The impact of insect herbivory on biogeochemical cycling in broadleaved forests varies with temperature

Herbivorous insects alter biogeochemical cycling within forests, but the magnitude of these impacts, their global variation, and drivers of this variation remain poorly understood. To address this knowledge gap and help improve biogeochemical models, we established a global network of 74 plots within 40 mature, undisturbed broadleaved forests. We analyzed freshly senesced and green leaves for carbon, nitrogen, phosphorus and silica concentrations, foliar production and herbivory, and stand-level nutrient fluxes. We show more nutrient release by insect herbivores at non-outbreak levels in tropical forests than temperate and boreal forests, that these fluxes increase strongly with mean annual temperature, and that they exceed atmospheric deposition inputs in some localities. Thus, background levels of insect herbivory are sufficiently large to both alter ecosystem element cycling and influence terrestrial carbon cycling. Further, climate can affect interactions between natural populations of plants and herbivores with important consequences for global biogeochemical cycles across broadleaved forests

Herbivore diversity effects on Arctic tundra ecosystems: a systematic review

Background: Northern ecosystems are strongly influenced by herbivores that differ in their impacts on the ecosystem. Yet the role of herbivore diversity in shaping the structure and functioning of tundra ecosystems has been overlooked. With climate and land-use changes causing rapid shifts in Arctic species assemblages, a better understanding of the consequences of herbivore diversity changes for tundra ecosystem functioning is urgently needed. This systematic review synthesizes available evidence on the effects of herbivore diversity on different processes, functions, and properties of tundra ecosystems. Methods: Following a published protocol, our systematic review combined primary field studies retrieved from bibliographic databases, search engines and specialist websites that compared tundra ecosystem responses to different levels of vertebrate and invertebrate herbivore diversity. We used the number of functional groups of herbivores (i.e., functional group richness) as a measure of the diversity of the herbivore assemblage. We screened titles, abstracts, and full texts of studies using pre-defined eligibility criteria. We critically appraised the validity of the studies, tested the influence of different moderators, and conducted sensitivity analyses. Quantitative synthesis (i.e., calculation of effect sizes) was performed for ecosystem responses reported by at least five articles and meta-regressions including the effects of potential modifiers for those reported by at least 10 articles. Review findings: The literature searches retrieved 5944 articles. After screening titles, abstracts, and full texts, 201 articles including 3713 studies (i.e., individual comparisons) were deemed relevant for the systematic review, with 2844 of these studies included in quantitative syntheses. The available evidence base on the effects of herbivore diversity on tundra ecosystems is concentrated around well-established research locations and focuses mainly on the impacts of vertebrate herbivores on vegetation. Overall, greater herbivore diversity led to increased abundance of feeding marks by herbivores and soil temperature, and to reduced total abundance of plants, graminoids, forbs, and litter, plant leaf size, plant height, and moss depth, but the effects of herbivore diversity were difficult to tease apart from those of excluding vertebrate herbivores. The effects of different functional groups of herbivores on graminoid and lichen abundance compensated each other, leading to no net effects when herbivore effects were combined. In turn, smaller herbivores and large-bodied herbivores only reduced plant height when occurring together but not when occurring separately. Greater herbivore diversity increased plant diversity in graminoid tundra but not in other habitat types. Conclusions: This systematic review underscores the importance of herbivore diversity in shaping the structure and function of Arctic ecosystems, with different functional groups of herbivores exerting additive or compensatory effects that can be modulated by environmental conditions. Still, many challenges remain to fully understand the complex impacts of herbivore diversity on tundra ecosystems. Future studies should explicitly address the role of herbivore diversity beyond presence-absence, targeting a broader range of ecosystem responses and explicitly including invertebrate herbivores. A better understanding of the role of herbivore diversity will enhance our ability to predict whether and where shifts in herbivore assemblages might mitigate or further amplify the impacts of environmental change on Arctic ecosystems

Location of studies and evidence of effects of herbivory on Arctic vegetation: a systematic map

Herbivores modify the structure and function of tundra ecosystems. Understanding their impacts is necessary to assess the responses of these ecosystems to ongoing environmental changes. However, the effects of herbivores on plants and ecosystem structure and function vary across the Arctic. Strong spatial variation in herbivore effects implies that the results of individual studies on herbivory depend on local conditions, i.e., their ecological context. An important first step in assessing whether generalizable conclusions can be produced is to identify the existing studies and assess how well they cover the underlying environmental conditions across the Arctic. This systematic map aims to identify the ecological contexts in which herbivore impacts on vegetation have been studied in the Arctic. Specifically, the primary question of the systematic map was: “What evidence exists on the effects of herbivores on Arctic vegetation?”

Reviews and syntheses : Impacts of plant-silica-herbivore interactions on terrestrial biogeochemical cycling

Researchers have known for decades that silicon plays a major role in biogeochemical and plant-soil processes in terrestrial systems. Meanwhile, plant biologists continue to uncover a growing list of benefits derived from silicon to combat abiotic and biotic stresses, such as defense against herbivory. Yet despite growing recognition of herbivores as important ecosystem engineers, many major gaps remain in our understanding of how silicon and herbivory interact to shape biogeochemical processes, particularly in natural systems. We review and synthesize 119 available studies directly investigating silicon and herbivory to summarize key trends and highlight research gaps and opportunities. Categorizing studies by multiple ecosystem, plant, and herbivore characteristics, we find substantial evidence for a wide variety of important interactions between plant silicon and herbivory but highlight the need for more research particularly in non-graminoid-dominated vegetation outside of the temperate biome as well as on the potential effects of herbivory on silicon cycling. Continuing to overlook silicon-herbivory dynamics in natural ecosystems limits our understanding of potentially critical animal-plant-soil feedbacks necessary to inform land management decisions and to refine global models of environmental change

Reviews and Syntheses: Impacts of plant silica – herbivore interactions on terrestrial biogeochemical cycling

Abstract. Researchers have known for decades that silicon plays a major role in biogeochemical and plant-soil processes in terrestrial systems. Meanwhile, plant biologists continue to uncover a growing list of benefits derived from silicon to combat abiotic and biotic stresses, such as defense against herbivory. Yet despite growing recognition of herbivores as important ecosystem engineers, many major gaps remain in our understanding of how silicon and herbivory interact to shape biogeochemical processes, particularly in natural systems. We review and synthesize 119 available studies directly investigating silicon and herbivory to summarize key trends and highlight research gaps and opportunities. Categorizing studies by multiple ecosystem, plant, and herbivore characteristics, we find substantial evidence for a wide variety of important interactions between plant silicon and herbivory, but highlight the need for more research particularly in non-graminoid dominated vegetation outside of the temperate biome as well as on the potential effects of herbivory on silicon cycling. Continuing to overlook silicon-herbivory dynamics in natural ecosystems limits our understanding of potentially critical animal-plant-soil feedbacks necessary to inform land management decisions and to refine global models of environmental change. </jats:p

Reviews and syntheses: Impacts of plant-silica–herbivore interactions on terrestrial biogeochemical cycling

Abstract. Researchers have known for decades that silicon plays a major role in biogeochemical and plant–soil processes in terrestrial systems. Meanwhile, plant biologists continue to uncover a growing list of benefits derived from silicon to combat abiotic and biotic stresses, such as defense against herbivory. Yet despite growing recognition of herbivores as important ecosystem engineers, many major gaps remain in our understanding of how silicon and herbivory interact to shape biogeochemical processes, particularly in natural systems. We review and synthesize 119 available studies directly investigating silicon and herbivory to summarize key trends and highlight research gaps and opportunities. Categorizing studies by multiple ecosystem, plant, and herbivore characteristics, we find substantial evidence for a wide variety of important interactions between plant silicon and herbivory but highlight the need for more research particularly in non-graminoid-dominated vegetation outside of the temperate biome as well as on the potential effects of herbivory on silicon cycling. Continuing to overlook silicon–herbivory dynamics in natural ecosystems limits our understanding of potentially critical animal–plant–soil feedbacks necessary to inform land management decisions and to refine global models of environmental change. </jats:p

Impacts of insect frass and cadavers on soil surface litter decomposition along a tropical forest temperature gradient

Insect herbivores play important roles in shaping many ecosystem processes, but how climate change will alter the effects of insect herbivory are poorly understood. To address this knowledge gap, we quantified for the first time how insect frass and cadavers affected leaf litter decomposition rates and nutrient release along a highly constrained 4.3°C mean annual temperature (MAT) gradient in a Hawaiian montane tropical wet forest. We constructed litterbags of standardized locally sourced leaf litter, with some amended with insect frass + cadavers to produce treatments designed to simulate ambient (Control = no amendment), moderate (Amended-Low = 2 × Control level), or severe (Amended-High = 11 × Control level) insect outbreak events. Multiple sets of these litterbags were deployed across the MAT gradient, with individual litterbags collected periodically over one year to assess how rising MAT altered the effects of insect deposits on litter decomposition rates and nitrogen (N) release. Increased MAT and insect inputs additively increased litter decomposition rates and N immobilization rates, with effects being stronger for Amended-High litterbags. However, the apparent temperature sensitivity (Q10) of litter decomposition was not clearly affected by amendments. The effects of adding insect deposits in this study operated differently than the slower litter decomposition and greater N mobilization rates often observed in experiments which use chemical fertilizers (e.g., urea, ammonium nitrate). Further research is required to understand mechanistic differences between amendment types. Potential increases in outbreak-related herbivore deposits coupled with climate warming will accelerate litter decomposition and nutrient cycling rates with short-term consequences for nutrient cycling and carbon storage in tropical montane wet forests

Insect herbivory releases more nutrients in warmer and drier forests

Climate, forest successional stage, and soil substrate age can alter herbivore communities and their effects on biogeochemical cycling, but the size and spatial variability of these effects are poorly quantified. To address this knowledge gap, we established a globally distributed network of 50 broadleaved old-growth forests across six continents, encompassing well-constrained local-scale gradients in mean annual temperature (MAT), mean annual precipitation (MAP), succession, and soil substrate age. We used this network to investigate how these variables impact insect foliar herbivory and the associated carbon, nitrogen, phosphorus, and silica fluxes in forest ecosystems. Over 1 to 2 years, we measured stand-level foliar biomass production, leaf-level herbivory, and foliar element concentrations. At the global scale, insect herbivores liberated higher amounts of elements from the canopies of warmer and drier sites than those of cooler and wetter sites with patterns for phosphorus being most pronounced. MAT exerted a stronger influence over insect-mediated element fluxes than MAP. Foliar biomass production and leaf-level herbivory responses to MAT and MAP were mainly responsible for the observed changes in insect-mediated element fluxes; we also observed minor effects of foliar phosphorus concentration on phosphorus fluxes. Local-scale trends were mixed and successional stage or soil substrate age did not appear to influence insect herbivore-mediated element fluxes. These results demonstrate that climate effects on plant-herbivore interactions are stronger at large than small scales, at which herbivory rates and nutrient fluxes appear to be more strongly affected by a diversity of non-climate factors

What are the effects of herbivore diversity on tundra ecosystems?

Northern ecosystems are strongly influenced by herbivores, but differences in diet composition, behaviour and energy requirements among herbivore species lead to contrasting impacts on ecosystem functioning. Arctic herbivore communities are changing rapidly in response to ongoing environmental and land use changes, and for these reasons, a better understanding of the consequences of changes in the diversity of herbivore assemblages in ecosystem functioning is needed. We used a systematic review to synthesize the available evidence on the effects of herbivore diversity on different processes, functions, and properties of Arctic tundra ecosystems. We searched primary field studies in databases, search engines and specialist websites and retrieved 5891 articles. After screening titles, abstracts, and full texts, 201 articles including 3955 studies were considered relevant for the systematic review. There was a geographical bias in the distribution of studies across the tundra biome, with most studies concentrated around well-established research locations. Most studies focused on the effects of a single herbivore species, while only a few studies specifically addressed the separate and combined effects of different herbivore species. The majority of studies focused on impacts of vertebrate herbivores on different aspects of arctic tundra vegetation, while other ecosystem processes, functions and properties have received less attention. Our results indicate that the available knowledge on the effects of herbivore diversity is relatively limited, so the consequences of ongoing changes in herbivore communities may be difficult to forecast. Future studies should explicitly address the role of herbivore diversity to understand the responses of tundra ecosystems to ongoing environmental changes