Foraging behaviour and ecology of transient killer whales within a deep submarine canyon system.

Transient killer whales have been documented hunting marine mammals across a variety of habitats. However, relatively little has been reported about their predatory behaviours near deep submarine canyons and oceanic environments. We used a long-term database of sightings and encounters with these predators in and around the Monterey Submarine Canyon, California to describe foraging behaviour, diet, seasonal occurrence, and habitat use patterns. Transient killer whales belonging to the outer coast subpopulation were observed within the study area 261 times from 2006-2021. Occurrences, behaviours, and group sizes all varied seasonally, with more encounters occurring in the spring as grey whales migrated northward from their breeding and calving lagoons in Mexico (March-May). Groups of killer whales foraged exclusively in open water, with individuals within the groups following the contours of the submarine canyon as they searched for prey. Focal follows revealed that killer whales spent 51% of their time searching for prey (26% of their time along the shelf-break and upper slope of the canyon, and 25% in open water). The remainder of their time was spent pursuing prey (10%), feeding (23%), travelling (9%), socializing (6%), and resting (1%). Prey species during 87 observed predation events included California sea lions, grey whale calves, northern elephant seals, minke whales, common dolphins, Pacific white-sided dolphins, Dall's porpoise, harbour porpoise, harbour seals, and sea birds. The calculated kill rates (based on 270 hours of observing 50 predation events) were 0.26 California sea lions per killer whale over 24 hours, 0.11 grey whale calves, and 0.15 for all remaining prey species combined. These behavioural observations provide insights into predator-prey interactions among apex predators over submarine canyons and deep pelagic environments.

Oust the louse: leaping behaviour removes sea lice from wild juvenile sockeye salmon Oncorhynchus nerka.

We conducted a manipulative field experiment to determine whether the leaping behaviour of wild juvenile sockeye salmon Oncorhynchus nerka dislodges ectoparasitic sea lice Caligus clemensi and Lepeophtheirus salmonis by comparing sea-lice abundances between O. nerka juveniles prevented from leaping and juveniles allowed to leap at a natural frequency. Juvenile O. nerka allowed to leap had consistently fewer sea lice after the experiment than fish that were prevented from leaping. Combined with past research, these results imply potential costs due to parasitism and indicate that the leaping behaviour of juvenile O. nerka does, in fact, dislodge sea lice.

Fear of large carnivores causes a trophic cascade.

The fear large carnivores inspire, independent of their direct killing of prey, may itself cause cascading effects down food webs potentially critical for conserving ecosystem function, particularly by affecting large herbivores and mesocarnivores. However, the evidence of this has been repeatedly challenged because it remains experimentally untested. Here we show that experimentally manipulating fear itself in free-living mesocarnivore (raccoon) populations using month-long playbacks of large carnivore vocalizations caused just such cascading effects, reducing mesocarnivore foraging to the benefit of the mesocarnivore's prey, which in turn affected a competitor and prey of the mesocarnivore's prey. We further report that by experimentally restoring the fear of large carnivores in our study system, where most large carnivores have been extirpated, we succeeded in reversing this mesocarnivore's impacts. We suggest that our results reinforce the need to conserve large carnivores given the significant "ecosystem service" the fear of them provides.

Human activities change marine ecosystems by altering predation risk.

In ocean ecosystems, many of the changes in predation risk - both increases and decreases - are human-induced. These changes are occurring at scales ranging from global to local and across variable temporal scales. Indirect, risk-based effects of human activity are known to be important in structuring some terrestrial ecosystems, but these impacts have largely been neglected in oceans. Here, we synthesize existing literature and data to explore multiple lines of evidence that collectively suggest diverse human activities are changing marine ecosystems, including carbon storage capacity, in myriad ways by altering predation risk. We provide novel, compelling evidence that at least one key human activity, overfishing, can lead to distinct, cascading risk effects in natural ecosystems whose magnitude exceeds that of presumed lethal effects and may account for previously unexplained findings. We further discuss the conservation implications of human-caused indirect risk effects. Finally, we provide a predictive framework for when human alterations of risk in oceans should lead to cascading effects and outline a prospectus for future research. Given the speed and extent with which human activities are altering marine risk landscapes, it is crucial that conservation and management policy considers the indirect effects of these activities in order to increase the likelihood of success and avoid unfortunate surprises.

Mammalian mesopredators on islands directly impact both terrestrial and marine communities.

Medium-sized mammalian predators (i.e. mesopredators) on islands are known to have devastating effects on the abundance and diversity of terrestrial vertebrates. Mesopredators are often highly omnivorous, and on islands, may have access not only to terrestrial prey, but to marine prey as well, though impacts of mammalian mesopredators on marine communities have rarely been considered. Large apex predators are likely to be extirpated or absent on islands, implying a lack of top-down control of mesopredators that, in combination with high food availability from terrestrial and marine sources, likely exacerbates their impacts on island prey. We exploited a natural experiment--the presence or absence of raccoons (Procyon lotor) on islands in the Gulf Islands, British Columbia, Canada--to investigate the impacts that this key mesopredator has on both terrestrial and marine prey in an island system from which all native apex predators have been extirpated. Long-term monitoring of song sparrow (Melospiza melodia) nests showed raccoons to be the predominant nest predator in the Gulf Islands. To identify their community-level impacts, we surveyed the distribution of raccoons across 44 Gulf Islands, and then compared terrestrial and marine prey abundances on six raccoon-present and six raccoon-absent islands. Our results demonstrate significant negative effects of raccoons on terrestrial, intertidal, and shallow subtidal prey abundance, and point to additional community-level effects through indirect interactions. Our findings show that mammalian mesopredators not only affect terrestrial prey, but that, on islands, their direct impacts extend to the surrounding marine community.

Patterns of top-down control in a seagrass ecosystem: could a roving apex predator induce a behaviour-mediated trophic cascade?

1. The loss of large-bodied herbivores and/or top predators has been associated with large-scale changes in ecosystems around the world, but there remain important questions regarding the contexts in which such changes are most likely and the mechanisms through which they occur, particularly in marine ecosystems. 2. We used long-term exclusion cages to examine the effects of large grazers (sea cows, Dugong dugon; sea turtles Chelonia mydas) on seagrass community structure, biomass and nutrient dynamics. Experiments were conducted in habitats with high risk of predation (interior of shallow banks) and lower risk (edges of banks) to elucidate whether nonconsumptive (risk) effects of tiger sharks (Galeocerdo cuvier), a roving predator, structure herbivore impacts on seagrasses. 3. In lower-risk habitats, excluding large herbivores resulted in increased leaf length for Cymodocea angustata and Halodule uninervis. C. angustata shoot densities nearly tripled when released from herbivory, while H. uninervis nearly disappeared from exclusion cages over the course of the study. 4. We found no support for the hypothesis that grazing increases seagrass nutrient content. Instead, phosphorus content was higher in seagrasses within exclosures. This pattern is consistent with decreased light availability in the denser C. angustata canopies that formed in exclosures, and may indicate that competition for light led to the decrease in H. uninervis. 5. Impacts of large grazers were consistent with a behaviour-mediated trophic cascade (BMTC) initiated by tiger sharks and mediated by risk-sensitive foraging by large grazers. 6, Our results suggest that large-bodied grazers likely played important roles in seagrass ecosystem dynamics historically and that roving predators are capable of initiating a BMTC. Conservation efforts in coastal ecosystems must account for such interactions or risk unintended consequences.

Effects of parasites from salmon farms on productivity of wild salmon.

The ecological risks of salmon aquaculture have motivated changes to management and policy designed to protect wild salmon populations and habitats in several countries. In Canada, much attention has focused on outbreaks of parasitic copepods, sea lice (Lepeophtheirus salmonis), on farmed and wild salmon in the Broughton Archipelago, British Columbia. Several recent studies have reached contradictory conclusions on whether the spread of lice from salmon farms affects the productivity of sympatric wild salmon populations. We analyzed recently available sea lice data on farms and spawner-recruit data for pink (Oncorhynchus gorbuscha) and coho (Oncorhynchus kisutch) salmon populations in the Broughton Archipelago and nearby regions where farms are not present. Our results show that sea lice abundance on farms is negatively associated with productivity of both pink and coho salmon in the Broughton Archipelago. These results reconcile the contradictory findings of previous studies and suggest that management and policy measures designed to protect wild salmon from sea lice should yield conservation and fishery benefits.

Fish farms, parasites, and predators: implications for salmon population dynamics.

For some salmon populations, the individual and population effects of sea lice (Lepeophtheirus salmonis) transmission from sea cage salmon farms is probably mediated by predation, which is a primary natural source of mortality of juvenile salmon. We examined how sea lice infestation affects predation risk and mortality of juvenile pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon, and developed a mathematical model to assess the implications for population dynamics and conservation. A risk-taking experiment indicated that infected juvenile pink salmon accept a higher predation risk in order to obtain foraging opportunities. In a schooling experiment with juvenile chum salmon, infected individuals had increased nearest-neighbor distances and occupied peripheral positions in the school. Prey selection experiments with cutthroat trout (O. clarkii) predators indicated that infection reduces the ability of juvenile pink salmon to evade a predatory strike. Group predation experiments with coho salmon (O. kisutch) feeding on juvenile pink or chum salmon indicated that predators selectively consume infected prey. The experimental results indicate that lice may increase the rate of prey capture but not the handling time of a predator. Based on this result, we developed a mathematical model of sea lice and salmon population dynamics in which parasitism affects the attack rate in a type II functional response. Analysis of the model indicates that: (1) the estimated mortality of wild juvenile salmon due to sea lice infestation is probably higher than previously thought; (2) predation can cause a simultaneous decline in sea louse abundance on wild fish and salmon productivity that could mislead managers and regulators; and (3) compensatory mortality occurs in the saturation region of the type II functional response where prey are abundant because predators increase mortality of parasites but not overall predation rates. These findings indicate that predation is an important component of salmon-louse dynamics and has implications for estimating mortality, reducing infection, and developing conservation policy.

Applying behavioral-ecological theory to plant defense: light-dependent movement in Mimosa pudica suggests a trade-off between predation risk and energetic reward.

Many animal species tolerate different amounts of predation risk based on environmental conditions and the individual's own condition, often accepting greater risk when energetically stressed. We studied the sensitive plant Mimosa pudica to see whether it too accepts greater risk of predation when less light energy is available. This plant displays a defensive behavior of rapidly folding its leaves when stimulated by touch, thereby decreasing visibility to herbivores. Averting herbivory involves a trade-off because leaf closure results in a reduction in light foraging. We manipulated the light environment of individual M. pudica plants and recorded the time it took a plant to reopen its leaves following stimulation as a measure of tolerance of predation risk. As predicted by theory, avoidance behavior was sustained longer under high light conditions than under more light-limited conditions. These findings suggest this species balances the risk and reward of antiherbivore behavior in relation to current environmental conditions and that behavioral-ecological theory is a useful framework for understanding plant responses to predators.

Towards a predictive framework for predator risk effects: the interaction of landscape features and prey escape tactics.

1. Risk effects of predators can profoundly affect community dynamics, but the nature of these effects is context dependent. 2. Although context dependence has hindered the development of a general framework for predicting the nature and extent of risk effects, recent studies suggest that such a framework is attainable if the factors that shape anti-predator behaviour, and its effectiveness, in natural communities are well understood. 3. One of these factors, the interaction of prey escape tactics and landscape features, has been largely overlooked. 4. We tested whether this interaction gives rise to interspecific variation in habitat-use patterns of sympatric large marine vertebrates at risk of tiger shark (Galeocerdo cuvier Peron and LeSueur, 1822) predation. Specifically, we tested the a priori hypothesis that pied cormorants (Phalacrocorax varius Gmelin, 1789) would modify their use of shallow seagrass habitats in a manner opposite to that of previously studied dolphins (Tursiops aduncus Ehrenberg, 1833), dugongs (Dugong dugon Müller, 1776), and green turtles (Chelonia mydas Linnaeus, 1758) because, unlike these species, the effectiveness of cormorant escape behaviour does not vary spatially. 5. As predicted, cormorants used interior and edge portions of banks proportional to the abundance of their potential prey when sharks were absent but shifted to interior portions of banks to minimize encounters with tiger sharks as predation risk increased. Other shark prey, however, shift to edge microhabitats when shark densities increase to take advantage of easier escape despite higher encounter rates with sharks. 6. The interaction of landscape features and escape ability likely is important in diverse communities. 7. When escape probabilities are high in habitats with high predator density, risk effects of predators can reverse the direction of commonly assumed indirect effects of top predators. 8. The interaction between landscape features and prey escape tactics can result in a single predator species having differential effects on their sympatric prey that could cascade through ecosystems and should be incorporated into a general framework for context dependence of risk effects.

Revisiting the classics: considering nonconsumptive effects in textbook examples of predator-prey interactions.

Predator effects on prey dynamics are conventionally studied by measuring changes in prey abundance attributed to consumption by predators. We revisit four classic examples of predator-prey systems often cited in textbooks and incorporate subsequent studies of nonconsumptive effects of predators (NCE), defined as changes in prey traits (e.g., behavior, growth, development) measured on an ecological time scale. Our review revealed that NCE were integral to explaining lynx-hare population dynamics in boreal forests, cascading effects of top predators in Wisconsin lakes, and cascading effects of killer whales and sea otters on kelp forests in nearshore marine habitats. The relative roles of consumption and NCE of wolves on moose and consequent indirect effects on plant communities of Isle Royale depended on climate oscillations. Nonconsumptive effects have not been explicitly tested to explain the link between planktonic alewives and the size structure of the zooplankton, nor have they been invoked to attribute keystone predator status in intertidal communities or elsewhere. We argue that both consumption and intimidation contribute to the total effects of keystone predators, and that characteristics of keystone consumers may differ from those of predators having predominantly NCE. Nonconsumptive effects are often considered as an afterthought to explain observations inconsistent with consumption-based theory. Consequently, NCE with the same sign as consumptive effects may be overlooked, even though they can affect the magnitude, rate, or scale of a prey response to predation and can have important management or conservation implications. Nonconsumptive effects may underlie other classic paradigms in ecology, such as delayed density dependence and predator-mediated prey coexistence. Revisiting classic studies enriches our understanding of predator-prey dynamics and provides compelling rationale for ramping up efforts to consider how NCE affect traditional predator-prey models based on consumption, and to compare the relative magnitude of consumptive and NCE of predators.

Environment-contingent sexual selection in a colour polymorphic fish.

Sexual selection could be a driving force in the maintenance of intraspecific variation, but supporting observations from nature are limited. Here, we test the hypothesis that spatial heterogeneity of the visual environment can influence sexual selection on colourful male secondary traits such that selective advantage is environment contingent. Using a small fish endemic to Sulawesi, Indonesia (Telmatherina sarasinorum) that has five male colour morphs varying in frequency between two visually distinct mating habitats, we used direct behavioural observations to test the environment-contingent selection hypothesis. These observations were combined with measurements of the visual environment, fish coloration and the sensitivity of visual photopigments to determine whether differential morph conspicuousness was associated with reproductive success across habitats. We found that blue and yellow males are most conspicuous in different habitats, where they also have the highest reproductive fitness. A less conspicuous grey morph also gained high reproductive success in both habitats, raising the possibility that alternative behaviours may also contribute to reproductive success. In a comprehensive analysis, conspicuousness was strongly correlated with reproductive success across morphs and environments. Our results suggest an important role for spatially heterogeneous environments in the maintenance of male colour polymorphism.

State-dependent risk-taking by green sea turtles mediates top-down effects of tiger shark intimidation in a marine ecosystem.

1. A predictive framework of community and ecosystem dynamics that applies across systems has remained elusive, in part because non-consumptive predator effects are often ignored. Further, it is unclear how much individual-level detail community models must include. 2. Previous studies of short-lived species suggest that state-dependent decisions add little to our understanding of community dynamics. Body condition-dependent decisions made by long-lived herbivores under risk of predation, however, might have greater community-level effects. This possibility remains largely unexplored, especially in marine environments. 3. In the relatively pristine seagrass community of Shark Bay, Australia, we found that herbivorous green sea turtles (Chelonia mydas Linnaeus, 1758) threatened by tiger sharks (Galeocerdo cuvier Peron and LeSueur, 1822) select microhabitats in a condition-dependent manner. Turtles in poor body condition selected profitable, high-risk microhabitats, while turtles in good body condition, which are more abundant, selected safer, less profitable microhabitats. When predation risk was low, however, turtles in good condition moved into more profitable microhabitats. 4. Condition-dependent use of space by turtles shows that tiger sharks modify the spatio-temporal pattern of turtle grazing and their impacts on ecosystem dynamics (a trait-mediated indirect interaction). Therefore, state-dependent decisions by individuals can have important implications for community dynamics in some situations. 5. Our study suggests that declines in large-bodied sharks may affect ecosystems more substantially than assumed when non-lethal effects of these top predators on mesoconsumers are not considered explicitly.

Fear factor: do dugongs (Dugong dugon) trade food for safety from tiger sharks (Galeocerdo cuvier)?

Predators can influence plants indirectly by altering spatial patterns of herbivory, so studies assessing the relationship between perceived predation risk and habitat use by herbivores may improve our understanding of community organization. In marine systems, the effects of predation danger on space use by large herbivores have received little attention, despite the possibility that predator-mediated alterations in patterns of grazing by these animals influence benthic community structure. We evaluated the relationship between habitat use by foraging dugongs (Dugong dugon) and the threat of tiger shark predation in an Australian embayment (Shark Bay) between 1997 and 2004. Dugong densities were quantified in shallow (putatively dangerous) and deep (putatively safe) habitats (seven survey zones allocated to each habitat), and predation hazard was indexed using catch rates of tiger sharks (Galeocerdo cuvier); seagrass volume provided a measure of food biomass within each zone. Overall, dugongs selected shallow habitats, where their food is concentrated. Foragers used shallow and deep habitats in proportion to food availability (input matching) when large tiger sharks were scarce and overused deep habitats when sharks were common. Furthermore, strong synchrony existed between daily measures of shark abundance and the extent to which deep habitats were overused. Thus, dugongs appear to adaptively manage their risk of death by allocating time to safe but impoverished foraging patches in proportion to the likelihood of encountering predators in profitable but more dangerous areas. This apparent food-safety trade-off has important implications for seagrass community structure in Shark Bay, as it may result in marked temporal variability in grazing pressure.

Can measures of prey availability improve our ability to predict the abundance of large marine predators?

Apex marine predators can structure marine communities, so factors underlying their abundance are of broad interest. However, such data are almost completely lacking for large sharks. We assessed the relationship between tiger shark abundance, water temperature, and the availability of a variety of known prey over 5 years in Western Australia. Abundance of sharks in four size categories and the density of prey (cormorants, dugongs, sea snakes, sea turtles) were indexed using daily catch rates and transects, respectively. Across all sizes, thermal conditions were a determinant of abundance, with numerical peaks coinciding with periods of high water temperature. However, for sharks exceeding 300 cm total length, the inclusion of dugong density significantly improved temperature-based models, suggesting that use of particular areas by large tiger sharks is influenced by availability of this sirenian. We conclude that large marine predator population models may benefit from the inclusion of measures of prey availability, but only if such measures consider prey types separately and account for ontogenetic shifts in the diet of the predator in question.

Cuckoldry incites cannibalism: male fish turn to cannibalism when perceived certainty of paternity decreases.

Perceived certainty of paternity is expected to influence a male's behavior toward his offspring: if he is uncertain of his reproductive success with a current brood due to the presence of cuckolders, it may benefit him to invest instead in future reproduction. A decrease in perceived certainty of paternity incites filial cannibalism (the eating of one's own offspring) in some teleost fishes that provide parental care; however, no work has demonstrated that cannibalism increases proportionately with increased levels of cuckoldry. Here we show for the first time in a fish with no parental care that as the number of cuckolders at a spawning event increases, so does the probability that a male will cannibalize eggs. In field observations of Telmatherina sarasinorum, a small fish endemic to Sulawesi, Indonesia, males increased filial cannibalism behavior threefold in the presence of one cuckolder and nearly sixfold in the presence of two or more cuckolders. This suggests that males may use detection of cuckolders as an indication that the paternity of current offspring has been compromised.

Validation of a randomization procedure to assess animal habitat preferences: microhabitat use of tiger sharks in a seagrass ecosystem.

1. Tiger sharks Galeocerdo cuvier are important predators in a variety of nearshore communities, including the seagrass ecosystem of Shark Bay, Western Australia. Because tiger sharks are known to influence spatial distributions of multiple prey species, it is important to understand how they use habitats at a variety of spatial scales. We used a combination of catch rates and acoustic tracking to determine tiger shark microhabitat use in Shark Bay. 2. Comparing habitat-use data from tracking against the null hypothesis of no habitat preference is hindered in Shark Bay, as elsewhere, by the difficulty of defining expected habitat use given random movement. We used randomization procedures to generate expected habitat use in the absence of habitat preference and expected habitat use differences among groups (e.g. males and females). We tested the performance of these protocols using simulated data sets with known habitat preferences. 3. The technique correctly classified sets of simulated tracks as displaying a preference or not and was a conservative test for differences in habitat preferences between subgroups of tracks (e.g. males vs. females). 4. Sharks preferred shallow habitats over deep ones, and preferred shallow edge microhabitats over shallow interior ones. The use of shallow edges likely increases encounter rates with potential prey and may have profound consequences for the dynamics of Shark Bay's seagrass ecosystem through indirect effects transmitted by grazers that are common prey of tiger sharks. 5. Females showed a greater tendency to use shallow edge microhabitats than did males; this pattern was not detected by traditional analysis techniques. 6. The randomization procedures presented here are applicable to many field studies that use tracking by allowing researchers both to determine overall habitat preferences and to identify differences in habitat use between groups within their sample.

Pacific and Atlantic herring produce burst pulse sounds.

The commercial importance of Pacific and Atlantic herring (Clupea pallasii and Clupea harengus) has ensured that much of their biology has received attention. However, their sound production remains poorly studied. We describe the sounds made by captive wild-caught herring. Pacific herring produce distinctive bursts of pulses, termed Fast Repetitive Tick (FRT) sounds. These trains of broadband pulses (1.7-22 kHz) lasted between 0.6 s and 7.6 s. Most were produced at night; feeding regime did not affect their frequency, and fish produced FRT sounds without direct access to the air. Digestive gas or gulped air transfer to the swim bladder, therefore, do not appear to be responsible for FRT sound generation. Atlantic herring also produce FRT sounds, and video analysis showed an association with bubble expulsion from the anal duct region (i.e. from the gut or swim bladder). To the best of the authors' knowledge, sound production by such means has not previously been described. The function(s) of these sounds are unknown, but as the per capita rates of sound production by fish at higher densities were greater, social mediation appears likely. These sounds may have consequences for our understanding of herring behaviour and the effects of noise pollution.

Three-player social parasitism games: implications for resource defense and group formation.

Abstract: Individuals that produce resources are often exploited by several individuals; such exploitation may or may not be tolerated. We modeled the decision of a resource owner to accept one scrounger (the "satellite") and of both of these to accept being joined by another (the "floater"). In general, satellites tolerated floaters when competition between them was low, while owners tolerated satellites when facilitation between satellite and floater was high. When floaters were likely to find resources without joining, owners were more likely to resist satellites. In some cases, Nash equilibria were also mutually beneficial for two of the three individuals. Our model makes the counterintuitive prediction that mutually beneficial coalitions between satellites and floaters can only arise when the net benefits arising from the other's presence are low. When facilitation between satellites and floaters is high, satellites and owners may form mutually beneficial groups and groups with division of labor, or alternatively, owners may benefit from encouraging floaters to join. Finally, our model suggests there must be differences in competitive ability or some benefit of familiarity for owners to tolerate satellites but not floaters. We discuss empirical evidence for these and other predictions of the model.

Possible indirect interactions between transient and resident killer whales: implications for the evolution of foraging specializations in the genus Orcinus.

Two distinct forms of killer whale (Orcinus orca) occur off the coast of British Columbia, Alaska and Washington State. These have different diets, and may be reproductively isolated. Because the primary food of transient whales (pinnipeds) is a potential competitor for the primary food of resident whales (salmon), or for the smaller fishes on which salmon feed, there should be an indirect interaction between the two forms of killer whale. We use simple mathematical models to show that this interaction will be either of a "plus-minus" type, or a "plus-plus" type (indirect mutualism), depending on whether or not pinnipeds and residents are on the same trophic level. In the case of the "plus-minus" interaction, increasing the population density or improving the environmental conditions of transients will increase the population density of residents, while increasing resident populations will reduce the equilibrium population size of transients. In the case of the "plus-plus" interaction, increasing the population density or improving the environmental conditions of transients will increase the population density of residents, while increasing resident populations will reduce the equilibrium population size of transients. In the case of the "plus-plus" interaction, increasing the population density or improving the environmental conditions of transients will increase the population density of residents, and vice versa. Such effects may not be currently manifest due to reduced populations at most levels in the food web. Regardless, considering such indirect interactions may be important for the management of many of the species involved, and can also provide a valuable framework for examining the evolution of the two forms of killer whales. Frequency-dependent indirect interactions, acting in concert with density-dependence within populations and disruptive selection on prey-type specific foraging characteristics, may have favoured reproductive isolation of the two forms of killer whales. We suggest that these two forms of whale are in the process of speciating, i.e., the two forms are incipient species.