, respectively. We show that more preferred species in the feeding experiments were those that increased in cover after herbivore exclusion in the field, whereas less preferred ones decreased. Herbivore performance and several measured leaf traits were not related to the change in plant cover in the field in response to herbivore removal. Additionally, the generalist slug and the generalist caterpillar preferred and disliked the same plant species, indicating that they perceive the balance between defense and nutritional value similarly. We conclude that the growth-defense trade-off in grassland species acts via the preference of herbivores and that among-species variation in plant growth and preference to herbivores drives plant community composition.

Tradeoffs associated with constitutive and induced plant resistance against herbivory.

Several prominent hypotheses have been posed to explain the immense variability among plant species in defense against herbivores. A major concept in the evolutionary ecology of plant defenses is that tradeoffs of defense strategies are likely to generate and maintain species diversity. In particular, tradeoffs between constitutive and induced resistance and tradeoffs relating these strategies to growth and competitive ability have been predicted. We performed three independent experiments on 58 plant species from 15 different plant families to address these hypotheses in a phylogenetic framework. Because evolutionary tradeoffs may be altered by human-imposed artificial selection, we used 18 wild plant species and 40 cultivated garden-plant species. Across all 58 plant species, we demonstrate a tradeoff between constitutive and induced resistance, which was robust to accounting for phylogenetic history of the species. Moreover, the tradeoff was driven by wild species and was not evident for cultivated species. In addition, we demonstrate that more competitive species-but not fast growing ones-had lower constitutive but higher induced resistance. Thus, our multispecies experiments indicate that the competition-defense tradeoff holds for constitutive resistance and is complemented by a positive relationship of competitive ability with induced resistance. We conclude that the studied genetically determined tradeoffs are indeed likely to play an important role in shaping the high diversity observed among plant species in resistance against herbivores and in life history traits.

Are rare plant species less resistant than common ones to herbivores? A multi-plant species study using above- and below-ground generalist herbivores.

Rare plant species are suggested to be less resistant to herbivores than common species. Their lower apparency and the fact that they often live in isolated populations, resulting in fewer herbivore encounters, might have led to the evolution of reduced defences. Moreover, their frequent lower levels of genetic diversity compared with common species could negatively affect their resistance against enemies. However, the hypothesis that plant resistance depends on plant regional and local rarity, independently of habitat and competitive and growth strategy, lacks evidence. To test this hypothesis, we assessed the performance and preference of one belowground and three aboveground generalist invertebrate herbivores from different taxonomic groups as indicators of plant resistance. Herbivores were fed a total of 62 regionally and locally rare and common plant species from Switzerland. We accounted for differences in a plant's growth and competitive strategy and habitat resource availability. We found that regionally and locally rare and common plant species did not generally differ in their resistance to most generalist herbivores. However, one herbivore species even performed better and preferred locally and regionally common plant species over rarer ones, indicating that common species are not more resistant, but tend to be less resistant. We also found that all herbivore species consistently performed better on competitive and large plant species, although different herbivore species generally preferred and performed better on different plant species. The latter indicates that the use of generalist herbivores as indicators of plant-resistance levels can be misleading. Synthesis: Our results show that rare plant species are not inherently less resistant than common ones to herbivores. Instead, our results suggest that the ability of plants to allocate resources away from defence towards enhancing their competitive ability might have allowed plants to tolerate herbivory, and to become locally and regionally common.

Unexpected sensitivity of the highly invasive spider Mermessus trilobatus to soil disturbance in grasslands.

The dwarf spider Mermessus trilobatus (Araneae: Linyphiidae), native to North America, has expanded its range over large parts of Europe within less than fifty years. It is notable for occurring in a wide range of mostly agricultural habitats, while most other invasive spiders in Europe are associated with human buildings. As in other invasive invertebrates and plants, the tremendous colonisation success of Mermessus trilobatus might be related to anthropogenic habitat disturbance. Here we aim to test if the invasion success of Mermessus trilobatus in Europe is associated with high tolerance towards soil disturbance. We sampled spiders from eight grasslands experimentally disturbed with superficial soil tillage and eight undisturbed grasslands without tillage. Opposite to our expectation, Mermessus trilobatus densities decrease sharply with soil disturbance. This is in contrast to several native species such as Oedothorax apicatus, which becomes more abundant in the fields after superficial soil tillage. Our study suggests that invasion success of Mermessus trilobatus is not connected to a ruderal strategy. The ecological and evolutionary processes behind colonisation success of Mermessus trilobatus need to be further investigated.