Tree species richness attenuates the positive relationship between mutualistic ant-hemipteran interactions and leaf chewer herbivory

Omnivorous ants are less carnivorous and more protein-limited in exotic plantations

Diets of species are crucial in determining how they influence food webs and community structures, and how their populations are regulated by different bottom-up processes. Omnivores are able to adjust their diet flexibly according to environmental conditions, such that their impacts on food webs and communities, and the macronutrients constraining their population, can be plastic. In particular, omnivore diets are known to be influenced by prey availability, which exhibits high spatial and temporal variation. To examine the plasticity of diet and macronutrient limitation in omnivores, we compared trophic positions, macronutrient preferences and food exploitation rates of omnivorous ants in invertebrate-rich (secondary forests) and invertebrate-poor (Lophostemon confertus plantations) habitats. We hypothesized that omnivorous ants would have lower trophic positions, enhanced protein limitation and reduced food exploitation rates in L. confertus plantations relative to secondary forests. We performed cafeteria experiments to examine changes in macronutrient limitation and food exploitation rates. We also sampled ants and conducted stable isotope analyses to investigate dietary shifts between these habitats. We found that conspecific ants were less carnivorous and had higher preferences for protein-rich food in L. confertus plantations compared to secondary forests. However, ant assemblages did not exhibit increased preferences for protein-rich food in L. confertus plantations. At the species-level, food exploitation rates varied idiosyncratically between habitats. At the assemblage-level, food exploitation rates were reduced in L. confertus plantations. Our results reveal that plantation establishments alter the diet and foraging behaviour of omnivorous ants. Such changes suggest that omnivorous ants in plantations will have reduced top-down impacts on prey communities but also see an increased importance of protein as a bottom-up force in constraining omnivore population sizes.

Tree diversity increases robustness of multi-trophic interactions

Multi-trophic interactions maintain critical ecosystem functions. Biodiversity is declining globally, while responses of trophic interactions to biodiversity change are largely unclear. Thus, studying responses of multi-trophic interaction robustness to biodiversity change is crucial for understanding ecosystem functioning and persistence. We investigate plant-Hemiptera (antagonism) and Hemiptera-ant (mutualism) interaction networks in response to experimental manipulation of tree diversity. We show increased diversity at both higher trophic levels (Hemiptera and ants) and increased robustness through redundancy of lower level species of multi-trophic interactions when tree diversity increased. Hemiptera and ant diversity increased with tree diversity through non-additive diversity effects. Network analyses identified that tree diversity also increased the number of tree and Hemiptera species used by Hemiptera and ant species, and decreased the specialization on lower trophic level species in both mutualistic and antagonist interactions. Our results demonstrate that bottom-up effects of tree diversity ascend through trophic levels regardless of interaction type. Thus, local tree diversity is a key driver of multi-trophic community diversity and interaction robustness in forests.

A multitrophic perspective on biodiversity-ecosystem functioning research

Concern about the functional consequences of unprecedented loss in biodiversity has prompted biodiversity-ecosystem functioning (BEF) research to become one of the most active fields of ecological research in the past 25 years. Hundreds of experiments have manipulated biodiversity as an independent variable and found compelling support that the functioning of ecosystems increases with the diversity of their ecological communities. This research has also identified some of the mechanisms underlying BEF relationships, some context-dependencies of the strength of relationships, as well as implications for various ecosystem services that mankind depends upon. In this paper, we argue that a multitrophic perspective of biotic interactions in random and non-random biodiversity change scenarios is key to advance future BEF research and to address some of its most important remaining challenges. We discuss that the study and the quantification of multitrophic interactions in space and time facilitates scaling up from small-scale biodiversity manipulations and ecosystem function assessments to management-relevant spatial scales across ecosystem boundaries. We specifically consider multitrophic conceptual frameworks to understand and predict the context-dependency of BEF relationships. Moreover, we highlight the importance of the eco-evolutionary underpinnings of multitrophic BEF relationships. We outline that FAIR data (meeting the standards of findability, accessibility, interoperability, and reusability) and reproducible processing will be key to advance this field of research by making it more integrative. Finally, we show how these BEF insights may be implemented for ecosystem management, society, and policy. Given that human well-being critically depends on the multiple services provided by diverse, multitrophic communities, integrating the approaches of evolutionary ecology, community ecology, and ecosystem ecology in future BEF research will be key to refine conservation targets and develop sustainable management strategies.

Multiple plant diversity components drive consumer communities across ecosystems

Humans modify ecosystems and biodiversity worldwide, with negative consequences for ecosystem functioning. Promoting plant diversity is increasingly suggested as a mitigation strategy. However, our mechanistic understanding of how plant diversity affects the diversity of heterotrophic consumer communities remains limited. Here, we disentangle the relative importance of key components of plant diversity as drivers of herbivore, predator, and parasitoid species richness in experimental forests and grasslands. We find that plant species richness effects on consumer species richness are consistently positive and mediated by elevated structural and functional diversity of the plant communities. The importance of these diversity components differs across trophic levels and ecosystems, cautioning against ignoring the fundamental ecological complexity of biodiversity effects. Importantly, plant diversity effects on higher trophic-level species richness are in many cases mediated by modifications of consumer abundances. In light of recently reported drastic declines in insect abundances, our study identifies important pathways connecting plant diversity and consumer diversity across ecosystems.

Here, Schuldt et al. collate data from two long-term grassland and forest biodiversity experiments to ask how plant diversity facets affect the diversity of higher trophic levels. The results show that positive effects of plant diversity on consumer diversity are mediated by plant structural and functional diversity, and vary across ecosystems and trophic levels.

Landscape-level bird loss increases the prevalence of honeydew-producing insects and non-native ants

Bird exclusion experiments consistently show that birds exhibit strong top-down control of arthropods, including ants and the honeydew-producing insects (HPIs) that they tend. However, it remains unclear whether the results of these small-scale bird exclosure experiments can be extrapolated to larger spatial scales. In this study, we use a natural bird removal experiment to compare the prevalence of ants and HPIs between Guam, an island whose bird community has been extirpated since the 1980s due to the introduction of the brown tree snake, and two nearby islands (Rota and Saipan) that have more intact bird assemblages. Consistent with smaller-scale bird exclosure experiments, we show that (1) forest trees from Guam are significantly more likely to host HPIs than trees from Saipan and (2) ants are nearly four times as abundant on Guam than on both Saipan and Rota. The prevalence of HPIs varied slightly based on tree species identity, although these effects were not as strong as island-level effects associated with bird loss. Ant community composition differed between Guam and the other two islands. These results corroborate past observational studies showing increased spider densities on Guam and suggest that trophic changes associated with landscape-level bird extirpation may also involve alterations in the abundance of ants and HPIs. This study also provides a clear example of the strong indirect effects that invasive species can have on natural food webs.