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The composition and richness of herbivore and plant assemblages change along climatic gradients, but knowledge about associated shifts in specialization is scarce and lacks controlling for the abundance and phylogeny of interaction partners. Thus, we aimed to test whether the specialization of phytophagous insects in insect‐plant interaction networks decreases toward cold habitats as predicted by the ‘altitude niche‐breadth hypothesis’ to forecast possible consequences of interaction rewiring under climate change. We used a non‐invasive, standardized metabarcoding approach to reconstruct dietary relationships of Orthoptera species as a major insect herbivore taxon along a broad temperature gradient (~12°C) in Southern Germany. Based on Orthoptera surveys, feeding observations, collection of fecal pellets from >3,000 individuals of 54 species, and parallel vegetation surveys on 41 grassland sites, we quantified plant resource availability and its use by herbivores. Herbivore assemblages were richer in species and individuals at sites with high summer temperatures, while plant richness peaked at intermediate temperatures. Corresponding interaction networks were most specialized in warm habitats. Considering phylogenetic relationships of plant resources, however, the specialization pattern was not linear but peaked at intermediate temperatures, mediated by herbivores feeding on a narrow range of phylogenetically related resources. Our study provides empirical evidence of resource specialization of insect herbivores along a climatic gradient, demonstrating that resource phylogeny, availability, and temperature interactively shape the specialization of herbivore assemblages. Instead of low specialization levels only in cold, harsh habitats, our results suggest increased generalist feeding due to intraspecific changes and compositional differences at both ends of the microclimatic gradient. We conclude that this nonlinear change of phylogeny‐based resource specialization questions predictions derived from the ‘altitude‐niche breadth hypothesis’ and highlights the currently limited understanding of how plant‐herbivore interactions will change under future climatic conditions.
Despite sometimes strong codependencies of insect herbivores and plants, the responses of individual taxa to accelerating climate change are typically studied in isolation. For this reason, biotic interactions that potentially limit species in tracking their preferred climatic niches are ignored. Here, we chose butterflies as a prominent representative of herbivorous insects to investigate the impacts of temperature changes and their larval host plant distributions along a 1.4‐km elevational gradient in the German Alps. Following a sampling protocol of 2009, we revisited 33 grassland plots in 2019 over an entire growing season. We quantified changes in butterfly abundance and richness by repeated transect walks on each plot and disentangled the direct and indirect effects of locally assessed temperature, site management, and larval and adult food resource availability on these patterns. Additionally, we determined elevational range shifts of butterflies and host plants at both the community and species level. Comparing the two sampled years (2009 and 2019), we found a severe decline in butterfly abundance and a clear upward shift of butterflies along the elevational gradient. We detected shifts in the peak of species richness, community composition, and at the species level, whereby mountainous species shifted particularly strongly. In contrast, host plants showed barely any change, neither in connection with species richness nor individual species shifts. Further, temperature and host plant richness were the main drivers of butterfly richness, with change in temperature best explaining the change in richness over time. We concluded that host plants were not yet hindering butterfly species and communities from shifting upwards. However, the mismatch between butterfly and host plant shifts might become a problem for this very close plant–herbivore relationship, especially toward higher elevations, if butterflies fail to adapt to new host plants. Further, our results support the value of conserving traditional extensive pasture use as a promoter of host plant and, hence, butterfly richness.
Context
Habitat loss and degradation impose serious threats on biodiversity. However, not all habitats receive the attention commensurate with their ecological importance. Shrub ecotones (successional stages between grasslands and forests) can be highly species-diverse but are often restricted to small areas as prevalent management practices either promote open grassland or forest habitats, threatening the effective conservation of ecotone species.
Objectives
In this study, we assessed the importance of habitat and landscape features of shrub ecotones for the rarely studied true bugs (Heteroptera), a functionally diverse taxon that comprises highly specialized species and broad generalists.
Methods
True bugs were sampled with a beating tray in 118 spatially independent shrub ecotones in a region of 45,000 square kilometers in Germany. In addition to habitat area and landscape context, we used a hedge index to evaluate habitat quality.
Results
Shrub ecotones in open habitats harbored a greater species richness and abundance compared to shaded ones in later seral stages, and species composition differed. Richness and abundance were positively affected by increasing habitat area and quality, whereas an increase in the proportion of semi-natural habitats within 1 km only enhanced richness. While feeding and habitat specialists were more sensitive to habitat area reduction than generalists, this was not the case for weak dispersers and carnivores.
Conclusions
Our findings emphasize the importance of large and high-quality ecotones that form a patchy mosaic of shrubs and herbaceous plants. Such ecotones can benefit both grassland species and species depending on woody plants. Conservation authorities should balance between promoting shrubs and keeping such habitats open to maximize species diversity.