70 Matching Annotations
  1. Mar 2020
    1. J. Terborgh et al., Science 294, 1923 (2001)

      In this natural experiment, as a result of a hydroelectric dam flooding a rainforest in Venezuela, researchers were able to measure the results of predator removal in isolated communities. Top-down regulation of these communities were discovered and drastic trophic cascades were observed. 

    2. J. A. Estes, D. O. Duggins, Ecol. Monogr. 65, 75 (1995).

      This observational study of 153 sites demonstrated, over the course of 3-15 years, the importance of sea otters to the community structure of the kelp forests of the Aleutian Islands and Alaska. Without sea otters, the kelp forests collapsed due to overgrazing by urchins and other herbivores, leading to implications for numerous other organisms in the system. With sea otter predation on urchins, the kelp system was stable and supported a much higher diversity of organisms at all trophic levels (Fig 3).

    3. N. G. Hairston, F. E. Smith, L. B. Slobodkin, Am. Nat. 94, 421 (1960)

      The authors highlight 5 lines of reasoning to underscore the importance of predators as top-down controls:

      1) the rate of planetary fossil fuel accumulation over time has not been minuscule as compared to the rate of photosynthesis in the same systems;

      2) given this, decomposers must be food-limited; otherwise, fossil fuels would build up at higher rates;

      3) in terrestrial systems, plants are typically not herbivore-controlled, nor are they regularly destroyed by weather, but are controlled by bottom-up factors such as light, water, and nutrients;

      4) terrestrial herbivores are therefore typically not limited by their food supply, even in areas where the primary consumers are overabundant;

      5) herbivore populations are therefore controlled by predators.

    4. Many practicing ecologists still view large animals in general, and apex consumers in particular, as ecological passengers riding atop the trophic pyramid but having little impact on the structure below.

      In the past, the role that top-down forces can have within a system has not been appreciated. The role that apex predators play within a complex ecosystem is not often recognized until after those organisms are removed.

      The authors are calling for a change in the lens through which ecosystem ecologists view the role of these species, specifically, that the working hypothesis of ecosystem ecologists should be that apex consumers play a fundamental role in any particular ecosystem's structure and stability until empirical research indicates otherwise.

    5. graminoid

      Grasslike plants.

    6. white-tailed deer (Odocoileus virginianus) have persisted in the absence of predators for more than a century, causing the successive elimination of saplings of less and less palatable trees and shrubs

      In this study, it was discovered that balsam fir, the island's original dominant tree species, was not able to recruit, as it was a favorite food for deer. White spruce, and several companion species, have replaced the fir, as deer do not find them palatable.

    7. Other examples include the spread of the invasive brown tree snake (Boiga irregularis) on the otherwise vertebrate predator–free island of Guam

      A number of different factors led to the success of the brown tree snake in Guam: barrier-free dispersal, few safe areas for avian prey, lifestyle of the predator (nocturnal and arboreal), and the ability of the predator to find prey under varying conditions.

    8. In contrast, introduced rats (46) and arctic foxes (Fig. 4) (47) have reduced soil fertility and plant nutrition on high-latitude islands by disrupting seabirds and their sea-to-land nutrient subsidies, with striking effects on plant community composition.

      A metastudy of 45 replicated and 35 unreplicated field experiments found that introduced predators had double the negative effect than of native predators on their vertebrate prey:

      Read more: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1950296/

    9. From land, the demise of Pleistocene megaherbivores may have contributed to or even largely accounted for the reduced atmospheric methane concentration and the resulting abrupt 9°C temperature decline that defines the Younger-Dryas period

      Herbivores produce large amounts of methane, a potent greenhouse gas, as a byproduct of their diet. By losing so many of these organisms during the Pleistocene and early Holocene, the methane concentration in the atmosphere would have dropped rapidly, helping to cause the global cooling of the Younger-Dryas.

    10. Further examples of the interplay between predation and disease exist for aquatic systems. The establishment of no-take marine reserves in the Channel Islands of southern California led to increases in the size and abundance of spiny lobsters (Panulirus interruptus) and declines in population densities of sea urchins, which are preyed on by the lobsters. The reduced urchin densities thwarted the spread of disease among individual sea urchins, which led to a lowered frequency of epidemics of sea urchin wasting disease within the reserves

      By allowing the expansion of the predator population (spiny lobster), the size of the prey population (sea urchins) was held in check and individual urchins suffered from disease less often.

    11. Apart from small oceanic islands, all regions of our planet supported a megafauna before the rise of Homo sapiens

      Scientists are currently debating how much influence early human migrants had on the extinction of North American megafauna in the Pleistocene and Holocene:


    12. The sea otter/kelp forest system in the North Pacific Ocean

      The kelp forests of Alaska and the Aleutian Islands were monitored for up to 15 years to evaluate the impacts of top-down controls by sea otters on the community structure of the region.

    13. For example, empirical research in Serengeti, Tanzania, showed that the presence or absence of apex predators had little short-term effect on resident megaherbivores [elephant (Loxodonta africana), hippopotamus (Hippopotamus amphibius), and rhinoceros (Diceros bicornis)] because these herbivores were virtually invulnerable to predation (24). Conversely, predation accounted for nearly all mortality in smaller herbivores [oribi (Ourebia ourebi), Thompson’s gazelle (Eudorcas thomsonii), and impala (Aepyceros melampus)], and these species showed dramatic increases in abundance and distribution after the local extinction of predators.

      In this study, it was noted that ungulates above 150 kg were regularly limited by food availability instead of predation, due to their large body sizes.

      Conversely, small ungulates (<150 kg) were regular prey of the several large carnivores on the Serengeti, and their populations, and the trophic levels below them, were controlled by the apex predators.

      Loss of predators in these systems caused much greater impacts on the smaller herbivores than the larger ones.

    14. Although the extent and quality of evidence differs among species and systems, top-down effects over spatial scales that are amenable to experimentation have proven robust to alternative explanations

      A metastudy of 41 papers and 60 independent tests found that terrestrial trophic cascades occurred more frequently than previously thought across systems and were on par with those in aquatic environments. While the strength of the cascade response varied across systems based on several factors (type of carnivore, plant antiherbivore defenses, type of damage measured, etc.), trophic cascades were common, although not universal.

    15. When the impacts of apex consumers are reduced or removed or when systems are examined over sufficiently large scales of space and time, their influences are often obvious

      One of the most widely publicized long-term studies was that of the Greater Yellowstone system. By the early 1900s, wolves were largely extirpated from the Yellowstone basin. After wolves were reintroduced in the 1990s, the region experienced a very quick recovery in biodiversity.

      Read more here: https://www.nps.gov/yell/learn/nature/wolf-restoration.htm

    16. The omnipresence of top-down control in ecosystems is not widely appreciated because several of its key components are difficult to observe.

      To see the effects of one such natural experiment, read "Ecological meltdown in predator-free forest fragments," from Terborgh et al. in Science.

      Also can be found in the "Related Content" sidebar.

    17. physicochemical

      Interactions of physics and chemistry.

    18. hysteresis

      When the direction of an ecosystem change cannot be reversed, i.e., an ecosystem cannot be returned to its previous state once it has gone through a phase shift.

    19. predation

      Killing other organisms for food.

    20. interdisciplinary

      Research involving more than one field of science, e.g., biology and chemistry.

    21. biogeochemical cycles

      Complex cycles of nitrogen, sulfur, phosphorus, carbon, and water involving the atmosphere, land, water, and organisms.

    22. ecosystems

      The interactions between organisms and the nonliving environment (i.e., rocks, water) within an area.

    23. top-down forcing

      The impacts that top-level predators have on the food web levels below them.

    24. 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.

      To learn more about the interesting work on the complex impacts of trophic cascades, see research in Science (Ngai & Srivastava, 2006; Chapin III et al., 1997; Myers et al., 2007).

      PDF downloads of these papers can be found under "Related Content."

    25. The loss of these animals may be humankind’s most pervasive influence on nature

      Humans have a number of different impacts on the biosphere, but loss of apex predators is thought to be one of the widest ranging issues of the planet.

      An overview of the threats to the planet's largest 31 carnivores and implications can be found here.

      The study (Ripple et al.) can also be found in the "Related Content" sidebar.

    26. carbon sequestration

      The storage of carbon atoms in organisms, soil, and water from the atmosphere

  2. Jun 2019
    1. trophic downgrading

      Impacts from the loss of the top-level consumers.

    2. pervasive

      Widely felt.

    3. trophic

      Feeding relationships.

    4. function

      How a system works.

    5. resilience

      How quickly a community is able to recover from a change in the environment.

    6. recent research reveals extensive cascading effects of their disappearance in marine, terrestrial, and freshwater ecosystems worldwide

      The United Nations Convention on Biological Diversity completed a metastudy in 2010, examining biodiversity targets and projecting biodiversity losses into the future. Read more: https://www.cbd.int/gbo3/

    7. empirical

      Based on data.

    8. mass extinction events

      Periods of Earth's history when vast numbers of species went extinct in a short period of time.

    9. herbivory

      Eating only vegetation for food.

    10. abundance


    11. distribution


    12. topology


    13. basins of attraction

      Conditions that allow for stability in an ecosystem.

    14. flux


    15. perturbed


    16. extirpated


    17. weakly motile

      Not able to move far.

    18. autotrophs

      Self-feeding organisms such as plants, algae, many protists, and some bacteria.

    19. regime shifts

      Changes in abundance or dominance of species within an ecosystem.

    20. “natural experiments”

      Data collected from unintended consequences seen in nature.

    21. mesopredators

      Predators found in the middle of the food web—that is, they both eat prey and are eaten as prey.

    22. megaherbivores

      Large, plant-eating organisms.

    23. aggregate


    24. R. T. Paine, Am. Nat. 103, 91 (1969)

      In this letter, Paine notes the importance of predators to community stability not only to the system that he studied (the intertidal of the Pacific Northwest) but also that of other simple or complex systems worldwide.

    25. Tipping points (also known as thresholds or breakpoints), around which abrupt changes in ecosystem structure and function (a.k.a. phase shifts) occur, often characterize transitions between alternative stable states.

      These tipping points are notoriously difficult to predict but can be recognized after the fact. Models are being developed to identify tipping points ahead of time, for example here.

    26. The history of life on Earth is punctuated by several mass extinction events (2), during which global biological diversity was sharply reduced. These events were followed by novel changes in the evolution of surviving species and the structure and function of their ecosystems. Our planet is presently in the early to middle stages of a sixth mass extinction (3), which, like those before it, will separate evolutionary winners from losers.

      There have been five mass extinctions on Earth in the past. These were caused by various natural events, from gamma-ray bursts to megavolcanic eruptions to meteor strikes. During each mass extinction, species diversity diminished significantly in a short period of time. Each mass extinction was followed by the evolution of novel forms of organisms that transformed their habitats in new ways.

      Most scientists agree that we are currently in the 6th mass extinction event, this one largely caused by human activities such as climate change, habitat loss/fragmentation, pollution, invasive species, and human consumption/overexploitation.

    27. alternative stable states

      A different persistent community structure from the original in an ecosystem, typically resulting from a disturbance.

    28. Such interdependencies are well illustrated in East Africa, where the introduction of rinderpest in the late 1800s decimated many native ungulate populations, including wildebeest (Connochaetes taurinus) and buffalo (Syncerus caffer). Reductions of these large herbivores caused an increase in plant biomass, which fueled wildfires during the dry season. Rinderpest was eliminated from East Africa in the 1960s through an extensive vaccination and control program. Because of this, wildebeest and buffalo populations had recovered to what was thought to be historically high levels by the early 1980s. The resulting increase in herbivory drove these systems from shrublands to grasslands, thus decreasing the fuel loads and reducing the frequency and intensity of wildfires

      This top-down forcing by disease and loss of herbivores has also been linked to the complex carbon cycle in the region:


    29. For example, the reduction of lions and leopards from parts of sub-Saharan Africa has led to population outbreaks and changes in behavior of olive baboons

      In this case, the apex predators have been overhunted by humans for bushmeat, for trophies, or as pests to their livestock.

    30. A similar result, involving different species and processes, occurred in India, where the decline of vultures also led to increased health risks from rabies and anthrax

      Because of overuse of antibiotics in livestock, the vulture population declined, leading to more scavenging by feral dogs of carcasses throughout the region. As the dog populations grew, and due to their proximity to humans, the risk of transmission of rabies to humans has increased greatly.

    31. In freshwater systems, the localized rise and fall of human malaria is associated with the impacts of predatory fishes on planktivores, which are in turn important consumers of mosquito larvae

      In this case, researchers found that certain types of predatory fish, while consumers of mosquito larvae, also regularly consumed the mosquito's main predators, leading to increased malaria infections.

    32. Trophic cascades associated with the presence or absence of apex predatory fishes in lakes can affect phytoplankton density, in turn affecting the rate of primary production, the uptake rate of CO2, and the direction of carbon flux between lakes and the atmosphere

      This article from the Chicago Tribune describes how scientists on the Great Lakes are trying to determine whether these large bodies of water are carbon sources or sinks.


    33. even today whale feces return various limiting nutrients from the aphotic to photic zones, thereby directly enhancing primary productivity

      Certain island nations are looking to use this carbon "sink" as a way to meet their own carbon emissions targets:


      Climate change may hamper these efforts by making it difficult for whales to survive in these regions, however.

    34. For example, the reintroduction of wolves to Yellowstone National Park has reduced the positive indirect effects of ungulates on soil nitrogen mineralization and potentially the nitrogen supply for plant growth

      Researchers found that after wolves were reintroduced to Yellowstone, there was a net decline in soil nitrogen, likely due to fewer large herbivores. In addition, the spatial nature of nitrogen renewal changed due to new use patterns by the herbivores.

    35. Similarly, in terrestrial systems wolves protect riparian trees and shrubs from overbrowsing by large ungulates, in turn shading and cooling the adjacent streams, reducing stream bank erosion, and providing cover for fish and other aquatic life

      The 2007 Final Environmental Impact Statement for the Elk and Vegetation Management Plan for Rocky Mountain National Park lists, as Alternative 5, the reintroduction of wolves to the system to manage elk populations and to restore willow and aspen communities throughout the park. Alternative 5 is listed as the Environmentally Preferred Alternative at the end of the report.


    36. predatory seastars in the rocky intertidal

      Predatory sea stars regularly graze the rocky intertidal regions and are not prey-specific. Their grazing opens new substrate for multiple species, preventing the dominant species from colonizing all of the rocks.

    37. mesopredator release [coyotes (Canis latrans) maintaining small vertebrate species in chaparral habitats

      It was observed that the loss of coyotes as an apex predator control on smaller vertebrate predators, along with habitat fragmentation, led to trophic cascades in the ecosystem.

    38. recruitment failure

      Inability of seeds to germinate.

    39. We propose that many of the ecological surprises that have confronted society over past centuries—pandemics, population collapses of species we value and eruptions of those we do not, major shifts in ecosystem states, and losses of diverse ecosystem services—were caused or facilitated by altered top-down forcing regimes associated with the loss of native apex consumers or the introduction of exotics.

      The disruption of many biological processes and natural ecosystem functions can be traced to the loss of predators shifting systems to a new state. Alternatively, introducing invasive exotic species that may outcompete native organisms, removing food sources of predators/herbivores, may cause ecosystem collapse via trophic cascade.

    40. Bottom-up forces are ubiquitous and fundamental, and they are necessary to account for the responses of ecosystems to perturbations, but they are not sufficient. Top-down forcing must be included in conceptual overviews if there is to be any real hope of understanding and managing the workings of nature

      Given the numerous instances of trophic cascades noted in the literature, and the critical impacts that top-down forcing can have as noted above, it is necessary for researchers to consider both bottom-up and top-down controls when looking at ecosystem function, change, and decline.

    41. R. T. Paine, J. Anim. Ecol. 49, 667 (1980)

      A discussion of food webs, trophic relationships, species connectedness, and whether community structure and stability could be modeled based on these ideas.

    42. trophic cascades

      Also known as top-down controls, these refer to the effects of predators that propagate downward through food webs across multiple trophic levels—where trophic level refers to an organism's position in the food chain.

    43. The impacts of trophic cascades on communities are far-reaching, yet the strength of these impacts will likely differ among species and ecosystems.

      An additional example of top-down forcing can be seen in this HHMI BioInteractive Scientist at Work video featuring Dr. Brian Silliman and his work in salt marshes here.

    44. M. E. Power, W. J. Matthews, A. J. Stewart, Ecology 66, 1448 (1985)

      This seminal paper on the indirect effects of predation in freshwater rivers demonstrated that the trophic cascades previously seen in marine and terrestrial systems also held true for river ecosystems.