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1. Global Connectivity of Southern Ocean Ecosystems

   https://doi.org/10.3389/fevo.2021.624451  

   Murphy, Eugene J. ; Johnston, Nadine M. ; Hofmann, Eileen E. ; Phillips, Richard A. ; Jackson, Jennifer A. ; Constable, Andrew J. ; Henley, Sian F. ; Melbourne-Thomas, Jessica ; Trebilco, Rowan ; Cavanagh, Rachel D. ; et al ( August 2021 , Frontiers in Ecology and Evolution)

   null (Ed.)

   Southern Ocean ecosystems are globally important. Processes in the Antarctic atmosphere, cryosphere, and the Southern Ocean directly influence global atmospheric and oceanic systems. Southern Ocean biogeochemistry has also been shown to have global importance. In contrast, ocean ecological processes are often seen as largely separate from the rest of the global system. In this paper, we consider the degree of ecological connectivity at different trophic levels, linking Southern Ocean ecosystems with the global ocean, and their importance not only for the regional ecosystem but also the wider Earth system. We also consider the human system connections, including the role of Southern Ocean ecosystems in supporting society, culture, and economy in many nations, influencing public and political views and hence policy. Rather than Southern Ocean ecosystems being defined by barriers at particular oceanic fronts, ecological changes are gradual due to cross-front exchanges involving oceanographic processes and organism movement. Millions of seabirds and hundreds of thousands of cetaceans move north out of polar waters in the austral autumn interacting in food webs across the Southern Hemisphere, and a few species cross the equator. A number of species migrate into the east and west ocean-basin boundary current and continental shelf regions of the major southern continents. Human travel in and out of the Southern Ocean region includes fisheries, tourism, and scientific vessels in all ocean sectors. These operations arise from many nations, particularly in the Northern Hemisphere, and are important in local communities as well as national economic, scientific, and political activities. As a result of the extensive connectivity, future changes in Southern Ocean ecosystems will have consequences throughout the Earth system, affecting ecosystem services with socio-economic impacts throughout the world. The high level of connectivity also means that changes and policy decisions in marine ecosystems outside the Southern Ocean have consequences for ecosystems south of the Antarctic Polar Front. Knowledge of Southern Ocean ecosystems and their global connectivity is critical for interpreting current change, projecting future change impacts, and identifying integrated strategies for conserving and managing both the Southern Ocean and the broader Earth system. 

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2. Hurricane disturbance drives trophic changes in neotropical mountain stream food webs

   https://doi.org/10.1002/ecy.4202  

   Gutiérrez‐Fonseca, Pablo E. ; Pringle, Catherine M. ; Ramírez, Alonso ; Gómez, Jesús E. ; García, Pavel ( November 2023 , Ecology)

   Food webs are complex ecological networks that reveal species interactions and energy flow in ecosystems. Prevailing ecological knowledge on forested streams suggests that their food webs are based on allochthonous carbon, driven by a constant supply of organic matter from adjacent vegetation and limited primary production due to low light conditions. Extreme climatic disturbances can disrupt these natural ecosystem dynamics by altering resource availability, which leads to changes in food web structure and functioning. Here, we quantify the response of stream food webs to two major hurricanes (Irma and María, Category 5 and 4, respectively) that struck Puerto Rico in September 2017. Within two tropical forested streams (first and second order), we collected ecosystem and food web data 6 months prior to the hurricanes and 2, 9, and 18 months afterward. We assessed the structural (e.g., canopy) and hydrological (e.g., discharge) characteristics of the ecosystem and monitored changes in basal resources (i.e., algae, biofilm, and leaf litter), consumers (e.g., aquatic invertebrates, riparian consumers), and applied Layman's community‐wide metrics using the isotopic composition of¹³C and¹⁵N. Continuous stream discharge measurements indicated that the hurricanes did not cause an extreme hydrological event. However, the sixfold increase in canopy openness and associated changes in litter input appeared to trigger an increase in primary production. These food webs were primarily based on terrestrially derived carbon before the hurricanes, but most taxa (includingAtyaandXiphocarisshrimp, the consumers with highest biomass) shifted their food source to autochthonous carbon within 2 months of the hurricanes. We also found evidence that the hurricanes dramatically altered the structure of the food web, resulting in shorter (i.e., smaller food‐chain length), narrower (i.e., lower diversity of carbon sources) food webs, as well as increased trophic species packing. This study demonstrates how hurricane disturbance can alter stream food webs, changing the trophic base from allochthonous to autochthonous resources via changes in the physical environment (i.e., canopy defoliation). As hurricanes become more frequent and severe due to climate change, our findings greatly contribute to our understanding of the mechanisms that maintain forested stream trophic interactions amidst global change.

    

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3. 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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4. Bioenergy landscapes drive trophic shifts in generalist ants

   https://doi.org/10.1111/1365-2656.13407  

   Helms, IV, Jackson A. ; Roeder, Karl A. ; Ijelu, Selassie E. ; Ratcliff, Ian ; Haddad, Nick M. ; Shik, ed., Jonathan Zvi ( December 2020 , Journal of Animal Ecology)

   Changes in trophic niche—the pathways through which an organism obtains energy and nutrients—are a fundamental way in which organisms respond to environmental conditions. But the capacity for species to alter their trophic niches in response to global change, and the ways they do so when able, remain largely unknown.

   Here we examine food webs in three long‐term and large‐scale experiments to test how resource availability and nutritional requirements interact to determine an organism's trophic niche in the context of one of the largest global trends in land use—the rise in bioenergy production.

   We use carbon and nitrogen stable isotope analyses to characterize arthropod food webs across three biofuel crops representing a gradient in plant resource richness (corn monocultures, fields dominated by native switchgrass and restored prairie), and to quantify changes in the trophic niche of a widespread generalist ant species across habitats. In doing so, we measure the effects of basal resource richness on food chain length, niche breadth and trophic position. We frame our results in the context of two hypotheses that explain variation in trophic niche—the niche variation hypothesis which emphasizes the importance of resource diversity and ecological opportunity, and the optimal diet hypothesis which emphasizes dietary constraints and the availability of optimal resources.

   Increasing plant richness lengthened food chains by 10%–20% compared to monocultures. Niche breadths of generalist ants did not vary with resource richness, suggesting they were limited by optimal diet requirements and constraints rather than by ecological opportunity. The ants instead responded to changes in plant richness by shifting their estimated trophic position. In resource‐poor monocultures, the ants were top predators, sharing a trophic position with predatory spiders. In resource‐rich environments, in contrast, the ants were omnivores, relying on a mix of animal prey and plant‐based resources.

   In addition to highlighting novel ecosystem impacts of alternate bioenergy landscapes, our results suggest that niche breadth and trophic diversification depend more on the presence of optimal resources than on ecological opportunity alone.

    

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5. Biodiversity loss leads to reductions in community‐wide trophic complexity

   https://doi.org/10.1002/ecs2.3361  

   Gabara, Scott S. ; Konar, Brenda H. ; Edwards, Matthew S. ( February 2021 , Ecosphere)

   With increasing biodiversity loss occurring worldwide, there is a need to understand how these losses will affect ecosystem structure and function. Biodiversity loss leads to changes in species interactions and alters the trophic complexity of food webs. These alterations to trophic complexity can be described by changes to the diversity of food resources and the diversity of trophic levels. To understand how biodiversity affects trophic complexity of food webs, we used 10 islands across the Aleutian Archipelago to compare the alternate state communities found in kelp forest ecosystems (kelp forest and urchin barren communities) and then compared these to natural reference communities without local benthic production (their associated offshore communities). We constructed food webs for each community across the Aleutian Archipelago using primary producer and consumer carbon (δ¹³C, a proxy for food sources to a consumer) and nitrogen (δ¹⁵N, a proxy for consumer trophic level) stable isotope values. Our findings suggest that biodiversity loss (i.e., phase change from kelp forest to urchin barren) leads to reductions in trophic complexity, which was similar to naturally occurring communities with low local resource biodiversity. This was expressed by lower consumer isotopic dietary niche areas, especially omnivores and herbivores, and lower omnivore and carnivore trophic levels within the urchin barren communities. We clarify how biodiversity promotes food resources and increases trophic levels and complexity through critical trophic conduits.

    

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