The geographical range structure of the holly leaf-miner. IV. Effects of variation in host-plant quality

Does spatial variation in insect herbivory match variations in plant quality? A meta-analysis

Variation in herbivore pressure has often been predicted from patterns in plant traits considered as antiherbivore defences. Here, we tested whether spatial variation in field insect herbivory is associated with the variation in plant quality by conducting a meta-analysis of 223 correlation coefficients between herbivory levels and the expression of selected plant traits. We found no overall correlation between herbivory and either concentrations of plant secondary metabolites or values of physical leaf traits. This result was due to both the large number of low correlations and the opposing directions of high correlations in individual studies. Field herbivory demonstrated a significant association only with nitrogen: herbivore pressure increased with an increase in nitrogen concentration in plant tissues. Thus, our meta-analysis does not support either theoretical prediction, i.e., that plants possess high antiherbivore defences in localities with high herbivore pressure or that herbivory is low in localities where plant defences are high. We conclude that information about putative plant defences is insufficient to predict plant losses to insects in field conditions and that the only bottom-up factor shaping spatial variation in insect herbivory is plant nutritive value. Our findings stress the need to improve a theory linking plant putative defences and herbivory.

Elevational Pattern of Leaf Mine Diversity on Quercus variabilis Blume at Baotianman, Henan, China

The species composition and diversity pattern of leaf miners on dominant trees in China are poorly understood. Using Hill-based diversity metrics, the elevational patterns of taxonomic, phylogenetic, and functional diversity for leaf miners on Quercus variabilis Blume at Baotianman were systematically analyzed. Leaf mine types belonged to ten genera and seven families. Different leaf miners had different elevational preferences. Most taxonomic and phylogenetic Hill diversity indices had typical hump-shaped elevational patterns, with a peak at the middle elevation of approximately 875 m. No functional Hill diversity indices presented significant linear or nonlinear trends with altitude. The driving factors behind the elevational distribution patterns of leaf miners require further work.

The patterns of co-occurrence variation are explained by the low dependence of bark beetles (Coleoptera: Scolytinae and Platypodinae) on hosts along altitude gradients

Background

Separation of biotic and abiotic impacts on species diversity distribution patterns across a significant climatic gradient is a challenge in the study of diversity maintenance mechanisms. The basic task is to reconcile scale-dependent effects of abiotic and biotic processes on species distribution models. Here, we used a hierarchical modeling method to detect the host specificities of bark beetles (Scolytinae and Platypodinae) with their dependent tree communities across a steep climatic gradient, which was embedded within a relatively homogenous spatial niche.

Results

Species turnover of both trees and bark beetles have an opposite pattern along the climatic proxy (represented by the elevation gradients) at the regional scale, but not at local spatial scales. This pattern confirmed the hypothesis wherein emphasis was on influences of macro-climate on local biotic interactions between trees and hosted bark beetle communities, whereas local biotic relations, represented by host specificity dependence, were regionally conserved.

Conclusions

At a confined spatial scale, cross-taxa comparisons of β-diversity highlighted the importance of simultaneous impacts from both extrinsic factors related to geography and environment, and intrinsic factors related to organism characteristics. The effects of tree abundance and phylogeny diversity on bark beetle diversity were, to a large extent, indirect, operating via changes in bark beetle abundance through spatial and temporal dynamics of resources distribution. Tree host dependence, which was considered and represented by host specificities, plays a minor role on the hosted beetle community in this concealed wood decomposing interacting system.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-022-00455-y.

Spatial covariance of herbivorous and predatory guilds of forest canopy arthropods along a latitudinal gradient

In arthropod community ecology, species richness studies tend to be prioritised over those investigating patterns of abundance. Consequently, the biotic and abiotic drivers of arboreal arthropod abundance are still relatively poorly known. In this cross-continental study, we employ a theoretical framework in order to examine patterns of covariance among herbivorous and predatory arthropod guilds. Leaf-chewing and leaf-mining herbivores, and predatory ants and spiders, were censused on > 1000 trees in nine 0.1 ha forest plots. After controlling for tree size and season, we found no negative pairwise correlations between guild abundances per plot, suggestive of weak signals of both inter-guild competition and top-down regulation of herbivores by predators. Inter-guild interaction strengths did not vary with mean annual temperature, thus opposing the hypothesis that biotic interactions intensify towards the equator. We find evidence for the bottom-up limitation of arthropod abundances via resources and abiotic factors, rather than for competition and predation.

The effect of multiple natural enemies on a shared herbivore prey

Natural enemy diversity is thought to be important for effective suppression of herbivores in production systems. Studies investigating the importance of the diversity and composition of the natural enemy complex often use within-year empirical studies or experimental exclusion setups.However, within-year population suppression might not translate in long-term population regulation. Therefore, I used a combination of long-term data collection and an exclusion experiment to investigate mechanisms behind year-to-year population changes and potential effects of disturbance of the natural enemy complex.Using the holly leaf miner study system in Wytham Woods, I find that the dominant predator in the system does not necessarily contribute the most to the reduction in year-to-year changes in mine density or within-patch fluctuations. Using the exclusion experiment, it becomes clear that parasitism later in the prey life cycle can to a certain level compensate for disruption of mortality in the earlier life stage of the prey.Thus, for host suppression in perennial systems the mortality pressure over the whole life cycle is important and disturbance during one part of the life cycle might not necessarily be buffered by mortality in other parts of the life cycle, especially if the natural enemy complex consists of multiple predator guilds.

Reproductive isolation between host races of Phytomyza glabricola on Ilex coriacea and I. glabra

Recently diverged taxa often show discordance in genetic divergence among genomic loci, where some loci show strong divergence and others show none at all. Genetic studies alone cannot distinguish among the possible mechanisms but experimental studies on other aspects of divergence may provide guidance in the inference of causes of observed discordances. In this study, we used no-choice mating trials to test for the presence of reproductive isolation between host races of the leaf-mining fly, Phytomyza glabricola on its two holly host species, Ilex coriacea and I. glabra. These trials inform our effort to determine the cause of significant differences in the degree of divergence of nuclear and mitochondrial loci of flies collected from the two host plants. We present evidence of reproductive isolation between host races in a controlled greenhouse setting: significantly more mate pairs consisting of flies from the same host plant species produced offspring than inter-host mate pairs, which produced no offspring. We also tested whether the presence of the natal or non-natal host plant affects reproductive success. Flies collected from I. coriacea were more likely to produce offspring when in the presence of the natal host, whereas the presence or absence of either the natal or non-natal host had no effect on flies collected from I. glabra. The results indicate discordant patterns of nuclear and mitochondrial divergence among host races of P. glabricola are likely due to incomplete lineage sorting, and the host races may be well on their way to becoming biological species.

Putting plant resistance traits on the map: a test of the idea that plants are better defended at lower latitudes

• It has long been believed that plant species from the tropics have higher levels of traits associated with resistance to herbivores than do species from higher latitudes. A meta-analysis recently showed that the published literature does not support this theory. However, the idea has never been tested using data gathered with consistent methods from a wide range of latitudes. • We quantified the relationship between latitude and a broad range of chemical and physical traits across 301 species from 75 sites world-wide. • Six putative resistance traits, including tannins, the concentration of lipids (an indicator of oils, waxes and resins), and leaf toughness were greater in high-latitude species. Six traits, including cyanide production and the presence of spines, were unrelated to latitude. Only ash content (an indicator of inorganic substances such as calcium oxalates and phytoliths) and the properties of species with delayed greening were higher in the tropics. • Our results do not support the hypothesis that tropical plants have higher levels of resistance traits than do plants from higher latitudes. If anything, plants have higher resistance toward the poles. The greater resistance traits of high-latitude species might be explained by the greater cost of losing a given amount of leaf tissue in low-productivity environments.