Geology topics
J. A. Estes
Publications and source records attributed to J. A. Estes.
Predators and ecosystem management
Explore the source record for details and available documents.
The influence of large, mobile predators in aquatic food webs: examples from sea otters and kelp forests
No abstract available at this time
Top-level carnivores and ecosystem effects: Questions and approaches
Top-level carnivores comprise a diverse array of taxa and occur in most natural ecosystems. This chapter considers three main questions: What are the ecological and evolutionary effects of top-level carnivores, how general are they, and under what conditions do they occur? To answer these questions one first must determine whether a food web is under top-down control, and ultimately, the species, systems, and circumstances for which top-down control exists. For systems affected by top-down forces, the cascading effect of carnivory on autotrophs is expected to vary with the order of trophic complexity. Given this conceptual framework, three more specific questions are explored. First, how general are the direct and indirect effects of top-level carnivores? Second, what is the breadth of trophic influence from the direct impact of carnivory? Broadly ranging effects are expected for systems in which (1) top-level carnivores occupy odd-numbered trophic levels and (2) the food web is strongly linked. (3) what are the evolutionary consequences of trophic cascades on species lower in the food web? These questions can best be answered via manipulative or natural experiments in which the presence or absence of carnivores varies in space or time. Natural experiments are advocated as the only practical means of addressing these questions for many species and systems, in particular those in which the key players are large and highly mobile, or those in which the manipulation of predators has legal, political, or social ramifications.
Repertoire, structure, and individual variation of vocalizations in the sea otter
Vocalizations of the California sea otter ( Enhydra lutris nereis ) were recorded from wild and captive adults and young and analyzed spectrographically. Parameters measured from the sonagrams included fundamental frequency, duration, maximum frequency, intercall interval, and the location and amplitude of energy peaks. We identified 10 basic vocal categories, one of which consisted of graded signals. The contexts for each call, when known or suspected, are described. Discriminant analysis of the spectrographic parameters for the scream call showed significant differences among individuals for adult females and young. Using only the parameters quantified, each call was assigned correctly to the individual that produced it with 80% accuracy for mothers and 75% for young, thus, indicating that the potential exists for individual vocal recognition in the sea otter. The sea otter's vocal repertoire is similar in complexity to that of certain pinnipeds, but may be less complex than that of several species of social cetaceans and primates. In general, the sea otter's vocal patterns have characteristics thought to be most suitable for short-range communication among familiar individuals.
Sea otters and kelp forests in Alaska: Generality and variation in a community ecological paradigm
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.
Evolutionary consequences of food chain length in kelp forest communities
Explore the source record for details and available documents.
Conservation of Marine Otters
Explore the source record for details and available documents.
Experimental evidence for the effects of polyphenolic compounds from Dictyoneurum californicum Ruprecht (Phaeophyta: Laminariales) on feeding rate and growth in the red abalone Haliotus rufescens Swainson
The effects of polyphenolic compounds from brown algae on grazing and growth rate of the California red abalone Haliotis rufescens Swainson were examined. Abalone consumed three phenolic-poor algal species, Laminaria sinclarii (Harvey) Farlow, Macrocystis pyrifera Agardh, and Nereocystis luetkeana Postels et Ruprecht (mean phenolic content = 0.52% dry mass), at a greater rate than two phenolic-rich species, Dictyoneurum californicum Ruprecht and Cystoseira osmundacea Agardh (mean phenolic content = 4.60% dry mass). This inverse relationship between phenolic content and consumption rate also existed after the algae were macerated and the liquid portion of the blended slurry incorporated in agar discs. However, the correlation between grazing rate and phenolic content imprpve d in this latter experiment, thus suggesting that abalone grazing was deterred significantly by the morphology of L. sinclarii and, to a lesser extent, of M. pyrifera . Polyphenolics extracted from D. californicum reduced abalone grazing rates by 90% when incorporated into agar discs at a concentration of 6 mg·ml −1 . Although abalone were unable to maintain body mass when fed ad libitum on macerated M. pyrifera incorporated into agar discs, polyphenolics from D. californicum further inhibited shell growth when added to the discs at 5 mg·ml −1 . The abalone ate less of the phenol-containing discs than of the discs lacking phenolics. Our results support findings of several prior studies that polyphenolic compounds from brown algae deter grazing by coastal zone herbivores in the northeast Pacific Ocean.
Growth and equilibrium in sea otter populations
(1) Counts through time were compiled for five sea otter ( Enhydra lutris ) populations in the north-east Pacific Ocean that were below equilibrium density: Attu Island, south-east Alaska, British Columbia, Washington State, and central California. Similar data were obtained from the equilibrium density population at Amchitka Island in 1971 and 1986. (2) Shorelines of Attu and Amchitka islands each were divided into forty-five segments, within which lineal (length of shore at mean higher high water) and areal (mean higher high water to the 10-fathom (18.3-m) depth contour) measures were made of the amount of habitat. (3) Rate of increase for the four northern populations was 17-20% year-1. Density- or size-dependent changes in rate of increase could not be demonstrated for any of these populations. The California population, in contrast, has undergone three apparent growth phases: the early 1900s to the mid-1970s when it increased about 5% year-1; the mid-1970s to the mid-1980s when it declined about 5% year-1; and the mid-1980s to 1988 when it increased about 7% year-1. An exponential growth model accounted for 92-98% of the variation in counts through time in all cases. (4) Population increase at Attu Island was achieved largely by range expansion as opposed to increased density. Range expansion in lineal and areal habitat occurred at 11% and 13% year-1, respectively; neither rate was lower (P > 0.25) than the observed rate of increase in numbers of animals counted. (5) Despite similarities in island size and physical environment, the most conservative estimates of population density at Amchitka Island were > 3 X greater than maximum density estimates for Attu Island. (6) Surveys of Amchitka Island from the mid-1930s through the mid-1980s indicate that the population increased to a peak in the 1940s; declined abruptly thereafter; and subsequently increased to a new and higher equilibrium in the 1960s, where it has since remained. (7) These population data, together with information on sea otter foraging and benthic community structure at Attu and Amchitka islands, suggest that multiple population equilibria exist in this system, emanating from complex trophic interactions low in the food web. I hypothesize that the lower population equilibrium is achieved largely or exclusively on an invertebrate diet consisting principally of herbivorous sea urchins. When unregulated by sea otter predation, the rocky benthos is deforested by sea urchin grazing. As growing otter populations compete increasingly for food, grazing intensity declines and the system shifts to one dominated by kelp beds, in turn leading to increased production, a shift in habitat structure, and population increases of kelp bed fishes. Apparently this new food resource elevates the sea otter population to a higher and more stable equilibrium.
The sea otter (Enhydra lutris): behavior, ecology, and natural history
No abstract available at this time
Magnification of secondary production by kelp detritus in coastal marine ecosystems
Kelps are highly productive seaweeds found along most temperate latitude coastlines, but the fate and importance of kelp production to nearshore ecosystems are largely unknown. The trophic role of kelp-derived carbon in a wide range of marine organisms was assessed by a natural experiment. Growth rates of benthic suspension feeders were greatly increased in the presence of organic detritus (particulate and dissolved) originating from large benthic seaweeds (kelps). Stable carbon isotope analysis confirmed that kelp-derived carbon is found throughout the nearshore food web.
Adaptations for aquatic living in carnivores. Carnivore behavior ecology and evolution
No abstract available at this time
A review of the history, distribution, and foraging ecology of sea otters
Explore the source record for details and available documents.