ReviewCONSERVATION ECOLOGY

Ecosystem context and historical contingency in apex predator recoveries

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Science Advances  27 May 2016:
Vol. 2, no. 5, e1501769
DOI: 10.1126/sciadv.1501769
  • Fig. 1 Three-species community modules: Food chain, exploitative competition, and IGP.

    These modules are generic descriptions of common configurations of predator-prey interactions in the natural world (left), each of which corresponds to a predator recovery example (center) that has followed a restoration trajectory corresponding to the module (right).

  • Fig. 2 Module shape alters how an apex predator’s abundance will respond to the restoration of basal resources, as indicated by the contrast of low (solid lines) and high (dashed lines) resource carrying capacities (red lines, apex predator resource-only state; blue lines, three-species coexistence state).

    When ω has intermediate values, increases in resource productivity benefit the apex predator’s abundance to the detriment of the mesopredator because the mesopredator’s competetive advantage becomes superseded by the predation pressure that it experiences from the apex predator. (A and B) A continuous gradient of predator’s prey preference (ω) (A) and discrete measures of apex predator equilibrium density for characteristic models, including exploitative competition, IGP, and a food chain (B). For additional model details, see fig. S1 (baseline parameters here are as follows: ω = 0.5, r = 1, e = 0.1, a = 1, and α = 3).

  • Fig. 3 Module shape alters how apex predator density will respond to the culling of mesopredators, as indicated by the contrast of low (solid lines) and high (dashed lines) mesopredator mortality rates.

    Culling will increase apex predator recovery success when competition is strong. In most cases, culling rates must be sufficiently high such that only the apex predator and the resource persist (red). In contrast, culling will negatively affect the apex predator’s density across most of the range of apex predator prey preference values (ω), when three-species coexistence is desired (blue). Culling of mesopredators only benefits the apex predator when competition is strong but sufficiently weak so as not to cause competitive exclusion (inset). (A) A gradient of predator’s prey preference. (B) Discrete measures of apex predator equilibrium density for discrete models: exploitative competition, IGP, and food chain. NA, not applicable.

  • Fig. 4 Priority effects occur when final equilibrium population sizes are dependent on initial population sizes, even though all other parameter values (that is, environmental conditions) remain unchanged.

    Such priority effects occur in the IGP module when competition between the apex predators and the mesopredators is strongest, illustrated here with two simulations that differ only in the initial abundance of the apex predator. (A) The dynamics illustrate the scenario where the apex predator’s initial population size (green, P0 > 0.1) is sufficient to affect the extinction of the mesopredator (blue, N0 = 0.01). (B) In contrast, the dynamics illustrate a scenario where the apex predator’s initial population size (P0 < 0.1) is insufficient to avoid extinction due to exclusion by the mesopredator (N0 = 0.01) (that is, a failed restoration). Parameters are as in Figs. 2 and 3 but with ω = 0.225 reflecting an IGP module in which exploitative competition is strong.

  • Fig. 5 Time-varying modules of riparian corridors along small streams within the northern range of the Greater Yellowstone Ecosystem from the 1920s to present.

    Northern-range riparian areas have exhibited (at least) three different major types of communities since 1920. Before wolf extinctions (1920s), riparian areas included wolves, elk, beavers, and willows. Following wolf extinctions (1930s to 1990s), these areas were reduced to just elk and willow. Most recently (1990 to present), wolf reintroductions have produced a system with wolves, elk, and willow but few beavers. Qualitatively, these different modules exhibit fundamentally different dynamics, exemplify temporal variability in a single system’s characteristic module, and meet different ecological and social services. [Illustration by Shannon Hennessey, Oregon State University].

  • Table 1 Empirical studies of successful predator recoveries.

    T, terrestrial; M, marine; F, freshwater; IUCN, International Union for Conservation of Nature; MMPA, Marine Mammal Protection Act.

    StudyRegionSystemPredatorSummary of research results
    (84)AfricaTCheetahCheetahs survive with larger predators by seeking areas with low predator densities (spatial segregation from predators
    and competitors).
    (84)AfricaTCheetahSuccessful reintroduction in Namibia, where larger carnivores were nearly extirpated by hunting [see also the work by Polis and
    Holt (84)].
    (85)North AmericaTWolfWolf-driven declines in coyotes led to a fourfold increase in survival of juvenile pronghorn antelope (Antilocapra americana)
    in wolf restoration areas in the Greater Yellowstone Ecosystem.
    (86)EuropeTLynxIn Europe, restored lynx and wolf populations suppress red foxes.
    (86)EuropeTWolfIn Europe, restored lynx and wolf populations suppress red foxes.
    (87)New ZealandTCook’s petrelReductions in predatory feral cats and rats and altitude-dependent resource availability promote petrel recovery.
    (88)North America
    (Colorado)
    TMountain
    lion
    Reduced exploitative and interference competition between mountain lions and other historically abundant predators
    (grizzly bears and wolves), combined with increased ungulate prey abundance, has facilitated mountain lion recovery.
    (89)T/MPolar bearAppreciation of social-ecological system allowed for subsistence harvest and reduced illegal hunting in shared population
    between the United States and Russia.
    (90)North AmericaFBassFollowing natural extirpation of bass (Micropterus salmoides) in 1978, reintroduced bass in 1986 led to the return of bass
    populations despite exploitative competition with dominant mesopredators.
    (91)AustraliaTDingoRestoration of dingoes in parts of Australia is now being advocated as a necessary condition for the large-scale rees
    tablishment of declined mammal species (91).
    (92)North AmericaTPeregrine
    falcon
    Populations declined globally because of exposure to contaminants and are listed in the United States in 1970 after being
    extirpated east of the Rockies. Declines of DDT and captive breeding led to rebound, and delisting in 1999.
    (93)EuropeTBrown bearPopulations hunted to near extinction in the 1800s in much of Europe, including Norway and Sweden. Economic in
    centives and conservation plans have led to a rebound in recent years.
    (94)GlobalT/MSea eagleBald and white-tailed eagles were either directly removed or negatively affected by pesticides until the latter half of the
    20th century. Since then, populations have recovered worldwide, to the point that these apex predators are having some
    worrying effects further down the food chain.
    (95)AsiaTAsiatic lionFollowing collapse, incentivized pastoral communities to move, which allowed forest/prey populations to recover and
    lion populations to rebound
    (96)North AmericaFAlligatorAlligators were depleted as a result of habitat loss and hunting; following protection in 1967, alligators increased nearly
    exponentially and were delisted in 1987.
    (97)BelizeFMorelet’s
    crocodile
    Like alligators in North America, these crocodiles were affected by habitat destruction and hunting. Populations have
    been increasing since IUCN recognition and legal protection in 1981.
    (98)AustraliaMSaltwater
    crocodile
    Intense commercial hunting in the mid-20th century led to a population collapse, from near 100,000 to 500. Legal protection in
    1971 and conservation actions have helped the population largely to recover, increasing interactions (mortality) with humans.
    (99)AfricaMFur sealLike many pinniped populations (30), fur seals were commercially hunted through part of the 20th century, and portions
    of this population continue to be harvested; following protection in part of the range, the species has shown increasing
    trends and range expansion.
    (100)Northeast Pacific
    Ocean
    MWhite sharkWhite sharks were either removed as pest species or taken incidentally in fisheries through most of the 20th century and,
    in a portion of their range, were negatively affected by contaminants. Recognition of declines and their importance led to
    reductions in mortality; over the last 20 years, indices of abundance and juvenile growth are increasing.
    (101)Northeast PacificMBlue whaleBlue whales were targeted during industrial whaling, leaving them at a fraction of carrying capacity. Following the cessation of whaling, and additional protection, this population is thought to nearly be at historic levels.
    (102)Northeast PacificMSea otterSea otters were hunted to local extinction through much of their range but, following protection under the MMPA, has
    largely rebounded in California.
    (103)New ZealandMSpiny lobsterMarine reserves were used as a tool to protect spiny lobster habitat. Older reserves were found to yield higher
    lobster density, as well as larger lobsters.
    (104)North AmericaMGray whaleFollowing the end of whaling and protection under the MMPA, gray whales largely rebounded and were the first
    marine mammal species delisted from the ESA.
    (105)AfricaTEthiopian
    wolf
    Wolves have largely declined as a result of habitat loss. These populations experienced a catastrophic
    disease-induced collapse 20 years ago but have fully recovered and are no longer affected by Allee dynamics.
  • Table 2 Empirical studies of failed and stalled predator recoveries.
    StudyRegionSystemPredatorSummary research results
    (84)AfricaTCheetahIn reserves of Kenya and South Africa, cheetahs have failed to recover because lions and
    hyenas kill the cheetah cubs.
    (106)AntarcticaMFur sealLeopard seal predation on fur seal pups limited recovery.
    (107)Southwest AlaskaMSea otterPredation by killer whales on sea otters depressed population recovery.
    (108)North AmericaTBobcatSuggests that bobcats compete with coyotes for resources and are also killed by them.
    Need to verify that bobcat recovery is desired and that population size trajectory is not as
    positive as desired.
    (109)Eastern Scotian Shelf
    (Northwest Atlantic)
    MCodGray seal predation, not competition and predation related to forage fish abundance, may
    contribute to high mortality rates of older cod, preventing or slowing recovery.
    (110)North America (California)TMountain lionEspecially in southwestern California, urban development has fragmented mountain lion
    habitat, reducing genetic exchange and prey base available to facilitate population growth.
    (111)North America (Arizona
    and New Mexico)
    TGray wolfMexican wolf reintroductions have been minimally successful compared to other locations,
    in large part due to human hunting, purportedly in response to wolf predation on livestock.
    (112)North AmericaMCook Inlet belugaPopulation is small in number and geographically isolated and inhabits a core habitat
    that is shrinking
    (113)North AmericaMSouthern Resident
    killer whale
    Juveniles from this population were targeted for aquaria removals until the early 1970s.
    Because of its small population size, skewed age distribution, and various human threats
    (contaminants and lack of prey), it is not growing fast enough to meet recovery goals.
    (114)North America
    (Mexico)
    MVaquitaIUCN recognized this as the most endangered cetacean species in the world; like beluga,
    the population size is small (<250) and geographically isolated. Further, vaquita are also
    incidentally captured in gillnet fisheries and further affected by humans
    (115)North AmericaTNorthern spotted
    owl
    Stable in portions of the range, but continual declines in others, as a result of habitat loss and
    interspecific competition with barred owls
    (116)North AmericaT/FWood storkFederally listed wood storks consumed by nonnative introduced Burmese pythons
    (117)North AmericaFLake troutInvasive predator has had severe impacts on threatened Yellowstone cutthroat trout.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/5/e1501769/DC1

    fig. S1. Table of equilibrium solutions for three-species Lotka-Volterra model of IGP for the basal resource (R), the mesopredator (N), and the apex predator (P).

    fig. S2. Response of the equilibrium densities of the apex predator (P) and mesopredator (N) to increases in the resource’s carrying capacity (K) for stable equilibria in which all three species coexist (RNP), only the resource and mesopredator coexist (RN), and only the resource and the apex predator coexist (RP).

    fig. S3. Stable (solid line) and unstable (dashed line) equilibrium densities for the apex predator (P) and mesopredator (N) across a range of the apex predator’s prey preferences.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Table of equilibrium solutions for three-species Lotka-Volterra model of IGP for the basal resource (R), the mesopredator (N), and the apex predator (P).
    • fig. S2. Response of the equilibrium densities of the apex predator (P) and mesopredator (N) to increases in the resource’s carrying capacity (K) for stable equilibria in which all three species coexist (RNP), only the resource and mesopredator coexist (RN), and only the resource and the apex predator coexist (RP).
    • fig. S3. Stable (solid line) and unstable (dashed line) equilibrium densities for the apex predator (P) and mesopredator (N) across a range of the apex predator’s prey preferences.

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