Although plants are sessile organisms, they can modulate their phenotype so as to cope with environmental stresses such as herbivore attack and competition with neighbouring plants. Plant‐produced ...volatile compounds mediate various aspects of plant defence. The emission of volatiles has costs and benefits. Research on the role of plant volatiles in defence has focused primarily on the responses of individual plants. However, in nature, plants rarely occur as isolated individuals but are members of plant communities where they compete for resources and exchange information with other plants. In this review, we address the effects of neighbouring plants on plant volatile‐mediated defences. We will outline the various roles of volatile compounds in the interactions between plants and other organisms, address the mechanisms of plant neighbour perception in plant communities, and discuss how neighbour detection and volatile signalling are interconnected. Finally, we will outline the most urgent questions to be addressed in the future.
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Abstract
Insect herbivory plays a critical role in the regulation of plant populations and community structures. Although the strength of insect herbivory varies with neighbouring tree diversity and ...identity, it is not clear to what extent insect guilds and leaf traits of host plants mediate the relationships, particularly in diverse natural forests.
In this study, we measured leaf herbivory by chewing and mining insect herbivores in 442 saplings of nine species of deciduous trees in a warm‐temperate montane forest. We censused neighbouring trees and measured five leaf traits known to influence herbivory (i.e. specific leaf area‐SLA, thickness, carbon: nitrogen ratio‐C:N ratio, sodium‐Na and silicon‐Si content) on a subset of saplings.
Only herbivory by leaf‐mining insects was influenced by the identity and diversity of neighbouring trees. Specifically, the incidence of leaf‐miners decreased with the phylogenetic diversity of neighbouring trees and increased with conspecific adult density in the vicinity of focal trees. Furthermore, leaf mining herbivory was positively correlated with the amount of Si in the leaves of focal saplings, and leaf chewing herbivory decreased with increasing C:N ratio in the leaves. In contrast to our hypothesis, the leaf traits of focal saplings did not serve as a functional link between tree diversity and insect herbivory.
Synthesis
: Overall, this study highlights the importance of considering the specialisation of insect herbivores in understanding the impacts of the diversity and identity of neighbouring trees on the strength of insect herbivory.
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Consumer‐resource interactions are often influenced by other species in the community, such as when neighbouring plants increase or reduce herbivory to a focal plant species (known as associational ...effects). The many studies on associational effects between a focal plant and some neighbour have shown that these effects can vary greatly in strength and direction. But because almost all of these studies measure associational effects from only one or two neighbour species, we know little about the actual range of associational effects that a plant species might encounter in a natural setting. This makes it difficult to determine how important effects of neighbours are in real field settings, and how associational effects might interact with competition and other processes to influence plant community composition.
In this study, we used a field experiment with a focal species, Solanum carolinense, and 11 common neighbour species to investigate how associational effects vary among many co‐occurring neighbour species and to test whether factors such as neighbour plant apparency, phylogenetic proximity to the focal species, or effects on focal plant defence traits help to explain interspecific variation in associational effect strength.
We found that some neighbour species affected S. carolinense damage and attack by specialist herbivores, but associational effects of most neighbours were weak. Associational effects increased herbivore attack on average earlier in the season (associational susceptibility) and reduced herbivore attack on average later in the season (associational resistance) relative to S. carolinense in monoculture.
We found some evidence that a neighbour's associational effect was related to its biomass and phylogenetic proximity to the focal species. While neighbour species differed in their effects on physical leaf traits of focal plants (trichome density, specific leaf area, and leaf toughness), these traits did not appear to mediate the effects of neighbours on focal plant herbivory.
Synthesis. Our results suggest that the distribution of associational effect strengths in natural communities are similar to those observed for other interaction types, and that multiple mechanisms are likely acting simultaneously to shape associational effects of different neighbour species.
In a field experiment with one focal plant and 11 common neighbour species, we found that the distribution of associational effect strengths in natural communities are similar to those observed for other interaction types, and that multiple mechanisms are likely acting simultaneously to shape associational effects of different neighbour species.
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•Rewilding aims to restore ecological processes with minimum human intervention.•It can be applied on abandoned agricultural lands to restore wood pasture landscapes.•We studied ...effects on sapling establishment in Europe’s oldest rewilding area.•We found that grazing refuges are essential for sapling establishment.•Rewilding projects should warrant grazing refuges for wood pasture restoration.
Rewilding is a novel nature management type that aims at restoring natural processes with minimal human intervention. It is increasingly employed on abandoned agricultural lands in Europe, but empirical studies are scarce. Rewilding may lead to formation of wood-pastures, arguably the primeval landscape in parts of Europe before Neolithic times. We investigated sapling establishment, a key process for wood-pasture formation, in the Oostvaardersplassen: Europe’s oldest large-scale rewilding area, with high densities of free-roaming large herbivores. We transplanted saplings of pioneers, spiny shrubs, and hardwood species and studied how herbivore accessibility (grazed control, partial and full exclosure), vegetation type (tall roughs; short lawns) and soil-tillage (mimicking wild boar rooting) affected sapling survival for four years. No single sapling survived in grazed controls, while survival in exclosures was 25%. Differences in survival between partial and full exclosures were minor, indicating that reduced herbivore access is sufficient for sapling survival. Survival was higher in lawn than in rough in both exclosure types and for all species, indicating positive effects of preceding grazing. Soil tillage initially benefitted all species, but effects lasted for pioneers only, suggesting that – once introduced – wild boar rooting may affect woody species composition. We conclude that rewilding with herbivores can successfully form wood-pasture landscapes on abandoned agricultural land as long as grazing refuges are present that allow for sapling establishment. As grazing refuges are generally lacking on abandoned agricultural lands, where most rewilding is foreseen, we recommend that future projects consider the presence – or creation – of grazing refuges.
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Epibiosis is increasingly considered a survival strategy in space-limited environments. However, epibionts can create a new interface between its host, environment and potential predators which may ...alter predator-prey relationships and biological functioning. Ex-situ experiments investigated the potential costs and benefits of epibiont barnacles on mortality and feeding rate of the mussel, Mytilus edulis, and its predator, the whelk Nucella lapillus. Mussels with living epibiont barnacles suffered no mortality from whelk predation, but when barnacles were absent, mortality was ∼21% over 48 days. Further comparisons revealed the structural complexity of barnacles provided mussels with protection from whelk predation, while the presence of living barnacles increased predator-prey encounters but led to predators targeting barnacles over mussels. Feeding trials revealed feeding rate increased by ∼24% in mussels with living epibionts over mussels with dead or without epibionts, indicating potential costs of hosting epibionts. Our results show that epibionts provide important associational resistance for mussels against whelk predation but a potential cost to the mussel of hosting epibionts requiring increased energy acquisition. These findings advance our understanding of associational resistance derived from epibionts and serve to highlight the potential trade-offs affecting basibiont functioning while showing the importance of positive ecological interactions in ecosystem structure and functioning.
•Epibiosis can alter predator-prey interactions and biological functioning.•Epibionts provide structural protection to basibionts from predation.•Predator-prey interactions increase but reduce predation mortality.•Hosting living epibionts come at a physiological cost through increased feeding.•Epibionts provide associational resistance to hosts mediating biotic interactions.
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The transition toward more sustainable plant protection with reduced pesticide use is difficult, because there is no “silver bullet” available among nonchemical tools. Integrating several plant ...protection approaches may thus be needed for efficient pest management. Recently, increasing the genetic diversity of plantations
via
cultivar mixing has been proposed as a possible method to reduce pest damage. However, previous studies have not addressed either the relative efficiency of exploiting cultivar mixing and intrinsic plant herbivore resistance or the potential utility of combining these approaches to increase cropping security. Here, using a full factorial experiment with 60 woodland strawberry plots, we tested for the relative and combined effect of cultivar mixing and intrinsic plant resistance on herbivore damage and yield. The experiment comprised two levels of diversity (“high” with 10 varieties and “low” with two varieties) and three levels of resistance (“resistant” comprising only varieties intrinsically resistant against strawberry leaf beetle
Galerucella tenella
; “susceptible” with susceptible varieties only; and “resistance mixtures” with 50:50 mixtures of resistant and susceptible varieties). The experiment was carried out over two growing seasons. Use of resistant varieties either alone or intermixed with susceptible varieties in “resistance mixtures” reduced insect herbivory. Interestingly, resistant varieties not only reduced the mean damage in “resistance mixtures” by themselves being less damaged, but also protected intermixed susceptible varieties
via
associational resistance. The effect of higher genetic diversity was less evident, reducing herbivory only at the highest level of herbivore damage. In general, herbivory was lowest in plots with high diversity that included at least some resistant varieties and highest in low diversity plots consisting only of susceptible varieties. Despite this, no significant difference in yield (fruit biomass) was found, indicating that strawberry may be relatively tolerant. Our results demonstrate that combined use of high genetic diversity and resistant varieties can help reduce pest damage and provide a useful tool for sustainable food production. “Resistance mixtures” may be particularly useful for sensitive food crops where susceptible varieties are high yielding that could not be completely replaced by resistant ones.
Associational resistance theory predicts that insect herbivory decreases with increasing tree diversity in forest ecosystems. However, the generality of this effect and its underlying mechanisms are ...still debated, particularly since evidence has accumulated that climate may influence the direction and strength of the relationship between diversity and herbivory.
We quantified insect leaf herbivory and leaf chemical defences (phenolic compounds) of silver birch Betula pendula in pure and mixed plots with different tree species composition across 12 tree diversity experiments in different climates. We investigated whether the effects of neighbouring tree species diversity on insect herbivory in birch, that is, associational effects, were dependent on the climatic context, and whether neighbour‐induced changes in birch chemical defences were involved in associational resistance to insect herbivory.
We showed that herbivory on birch decreased with tree species richness (i.e. associational resistance) in colder environments but that this relationship faded as mean annual temperature increased.
Birch leaf chemical defences increased with tree species richness but decreased with the phylogenetic distinctiveness of birch from its neighbours, particularly in warmer and more humid environments.
Herbivory was negatively correlated with leaf chemical defences, particularly when birch was associated with closely related species. The interactive effect of tree diversity and climate on herbivory was partially mediated by changes in leaf chemical defences.
Our findings confirm that tree species diversity can modify the leaf chemistry of a focal species, hence its quality for herbivores. They further stress that such neighbour‐induced changes are dependent on climate and that tree diversity effects on insect herbivory are partially mediated by these neighbour‐induced changes in chemical defences.
A free Plain Language Summary can be found within the Supporting Information of this article.
A free Plain Language Summary can be found within the Supporting Information of this article.
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Apparency revisited Strauss, Sharon Y.; Cacho, N. Ivalú; Schwartz, Mark W. ...
Entomologia experimentalis et applicata,
October 2015, Volume:
157, Issue:
1
Journal Article
Peer reviewed
Open access
How easy a plant is to find, or its apparency, is thought to shape plant defenses. Recent meta‐analyses suggest that the types of plant defenses employed are not well‐predicted by apparency, or ...apparency can be confounded with life history traits like woodiness and stature. Here, we suggest that the searching environments in which plants grow also influence plant apparency and should thus affect investment in plant defense. Specifically, bare, unvegetated environments may result in greater apparency of inhabitants of all statures to enemies, as a result of loss of associational resistance. We make several predictions about plant defenses in simple searching environments. (1) Plants living in simple searching environments should be more highly defended than plants living in more vegetated, complex searching environments. (2) Plant defenses involving signals—both, signals serving to hide plants and aposematic signals—should be favored in simple searching environments. (3) Levels of damage from enemies in simple searching environments should be related to defensive strategy (resistance, aposematism, mimicry, or crypsis); apparent plants should have low damage, because, as they are easily found, they should be well‐defended though physical or chemical defense. In contrast, predictions about damage levels in cryptic plants are harder to make, as damage reflects both whether plants are encountered or not, as well as overall palatability. If crypsis is favored in more palatable species, as has been suggested previously, we predict that cryptic plants should have greater variance in damage and greater maximum damage, if, once found, plants are palatable. (4) Organisms from diverse evolutionary lineages inhabiting the same simple searching environments should adapt to selection from apparency by converging on similar background matching or aposematic defenses. We then test some of these predictions with descriptive data collections in two simple searching environments: largely unvegetated graywacke scree mountaintops of New Zealand and serpentine barrens of northern California (USA). We find that plants that are more apparent (i.e., do not match local rock color as measured across 300–700 nm wavelengths) are more defended, as inferred from mean damage received. In contrast, cryptic species in the same habitats get 7× more heavily damaged, once found, suggesting overall greater palatability. There was no evidence of greater variation in damage, as measured by coefficient of variation, but maximum damage was much greater on cryptic species in both habitats. Convergence on gray substrate is found in diverse species of plants in New Zealand, as well as by scree‐living grasshoppers; in California, grasshoppers have also converged on substrate color, and seed color of a non‐cryptic plant also matches local outcrops. Considering searching environment and enemy searching abilities when evaluating plant apparency to enemies may shed more light on this challenge to plants.
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There has been accumulating evidence for effects of tree species composition on herbivory with many examples of lower damage by specialist feeders in tree species rich forests. In a joint study in ...five Central and Southeastern European countries, we studied the effect of tree species richness on infestation intensity of the oak lace bug, Corythucha arcuata (Say) (Heteroptera, Tingidae), an invasive pest on oak trees that has spread rapidly across the Balkan Peninsula and Central Europe. Intensity of infestation by C. arcuata on oaks was assessed on three or four study plots with high (pure stands) and three or four plots with low percentage of oak (mixed stands) in each country. Ordinal regression analysis showed that intensity of infestation of trees by C. arcuata differed between countries; no significant effect of stand type (mixed or pure) on infestation levels was detected. When analysing the percentage of trees in the highest infestation class, stand type had a significant effect with more intensive infestation in pure stands. We conclude that mixed stands will not prevent severe infestation but may help mitigating the impact of established C. arcuata populations.
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