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Estes JA, Tinker MT, Williams TM, and Doak DF. Killer whale predation on sea otters linking oceanic and nearshore ecosystems. Science
Abstract
After nearly a century of recovery from overhunting, sea otter populations are in abrupt decline over large areas of western Alaska. Increased killer whale predation is the likely cause of these declines. Elevated sea urchin density and the consequent deforestation of kelp beds in the nearshore community demonstrate that the otter's keystone role has been reduced or eliminated. This chain of interactions was probably initiated by anthropogenic changes in the offshore oceanic ecosystem.
... Commercial harvesting is well known to reduce fish stocks and produce adverse cascading effects in marine ecosystems (Estes et al., 1998;Heithaus et al., 2008;Jackson et al., 2001). It is now widely recognized that an ecosystem-based management approach to fisheries is necessary to ensure sustainable fishing and lasting ecosystem health (Pikitch et al., 2004). ...
Climate change is causing rapid, unexpected changes to ecosystems through alteration to environmental regimes, modification of species interactions, and increased frequency and magnitude of disturbances. Yet, how the type of disturbance affects food webs remains ambiguous. Long‐term studies capturing ecosystem responses to extreme events are necessary to understand climate effects on species interactions and ecosystem resilience but remain rare. In the Gulf of Mexico, our 8‐year study captured two disturbances that had contrasting effects on predator abundance and cascading effects on estuarine food webs. In 2017, Hurricane Harvey destroyed fishing infrastructure, fishing activity declined, and sportfish populations increased ~40% while intermediate trophic levels that sportfish prey upon declined ~50%. Then, in 2021, a fish kill caused by freezing temperatures during Winter Storm Uri reduced sportfish populations by ~60% and intermediate trophic levels increased by over 250%. Sportfish abundance affected the abundance and size of oyster reef mesopredators. Excluding fish predators significantly altered oyster reef community structure. These results demonstrate how extreme events shape communities and influence their resilience based on their effects on top predators. Moreover, top‐down forces from sportfish are important in estuaries, persist through disturbances, and influence community resilience, highlighting the necessity of proper recreational fisheries management through extreme events.
... Meanwhile, anthropogenic factors such as overfishing, pollution, or physical damage, stand as the main drivers of resilience loss (Ling et al., 2009;Piola and Johnston, 2008;Maynard et al., 2015). For instance, overfishing of marine resources leads to important changes in food webs and ecosystem configuration (Estes et al., 1998); pesticides and chemical contaminants dumped in coastal areas impact the health of marine biological communities by increasing the likelihood of diseases (Folke et al., 2004;Piola and Johnston, 2008); and sea-related activities, such as diving or anchoring, physically impact the seafloor and benthic life by dismantling the benthic structure (Gladstone et al., 2013;Siciliano et al., 2016). While both environmental and anthropogenic factors can lead to a loss of resilience, we can only influence anthropogenic pressures in the short and medium term. ...
... In the present study, a non-experimental approach based on the analysis of landing data was adopted to investigate the effects determined by the invasion of C. sapidus on the fish assemblage of the Goro Lagoon. Observational data obtained from surveys and fishery landings have long been used for causal inference on large-scale ecological questions in marine systems where experimental manipulation cannot be easily performed (e.g., Estes et al., 1998;Aebischer et al., 1990). Noticeably, the fundamental challenge of non-experimental settings lies in the often-cited "correlation does not imply causation": any conclusive inference on the effect of a factor of interest remains open to the possibility that hidden, not explicated drivers might have influenced the observed outcomes in a non-random manner. ...
... 8). Barren formation triggers catastrophic ecological shifts, resulting in the local disappearance of sometimes hundreds of reef-dependent species, including economically important species such as abalone and rock lobster that rely on macroalgal habitats 8,9 , with similar shifts observed around the world [10][11][12][13] . Extensive barren coverage in Tasmania has increased from 3% at the first survey to 15% by the second survey 7 , threatening a broad range of reef species 2 along with associated recreational and commercial fisheries that take place in Tasmania, particularly the lucrative abalone and southern rock lobster fisheries 3 . ...
Climate change is driving species to colonize new ranges, sometimes causing uncontrollable damage. Here we present a remarkable scenario in which government-supported incentives have driven the establishment of a commercial fishery targeting a destructive urchin, intentionally encouraging overfishing to protect kelp ecosystems. This ecosystem management strategy is paradoxical in the objective to overfish sustainably. Due to consistent and increasing larval influx, the eradication of urchins in their extended range is implausible. Management, therefore, focuses on maintaining urchin density below a critical threshold while ensuring a viable commercial fishery for long-term species control. Our model dissects the fishery’s impact, offering practical strategies for controlling a destructive range extender given economic and spatial dynamics. Beyond the implications of subsidizing a commercial fishery to counteract the impacts of climate change, our study explores the conflict between exploitation and conservation, challenging traditional views and presenting practical pathways to sustainability.
... The historical reliance of killer whales on large-bodied mysticete prey set the stage for a more recent regime shift, offering insight into where the framework presented here may be usefully expanded. As great whale populations declined during the last 300 years of industrialscale overharvest (Baker and Clapham 2004), some have hypothesized that killer whale populations iteratively switched to smaller-bodied prey such as pinnipeds (Estes et al. 2009, Springer et al. 2003, only recently adopting contemporary diets, which in some populations consist largely of sea otters (Estes et al. 1998. While the coinciding effects of anthropogentic pressures on pinniped populations introduces some complexity, the dietary shift of killer whales to pinnipeds may have introduced strong trophic cascades, accelerating their decline, and setting the stage for killer whales switching to smaller prey. ...
Quantifying population dynamics is a fundamental challenge in ecology and evolutionary biology, particularly for species that are cryptic, microscopic, or extinct. Traditional approaches rely on continuous representations of population size, but in many cases, the precise number of individuals is unknowable. Here, we present a coarse-grained population model that simplifies population dynamics to binary states - high or low - determined by the balance of bottom-up resource availability and top-down predation pressure. This Boolean framework provides a minimal yet analytically tractable alternative to traditional Lotka-Volterra-based models, enabling direct insights into the role of food web structure in shaping community stability. Using this approach, we investigate how trophic interactions influence population persistence, cyclic dynamics, and extinction risk across model food webs. We find that top-down effects are a primary driver of cycling, aligning with theoretical expectations from traditional population models, and that trophic position strongly influences extinction risk, with higher-trophic species more prone to persistent low-population states. Additionally, we explore the role of trophic short-circuits -- direct interactions between apex predators and low-trophic prey -- and find that they can buffer cascades and alter extinction patterns in ways that are often overlooked in classical models. By simplifying population dynamics to a two-state system, this framework provides a powerful tool for disentangling the structural drivers of community stability. These results highlight the potential of coarse-grained approaches to complement existing models, offering new insights into trophic interactions, extinction risks, and the susceptibility of species to trophic cascades.
... A prime example is the orcas in the Aleutian Islands, which innovated by including otters in their diet. This shift affected the entire ecosystem: with otters preying on urchins, their decline caused urchin populations to surge, devastating the kelp forests crucial for local biodiversity (Estes et al. 1998). Even if the orcas were to revert to their previous diet, this dietary innovation will likely have profound evolutionary consequences. ...
In recent decades, agency has become a prominent concept for conceiving of biological phenomena, both in philosophy and in biology. The concept aims to capture the spontaneity of organisms and illuminate processes overlooked by the previously dominant reductionist framework. However, its definition varies widely across and within disciplines, whether it is employed to understand development, organisation, or evolution, or applied to genes, cells, organisms, or even natural selection. This ambiguity hinders distinguishing its appropriate from inappropriate uses and establishing a cohesive theoretical basis for its various manifestations. This paper reviews the existing literature on agency from philosophy, evolutionary theory, developmental biology, and animal studies. It identifies three core capacities that unify the concept of agency: Individuality, Playful flexibility, and Memory (IPM). These capacities interact in diverse ways, with their influence varying depending on the process or context. These quantitative differences give rise to qualitatively distinct forms of agency: autonomous organisation, goal-directed choice, and inventiveness. Drawing on the IPM framework, this paper explores the relationships between these forms of agency, and argues that this approach can rationalise and formalise our understanding of agency while accommodating its diverse expressions.
... Marine mammals play essential roles in ecosystems because of their wide range of movements, widespread presence in all seas, feeding patterns across most ocean layers (from epi-to bathy-pelagic), and trophic levels (Kiszka et al., 2015(Kiszka et al., , 2022. They contribute to ecological dynamics through "top-down" and "bottom-up", "whale pump" and "whale fall" processes playing a vital role in the biochemical cycling of nutrients and carbon in the water column (Estes et al., 1998(Estes et al., , 2009Becker et al., 2021). Thus, they are considered ecosystem engineers and sentinel species crucial for ecosystem monitoring (Roman and McCarthy, 2010;Martin et al., 2021;Pearson et al., 2023). ...
Parasitism is increasingly recognised as a significant determinant of composition and structure of natural communities across ecosystems. In this chapter, a large amount of evidence from the aquatic realm is brought together to highlight the multitude of processes through which parasites impact aquatic community structure. There are three main ways by which parasites affecting community structure are emphasised: (1) direct effects through differential regulation of host populations; (2) density-mediated indirect effects through modification of biotic processes such as predation, herbivory, facilitation and inhibition; and (3) trait-mediated indirect effects through altered phenotype of infected host species that often act as keystone species or ecosystem engineers in the community. Selected studies are treated in-depth to illustrate the ways parasites modify aquatic communities, involving a variety of host organisms such as monocotyledons, gastropods, bivalves, amphipods, echinoderms and fishes, as well as parasites ranging from protists to trematodes and nematodes, among others. This synthesis concludes by stressing that the available evidence does not equivocally support the contemporary theory that parasitism generally acts to maintain biodiversity. Rather, the qualitative effect is context-dependent, determined by the ecological roles of involved host species.
Resident killer whales fed more in areas of high relief subsurface topography along salmon migratory routes, and may use these geographic features to increase feeding efficiency. Transient whales fed in shallow protected areas around concentrations of their prey, harbor seals Phoca vitulina. Whales traveled across deep, featureless areas in moving from one feeding area to another. Whales rested depending on the previous sequence of behaviors and played in open water areas or adjacent to feeding areas. Location of food resources and habitats suitable for prey capture appears to be the prime determining factor in the behavioral ecology of these whales, most likely representing cultural mechanisms that have been learned through trial and error experiences leading to successful foraging strategies. -from Author
Food webs in nature have multiple, reticulate connections between a diversity of consumers and resources. Such complexity affect web dynamics: it first spreads the direct effects of consumption and productivity throughout the web rather than focusing them at particular 'trophic levels'. Second, consumer densities are often donor controlled with food from across the trophic spectrum, the herbivore and detrital channels, other habitats, life-history omnivory, and even trophic mutualism. Although consumers usually do not affect these resources, increased numbers often allow consumers to depress other resources to levels lower than if donor-controlled resources were absent. We propose that such donor-controlled and "multi-channel" omnivory is a general feature of consumer control and central to fooweb dynamics. This observation is contrary to the normal practice of inferring dynamics by simplifying webs into few linear "trophic levels" as per "green world" theories. Such theories do not accomodate common and dynamically important features of real webs such as the ubiquity of donor control and the importance and dynamics of detritus, omnivory, resources crossing habitats, life history, nutrients(as opposed to energy), pathogens, resources defenses, and trophic symbioses. We conclude that trophic cascades and top-down community regulation as envisioned by trophic-level theories are relatively uncommon in nature.
The impetus for this volume comes from two sources. The first is scientific: by virtue of a preference for certain large benthic invertebrates as food, sea otters have interesting and significant effects on the structure and dynamics of nearshore communities in the North Pacific. The second is political: be cause of the precarious status of the sea otter population in coastal California, the U.S. Fish and Wildlife Service (USFWS) announced, in June 1984, a proposal to establish a new population of sea otters at San Nicolas Island, off southern California. The proposal is based on the premise that risks of catastrophic losses of sea otters, due to large oil spills, are greatly reduced by distributing the population among two geographically separate locations. The federal laws of the U.S. require that USFWS publish an Environmental Impact Statement (ElS) regarding the proposed translocation of sea otters to San Nicolas Island. The EIS is intended to be an assessment of likely bio logical, social, and economic effects of the proposal. In final form, the EIS has an important role in the decision of federal management authority (in this case, the Secretary of the Interior of the U.S.) to accept or reject the proposal.
When apparent declines in population numbers are observed, biologists are confronted with the challenge of determining if such behavior conforms to the expected dynamics of the population or if, on the contrary, the decline reflects some pathological change in the environment that requires management actions. Experimental investigations are often impractical for large free-ranging populations such as fish or large mammals. Consequently, biologists need to get the best possible answer from the limited data available. In this paper we combine deterministic and stochastic modeling, together with statistical estimation techniques, to explore the likelihood of various hypotheses for the decline of Steller sea lions in the North Pacific. Our results show that deterministic transient population behavior, historical pup harvesting, and short-term environmental stochasticity are unlikely causes for the decline. The elimination of these alternatives leads us to agree with previous authors, who suggest that some long-term change in the environment or a novel catastrophe is responsible for the decline.
Multiscale patterns of spatial and temporal variation in density and population structure were used to evaluate the generality of a three-trophic-level cascade among sea otters (Enhydra lutris), invertebrate herbivores, and macroalgae in Alaska. The paradigm holds that where sea otters occur herbivores are rare and plants are abundant, whereas when sea otters are absent herbivores are relatively common and plants are rare. Spatial patterns were based on 20 randomly placed quadrats at 153 randomly selected sites distributed among five locations with and four locations without sea otters. Both sea urchin and kelp abundance differed significantly among locations with vs. without sea otters in the Aleutian Islands and southeast Alaska. There was little (Aleutian Islands) or no (southeast Alaska) overlap between sites with and without sea otters, in plots of kelp density against urchin biomass. Despite intersite variation in the abundance of kelps and herbivores, these analyses demonstrate that sea otter predation has a predictable and broadly generalizable influence on the structure of Alaskan kelp forests. The percent cover of algal turf and suspension feeder assemblages also differed significantly (although less dramatically) between locations with and without sea otters. Temporal variation in community structure was assessed over periods of from 3 to 15 yr at sites in the Aleutian Islands and southeast Alaska where sea otters were 1) continuously present, 2) continuously absent, or 3) becoming reestablished because of natural range expansion. Kelp and sea urchin abundance remained largely unchanged at most sites where sea otters were continuously present or absent, the one exception being at Torch Bay (southeast Alaska), where kelp abundance varied significantly through time and urchin abundance varied significantly among sites because of episodic and patchy disturbances. In contrast, kelp and sea urchin abundances changed significantly, and in the expected directions, at sites that were being recolonized by sea otters. Sea urchin biomass declined by 50% in the Aleutian Islands and by nearly 100% in southeast Alaska following the spread of sea otters into previously unoccupied habitats. In response to these different rates and magnitudes of urchin reduction by sea otter predation, increases in kelp abundance were abrupt and highly significant in southeast Alaska but much smaller and slower over similar time periods in the Aleutian Islands. The different kelp colonization rates between southeast Alaska and the Aleutian Islands appear to be caused by large-scale differences in echinoid recruitment coupled with size-selective predation by sea otters for larger urchins. The length of urchin jaws (correlated with test diameter, r^2 = 0.968) in sea otter scats indicates that sea urchins <15-20 mm test diameter are rarely eaten by foraging sea otters. Sea urchin populations in the Aleutian Islands included high densities of small individuals (<20 mm test diameter) at all sites and during all years sampled, whereas in southeast Alaska similarly sized urchins were absent from most populations during most years. Small (<30-35 mm test diameter) tetracycline-marked urchins in the Aleutian Islands grew at a maximum rate of @?10 mm/yr; thus the population must have significant recruitment annually, or at least every several years. In contrast, echinoid recruitment in southeast Alaska was more episodic, with many years to perhaps decades separating significant events. Our findings help explain regional differences in recovery rates of kelp forests following recolonization by sea otters.
In Torch Bay, Alaska, USA, sea urchins structure shallow subtidal communities by consuming most macroscopic algae. Experimental removal of urchins leads to the development, within 1 yr, of a kelp association of high biomass and diversity. In the 2nd yr, however, these attributes decrease as a single kelp species, Laminaria groenlandica, becomes dominant. Additional field experiments confirm the competitive superiority of this kelp. These manipulations lead to predictions regarding the ecological role of sea otters, a once common (but now absent) sea urchin predator. The predictions are tested and verified by examining areas into which sea otters have recently been transplanted. As expected, a rapid and extensive modification of algal species composition and a dramatic increase in kelp biomass follow the return of sea otters.
The heavily descriptive, detailed methods of the traditional naturalist and the modern system analyst are being complemented with models of Hutchinsonian simplicity, where idealized, easily understandable models are offered as an approximation to reality. The analysis of Hairston, Smith, and Slobodkin is the major turning point in this revolution, where key assumptions about trophic levels and population regulation are first used as a basis for analysis. Modeling by Rosenzweig put these assumptions on solid ground, and introduced a key variable, primary productivity. Fretwell and Oksanen et al. generalized and extended the logic used in these pioneering studies to provide a theory that could be central to all ecology. This theory is here reviewed, and is extended to include the field of community ecology. Arguments are offered defending the position that this research program (Food Chain Dynamics) could be regarded as the central theory of ecology, at least as important as the theory of evolution.-from Author
Summary of the sea otter in relation to the eastern Pacific Ocean, including physical characteristics, habitat requirements, general behavior, food and feeding behavior, distribution and numbers, reproduction, limiting factors, and behavior in captivity.