How species interactions shape habitat structure is a longstanding question in ecology. A curious phenomenon reflecting ecological self-organization around reef habitat structures exists on coral ...reefs: large-scale (hundreds to hundreds of thousands of m2) halo-like patterns surrounding patch reefs, i.e., individual coral reefs that are often separated by seagrass or macroalgal meadows. These “halos,” long known to occur in various locations worldwide, reflect a distinct band of unvegetated sediments surrounding coral patch reefs. However, the full suite of mechanisms controlling them have never been rigorously explored, perhaps due to the common assumption dating back nearly 50 years that they arise solely from reef-based herbivory patterns shaped by anti-predator behavior. Here we provide empirical evidence from a set of halos within Australia's Great Barrier Reef that risk-averse foraging and a previously unrecognized functional group contribute to halo formation, demonstrating that these halos cannot be explained by any one mechanism in isolation. Our results show that halos are a more complex ecological phenomenon than previously assumed by the majority of studies of halos. Specifically, risk-averse grazing by herbivores is likely a key mechanism behind the formation of halos, as generally assumed, but bioturbators also play a central role. This knowledge furthers our understanding of how small-scale species interactions can structure habitat at landscape scales. These large-scale habitat features are important because they affect at least one important ecosystem function, carbon storage, and potentially others (e.g., biological nutrient transfer). These results also raise the question of whether other self-organized ecological patterns may be more nuanced than is currently assumed. This study capitalizes on recent advances in high resolution satellite imagery accessibility that allow ecologists to measure landscape-scale habitat features nearly everywhere on land and in shallow seas. Our results suggest that halos may hold potential as the basis for a tool for remotely observing ecological interactions and measuring large-scale ecosystem change on coral reefs.
Benthic animals profoundly influence the cycling and storage of carbon and other elements in marine systems, particularly in coastal sediments. Recent climate change has altered the distribution and ...abundance of many seafloor taxa and modified the vertical exchange of materials between ocean and sediment layers. Here, we examine how climate change could alter animal-mediated biogeochemical cycling in ocean sediments. The fossil record shows repeated major responses from the benthos during mass extinctions and global carbon perturbations, including reduced diversity, dominance of simple trace fossils, decreased burrow size and bioturbation intensity, and nonrandom extinction of trophic groups. The broad dispersal capacity of many extant benthic species facilitates poleward shifts corresponding to their environmental niche as overlying water warms. Evidence suggests that locally persistent populations will likely respond to environmental shifts through either failure to respond or genetic adaptation rather than via phenotypic plasticity. Regional and global ocean models insufficiently integrate changes in benthic biological activity and their feedbacks on sedimentary biogeochemical processes. The emergence of bioturbation, ventilation, and seafloor-habitat maps and progress in our mechanistic understanding of organism–sediment interactions enable incorporation of potential effects of climate change on benthic macrofaunal mediation of elemental cycles into regional and global ocean biogeochemical models.
Trophic cascade theory predicts that predator effects should extend to influence carbon cycling in ecosystems. Yet, there has been little empirical evidence in natural ecosystems to support this ...hypothesis. Here, we use a naturally-occurring trophic cascade to provide evidence that predators help protect sedimentary organic carbon stocks in coral reef ecosystems. Our results show that predation risk altered the behavior of herbivorous fish, whereby it constrained grazing to areas close to the refuge of the patch reefs. Macroalgae growing in “riskier” areas further away from the reef were released from grazing pressure, which subsequently promoted carbon accumulation in the sediments underlying the macroalgal beds. Here we found that carbon stocks furthest away from the reef edge were ~24% higher than stocks closest to the reef. Our results indicate that predators and herbivores play an important role in structuring carbon dynamics in a natural marine ecosystem, highlighting the need to conserve natural predator-prey dynamics to help maintain the crucial role of marine sediments in sequestering carbon.
Differences in seed dispersal patterns can alter plant distributions, species persistence, plant community composition, and biotic carbon sequestered within a landscape. Though carnivorans are known ...to be frugivorous, their contribution to seed dispersal is marginally studied, especially compared to other sympatric dispersers such as passerines. This gap is important to understand because carnivorans may be better suited to assist plant dispersal in disturbed habitats and the longer distances necessary for climate migration. In this study, we evaluated how seed dispersal by a particular carnivoran, coyote Canis latrans, differed from passerines Passeriformes for juniper Juniperus sp. in the conterminous United States under future climate change. We modeled changes in juniper niche suitability starting in 2021 through the next 80 years of climate change by estimating the current niche with Maxent, and then using climate predictions to define spatial changes in suitable niches. Seed dispersal by both coyote and passerine dispersers was simulated to estimate total juniper dispersal, juniper encroachment into grasslands, and finally changes in above‐ground biotic carbon storage due to juniper encroachment. Our models indicate that over the next 80 years, suitable conditions for juniper will contract, but losses from the current range will be minimal. Our model suggests that coyote dispersal of juniper will result in a 54–59% increase in range, which is 2.5 times as much as the estimated increase provided by passerines. We estimate that coyotes will facilitate juniper encroachment into 170 000–185 000 km2 of current grasslands, 3.4 times as much as passerines. Modeling the effect of coyote‐mediated juniper encroachment of grasslands, we forecast that the addition of woody aboveground biomass will provide between 1.1 and 1.2 Pg of additional carbon storage over the next 80 years. Results highlight how coyotes and passerines provide different outcomes for changes in juniper ranges, plant community composition, and landscape carbon storage. Understanding the differences in outcomes provided by different seed dispersers is important for modeling plant species distributions and carbon storage.
Falcataria moluccana (albizia) is an exotic nitrogen (N)-fixing tree currently invading riparian forests in Hawai'i, U.S.A. This study examined how this invasion is impacting stream ecosystems by ...using naturally occurring stable isotopes of carbon (C) and N to compare food web structure between a noninvaded and an albizia-invaded stream reach on the island of Hawai'i. Our study provides some of the first evidence that invasive riparian N-fixing trees can potentially alter the structure of stream food webs. (Authors) Source: National Library of New Zealand Te Puna Matauranga o Aotearoa, licensed by the Department of Internal Affairs for re-use under the Creative Commons Attribution 3.0 New Zealand Licence.
Blue carbon ecosystems (BCE) include mangrove forests, tidal marshes, and seagrass meadows, all of which are currently under threat, putting their contribution to mitigating climate change at risk. ...Although certain challenges and trade-offs exist, remote sensing offers a promising avenue for transparent, replicable, and cost-effective accounting of many BCE at unprecedented temporal and spatial scales. The United Nations Framework Convention on Climate Change (UNFCCC) has issued guidelines for developing blue carbon inventories to incorporate into Nationally Determined Contributions (NDCs). Yet, there is little guidance on remote sensing techniques for monitoring, reporting, and verifying blue carbon assets. This review constructs a unified roadmap for applying remote sensing technologies to develop cost-effective carbon inventories for BCE – from local to global scales. We summarise and discuss (1) current standard guidelines for blue carbon inventories; (2) traditional and cutting-edge remote sensing technologies for mapping blue carbon habitats; (3) methods for translating habitat maps into carbon estimates; and (4) a decision tree to assist users in determining the most suitable approach depending on their areas of interest, budget, and required accuracy of blue carbon assessment. We designed this work to support UNFCCC-approved IPCC guidelines with specific recommendations on remote sensing techniques for GHG inventories. Overall, remote sensing technologies are robust and cost-effective tools for monitoring, reporting, and verifying blue carbon assets and projects. Increased appreciation of these techniques can promote a technological shift towards greater policy and industry uptake, enhancing the scalability of blue carbon as a Natural Climate Solution worldwide.
•Blue carbon ecosystems (BCE) play a key role in climate change mitigation.•Remote sensing techniques are cost-effective for monitoring BCE extent and carbon stocks.•A tradeoff exists between spatial resolution and historical coverage of remotely sensed data.•New technologies can facilitate the inclusion of BCE in national inventories and Nationally Determined Contributions (NDCs).•Our decision tree guides the choice of remote sensing methods for monitoring BCE.
Marine reserves can effectively restore harvested populations, and ‘mega-reserves’ increasingly protect large tracts of ocean. However, no method exists of monitoring ecological responses at this ...large scale. Herbivory is a key mechanism structuring ecosystems, and this consumer–resource interaction's strength on coral reefs can indicate ecosystem health. We screened 1372, and measured features of 214, reefs throughout Australia's Great Barrier Reef using high-resolution satellite imagery, combined with remote underwater videography and assays on a subset, to quantify the prevalence, size and potential causes of ‘grazing halos’. Halos are known to be seascape-scale footprints of herbivory and other ecological interactions. Here we show that these halo-like footprints are more prevalent in reserves, particularly older ones (approx. 40 years old), resulting in predictable changes to reef habitat at scales visible from space. While the direct mechanisms for this pattern are relatively clear, the indirect mechanisms remain untested. By combining remote sensing and behavioural ecology, our findings demonstrate that reserves can shape large-scale habitat structure by altering herbivores' functional importance, suggesting that reserves may have greater value in restoring ecosystems than previously appreciated. Additionally, our results show that we can now detect macro-patterns in reef species interactions using freely available satellite imagery. Low-cost, ecosystem-level observation tools will be critical as reserves increase in number and scope; further investigation into whether halos may help seems warranted. Significance statement: Marine reserves are a widely used tool to mitigate fishing impacts on marine ecosystems. Predicting reserves' large-scale effects on habitat structure and ecosystem functioning is a major challenge, however, because these effects unfold over longer and larger scales than most ecological studies. We use a unique approach merging remote sensing and behavioural ecology to detect ecosystem change within reserves in Australia's vast Great Barrier Reef. We find evidence of changes in reefs' algal habitat structure occurring over large spatial (thousands of kilometres) and temporal (40+ years) scales, demonstrating that reserves can alter herbivory and habitat structure in predictable ways. This approach demonstrates that we can now detect aspects of reefs' ecological responses to protection even in remote and inaccessible reefs globally.
Concern over accelerating rates of species invasions and losses have initiated investigations into how local and global changes to predator abundance mediate trophic cascades that influence CO sub(2) ...fluxes of aquatic ecosystems. However, to date, no studies have investigated how species additions or losses at other consumer trophic levels influence the CO sub(2) flux of aquatic ecosystems. In this study, we added a large predatory stonefly, detritivorous stonefly, or grazer tadpole to experimental stream food webs and over a 70-day period quantified their effects on community composition, leaf litter decomposition, chlorophyll-a concentrations, and stream CO sub(2) emissions. In general, streams where the large grazer or large detritivore were added showed no change in total invertebrate biomass, leaf litter loss, chlorophyll-a concentrations, or stream CO sub(2) emissions compared with controls; although we did observe a spike in CO sub(2) emissions in the large grazer treatment following a substantial reduction in chlorophyll-a concentrations on day 28. However, the large grazer and large detritivore altered the community composition of streams by reducing the densities of other grazer and detritivore taxa, respectively, compared with controls. Conversely, the addition of the large predator created trophic cascades that reduced total invertebrate biomass and increased primary producer biomass. The cascading effects of the predator additions on the food web ultimately led to decreased CO sub(2) emissions from stream channels by up to 95%. Our results suggest that stream ecosystem processes were more influenced by changes in large predator abundance than large grazer or detritivore abundance, because of a lack of functionally similar large predators. Our study demonstrates that the presence/absence of species with unique functional roles may have consequences for the exchange of CO sub(2) between the ecosystem and the atmosphere.
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