Not all non-natives are equally unequal: reductions in herbivore β-diversity depend on phylogenetic similarity to native plant community

Tree communities and functional traits determine herbivore compositional turnover

There are many factors known to drive species turnover, although the mechanisms by which these operate are less clear. Based on comprehensive datasets from the largest tree diversity experiment worldwide (BEF-China), we used shared herbivore species (zeta diversity) and multi-site generalized dissimilarity modelling to investigate the patterns and determinants of species turnover of Lepidoptera herbivores among study plots across a gradient in tree species richness. We found that zeta diversity declined sharply with an increasing number of study plots, with complete changes in caterpillar species composition observed even at the fine spatial scale of our study. Plant community characteristics rather than abiotic factors were found to play key roles in driving caterpillar compositional turnover, although these effects varied with an increasing number of study plots considered, due to the varying contributions of rare and common species to compositional turnover. Our study reveals details of the impact of phylogeny- and trait-mediated processes of trees on herbivore compositional turnover, which has implications for forest management and conservation and shows potential avenues for maintenance of heterogeneity in herbivore communities.

Introduction of non-native Douglas fir reduces leaf damage on beech saplings and mature trees in European beech forests

Recent ecological research suggests that, in general, mixtures are more resistant to insect herbivores and pathogens than monocultures. However, we know little about mixtures with non-native trees, where enemy release could lead to patterns that differ from commonly observed relationships among native species. This becomes particularly relevant when considering that adaptation strategies to climate change increasingly promote a larger share of non-native tree species, such as North American Douglas fir in Central Europe. We studied leaf damage on European beech (Fagus sylvatica) saplings and mature trees across a wide range of site conditions in monocultures and mixtures with phylogenetically distant conifers native Norway spruce (Picea abies) and non-native Douglas fir (Pseudotsuga menziesii). We analyzed leaf herbivory and pathogen damage in relation to tree diversity and composition effects, as well as effects of environmental factors and plant characteristics. We observed lower sapling herbivory and tree sucking damage on beech in non-native Douglas fir mixtures than in beech monocultures, probably due to a lower herbivore diversity on Douglas fir trees, and higher pathogen damage on beech saplings in Norway spruce than Douglas fir mixtures, possibly because of higher canopy openness. Our findings suggest that for low diversity gradients, tree diversity effects on leaf damage can strongly depend on tree species composition, in addition to modifications caused by feeding guild and tree ontogeny. Moreover, we found that nutrient capacity modulated the effects of tree diversity, composition, and environmental factors, with different responses in sites with low or high nutrient capacity. The existence of contrasting diversity effects based on tree species composition provides important information on our understanding of the relationships between tree diversity and plant-herbivore interactions in light of non-native tree species introductions. Especially with recent Norway spruce die-off, the planting of Douglas fir as replacement is likely to strongly increase in Central Europe. Our findings suggest that mixtures with Douglas fir could benefit the survival or growth rates of beech saplings and mature trees due to lower leaf damage, emphasizing the need to clearly identify and compare the potential benefits and ecological trade-offs of non-native tree species in forest management under ongoing environmental change.

Contrasting effects of tree origin and urbanization on invertebrate abundance and tree phenology

The ongoing wide-scale introduction of nonnative plants across the world may negatively influence native invertebrate fauna, due to a lack of coevolved traits related to the novel plants, e.g., unique phytochemicals or shifted phenology. Nonnative plants, specifically trees, are common in urban environments, areas that already pose novel habitats to plants and wildlife through a wide array of anthropogenic factors. For example, impervious surfaces contribute to increased ambient temperatures, the so-called urban heat island effect (UHI), which can affect local plant phenology. Yet, few studies have simultaneously studied the effects of urbanization and tree species origin on urban invertebrate communities. We measured the city-level UHI and phenology of nine native and seven nonnative tree species in five city-center parks in southern Sweden, as well as four common native species in a rural control forest. We quantified the abundance of invertebrates on a subset of native and nonnative tree species through shake sampling, sticky traps, and frass collection. In the urban environment, nonnative trees hosted significantly fewer invertebrates compared to native trees. Furthermore, the nonnative trees had a delayed phenology compared to native species, while the peak of caterpillars associated with the subset of trees surveyed for this measure was significantly earlier compared to that of the native species studied. The effect of tree species origin on urban invertebrate abundance was of a greater magnitude (effect size) than the effect of urbanization on invertebrate abundance in native tree hosts. Hence, the results indicate that the impact of nonnative vegetation may be a stronger driver of invertebrate declines in urban areas than other factors. As the effect of species origin on tree phenology was at a level comparable to the urban effect, increasing prevalence of nonnative vegetation can potentially obscure effects of urbanization on phenology in large-scale studies, as well as induce mismatches to invertebrate populations. Since parks harbor a large proportion of urban biodiversity, native trees play a crucial role in such habitats and should not be considered replaceable by nonnative species in terms of conservation value.

Urbanization as a disrupter and facilitator of insect herbivore behaviors and life cycles

Insect herbivores require a variety of habitats across their life cycle, with behavior often mediating transitions between life stages or habitats. Human management strongly alters urban habitats, yet herbivore behavior is rarely examined in cities. We review the existing literature on several key behaviors: host finding, feeding, egg placement and pupation location, and antipredator defense. We emphasize that unapparent portions of the life cycle, such as the habitat of the overwintering stage, may influence if urbanized areas act as population sources or sinks. Here, management of the soil surface and aboveground biomass are two areas with especially pressing research gaps. Lastly, high variability in urban environments may select for more plastic behaviors or greater generalism. We encourage future research that assesses both behavior and less apparent portions of insect life cycles to determine best practices for conservation and management.

Bloom evenness modulates the influence of bloom abundance on insect community structure in suburban gardens

As land use change drives global insect declines, the value of enhancing habitat in urban and suburban landscapes has become increasingly important for flower-visiting insects. In order to help identify best landscaping practices, we conducted plant surveys and insect bowl-trap surveys in 34 suburban yards for 21 months in Gainesville, FL, USA, which resulted in 274 paired days of plant and insect survey data. We assessed the impact of nearest greenspace size, distance to greenspace, yard area, plant richness, plant type, bloom abundance, bloom richness and bloom evenness on insect abundance and richness. Our samples include 34,972 insects captured, 485,827 blooms counted and 774 species of plants recorded. We found that bloom evenness had a modulating effect on bloom abundance-a more even sample of the same number of blooms would have a disproportionately greater positive impact on flower visitor richness, insect richness and insect abundance. Bloom abundance was also highly significant and positively associated with flower visitor abundance, but nearest greenspace size, distance to greenspace, plant type (native vs. non-native vs. Florida Friendly), and yard area were not found to be important factors. Plant richness was a highly significant factor, but its effect size was very small.

Evidence of a rapid and adaptive response of hemipteran mouthparts to a physical barrier

Animals have encountered novel foods at points throughout history, due to factors such as range expansions and niche shifts driven by competition. One of the first challenges presented by novel foods is how to eat them. Mouthpart morphology is thus critical during the process of host shifts. Developmental plasticity in mouthparts is one potential mechanism that may allow animals to tolerate new foods and eventually to thrive upon them. Here, we investigated the extent to which insect mouthparts from two geographically distant populations can converge in morphology when feeding on common resources. We conducted a common garden/reciprocal transplant experiment using two populations of the cactus bug, Narnia femorata, that differ in mouthpart length. This insect uses straw-like mouthparts (hereafter 'beak') to get through the cactus fruit wall to reach the pulp inside. Our experimental results revealed clear developmental plasticity in beak length. Insects from both populations grew longer beaks when they fed on the cactus fruit with the thicker walls, and they grew shorter beaks when they fed on the cactus fruit with the thinner walls. Thus, insects from distant populations exhibited immediate developmental responses to a new food, and in the predicted directions. These results suggest that some fauna may be able to respond more rapidly than predicted when they encounter novel plants.

Introduced plants as novel Anthropocene habitats for insects

Major environmental changes in the history of life on Earth have given rise to novel habitats, which gradually accumulate species. Human-induced change is no exception, yet the rules governing species accumulation in anthropogenic habitats are not fully developed. Here we propose that nonnative plants introduced to Great Britain may function as analogues of novel anthropogenic habitats for insects and mites, analysing a combination of local-scale experimental plot data and geographic-scale data contained within the Great Britain Database of Insects and their Food Plants. We find that novel plant habitats accumulate the greatest diversity of insect taxa when they are widespread and show some resemblance to plant habitats which have been present historically (based on the relatedness between native and nonnative plant species), with insect generalists colonizing from a wider range of sources. Despite reduced per-plant diversity, nonnative plants can support distinctive insect communities, sometimes including insect taxa that are otherwise rare or absent. Thus, novel plant habitats may contribute to, and potentially maintain, broader-scale (assemblage) diversity in regions that contain mixtures of long-standing and novel plant habitats.

Exotic urban trees conserve similar natural enemy communities to native congeners but have fewer pests

Urban trees serve a critical conservation function by supporting arthropod and vertebrate communities but are often subject to arthropod pest infestations. Native trees are thought to support richer arthropod communities than exotic trees but may also be more susceptible to herbivorous pests. Exotic trees may be less susceptible to herbivores but provide less conservation value as a consequence. We tested the hypotheses that native species in Acer and Quercus would have more herbivorous pests than exotic congeners and different communities of arthropod natural enemies. The density of scale insects, common urban tree pests, was greatest on a native Acer and a native Quercus than exotic congeners in both years of our research (2012 and 2016) and sometimes reached damaging levels. However, differences in predator and parasitoid abundance, diversity, and communities were not consistent between native and exotic species in either genus and were generally similar. For example, in 2012 neither predator nor parasitoid abundance differed among native and exotic Acer congeners but in 2016 a native species, A. saccharum, had the least of both groups. A native, Q. phellos, had significantly more predators and parasitoids in 2012 than its native and exotic congeners but no differences in 2016. Parasitoid communities were significantly different among Acer species and Quercus species due in each case to greater abundance of a single family on one native tree species. These native and exotic tree species could help conserve arthropod natural enemies and achieve pest management goals.

Spatiotemporal and demographic variation in the diet of New Zealand lesser short-tailed bats (Mystacina tuberculata)

Variation in the diet of generalist insectivores can be affected by site-specific traits including weather, habitat, and season, as well as demographic traits such as reproductive status and age. We used molecular methods to compare diets of three distinct New Zealand populations of lesser short-tailed bats, Mystacina tuberculata. Summer diets were compared between a southern cold-temperate (Eglinton) and a northern population (Puroera). Winter diets were compared between Pureora and a subtropical offshore island population (Hauturu). This also permitted seasonal diet comparisons within the Pureora population. Lepidoptera and Diptera accounted for >80% of MOTUs identified from fecal matter at each site/season. The proportion of orders represented within prey and the Simpson diversity index, differed between sites and seasons within the Pureora population. For the Pureora population, the value of the Simpson diversity index was higher in summer than winter and was higher in Pureora compared to Eglinton. Summer Eglinton samples revealed that juvenile diets appeared to be more diverse than other demographic groups. Lactating females had the lowest dietary diversity during summer in Pureora. In Hauturu, we found a significant negative relationship between mean ambient temperature and prey richness. Our data suggest that M. tuberculata incorporate a narrower diversity of terrestrial insects than previously reported. This provides novel insights into foraging behavior and ecological interactions within different habitats. Our study is the first from the Southern Hemisphere to use molecular techniques to examine spatiotemporal variation in the diet of a generalist insectivore that inhabits a contiguous range with several habitat types and climates.

Diversity and composition of herbaceous angiosperms along gradients of elevation and forest-use intensity

Terrestrial herbs are important elements of tropical forests; however, there is a lack of research on their diversity patterns and how they respond to different intensities of forest-use. The aim of this study was to analyze the diversity of herbaceous angiosperms along gradients of elevation (50 m to 3500 m) and forest-use intensity on the eastern slopes of the Cofre de Perote, Veracruz, Mexico. We recorded the occurrence of all herbaceous angiosperm species within 120 plots of 20 m x 20 m each. The plots were located at eight study locations separated by ~500 m in elevation and within three different habitats that differ in forest-use intensity: old-growth, degraded, and secondary forest. We analyzed species richness and floristic composition of herb communities among different elevations and habitats. Of the 264 plant species recorded, 31 are endemic to Mexico. Both α- and γ-diversity display a hump-shaped relation to elevation peaking at 2500 m and 3000 m, respectively. The relative contribution of between-habitat β-diversity to γ-diversity also showed a unimodal hump whereas within-habitat β-diversity declined with elevation. Forest-use intensity did not affect α-diversity, but β-diversity was high between old-growth and secondary forests. Overall, γ-diversity peaked at 2500 m (72 species), driven mainly by high within- and among-habitat β-diversity. We infer that this belt is highly sensitive to anthropogenic disturbance and forest-use intensification. At 3100 m, high γ-diversity (50 species) was driven by high α- and within-habitat β-diversity. There, losing a specific forest area might be compensated if similar assemblages occur in nearby areas. The high β-diversity and endemism suggest that mixes of different habitats are needed to sustain high γ-richness of terrestrial herbs along this elevational gradient.