Plant-Animal Interactions: Evolutionary Ecology in Tropical and Temperate Regions

Trophic regulations of the soil microbiome

Primary consumers are under strong selection from resource ('bottom-up') and consumer ('top-down') controls, but the relative importance of these selective forces is unknown. We performed a meta-analysis to compare the strength of top-down and bottom-up forces on consumer fitness, considering multiple predictors that can modulate these effects: diet breadth, feeding guild, habitat/environment, type of bottom-up effects, type of top-down effects and how consumer fitness effects are measured. We focused our analyses on the most diverse group of primary consumers, herbivorous insects, and found that in general top-down forces were stronger than bottom-up forces. Notably, chewing, sucking and gall-making herbivores were more affected by top-down than bottom-up forces, top-down forces were stronger than bottom-up in both natural and controlled (cultivated) environments, and parasitoids and predators had equally strong top-down effects on insect herbivores. Future studies should broaden the scope of focal consumers, particularly in understudied terrestrial systems, guilds, taxonomic groups and top-down controls (e.g. pathogens), and test for more complex indirect community interactions. Our results demonstrate the surprising strength of forces exerted by natural enemies on herbivorous insects, and thus the necessity of using a tri-trophic approach when studying insect-plant interactions.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/ele.12874

Bottom-up vs. top-down effects on terrestrial insect herbivores: a meta-analysis.

Human-induced biodiversity change impairs ecosystem functions crucial to human well-being. However, the consequences of this change for ecosystem multifunctionality are poorly understood beyond effects of plant species loss, particularly in regions with high biodiversity across trophic levels. Here we adopt a multitrophic perspective to analyze how biodiversity affects multifunctionality in biodiverse subtropical forests. We consider 22 independent measurements of nine ecosystem functions central to energy and nutrient flow across trophic levels. We find that individual functions and multifunctionality are more strongly affected by the diversity of heterotrophs promoting decomposition and nutrient cycling, and by plant functional-trait diversity and composition, than by tree species richness. Moreover, cascading effects of higher trophic-level diversity on functions originating from lower trophic-level processes highlight that multitrophic biodiversity is key to understanding drivers of multifunctionality. A broader perspective on biodiversity-multifunctionality relationships is crucial for sustainable ecosystem management in light of non-random species loss and intensified biotic disturbances under future environmental change.

/pdf/biodiversity-across-trophic-levels-drives-multifunctionality-dllew90q1c.pdf

Biodiversity across trophic levels drives multifunctionality in highly diverse forests

Increased frequency and severity of drought, as a result of climate change, is expected to drive critical changes in plant-insect interactions that may elevate rates of tree mortality. The mechanisms that link water stress in plants to insect performance are not well understood. Here, we build on previous reviews and develop a framework that incorporates the severity and longevity of drought and captures the plant physiological adjustments that follow moderate and severe drought. Using this framework, we investigate in greater depth how insect performance responds to increasing drought severity for: (i) different feeding guilds; (ii) flush feeders and senescence feeders; (iii) specialist and generalist insect herbivores; and (iv) temperate versus tropical forest communities. We outline how intermittent and moderate drought can result in increases of carbon-based and nitrogen-based chemical defences, whereas long and severe drought events can result in decreases in plant secondary defence compounds. We predict that different herbivore feeding guilds will show different but predictable responses to drought events, with most feeding guilds being negatively affected by water stress, with the exception of wood borers and bark beetles during severe drought and sap-sucking insects and leaf miners during moderate and intermittent drought. Time of feeding and host specificity are important considerations. Some insects, regardless of feeding guild, prefer to feed on younger tissues from leaf flush, whereas others are adapted to feed on senescing tissues of severely stressed trees. We argue that moderate water stress could benefit specialist insect herbivores, while generalists might prefer severe drought conditions. Current evidence suggests that insect outbreaks are shorter and more spatially restricted in tropical than in temperate forests. We suggest that future research on the impact of drought on insect communities should include (i) assessing how drought-induced changes in various plant traits, such as secondary compound concentrations and leaf water potential, affect herbivores; (ii) food web implications for other insects and those that feed on them; and (iii) interactions between the effects on insects of increasing drought and other forms of environmental change including rising temperatures and CO2 levels. There is a need for larger, temperate and tropical forest-scale drought experiments to look at herbivorous insect responses and their role in tree death.

How do herbivorous insects respond to drought stress in trees

Biotic interactions underlie ecosystem structure and function, but predicting interaction outcomes is difficult We tested the hypothesis that biotic interaction strength increases toward the equator, using a global experiment with model caterpillars to measure predation risk Across an 11,660-kilometer latitudinal gradient spanning six continents, we found increasing predation toward the equator, with a parallel pattern of increasing predation toward lower elevations Patterns across both latitude and elevation were driven by arthropod predators, with no systematic trend in attack rates by birds or mammals These matching gradients at global and regional scales suggest consistent drivers of biotic interaction strength, a finding that needs to be integrated into general theories of herbivory, community organization, and life-history evolution

/pdf/higher-predation-risk-for-insect-prey-at-low-latitudes-and-1ouzwpy7na.pdf

Alison Slade

Papers

Higher predation risk for insect prey at low latitudes and elevations