Insect exuviae as soil amendment affect flower reflectance and increase flower production and plant volatile emission.

Soil composition and herbivory are two environmental factors that can affect plant traits including flower traits, thus potentially affecting plant-pollinator interactions. Importantly, soil composition and herbivory may interact in these effects, with consequences for plant fitness. We assessed the main effects of aboveground insect herbivory and soil amendment with exuviae of three different insect species on visual and olfactory traits of Brassica nigra plants, including interactive effects. We combined various methodological approaches including gas chromatography/mass spectrometry, spectroscopy and machine learning to evaluate changes in flower morphology, colour and the emission of volatile organic compounds (VOCs). Soil amended with insect exuviae increased the total number of flowers per plant and VOC emission, whereas herbivory reduced petal area and VOC emission. Soil amendment and herbivory interacted in their effect on the floral reflectance spectrum of the base part of petals and the emission of 10 VOCs. These findings demonstrate the effects of insect exuviae as soil amendment on plant traits involved in reproduction, with a potential for enhanced reproductive success by increasing the strength of signals attracting pollinators and by mitigating the negative effects of herbivory.

Plant defence to sequential attack is adapted to prevalent herbivores.

Plants have evolved plastic defence strategies to deal with the uncertainty of when, by which species and in which order attack by herbivores will take place1-3. However, the responses to current herbivore attack may come with a cost of compromising resistance to other, later arriving herbivores. Due to antagonistic cross-talk between physiological regulation of plant resistance to phloem-feeding and leaf-chewing herbivores4-8, the feeding guild of the initial herbivore is considered to be the primary factor determining whether resistance to subsequent attack is compromised. We show that, by investigating 90 pairwise insect-herbivore interactions among ten different herbivore species, resistance of the annual plant Brassica nigra to a later arriving herbivore species is not explained by feeding guild of the initial attacker. Instead, the prevalence of herbivore species that arrive on induced plants as approximated by three years of season-long insect community assessments in the field explained cross-resistance. Plants maintained resistance to prevalent herbivores in common patterns of herbivore arrival and compromises in resistance especially occurred for rare patterns of herbivore attack. We conclude that plants tailor induced defence strategies to deal with common patterns of sequential herbivore attack and anticipate arrival of the most prevalent herbivores.

Intraspecific variation in plant-associated herbivore communities is phylogenetically structured in Brassicaceae.

As a result of co-evolution between plants and herbivores, related plants often interact with similar herbivore communities. Variation in plant-herbivore interactions is determined by variation in underlying functional traits and by ecological and stochastic processes. Hence, typically, only a subset of possible interactions is realised on individual plants. We show that insect herbivore communities assembling on individual plants are structured by plant phylogeny among 12 species in two phylogenetic lineages of Brassicaceae. This community sorting to plant phylogeny was retained when splitting the community according to herbivore feeding guilds. Relative abundance of herbivores as well as the size of the community structured community dissimilarity among plant species. Importantly, the amount of intraspecific variation in realised plant-herbivore interactions is also phylogenetically structured. We argue that variability in realised interactions that are not directly structured by plant traits is ecologically relevant and must be considered in the evolution of plant defences.

Predictability of Biotic Stress Structures Plant Defence Evolution.

To achieve ecological and reproductive success, plants need to mitigate a multitude of stressors. The stressors encountered by plants are highly dynamic but typically vary predictably due to seasonality or correlations among stressors. As plants face physiological and ecological constraints in responses to stress, it can be beneficial for plants to evolve the ability to incorporate predictable patterns of stress in their life histories. Here, we discuss how plants predict adverse conditions, which plant strategies integrate predictability of biotic stress, and how such strategies can evolve. We propose that plants commonly optimise responses to correlated sequences or combinations of herbivores and pathogens, and that the predictability of these patterns is a key factor governing plant strategies in dynamic environments.

Urbanization drives community shifts towards thermophilic and dispersive species at local and landscape scales.

The increasing conversion of agricultural and natural areas to human-dominated urban landscapes is predicted to lead to a major decline in biodiversity worldwide. Two conditions that typically differ between urban environments and the surrounding landscape are increased temperature, and high patch isolation and habitat turnover rates. However, the extent and spatial scale at which these altered conditions shape biotic communities through selection and/or filtering on species traits are currently poorly understood. We sampled carabid beetles at 81 sites in Belgium using a hierarchically nested sampling design wherein three local-scale (200 × 200 m) urbanization levels were repeatedly sampled across three landscape-scale (3 × 3 km) urbanization levels. First, we showed that communities sampled in the most urbanized locations and landscapes displayed a distinct species composition at both local and landscape scale. Second, we related community means of species-specific thermal preferences and dispersal capacity (based on European distribution and wing morphology, respectively) to the urbanization gradients. We showed that urban communities consisted on average of species with a preference for higher temperatures and with better dispersal capacities compared to rural communities. These shifts were caused by an increased number of species tolerating higher temperatures, a decreased richness of species with low thermal preference, and an almost complete depletion of species with very low-dispersal capacity in the most urbanized localities. Effects of urbanization were most clearly detected at the local scale, although more subtle effects could also be found at the scale of entire landscapes. Our results demonstrate that urbanization may fundamentally and consistently alter species composition by exerting a strong filtering effect on species dispersal characteristics and favouring replacement by warm-dwelling species.