Top-down pressure by generalist and specialist natural enemies in relation to habitat heterogeneity and resource availability

Bellone, Davide; Björkman, Christer; Klapwijk, Maartje J.

doi:10.1016/j.baae.2019.10.005

Cited by 12 publications

(6 citation statements)

References 43 publications

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“…Yet, populations of individual predator species may react differently to tree diversity. While overall predator abundance is predicted to increase with tree diversity, individual numbers for some enemy species may also decrease in more diverse forest stands, as has been shown in observational studies for parasitoids of spruce budworm [80] and for specialist vs. generalist enemies of pine sawfly [81]. Such species-specific responses are conceptionally equivalent to identity effects (this time exerted by predators), for example, when tree diversity benefits a dominant predator species with subsequent influences of predator identity on the entire species community (e.g., [61••, 106]).…”

Section: Identity Effectsmentioning

confidence: 89%

“…These findings were possibly due to the fact that the parasitoids were largely generalists able to use different herbivore hosts and therefore unaffected by changes in herbivore diversity or composition with changing tree diversity. This points to the effects of diet specialization as a further moderator of how the "enemies" hypothesis applies to natural enemy communities in forests [80,81] and emphasizes the need for more studies on specialized natural enemy taxa, which appear underrepresented in the literature so far.…”

Section: Parasitoidsmentioning

confidence: 99%

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The Influence of Tree Diversity on Natural Enemies—a Review of the “Enemies” Hypothesis in Forests

Staab

Schuldt

2020

Curr Forestry Rep

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Purpose of Review Natural enemies are an important component for forest functioning. By consuming herbivores, they can be effective top-down regulators of potential pest species. Tree mixtures are generally expected to have larger predator and parasitoid populations compared to monocultures. This assumption is based on the “enemies” hypothesis, a classical ecological concept predicting a positive relationship between plant diversity (and complexity) and natural enemies, which, in turn, should increase top-down control in more diverse environments. However, the “enemies” hypothesis has mostly been tested and supported in relatively simple agricultural ecosystems. Until recently, research in forests was sparse. We summarize the upcoming knowledge-base for forests and identify forest characteristics likely shaping relationships between tree diversity, natural enemies (abundance, species richness, diversity), and top-down control. We further identify possible implications for mixed species forestry and key knowledge gaps. Recent Findings Tree diversity (almost exclusively quantified as tree species richness) does not consistently increase enemy abundance, diversity, or result in herbivore control. Tests of the “enemies” hypothesis are largely based on aboveground natural enemies (mainly generalists) and have highly variable outcomes across taxa and study systems, sometimes even finding a decrease in predator diversity with increasing tree diversity. Recurrent effects of tree species identity and composition indicate that a closer focus on tree functional and phylogenetic diversity might help to foster a mechanistic understanding of the specific circumstances under which tree diversity can promote top-down control. Summary Our review suggests that the “enemies” hypothesis may not unambiguously apply to forests. With trees as structurally complex organisms, even low-diversity forests can maintain a high degree of habitat heterogeneity and may provide niches for many predator and parasitoid species, possibly blurring correlations between tree and natural enemy diversity. Several further factors, such as latitude, identity effects, intraguild predation, or functional and phylogenetic components of biodiversity, may confound the predictions of the “enemies” hypothesis. We identify topics needing more research to fully understand under which conditions tree diversity increases natural enemy diversity and top-down control—knowledge that will be crucial for forest management.

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Section: Identity Effectsmentioning

confidence: 89%

Section: Parasitoidsmentioning

confidence: 99%

The Influence of Tree Diversity on Natural Enemies—a Review of the “Enemies” Hypothesis in Forests

Staab

Schuldt

2020

Curr Forestry Rep

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“…richness vs. abundance) which may have led to the difference in the results observed and in the conclusion made on the natural enemies hypothesis. In recent years, studies using direct measures of predation and parasitism while testing for an effect of tree diversity have increased (Abdala-Roberts et al, 2016; Bellone et al, 2020) but they cannot be directly compared to those measuring only predator abundance. Methodological issues can also arise from the type of study used to test the effect of tree diversity on natural enemies.…”

Section: Introductionmentioning

confidence: 99%

Meta-analysis of tree diversity effects on the abundance, diversity and activity of herbivores’ enemies

Stemmelen

Jactel

Brockerhoff

et al. 2021

Preprint

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The natural enemies hypothesis predicts that the abundance and diversity of antagonists such as predators and parasitoids of herbivores increases with the diversity of plants, which can lead to more effective top-down control of insect herbivores. However, although the hypothesis has received large support in agricultural systems, fewer studies have been conducted in forest ecosystems and a comprehensive synthesis of previous research is still lacking. We conducted a meta-analysis of 65 publications comparing the diversity, abundance or activity of various groups of natural enemies (including birds, bats, spiders and insect parasitoids) in pure vs. mixed forest stands. We tested the effects of forest biome, natural enemy taxon and type of study (managed vs experimental forest). We found a significant positive effect of forest tree diversity on natural enemy abundance and diversity but not on their activity. The effect of tree diversity on natural enemies was stronger towards lower latitudes but was not contingent on the natural enemy taxon. Overall, our study contributes substantially toward a better understanding of the natural enemies hypothesis in forest systems and provides new insights about the mechanisms involved. Furthermore, we outline potential avenues for strengthening forest resistance to the growing threat of herbivorous insects.

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“…Many studies have shown the positive effects of habitat heterogeneity on the increase of abundance and diversity of natural enemies in agricultural ecosystems [ 12 , 20 , 21 ]. In this context, the management of crop areas to increase the heterogeneity of agricultural landscapes is a viable alternative in conservation biological control programs.…”

Section: Introductionmentioning

confidence: 99%

Refuge distributions and landscape connectivity affect host-parasitoid dynamics: Motivations for biological control in agroecosystems

et al. 2022

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Species distributions are affected by landscape structure at different spatial scales. Here we study how the interplay between dispersal at different spatial scales and landscape connectivity and composition affect local species dynamics. Using a host-parasitoid model, we assessed host density and host occupancy on the landscape, under different parasitoid dispersal ranges and three local distributions of non-crop habitats, areas where hosts are unable to grow but parasitoids are provided with alternative hosts and food resources. Our results show distinct responses of host density to increases in non-crop area, measured by differences in slopes for different distributions of non-crop habitats, and that the effect of local landscape composition on species dynamics depends on the landscape connectivity at the regional scale. Moreover, we show how host density and occupancy are affected by increasing parasitoid dispersal ranges depending on landscape structure. Our results demonstrate the role of local and regional scales on species distributions and the importance of the combined effects of species biological parameters and landscape structure on species dynamics. Finally, we highlight the relevance of these aspects for the development of better strategies of biological control.

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Top-down pressure by generalist and specialist natural enemies in relation to habitat heterogeneity and resource availability

Cited by 12 publications

References 43 publications

The Influence of Tree Diversity on Natural Enemies—a Review of the “Enemies” Hypothesis in Forests

The Influence of Tree Diversity on Natural Enemies—a Review of the “Enemies” Hypothesis in Forests

Meta-analysis of tree diversity effects on the abundance, diversity and activity of herbivores’ enemies

Refuge distributions and landscape connectivity affect host-parasitoid dynamics: Motivations for biological control in agroecosystems

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