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Trophic Downgrading of Planet Earth

Authors:
James A Estes
James A Estes
James A Estes
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John Terborgh at Duke University
John Terborgh
Justin S. Brashares at University of California, Berkeley
Justin S. Brashares
Mary Power at University of California, Berkeley
Mary Power
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Abstract and Figures

Until recently, large apex consumers were ubiquitous across the globe and had been for millions of years. The loss of these animals may be humankind’s most pervasive influence on nature. Although such losses are widely viewed as an ethical and aesthetic problem, recent research reveals extensive cascading effects of their disappearance in marine, terrestrial, and freshwater ecosystems worldwide. This empirical work supports long-standing theory about the role of top-down forcing in ecosystems but also highlights the unanticipated impacts of trophic cascades on processes as diverse as the dynamics of disease, wildfire, carbon sequestration, invasive species, and biogeochemical cycles. These findings emphasize the urgent need for interdisciplinary research to forecast the effects of trophic downgrading on process, function, and resilience in global ecosystems.
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www.sciencemag.org/cgi/content/full/333/6040/301/DC1
Supporting Online Material for
Trophic Downgrading of Planet Earth
James A. Estes,* John Terborgh, Justin S. Brashares, Mary E. Power, Joel Berger,
William J. Bond, Stephen R. Carpenter, Timothy E. Essington, Robert D. Holt, Jeremy B. C.
Jackson, Robert J. Marquis, Lauri Oksanen, Tarja Oksanen, Robert T. Paine, Ellen K.
Pikitch, William J. Ripple, Stuart A. Sandin, Marten Scheffer, Thomas W. Schoener,
Jonathan B. Shurin, Anthony R. E. Sinclair, Michael E. Soulé, Risto Virtanen, David A.
Wardle
*To whom correspondence should be addressed. E-mail: jestes@ucsc.edu
Published 15 July 2011, Science 333, 301 (2011)
DOI: 10.1126/science.1205106
This PDF file includes:
Table S1
References
1
SOM Table 1. Examples of regime shifts in terrestrial, freshwater, and marine ecosystems resulting from the loss or addition of
large-bodied vertebrates. Those based on linkages across 3 or more species are noted with an asterisk
Ecosystem Perturbation/Location Response Reference
Terrestrial
Arctic tundra Extinction of megaherbivores/Siberia Conversion of steppe to tundra (43)
*Arctic tundra Introduction of arctic fox/Aleutian Islands Conversion of grass-land to tundra (47, 74)
* Boreal forest Self-introduction of moose/Isle Royale Reduction of balsam fir (63)
* Temperate forest Extirpation of wolf/Yellowstone National
Park Overbrowsing of aspen, cottonwood, willow (75, 76)
*Temperate forest Restoration of wolf/ Yellowstone National
Park Recovery of riparian vegetation (25, 77)
2
Table 1 (continued)
*Temperate forest Extirpation of large predators/Europe,
Japan, and eastern United States Eruption of cervid populations, overbrowsing,
altered forest composition (78-80)
*Temperate forest Loss of cougar/Zion National Park Eruption of mule deer, loss of riparian
vegetation and associated biodiversity, altered
channel morphology
(61)
Temperate forest Introduction of red deer/New Zealand Overbrowsing, altered composition of forests (81)
*Tropical forest Loss of jaguar, cougar, Harpy
eagle/Venezuela Explosion of herbivores, suppression of tree
recruitment (82)
*Tropical forest Decimation of large birds and mammals by
hunting/neotropics Altered tree recruitment (83, 84)
*Tropical savanna Decimation of ungulates by Rinderpest
epidemic/Serengeti Increased extent and frequency of fires (30, 85)
Tropical savanna Recovery of white rhino/South Africa Appearance of grazing lawns, reduced
incidence of fire (86)
Subtropical bush Predator control of dingo/Australia Mesopredator release/ proliferation of exotic
mesopredators and herbivores/reduced
biodiversity
(87)
3
Table 1 (continued)
Freshwater
*Tropical river Exclosures and enclosures/Panama Fishing birds protect algae from grazing catfish (88, 89)
* Temperate stream Exclosures and enclosures/Oklahoma Piscivorous bass protect algae from grazing
minnows (69)
*Temperate stream Predator introduction/New Zealand Invasive trout protect algae from grazing
insects (90, 91)
*Boreal stream Predator introduction/Hokkaido Invasive trout protect algae from grazing
insects (92)
* Fresh water lake Remove piscivore trophic level Reduced water clarity, increase in
phytoplankton and primary production,
increased N:P ratio, increased response to
nutrient inputs, net flux of CO2 into the lake
(26, 93)
*Fresh water lake Introduction of non-native top piscivore Reorganization of fish community, species loss (94)
4
Table 1 (continued)
Marine
*Temperate subtidal Sea otter recovery Recovery of kelp forest (95)
*Temperate subtidal Overharvest of cod Urchin outbreak; collapse of kelp forests (96)
*Temperate estuarine Decimation of apex sharks Outbreak of cow-nosed rays, shellfish decline (19)
Tropical coastal Reduced grazing by green turtles Reduced recycling of turtle grass (97)
*Coral reefs
Overfishing Reduction in crustose coralline algae resulting
in reduced reef calcification, (98)
*Coral reefs Overfishing followed by disease Reef overgrowth by algae (99)
Continental shelf Overharvest of cod Shrimp increases (100)
*Open ocean (North Pacific) Pink salmon fluctuation zooplankton decline, chlorophyll increase (101)
*Open ocean (North Atlantic) Overharvest of cod Planktivorous fish, zooplankton, chlorophyll (102)
Table 1 (continued)
*Open ocean (North Atlantic) Overharvest of cod Sprat increase, zooplankton decline (103, 104)
*Open ocean (North Pacific) Overharvest of great whales Killer whale diet shift, pinniped population
declines (105)
*Open ocean (Southern Ocean) Overharvest of great whales Krill increase, adelie penguin diet shift (106)
*Open ocean (Black Sea) Overfishing Planktivorous fish, gelatanous plankton,
zooplankton, phytoplankton (107)
5
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... Similarly to other apex predators, the loss of raptors, i.e. trophic downgrading, affects a plethora of other species such as prey that are released from predation and overall alters ecosystem services (e.g. disease spreading, Estes et al. 2011). Raptors are thus commonly targeted by conservation translocations (Griffith et al. 1989;Fischer & Lindenmayer 2000;Seddon et al. 2005) The hen harrier (Circus cyaneus) ...
... In conservation science, increased awareness of the complexity of delivering effective conservation measures has led to an increased considerations of the human dimensions through means of social research (Bennett et al. 2017a). Nevertheless, after more than a century of conservation translocations and 30 years after the definition of reintroduction biology as a discipline largely based on project management (Seddon et al. 2007 (Estes et al. 2011). Therefore, conservation translocations have often targeted raptors (Griffith et al. 1989;Fischer & Lindenmayer 2000;Seddon et al. 2005) In this review, we explore the research literature on both conservation and nonconservation translocations and also reintroductions reported in the IUCN Global Reintroduction Perspectives (GRPs). ...
... Birds of prey (raptors) form a polyphyletic group of birds that includes species such as eagles, hawks, vultures and owls(McClure et al. 2019). These represent an optimal choice for several reasons.Raptors have fundamental roles in socio-ecological systems(Estes et al. 2011;Donázar et al. 2016). Specifically, these carry cultural values associated, for example, with religious, aesthetic, and recreational experiences as well as essential providers of ecosystem services(Donázar et al. 2016). ...
Ecological and Social Dimensions of Raptor Translocations
Thesis
  • Aug 2023
Conservation science is a relatively young crisis discipline facing a rapid and increasing global loss of biodiversity with anthropogenic causes. Despite its constant evolution, conservation science is still riddled with challenges and uncertainties related to understanding and measuring success as well as achieving effective multidisciplinarity. Conservation translocations, the human mediated movements of wildlife species for conservation benefits, embody conservation science paradigms and the challenge of delivering positive outputs. Specifically, a wide range of socio-ecological factors and difficulties shape the outcomes of conservation translocations and reintroduction biology - the recent sub-discipline born to study these measures - is still struggling in engaging effectively with decision-making and considering their socio-ecological dimensions in an integrated way. Here, we investigated conservation translocations in their ecological and social dimensions, seeking to build a comprehensive understanding of how diverse factors can synergically shape translocation outcomes and, hence, success. We focused on conservation translocations targeting birds of prey (hereafter raptors), given their role in socio-ecological systems, their often-endangered status and, therefore, their resulting inclusion in many conservation translocations. In particular, we studied the proposed reintroduction of hen harriers Circus cyaneus to southern England (hereafter the Southern reintroduction), a locally endangered species at the centre of a long-standing conservation conflict in the United Kingdom. We developed a multidimensional framework to study conservation translocations, where social and ecological dimensions are addressed at the individual and population levels. We used a multi-pronged approach based on the literature on raptor translocations as well as qualitative and quantitative data related to socio-political and ecological aspects of the Southern reintroduction. We found that success is yet to be fully conceptualised and investigated in a holistic way in the literature on raptor translocations. Rare definitions and assessments of success are biased towards ecological considerations even though the challenges most frequently reported by practitioners are socio-political. We advised on the need for a new evaluation framework, that considers the multiple dimensions of translocations while also acknowledging the subjective component of success definition and evaluation. Using Discourse Network Analysis, we identified characteristics and dynamics of the stakeholder debate on the conservation and management of hen harriers in the UK in national newspaper media. The debate is characterised by the presence of vocal stakeholders and has become more polarised over time. Stakeholder coalitions diverge, especially over a conservation measure (i.e. the brood management scheme), but also share common discursive ground in the form of emotional reactions associated with hen harriers and the acknowledged need for collaboration. Through means of participant observation and semi-structured interviews, we disclosed generally favourable perspectives of local stakeholders in Salisbury Plain on the Southern reintroduction. We found key aspects of the engagement practice such as types of engagement activities, timing, and team composition shape engagement and social processes. We showed how these aspects are linked to the positive transformative potential of the Southern reintroduction for the conservation conflict in the UK. We explored the movement behaviour of hen harriers from the reintroduction source population in France. Using Hidden Markov Models (HMMs) and Integrated Step Selection Analysis (ISSA) to analyse, we determined land cover preferences. These showed an overall preference for arable land with differences based on behavioural states (resident and dispersal) and sex. Arable land is also associated with hen harriers transitioning to a resident state. Finally, we developed a population viability modelling approach aimed at simulating socio-ecological realism. By using this novel individual-based model, we showed the potential viability and spatial distribution of the reintroduced population of hen harriers under increasingly informed and complex scenarios where female demographic traits and farming operations significantly affect population establishment. Our findings provide decision-makers, practitioners, and stakeholders involved in the Southern reintroduction with essential information that can be used to inform current and future stages of this conservation programme. Most importantly, we provided conservation practitioners and researchers with a novel multidimensional framework that can be used in conservation programmes, including but not limited to translocations, to investigate their socio-ecological dimensions at different scales. Overall, this work contributes to a better understanding of the roles of both ecological and social drivers in conservation translocations, concluding with the recommendation that redefining reintroduction biology to translocation science would result in a more representative acknowledgement of the broad range of factors influencing translocation success.
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... Además, son señalados como especies indicadoras de la calidad de su hábitat (Crooks, 2002;Benchimol, 2016;Botero-Cañola et al., 2018); su ausencia puede provocar un crecimiento descontrolado de las poblaciones de herbívoros, aumentando la presión de herbivoría sobre la vegetación. Esta sobreexplotación de vegetación altera el hábitat y reduce la disponibilidad de recursos para otras especies, lo que desencadena una serie de impactos a nivel del paisaje, incluyendo cambios en la estructura y uso del hábitat, y la disminución de la biodiversidad (Di Bitetti, 2008;Beschta & Ripple, 2009;Estes et al., 2011;Ripple & Beschta, 2012;Ripple et al., 2014;Wallach et al., 2015). ...
... Por otro lado, las presas directas de los depredadores tope tienden a incrementar sus poblaciones debido a la reducción en la presión de depredación (Pérez-Irineo & Santos-Moreno, 2015;Amaya, 2020;Burgos et al., 2023), ambos procesos afectan de manera significativa el equilibrio de los ecosistemas. Este fenómeno se ha observado en regiones de África, Australia, Europa Occidental, y Norte América, tras la desaparición de los leones (Panthera leo), leopardos (Panthera pardus), dingos (Canis dingo), lince euroasiático (Lynx lynx), lobo gris (Canis lupus), oso grizzly (Ursus arctos) y puma (Puma concolor) (Estes et al., 2011;Ripple et al., 2014). ...
... La falta de registros de jaguar y puma en nuestra área de estudio, a pesar de que han sido reportados en los últimos 6 años al norte de la carretera Las Varas-Puerto Vallarta (Illescas, 2019;Cortés, 2022), podría explicar los elevados valores de la abundancia relativa de sus presas principales (pecarí de collar, coatí y mapache), lo cual es consistente con estudios previos que sugieren que la falta de depredadores tope puede generar un aumento en las poblaciones de presas (Estes et al., 2011;Ripple et al., 2014). ...
Ausencia de depredadores tope: sin pistas de jaguares (Panthera onca) ni pumas (Puma concolor) en Punta de Mita, Nayarit, México
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Los depredadores tope tienen un papel fundamental para la conservación de los ecosistemas; su ausencia puede desencadenar cascadas tróficas en el ecosistema. En Punta Mita, municipio de Bahía de Banderas, Nayarit, México, la urbanización turística, ganadera y la infraestructura vial, representan una amenaza para la presencia del jaguar (Panthera onca) y el puma (Puma concolor), alterando sus patrones de diversidad y abundancia. Entre abril y junio de 2023, se colocaron 36 cámaras trampa siguiendo la metodología del Censo Nacional del Jaguar para determinar la presencia y abundancia relativa de jaguar y puma en un área rodeada por carreteras y sometida a intensa perturbación humana en Punta Mita. Con un esfuerzo de muestreo de 1,291 días-trampa, se obtuvieron 1,975 registros fotográficos independientes. No se obtuvo ni un solo registro ni de jaguar ni de puma. Se registraron un total de 11 especies de mamíferos silvestres medianos y grandes. Las especies más abundantes fueron el coatí (Nasua narica, n = 252, RAI = 12.75) y el venado cola blanca (Odocoileus virginianus, n = 85, RAI = 4.30). El coatí, el pecarí y el mapache fueron más abundantes que en otras investigaciones donde se registraron grandes depredadores. La ausencia de jaguares y pumas podría estar relacionada con actividades ganaderas y la fragmentación del hábitat por carreteras. Esto resalta la necesidad de estudios a largo plazo para comprender los efectos de la falta de grandes depredadores en ecosistemas fragmentados y el impacto de la actividad humana en la fauna local.
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... For instance, agriculture transforms complex, mature forests into pastures or grain fields that, eventually, may return to secondary forests which are structurally simpler than mature forests (Colorado Zuluaga & Rodewald, 2015). In aquatic or terrestrial habitats, top-predator extirpation may have cascading effects resulting in a loss of the large autotrophic organisms that structure space (Estes et al., 2011). In oceans, eutrophication, acidification, bottom trawling and dredging all tend to destroy the complex structure provided by benthic habitats such as coral reefs (National Research Council, 2002;Rogers et al., 2014). ...
Habitat structural complexity increases age-class coexistence and population growth rate through relaxed cannibalism in a freshwater fish
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  • May 2025
Structurally-complex habitats harbour more taxonomically-diverse and more productive communities, a phenomenon generally ascribed to habitat complexity relaxing the strength of inter-specific predation and competition. Here, we extend this classical, community-centred view by showing that positive complexity-diversity and complexity-productivity relationships may also emerge from between-age-class, intra-specific interactions at a single-population level. In the laboratory, we show that medaka fish ( Oryzias latipes ) are strongly cannibalistic in complexity-free habitats, and that cannibalism may occur over a wide range of victim/cannibal body size ratios. In replicated outdoor pond populations, we manipulated habitat structural complexity using floating artificial structures, which selectively hampered movements of large-bodied medaka. Habitat complexity relaxed the strength of cannibalism, resulting in (1) increased survival of age-0+ individuals, (2) elevated age-class diversity, (3) increased population growth rate, and (4) dampened negative density-dependence in the stock-recruitment relationship reflecting elevated habitat carrying capacity. The resultant higher population density in complex habitats was associated with increased competition for food among both age-0+ and age-1+ individuals. Our results highlight that positive complexity-diversity and complexity-productivity relationships may be considered as a generally-emergent property of size-structured populations and communities in which a larger body size brings a predation or interference advantage. Hence, enhancement of habitat structural complexity may be seen as a pivotal management strategy not only in favour of taxonomic diversity, but also to increase the productivity and resilience of exploited populations and to improve the conservation status of endangered species.
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... Previous camera trapping data in Bluegums indicate that feral cats and potoroos overlap spatially and temporally, and images have recorded potoroo predation by cats (Miritis et al. 2020). Top-down population regulation is well documented in other species and ecosystems (Estes et al. 2011;Ritchie and Johnson 2009) and, given that cats fill the apex predator role on French Island (Miritis et al. 2020), it is possible that our observations follow a similar pattern. ...
Habitat Selection and Movement Behaviour of Long-Nosed Potoroo in the Presence of Feral Cats
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  • May 2025
  • AUSTRAL ECOL
The global biodiversity extinction crisis is attributed to a series of key threats, with the introduction and impacts of invasive predators considered among the most damaging. Given that effective large-scale lethal control is often not logistically or financially feasible, alternative solutions to promote the persistence of native wildlife most at risk of predation-critical weight range mammals must be sought. Understanding habitat use and selection of native prey under predation pressure with widespread and common invasive predators, such as feral cats, can quantify habitat elements that may promote survival. We aimed to determine the movement behaviour of a population of critical weight range mammals persisting in the presence of feral cats. We established a trapping grid across 'Bluegums', French Island, in southeastern Australia to collect morphometric, demographic and movement data by deploying GPS tracking devices on Long-nosed Potoroo (Potorous tridactylus trisulcatus). We used spatially explicit capture-recapture models to generate a potoroo density estimate, and autocorrelated kernel density estimators and dynamic Brownian Bridge Movement Models to examine home range and habitat use. Spatial overlap and habitat selection analyses were used to draw inferences about habitat selection and territoriality. Potoroos persisted at low densities and had large home ranges (7.3-12.5 ha). Individuals selected for structurally complex habitat and appeared reluctant to move across open areas, potentially in response to greater perceived predation risk. The fine-scale movement data in our study elucidate the extent to which potoroos rely on vegetation structure, suggesting that maintaining habitat cover and connectivity is likely to build resilience and aid po-toroos and similar species to co-exist with feral cats. Given the ongoing threat feral cats pose to biodiversity, our results support the need for maintaining diverse, structurally complex vegetation to build ecosystem resilience to support improved conservation outcomes in the presence of invasive species.
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... Apex predators play an important role in maintaining biodiversity and ecosystem function (Ripple et al. 2014), and the depletion of their populations can have wide-ranging ecosystem consequences (Estes et al. 2011;Ripple et al. 2016;Van Cleave et al. 2018). Due to the high metabolic demands driven by endothermy and body size, large carnivores require access to abundant prey and expansive habitats (Cardillo et al. 2004;Ripple et al. 2016). ...
Unusual Suspects: Detection Probability Covaries With Vegetation Productivity and Rainfall for Camera Survey of African Leopards (Panthera pardus pardus)
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Maximizing detection probability of elusive species enhances the robustness of population and occupancy estimates, which are essential for supporting impactful conservation strategies. Although the number of camera trap studies on leopards is increasing, few have assessed the drivers influencing leopard detection specifically. We used occupancy modeling embedded in a causal inference framework to provide four biologically robust site use models against which to test the influence of six factors likely influencing leopard detectability at the level of encounter probability, trigger probability, and image quality. In this study, vegetation productivity moderated by rainfall was the top predictor of leopard detectability associated with three of the four site use models. While optimizing detection probability improves estimates of population parameters, the cost‐effectiveness of the study designs is also an essential criterion to consider for long‐term monitoring of elusive species. Camera trap placement involves minimal cost in the early stages of the grid design. Our results suggest that setting cameras in microhabitats of moderate productivity improved leopard detectability in the wet season. This study can inform the design of camera trap studies occurring in semi‐arid bushland ecosystems to improve estimates of leopard population and occupancy.
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Global biodiversity has been in decline for several decades, marked by species extinctions and population losses that have cascading negative impacts on ecosystems. Despite the growing list of described species, much of the actual threatened biodiversity remains unknown, particularly among invertebrates, including the ones with a parasitic mode of life. Parasites, which may outnumber free-living species and constitute a significant proportion of ecosystem biomass, have largely been overlooked in conservation efforts. The extinction of free-living host species often leads to co-extinction events, where dependent parasite species also face extinction. This chapter highlights the need to integrate parasite conservation into broader conservation science and management practices. It provides an overview of the evolution of species conservation from a focus solely on free-living species to one that includes associated parasitic species. A case study on chondrichthyan parasites illustrates the critical role of parasites in ecosystem health and the importance of their conservation. The chapter also outlines future research directions to better understand and conserve aquatic parasites, advocating for their inclusion in biodiversity monitoring programmes and conservation agendas. Addressing these gaps is essential for maintaining the integrity of natural systems and the biodiversity they support.
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Full-text available
  • Dec 2003
The purpose of this data set was to compile body mass information for all mammals on Earth so that we could investigate the patterns of body mass seen across geographic and taxonomic space and evolutionary time. We were interested in the heritability of body size across taxonomic groups (How conserved is body mass within a genus, family, and order?), in the overall pattern of body mass across continents (Do the moments and other descriptive statistics remain the same across geographic space?), and over evolutionary time (How quickly did body mass patterns iterate on the patterns seen today? Were the Pleistocene extinctions size specific on each continent, and did these events coincide with the arrival of man?). These data are also part of a larger project that seeks to integrate body mass patterns across very diverse taxa (NCEAS Working Group on Body Size in Ecology and Paleoecology: linking pattern and process across space, time, and taxonomic scales). We began with the updated version of D. E. Wilson and D. M. Reeder's taxonomic list of all known Recent mammals of the world (N = 4629 species) to which we added status, distribution, and body mass estimates compiled from the primary and secondary literature. Whenever possible, we used an average of male and female body mass, which was in turn averaged over multiple localities to arrive at our species body mass values. The sources are line referenced in the main data set, with the actual references appearing in a table within the metadata. Mammals have individual records for each continent they occur on. Note that our data set is more than an amalgamation of smaller compilations. Although we relied heavily on a data set for Chiroptera by K. E. Jones (N = 905), the CRC handbook of Mammalian Body Mass (N = 688), and a data set compiled for South America by P. Marquet (N = 505), these represent less than half the records in the current database. The remainder are derived from more than 150 other sources. Furthermore, we include a comprehensive late Pleistocene species assemblage for Africa, North and South America, and Australia (an additional 230 species). “Late Pleistocene” is defined as approximately 11 ka for Africa, North and South America, and as 50 ka for Australia, because these times predate anthropogenic impacts on mammalian fauna. Estimates contained within this data set represent a generalized species value, averaged across sexes and geographic space. Consequently, these data are not appropriate for asking population-level questions where the integration of body mass with specific environmental conditions is important. All extant orders of mammals are included, as well as several archaic groups (N = 4859 species). Because some species are found on more than one continent (particularly Chiroptera), there are 5731 entries. We have body masses for the following: Artiodactyla (280 records), Bibymalagasia (2 records), Carnivora (393 records), Cetacea (75 records), Chiroptera (1071 records), Dasyuromorphia (67 records), Dermoptera (3 records), Didelphimorphia (68 records), Diprotodontia (127 records), Hydracoidea (5 records), Insectivora (234 records), Lagomorpha (53 records), Litopterna (2 records), Macroscelidea (14 records), Microbiotheria (1 record), Monotremata (7 records), Notoryctemorphia (1 record), Notoungulata (5 records), Paucituberculata (5 records), Peramelemorphia (24 records), Perissodactyla (47 records), Pholidota (8 records), Primates (276 records), Proboscidea (14 records), Rodentia (1425 records), Scandentia (15 records), Sirenia (6 records), Tubulidentata (1 record), and Xenarthra (75 records).
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Stream-bed scour, egg burial depths, and the influence of salmonid spawning on bed surface mobility and embryo survival
Article
Bed scour, egg pocket depths, and alteration of stream-bed surfaces by spawning chum salmon (Onchorhynchus keta) were measured in two Pacific Northwest gravel-bedded streams. Close correspondence between egg burial depths and scour depths during the incubation period suggests an adaptation to typical depths of bed scour and indicates that even minor increases in the depth of scour could significantly reduce embryo survival. Where egg burial depths are known, expressing scour depth in terms of bed-load transport rate provides a means for predicting embryo mortality resulting from changes in watershed processes that alter shear stress or sediment supply. Stream-bed alteration caused by mass spawning also may influence embryo survival. Theoretical calculations indicate that spawning-related bed surface coarsening, sorting, and form drag reduce grain mobility and lessen the probability of stream-bed scour and excavation of buried salmon embryos. This potential feedback between salmon spawning and bed mobility implies that it could become increasingly difficult to reverse declines in mass-spawning populations because decreased spawning activity would increase the potential for bed scour, favoring higher embryo mortality. Further analysis of this effect is warranted, however, as the degree to which spawning-related bed loosening counteracts reduced grain mobility caused by surface coarsening, sorting, and redd form drag remains uncertain.
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Foraging Strategies of Glaucous-Winged Gulls in a Rocky Intertidal Community
Article
  • Dec 1986
Foraging strategies of Larus glaucescens were studied in rocky intertidal habitats of the W Aleutian Islands, Alaska. Daily foraging activity was most intense at maximum low tide, and was concentrated in the lowest intertidal zones available to the birds. Barnacles Balanus glandula and mussels Mytilus edulis comprised most of the gull's diet during neap low tides, but these species were almost entirely abandoned during spring low tides in favor of sea urchins Strongylocentrotus polyacanthus, chitons Katharina tunicata and limpets Collisella pelta and Notoacmaea scutum. Sea urchins, chitons and limpets, which had positive prey selection indices, were most abundant in the lower intertidal zones; barnacles and mussels, which had negative prey selection indices, were most abundant in the upper zones. Gulls also generally selected the larger individuals from each prey species, although sea urchins larger than the commissural bill width were avoided and limpets were selected in proportion to availability. Variation in prey availability also occurred among study areas with varying densities of sea otters Enhydra lutris. With increasing depression of invertebrate prey by sea otter predation, gulls fed on a more diverse prey resource, and they switched to neritic fishes under intense sea otter predation. Data support the main predictions of optimal foraging theory in that 1) foraging patches (intertidal zones) and diets were selected such that net rates of energy gain were maximized, and 2) gulls became more selective foragers when energetically more profitable prey were more available.-from Authors
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Sea Otters and Kelp Forests in Alaska: Generality and Variation in a Community Ecological Paradigm
Article
  • Feb 1995
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.
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Food Chain Dynamics: The Central Theory of Ecology?
Article
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
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Strong effects of predation by fishes on an invasive macroinvertebrate in a large floodplain river
Article
  • Mar 2005
We assessed the effects of fish predation on zebra mussels (Dreissena polymorpha) in Navigation Pool 8 of the Upper Mississippi River from 13 May to 5 October, 1998. Concrete-block samplers were deployed at 18 randomly chosen sites in the main-channel border, with 6 sites in the upper, middle, and lower segments of the pool. Two blocks, 1 of which was enclosed in a cage to exclude large predatory fishes, were deployed at each site. After 145 d, blocks were retrieved from 12 of the 18 sites, and zebra mussels were found on all blocks. Densities of zebra mussels were higher on caged blocks than uncaged blocks, and the magnitudes of the differences varied spatially. Mean mussel densities on uncaged blocks were reduced by 66%, 86%, and 20% compared to caged blocks in the upper, middle, and lower pool segments, respectively, over the 145-d interval. Mean daily instantaneous zebra mussel mortality rates from large predators ranged from 0.0016 to 0.0138. Similarly, biomass of zebra mussels was higher on caged than uncaged blocks. Mean mussel biomass on uncaged blocks was reduced by 64% pool-wide, relative to biomass on caged blocks. Zebra mussels were consumed by at least 6 fish taxa including redhorse suckers (Moxostoma spp.), common carp (Cyprinus carpio), bluegill (Lepomis macrochirus), quillback carpsucker (Carpiodes cyprinus), flathead catfish (Pylodictis olivaris), and freshwater drum (Aplodinotus grunniens). Fish predation had an important moderating effect on zebra mussel demography in Pool 8.
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Extinction of an Island Forest Avifauna by an Introduced Snake
Article
  • Jun 1987
.Abstract. The island of Guam has experienced a precipitous decline of its native forest birds, and several lines of evidence implicate the introduced brown tree snake (Boiga irregularis) as the cause of the range reductions and extinctions. The range expansion of B. ~rregularis correlated well with the range contraction of the forest avifauna. Exceptionally high predation by snakes on bird-baited funnel traps occurred in areas where bird popu- lations had declined. Little or no snake predation occurred in areas with stable bird pop- ulations. Several factors have contributed to Boiga's success in decimating the avifauna on Guam. Although most common in forests. B. irregularis has occupied a variety of habitats on Guam. and few effective barriers to its dispersal exist. Prey refuges are present only in urban areas, on concrete or metal structures, and in the savanna. Boiga's nocturnal and arboreal habits and an apparent keen ability to locate prey. make roosting and nesting birds, eggs, and nestlings vulnerable. Besides birds, B. irregularis feeds on small mammals and lizards. By including the abundant small reptiles as a major component in its diet, Boiga has maintained high densities in forest and second-growth habitats while extermi- nating its more vulnerable prey. This is the first time a snake has been implicated as an agent of extinction.
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