Primary consumers are under strong selection from resource ('bottom-up') and consumer ('topdown') controls, but the relative importance of these selective forces is unknown. We performed a meta-analysis to compare the strength of top-down and bottom-up forces on consumer fitness, considering multiple predictors that can modulate these effects: diet breadth, feeding guild, habitat/environment, type of bottom-up effects, type of top-down effects and how consumer fitness effects are measured. We focused our analyses on the most diverse group of primary consumers, herbivorous insects, and found that in general top-down forces were stronger than bottom-up forces. Notably, chewing, sucking and gall-making herbivores were more affected by top-down than bottom-up forces, top-down forces were stronger than bottom-up in both natural and controlled (cultivated) environments, and parasitoids and predators had equally strong top-down effects on insect herbivores. Future studies should broaden the scope of focal consumers, particularly in understudied terrestrial systems, guilds, taxonomic groups and top-down controls (e.g. pathogens), and test for more complex indirect community interactions. Our results demonstrate the surprising strength of forces exerted by natural enemies on herbivorous insects, and thus the necessity of using a tri-trophic approach when studying insect-plant interactions.

No abstract

Population divergence can occur due to mechanisms associated with geographic isolation and/or due to selection associated with different ecological niches. Much of the evidence for selection‐driven speciation has come from studies of specialist insect herbivores that use different host plant species; however, the influence of host plant use on population divergence of generalist herbivores remains poorly understood. We tested how diet breadth, host plant species and geographic distance influence population divergence of the fall webworm (Hyphantria cunea; FW). FW is a broadly distributed, extreme generalist herbivore consisting of two morphotypes that have been argued to represent two different species: black‐headed and red‐headed. We characterized the differentiation of FW populations at two geographic scales. We first analysed the influence of host plant and geographic distance on genetic divergence across a broad continental scale for both colour types. We further analysed the influence of host plant, diet breadth and geographic distance on divergence at a finer geographic scale focusing on red‐headed FW in Colorado. We found clear genetic and morphological distinction between red‐ and black‐headed FW, and Colorado FW formed a genetic cluster distinct from other locations. Although both geographic distance and host plant use were correlated with genetic distance, geographic distance accounted for up to 3× more variation in genetic distance than did host plant use. As a rare study investigating the genetic structure of a widespread generalist herbivore over a broad geographic range (up to 3,000 km), our study supports a strong role for geographic isolation in divergence in this system.

Titles from entomology and ecology journals were analysed, testing the effect of Latin and common names, functional groups, geographic location, question marks, humour, and title length on citation rate. Using the Latin names of study organisms in a title decreases a paper's citation rate. There was no effect of the use of common names, question marks, humour, or title length on citation rate. Effects of functional group and geographic location were variable.

Herbivore‐plant interactions should be studied using a tri‐trophic approach, but we lack a quantitative measure of the combined effect of top‐down and bottom‐up forces on herbivore fitness. We propose the combination of the bi‐trophic fitness slopes as a tri‐trophic fitness measure. We use the relationship between fitness associated with top‐down and bottom‐up forces and the frequency of host plant use to calculate the top‐down and bottom‐up fitness slopes, which we then combine to obtain three possible directions of tri‐trophic slopes. A positive tri‐trophic slope indicates that herbivores have overall greater tri‐trophic fitness on the more frequently used hosts. A null tri‐trophic fitness slope indicates that herbivores have similar fitness on all host plants. A negative tri‐trophic slope indicates that herbivores have generally lower fitness on the more frequently used hosts. We tested the explanation power of our method using data from the literature that tested herbivore host shifts and experimentally using a generalist herbivore with variable diet breadth across populations. We found that in host shifts, herbivores have higher tri‐trophic fitness on the novel host, while in generalist populations, herbivores use most frequently the best host available. We present applications in other research areas and consider the limitations of our approach. Our approach is a first step towards a comprehensive model of multiple selective forces acting on the evolution of interactions.

Mutualisms, or reciprocally beneficial interspecific interactions, constitute the foundation of many ecological communities and agricultural systems. Mutualisms come in different forms, from pairwise interactions to extremely diverse communities, and they are continually challenged with exploitation by nonmutualistic community members (exploiters). Thus, understanding how mutualisms persist remains an essential question in ecology. Theory suggests that high species richness and functional redundancy could promote mutualism persistence in complex mutualistic communities. Using a yeast system (Saccharomyces cerevisiae), we experimentally show that communities with the greatest mutualist richness and functional redundancy are nearly two times more likely to survive exploitation than are simple communities. Persistence increased because diverse communities were better able to mitigate the negative effects of competition with exploiters. Thus, large mutualistic networks may be inherently buffered from exploitation.

1. Animal ecology and evolution are shaped by environmental perturbations, which are undergoing unprecedented alterations due to climate change. Fire is one such perturbation that causes significant disruption by causing mortality and altering habitats and resources for animals. Fire regimes are changing on a global scale, but the effects of these changes on animal communities are poorly understood. Arthropods are one of the most ubiquitous and diverse animal taxa on the planet and their populations are sensitive to environmental change. Given their wide-ranging impacts on ecosystem functioning, a better understanding of arthropod responses to changing fire regimes is critical and may also provide more general insights into how other groups might respond to fire.2. Here, we provide a comprehensive meta-analytical assessment of how fire influences the arthropod community across habitats and functional groups. Using data from 130 peer-reviewed papers across the globe, we tested how a variety of fire characteristics, including management regime, severity and time-sincefire affect arthropod populations and communities across habitats.3. Our results show that arthropod communities display substantial variation in response to fire and that community-level responses are most likely to be detected within the first year. Responses also vary depending on fire characteristics and habitat. Specifically, while community metrics such as diversity were increased by low severity fires, they were reduced by high severity fires. Likewise, evenness increased after prescribed burns but was reduced after wildfire. Measures of arthropod community structure decreased following fires in deserts and forests. 4. Across the entire arthropod community, fire also had variable effects on community diversity. Fire tended to have a negative effect size on arthropods across life stages, but responses did vary among groups. Nearly all functional groups exhibited a negative response to fire with the exception of herbivores, for which abundance, diversity and richness increased after fire.

Understanding how mutualisms persist over time requires investigations of how mutualist species coevolve and adapt to the interaction. In particular, the key factors in the evolution of mutualisms are the costs and benefits mutualists experience during the interaction. Here, we used a yeast nutritional mutualism to test how mutualists coevolve and adapt in an obligate mutualism. We allowed two yeast mutualists to evolve together for 15 weeks (about 150 generations), and then we tested if the mutualists had coevolved using time‐shift assays. We also examined two mutualistic traits associated with the costs and benefits: resource use efficiency and commodity production. We found that the mutualists quickly coevolved. Furthermore, the changes in benefits and costs were nonlinear and varied with evolutionary changes occurring in the mutualist partner. One mutualist initially evolved to reduce mutualistic commodity production and increase efficiency in mutualistic resource use; however, this negatively affected its mutualist partner that evolved reduced commodity production and resource use efficiency. As a result, the former increased commodity production, resulting in an increase in benefits for its partner. The quick, nonlinear, and asynchronous evolution of yeast mutualists closely resembles antagonistic coevolutionary patterns, supporting the view that mutualisms should be considered as reciprocal exploitation.