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1. Species interactions across trophic levels mediate rainfall effects on dryland vegetation dynamics

   https://doi.org/10.1002/ecm.1441  

   Farías, Ariel_A ; Armas, Cristina ; Gaxiola, Aurora ; Cea, Alex_P ; Luis_Cortés, Jose ; López, Ramiro_P ; Casanoves, Fernando ; Holmgren, Milena ; Meserve, Peter_L ; Gutiérrez, Julio_R ; et al ( February 2021 , Ecological Monographs)

   Arid ecosystems are strongly limited by water availability, and precipitation plays a major role in the dynamics of all species in arid regions, as well as the ecosystem processes that occur there. However, understanding how biotic interactions mediate long‐term responses of dryland ecosystems to rainfall remains very fragmented. We report on a unique large‐scale field experiment spanning 25 yr and three trophic levels (plants, small mammal herbivores, predators) in a dryland ecosystem in the northern Chilean Mediterranean Region where we assessed how biotic interactions influence the long‐term plant community responses to precipitation. As the most persistent ecological changes in dryland systems may result from changes in the structure, cover, and composition of the perennial vegetation, we emphasized the interplay between bottom‐up and top‐down controls of perennial plants in our analyses. Rainfall was the primary factor affecting the dynamics of, and interactions among, plants and small mammals. Ephemeral plant cover dynamics closely tracked short‐term annual rainfall, but seemed unaffected by top‐down controls (herbivory). In contrast, the response of the perennial plant cover to precipitation was mediated by (1) a complex interplay between subtle top‐down (herbivory) controls that become more apparent in the long‐term, (2) competition with ephemeral plants during wet years, and (3) an indirect effect of predators on subdominant shrubs and perennial herbs. This long‐term field experiment highlights how climate‐induced responses of arid perennial vegetation are influenced by interactions across trophic levels and temporal scales. In the face of global change, understanding how multi‐trophic controls mediate dryland vegetation responses to climate is essential to properly managing the conservation of biodiversity in arid systems.

    

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2. Insectivorous birds reduce herbivory but do not increase mangrove growth across productivity zones

   https://doi.org/10.1002/ecy.3768  

   Forde, Alexander J. ; Feller, Ilka C. ; Parker, John D. ; Gruner, Daniel S. ( July 2022 , Ecology)

   Top‐down effects of predators and bottom‐up effects of resources are important drivers of community structure and function in a wide array of ecosystems. Fertilization experiments impose variation in resource availability that can mediate the strength of predator impacts, but the prevalence of such interactions across natural productivity gradients is less clear. We studied the joint impacts of top‐down and bottom‐up factors in a tropical mangrove forest system, leveraging fine‐grained patchiness in resource availability and primary productivity on coastal cays of Belize. We excluded birds from canopies of red mangrove (Rhizophoraceae:Rhizophora mangle) for 13 months in zones of phosphorus‐limited, stunted dwarf mangroves, and in adjacent zones of vigorous mangroves that receive detrital subsidies. Birds decreased total arthropod densities by 62%, herbivore densities more than fivefold, and reduced rates of leaf and bud herbivory by 45% and 52%, respectively. Despite similar arthropod densities across both zones of productivity, leaf and bud damage were 2.0 and 4.3 times greater in productive stands. Detrital subsidies strongly impacted a suite of plant traits in productive stands, potentially making leaves more nutritious and vulnerable to damage. Despite consistently strong impacts on herbivory, we did not detect top‐down forcing that impacted mangrove growth, which was similar with and without birds. Our results indicated that both top‐down and bottom‐up forces drive arthropod community dynamics, but attenuation at the plant‐herbivore interface weakens top‐down control by avian insectivores.

    

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

   https://doi.org/10.1111/ele.12874  

   Vidal, Mayra C. ; Murphy, Shannon M. ; Scherber, ed., Christoph ( November 2017 , Ecology Letters)

   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.

    

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4. Transient top‐down and bottom‐up effects of resources pulsed to multiple trophic levels

   https://doi.org/10.1002/ecy.3197  

   McCary, Matthew A. ; Phillips, Joseph S. ; Ramiadantsoa, Tanjona ; Nell, Lucas A. ; McCormick, Amanda R. ; Botsch, Jamieson C. ( November 2020 , Ecology)

   Pulsed fluxes of organisms across ecosystem boundaries can exert top‐down and bottom‐up effects in recipient food webs, through both direct effects on the subsidized trophic levels and indirect effects on other components of the system. While previous theoretical and empirical studies demonstrate the influence of allochthonous subsidies on bottom‐up and top‐down processes, understanding how these forces act in conjunction is still limited, particularly when an allochthonous resource can simultaneously subsidize multiple trophic levels. Using the Lake Mývatn region in Iceland as an example system of allochthony and its potential effects on multiple trophic levels, we analyzed a mathematical model to evaluate how pulsed subsidies of aquatic insects affect the dynamics of a soil–plant–arthropod food web. We found that the relative balance of top‐down and bottom‐up effects on a given food web compartment was determined by trophic position, subsidy magnitude, and top predators’ ability to exploit the subsidy. For intermediate trophic levels (e.g., detritivores and herbivores), we found that the subsidy could either alleviate or intensify top‐down pressure from the predator. For some parameter combinations, alleviation and intensification occurred sequentially during and after the resource pulse. The total effect of the subsidy on detritivores and herbivores, including top‐down and bottom‐up processes, was determined by the rate at which predator consumption saturated with increasing size of the allochthonous subsidy, with greater saturation leading to increased bottom‐up effects. Our findings illustrate how resource pulses to multiple trophic levels can influence food web dynamics by changing the relative strength of bottom‐up and top‐down effects, with bottom‐up predominating top‐down effects in most scenarios in this subarctic system.

    

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5. Do experimental drought stress and species' drought sensitivity influence herbivory in tropical tree seedlings?

   https://doi.org/10.1111/btp.13075  

   Muehleisen, Andrew J. ; Manzané‐Pinzón, Eric ; Engelbrecht, Bettina M. J. ; Jones, F. Andrew ; Comita, Liza S. ( February 2022 , Biotropica)

   In tropical forests, drought and herbivory represent two potent stresses on seedlings. Climate change is expected to increase the frequency of severe droughts in many tropical forests, which may influence seedling vulnerability to herbivores if drought stress affects seedling palatability. Furthermore, contrasting selective pressures in wetter vs drier forests could mean that species well‐adapted to herbivores are less drought resistant and vice versa. In this study, we measured seedling performance and herbivory in a common garden experiment where seedlings of 15 tree species were subjected to irrigation or rainfall exclusion treatments across two dry seasons in Panama. Water manipulation had no effects on foliar herbivory during the experiment for all species combined and for 14 of the 15 focal species when analyzed separately. There was large variation among species in herbivore damage, but no relationship between the sensitivity of species to drought and the amount of herbivory they experienced. Altogether, our findings suggest that increasing drought stress is unlikely to directly alter tropical tree seedling susceptibility to herbivore attack in this forest. Additional studies are needed to determine whether drought alters tropical plant‐herbivore interactions via other mechanisms, such as through changes in herbivorous insect communities and/or increases in fitness costs of herbivory.

   Abstract in Spanish is available with online material.

    

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