The number of invasive non-native pests is increasing rapidly in forests as a result of global change. It is therefore important to prevent their damage in order to preserve the integrity of forest ...ecosystems and the associated services. According to the biotic resistance hypothesis, species-rich communities are less likely to be invaded. The associational resistance hypothesis states that insect herbivores are more likely to colonise and exploit plants surrounded by conspecific neighbours than heterospecific, non-host plant species. More diverse forests would therefore be less damaged by non-native pests than tree monocultures. We tested these hypotheses by comparing the damage caused to seeds by the western conifer bug Leptoglossus occidentalis, an invasive insect native to North America, in plots of pure maritime pine and mixed plots of maritime pine and birch. These plots were in two tree diversity experiments in Europe. Mixed pine plots differed in terms of pine density (1250 versus 625 pines/ha), proportion of pine and birch (25 %, 50 %, 75 %) and spatial mixing pattern (aggregated versus dispersed). We sampled 635 cones in 37 plots. Overall, the proportion of seeds damaged by the invasive bug was significantly lower in mixed plots than in pure pine plots at both experimental sites. There was no significant effect of pine density or relative proportion in the mixtures on seed damage. Aggregated pines in the mixed plots were significantly more damaged than dispersed pines, suggesting that bugs may have done less damage when they had more difficulty locating their host trees. This is consistent with the hypothesis of lower plant apparency. These results support the view that forest plantations of mixed species, which are known to be less vulnerable to attack by native insects, may also be more resistant to infestation by exotic insects. Row-wise, intimate mixtures of two species represent a promising option for designing tree plantations that are more resistant and easier to manage.
•The seed bug Leptoglossus occidentalis is invading coniferous forest in Europe.•Its damage to maritime pine is significantly lower in mixtures with birch.•Intimate mixing of pine with birch reduces damage even more.•The diversity of trees provides biotic resistance to plantations.
Energy allocation strategies have been widely documented in insects and were formalized in the context of the reproduction process by the terms ‘capital breeder’ and ‘income breeder’. We propose here ...the extension of this framework to dispersal ability, with the concepts of ‘capital disperser’ and ‘income disperser’, and explore the trade-off in resource allocation between dispersal and reproduction. We hypothesized that flight capacity was sex-dependent, due to a trade-off in energy allocation between dispersal and egg production in females. We used Monochamus galloprovincialis as model organism, a long-lived beetle which is the European vector of the pine wood nematode. We estimated the flight capacity with a flight mill and used the number of mature eggs as a proxy for the investment in reproduction. We used the ratio between dry weights of the thorax and the abdomen to investigate the trade-off. The probability of flying increased with the adult weight at emergence, but was not dependent on insect age or sex. Flight distance increased with age in individuals but did not differ between sexes. It was also positively associated with energy allocation to thorax reserves, which increased with age. In females, the abdomen weight and the number of eggs also increase with age with no negative effect on flight capacity, indicating a lack of trade-off. This long-lived beetle has a complex strategy of energy allocation, being a ‘capital disperser’ in terms of flight ability, an ‘income disperser’ in terms of flight performance and an ‘income breeder’ in terms of egg production.
•Trophic interactions in novel forests vary with stand size and connectivity.•Insect herbivory varied with stand size and stand connectivity.•Bird abundance but not richness increased with stand size ...and decreased with stand connectivity.•Avian predation increased with stand size, regardless of connectivity.•Avian predation was not determined by bird richness or abundance in novel forests.•Insect herbivory was not affected by avian predation in novel forests.
The value of novel native broadleaf woodlands for biodiversity conservation is important to consider for adequate forest management in rural landscapes. Passive reforestation has been proposed as a cost-efficient tool for creating networks of novel native forest stands that would help restoring biodiversity and associated ecosystem services. Yet to date the ecological functioning of such stands remains strongly understudied compared to forest remnants resulting from longer-term fragmentation. We assessed how the size and connectivity of newly established Pedunculate oak (Quercus robur L.) stands in rural landscapes of SW France affect rates of herbivory by different insect guilds as well as rates of avian insectivory and the abundance and richness of insectivorous birds. Comparing 18 novel forest stands along a gradient of size (0.04–1.15 ha) and cover of broadleaf forests in the surroundings (0–30% within a 500 m radius), we found that even the smallest stands are colonised by leaf miners and chewers/skeletonizers, and that rates of herbivory are globally comparable to those reported from older and larger oak forests. The size of stands had a relatively minor effect on herbivory, whereas it increased the abundance of insectivorous bird. It also determined rates of avian insectivory as estimated by an experiment with plasticine caterpillars. These rates were however rather low and unrelated with the extent of herbivory in the stand. Overall, our study indicates that insect herbivores tend to react more rapidly to the establishment of novel native forests than their avian predators as the latter may depend on the development of larger patches of suitable habitat in the surrounding landscape. To favour a rapid build-up of diverse, and hence stable, trophic networks involving insect herbivores and their predators, woodland creation schemes should therefore primarily focus on habitat size and quality.
Edge contrast, is one of the main determinants of edge effects. This study examines the response of plant and pollinator diversity (bees and butterflies) to forest edge contrast, i.e. the difference ...between forests and adjacent open habitats with different disturbance regimes. We also investigated a potential cascading effect from plants to pollinators and whether edge structure and landscape composition mediate the relationship between edge contrast and beta diversity of pollinators. We sampled 51 low-contrast edges where forests were adjacent to habitats showing low levels of disturbance (i.e. grey dunes, mowed fire-breaks, orchards, grasslands) and 29 high-contrast edges where forests were adjacent to more intensively disturbed habitats (i.e. tilled firebreaks, oilseed rape) in three regions of France. We showed that plant diversities were higher in edges than in adjacent open habitat, whatever the edge contrast. However, plant beta diversity did not differ significantly between low and high-contrast edges. While we observed higher pollinator diversities in adjacent habitats than in low-contrast edges, there were no significant differences in pollinator beta diversity depending on edge contrast. We did not observe a cascading effect from plants to pollinators. Plant and bee beta diversities were mainly explained by local factors (edge structure and flower cover) while butterfly beta diversity was explained by surrounding landscape characteristics (proportion of land cover in grassland).
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•Pine stand colonization by PPM increases with pine density.•The proportion of attacked pines decreases with pine density.•Birch provides pure associational resistance by reducing ...pine apparency.•Apparency effects occur within and between stands.•Associational resistance fades with time.
Mixed forests are thought to be less prone to pest insect damage than monocultures. This may result from reduced host availability (i.e., density effect) or from non-host trees reducing the physical or chemical apparency of host trees (i.e., associational resistance, AR). However, associational and density effects are often confounded in mixed forests. We aimed to disentangle their relative contribution to attacks of pine trees by a specialist pest, the pine processionary moth (PPM, Thaumetopoea pityocampa). We assessed pine infestation by PPM by counting the number of winter nests during three consecutive years along an experimental gradient of pine density in presence or absence of a fast growing species, namely birch. The total number of PPM nests per plot increased with pine density (maximum in high density monocultures), while the proportion of attacked pine trees decreased along the same gradient. Birch provided associational resistance via reduced pine apparency due to their greatest higher. This mechanism occurred at two spatial scales, whenever birch was planted within pine plots or in adjacent plots. Associational resistance was stronger in dense stands, probably due to reduced distance between pines and neighboring birches. But AR faded with time, pines becoming taller than birches, making density effects preeminent over apparency effects. Our findings suggest that mixing tree species to trigger resistance to pest insects requires taking into account the relative growth rate of associated species together with the relative proportion of associated species, both within and between stands.
•Ixodes ticks are more abundant in forests than in other types of habitat.•This difference is greater with mixed forests than with deciduous forests.•Exophilic Ixodes abundance is positively ...correlated with the abundance of ungulates.
With increasing deforestation, questions are being raised about the risk of zoonotic disease to humans. To better assess the role of forest in the emergence of tick-borne diseases, we conducted a meta-analysis of the scientific literature to compare the abundance or diversity of ticks between forest and open habitats (natural or anthropogenic) and a meta-regression to test how tick abundance is influenced by the abundance of their vertebrate hosts in forest habitats.
We found that Ixodes ticks were on average more abundant in forests than in any other non-forested habitats, the difference being more pronounced with mixed deciduous- coniferous than with deciduous forests. At the forest scale, exophilic Ixodes tick abundance was positively influenced by the abundance of their ungulate hosts.
Our results suggest that mixed forests represent the habitats with the highest level of tick hazard. However, more studies are needed to assess the risk of transmission of tick-borne diseases in forests, which also depends on the prevalence of pathogens and the exposure of people.
The conservation of biodiversity has gained prominence in ecological research for the last decades. Conservation actions require a measure of biodiversity such as species richness, but its assessment ...is very difficult, even for small areas and therefore the search for surrogates (i.e. indicators) of biodiversity has emerged as an active research topic. We investigated the relationships between butterfly species richness and landscape structure and composition in two pine plantation sites in Southwest France. We assessed the correlation between butterfly species richness and a set of 15 landscape metrics computed for 18 land-uses at 10 different spatial scales. Spatial scales were accounted for by computing landscape metrics for circular buffers with radius ranging from 100 to 1000
m. The joint use of the Partial Least Squares Regression (PLSR) and a stepwise regression procedure revealed strong correlations between butterfly species richness and various landscape metrics in both study sites. The selected landscape metrics differed from one site to another and mostly involved measures of landscape fragmentation. We found a very strong effect of the spatial scale of investigation upon the perception of the landscape–butterfly richness relationship. Our main conclusions are that (i) certain landscape attributes can potentially serve as indicators for butterfly species richness at the landscape scale; (ii) future indicators of biodiversity based on landscape features should consider various spatial scales.
Aim The role of bird–insect interactions in shaping bird distribution patterns at the landscape scale has been seldom investigated. In mosaic landscapes, bird functional diversity is considered to be ...an important driver of avian insectivory, but depends on forest fragmentation and edge effects from adjacent, non-forest habitats. In a transcontinental experiment, we investigated edge and landscape effects on bird functional diversity and insectivory in mosaic landscapes of mixed forests and open habitats. Location New Zealand and France. Methods We paired edge and interior plots in native forest fragments in New Zealand and native plantation forests in France. We sampled bird communities using point-counts and linear transects respectively and simultaneously quantified avian insectivory as the rate of bird attacks on plasticine models mimicking tree-feeding Lepidoptera larvae. The same seven life traits and attributes were compiled for French and New Zealand birds, including biogeographic origin, body mass, mobility, foraging method, adult diet, nest location and clutch size. Bird functional diversity was quantified on this multitrait basis by four indices: functional richness, evenness, divergence and dispersion. We used mixed models to test for the effects of forest edges, study area, surrounding landscape diversity and native forest cover on bird functional diversity and insectivory. Results We found higher bird functional richness at forest edges than interiors in New Zealand and lower functional richness at edges in France. However, bird functional evenness and divergence were significantly higher at forest edges in the two countries. Functional evenness and dispersion both increased with landscape diversity and evenness increased with native forest cover. Moreover, bird insectivory increased at forest edges with functional evenness, irrespective of the study area. Main conclusions We suggest that intermediate levels of forest fragmentation and edge effects increase avian insectivory in mosaic landscapes, through enhanced functional evenness and trait complementation within predatory bird assemblages.
•Mistletoe infestation was higher in pure Scots pine stands than in mixtures.•Associational resistance was partly driven by relative tree height.•Our study is the first to quantify associational ...resistance to a plant parasite.•In the study area Maritime pine was almost never infested with mistletoe.
The pattern that a given tree species suffers less damage when growing with heterospecific neighbors than amongst conspecific plants, i.e. associational resistance, is common for insect herbivores and many fungal pathogens. However, associational resistance to parasitic plants has never been tested in a replicated study. Using paired forest plots, we investigated whether tree diversity triggered associational resistance to a tree parasite, the European mistletoe Viscum album ssp. austriacum, by comparing pure stands of Scots pine (Pinus sylvestris) with mixtures of Scots pine and Maritime pine (Pinus pinaster) in northern Spain. Maritime pine, with 1.2% of trees being infested, was considered a non-host species in the study area. The infestation level of Scots pines was significantly higher in pure plots (45.1%) than in mixed plots of Scots pines and Maritime pines (25.4%). Our study is the first to quantify associational resistance to a plant parasite in mixed vs. pure forest stands and suggests that mechanisms proposed to explain associational resistance to insects and pathogens also apply to plant parasites. Scots pine trees that were taller than the surrounding trees had a higher infestation probability, in both pure and mixed stands. Scots pine trees growing in mixtures were slightly lower than Maritime pines, suggesting that associational resistance was partly driven by reduced relative tree height. However, the effect of plot type (pure vs. mixed) remained significant after the effect of tree height was accounted for, thus indicating that other factors also contributed to lower mistletoe infestation in mixed plots. In particular, the behavior of birds dispersing mistletoe seeds might differ in mixed vs. pure stands.
► Pine processionary moth pupae survive better in open areas that in pine or broadleaved forests. ► Pupae survival increases with temperature and humidity in the weeks following pupation in sandy ...soils. ► Combinations of pine forests and open areas provide the pest with habitat complementation.
Little attention has been given to the relevance of habitat complementation concept to the population dynamics of insect herbivores. Late instar larvae of the pine processionary moth (PPM)
Thaumetopoea pityocampa move in late winter from pine stands, their feeding habitat, to neighbouring habitats where they pupate until next summer. They search for sunny exposed soil which they can find in open areas. We investigated the effect of both forest cover and soil origin, with three matching types (pine stand, broadleaved stand and open area) on the survival of PPM pupae. The microclimatic soil variables which significantly differentiated cover types and soil origins were the maximum temperature and the mean relative humidity in spring, soon after pupation has occurred. A significant effect of the cover type, but not of the soil origin, was detected on the proportion of dead, emerged or diapausing pupae. Open areas were more suitable for pupae survival than forest covers (pine or broadleaved stands), due to warmer and more humid soil conditions. In this study, we provide one of the first examples of habitat complementation for an insect herbivore, as PPM population can benefit from the combination of pine habitats for the feeding of larvae with open habitats for pupation. The presence of broadleaved stands next to pine stands might also represent an ecological trap for PPM pupae, as broadleaved habitats may turn to be less suitable for pupae survival in spring when the apparition of leaves creates cooler conditions.
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