2024-08-01 カリフォルニア大学サンタバーバラ校(UCSB)
<関連情報>
- https://news.ucsb.edu/2024/021569/big-sharks-have-big-impact-and-big-problem
- https://www.science.org/doi/10.1126/science.adl2362
人新世の海におけるサメの生態学的役割と重要性 Ecological roles and importance of sharks in the Anthropocene Ocean
Simon Dedman, Jerry H. Moxley, Yannis P. Papastamatiou, Matias Braccini, […], and Michael R. Heithaus
Science Published:2 Aug 2024
DOI:https://doi.org/10.1126/science.adl2362
Editor’s summary
Sharks are well known as massive top predators with large pelagic ranges. This conception is accurate but too narrow. Sharks occupy a wide variety of niches and come in a range of sizes and shapes. Dedman et al. reviewed the ecological roles that sharks play and their importance in terms of how they affect their ecosystems. Many sharks are essential to ecosystem functioning, and many of these effects have been altered by human impacts on shark populations. Rebuilding these populations is essential not only for conserving sharks, but also for conserving their ecological function. —Sacha Vignieri
Structured Abstract
BACKGROUND
Pervasive losses of predators on land, in freshwater habitats, and in oceans have disrupted ecosystems, prompting interest in rebuilding their populations to ecologically functional levels. Predator restoration may also facilitate nature-based climate solutions by indirectly increasing carbon sequestration through sharks’ direct predation and risk effects on herbivores and herbivores’ predators and by enhancing ecosystem resilience. However, selecting target species for these efforts requires a functional understanding of their ecosystem roles.
Sharks are a diverse (more than 500 species) group of predators found within marine, estuarine, and freshwater ecosystems. Although there is considerable variation in feeding modes and body sizes, sharks are often presumed to be critical to ecosystem structure, function, and resilience through top-down forcing of ecological communities. Although often valid, this presumption oversimplifies the many roles played by sharks. It also minimizes examples of functional redundancy and small ecological effects. Precipitous population declines in many species are a cause for concern and an impetus to investigate whether reversing declines could benefit ecosystems. Yet a functional understanding of ecological roles and importance of sharks is lacking because of the inherent difficulties of studying their interactions and the mechanisms through which sharks may—or may not—affect ecosystems.
In this Review, we have evaluated historical and ongoing global depletions—and occasional recoveries—of sharks to elucidate their diverse ecological roles, and we highlight the value of understanding their past, present, and future roles. We investigated where sharks play important roles, identified where population restorations may be particularly beneficial, and evaluated policies that can support role recovery.
ADVANCES
Empirical studies of the ecological roles and importance of sharks have revealed considerable cascading effects of macropredatory sharks [such as the tiger shark (Galeocerdo cuvier) and white shark (Carcharodon carcharias)] in coastal seagrass and kelp ecosystems, which influence habitat quality and carbon sequestration. These shark-initiated indirect effects may enhance the resilience of ecosystems that are increasingly experiencing extreme climate events (such as marine heat waves). However, not all sharks exert large top-down effects on prey or wider communities, although long-term overfishing causing large-scale depletions may obscure historical roles, particularly in difficult-to-study habitats such as pelagic systems or deep waters. Previously unappreciated roles of sharks (for example, facilitating ecosystems through nutrient transport) have recently become apparent in multiple ecosystems and taxa.
Climate change and industrializing oceans (such as overfishing, resource extraction, and tourism) are creating new roles for sharks and modifying the spatiotemporal patterns and importance of their effects. Warming oceans are expanding some shark ranges to higher latitudes, suggesting broadening importance across wider geographies, and thermal asymmetries in the metabolic costs and performance of sharks and marine mammals could affect competitive and predator-prey interactions. Range shifts and the recovery of white sharks, for example, are limiting the recolonization of sea otter historic ranges, which necessitates understanding how shark ecologies and species interactions affect broader ecosystem recovery. In multiple ocean basins, killer whale (Orcinus orca) predation risk has shifted distributions of white sharks, disrupting shark feeding patterns and ecological roles. Other accelerating anthropogenic pressures such as aquaculture, wildlife tourism, and extractive activities such as fishing are diversifying shark-human interactions and altering the manner and magnitude of spatiotemporal impacts of sharks on ecosystems.
OUTLOOK
Gaps remain in our understanding of the ecological importance of sharks in today’s oceans, necessitating research especially on small-bodied and deepwater sharks and shark-driven nutrient transport. Concurrently, management should aim to maintain ecological function rather than just maximum sustainable yield or population persistence, especially for influential and threatened macropredatory species. Given the potential for diverse and hidden roles, managing for shark biodiversity is important. Although transitions in fisheries and recovery goals will be challenging (for example, owing to commercial value and fisheries depredation), they are necessary to ensure healthy ecosystems in a changing ocean.
Sharks influence ecosystems through various mechanisms.
Tiger sharks and other large species are important to the development, maintenance, and resilience of seagrass and other macrophyte communities through top-down effects on their prey. Other species transport limiting nutrients or play less understood roles. Management must preserve diverse shark roles in a rapidly developing ocean and changing climate.
PHOTO: MICHAEL R. HEITHAUS
Abstract
In ecosystems, sharks can be predators, competitors, facilitators, nutrient transporters, and food. However, overfishing and other threats have greatly reduced shark populations, altering their roles and effects on ecosystems. We review these changes and implications for ecosystem function and management. Macropredatory sharks are often disproportionately affected by humans but can influence prey and coastal ecosystems, including facilitating carbon sequestration. Like terrestrial predators, sharks may be crucial to ecosystem functioning under climate change. However, large ecosystem effects of sharks are not ubiquitous. Increasing human uses of oceans are changing shark roles, necessitating management consideration. Rebuilding key populations and incorporating shark ecological roles, including less obvious ones, into management efforts are critical for retaining sharks’ functional value. Coupled social-ecological frameworks can facilitate these efforts.
