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Thread: Science education: Food webs, pyramids & ecosystems (20 february 2015)

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    Science education: Food webs, pyramids & ecosystems (20 february 2015)

    The next science education post deals with food webs, and energy pyramids, and the basic rules governing ecosystems. Every day we hear of more species being endangered, or some environmental accident or disaster. How vulnerable are ecosystems? What are the consequences for other species (including humans) if certain species are eliminated? Should we cull seals, deer or other such animals? How did this situation arise in the first place? Is there a place for wolves and bears in our wildernesses or are we better off without them? We talk of pollution but what is the impact of introducing insecticides and herbicides into the environment for creatures other than the intended target for these chemicals. If you want to understand many of these issues you should read this coming Friday's piece on FOOD WEBS, PYRAMIDS & ECOSYSTEMS.

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    Science education: Food webs, pyramids & ecosystems

    FOOD WEBS, PYRAMIDS & ECOSYSTEMS



    Living organism within any one locality or region are interconnected. At a very crude level we can understand that cows or sheep graze on pasture which is dependent upon sunshine (and rain), and we humans feed on the milk or meat of such animals. This chain of feeding (sunshine feeding the plants which in turn feed the animals which ultimately feed humans) is called a food chain. Food chains are connected to form complex relationships best described as a food web. We are going to examine some food chains and webs in more detail to discover what basic science principles underlie their operation, and thereby sustains life on planet Earth.

    Generally living organisms are divided into the rough categories of plants and animals. A useful distinction between plants and animals is the fact that only plants are capable of deriving the energy for living and growing, directly from the sun. Plants (and this includes single celled varieties) achieve this through photosynthesis. Organisms like plants (and certain varieties of bacteria) do not rely on other living organisms for their energy supply, and are described as autotrophs. Such autotrophs are also called producers, since they produce their own food using the energy of sunlight.

    As animals are not capable of photosynthesis, they consume plants, or other animals, to gain the energy to live, grow and reproduce. We describe such organisms as heterotrophs. Heterotrophs since they need to eat other organisms to sustain themselves are also called consumers. A rabbit is a consumer since it eats vegetation, but the fox that eats the rabbit is also a consumer. To distinguish between the two, we describe the rabbit as a primary consumer and the fox as a secondary consumer.

    In addition food chains and webs usually contain organisms that cause plant or animal material to decay or decompose e.g. fungi and bacteria. Such organisms are called decomposers and are vital for recycling the energy and chemicals in the food web.

    All the rich and diverse variety of life on Earth is ultimately dependent on the sunlight falling on the planet. This energy passes from one organism to another and is constantly recycled. Like all mechanisms, this energy transfer is not totally efficient and much is lost in the form of heat.

    All food chains/webs include both autotrophs and heterotrophs. Look at the example below:

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    Note: In writing a food chain or food web, the convention is that the head of the arrow points to where the energy (of the organism) goes or ends up.

    Now imagine what would happen if the plants were to diminish in number. The number of insects that could be supported by the smaller supply of plants, would decrease, and hence the mouse population would also decrease proportionately. The energy in the food chain may be so low by the time it reaches the mouse, a population of owls could not be sustained by it i.e. the owl would become extinct (unless it changed to a different food source.)


    THE ENERGY/FOOD PYRAMID

    Before returning to the matter of the interrelationships of food chains to form food webs, let us look at this matter of energy transfers between living organisms.

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    Each level in the pyramid represents a feeding level or trophic level. The amount of energy available at each level is limited. As we proceed up towards the apex of the pyramid, the amount of energy available to living organisms diminishes rapidly. This is due to the inefficiency of energy transfer from one organism to another in the process of being eaten. Roughly 10% of the energy is transferred from one trophic level to another. The result of this is that the population of owls will be less than the population the animals that owls consume. That is why in any locality the number of top predators that can be sustained is always less than the number of herbivores that can be supported. Instead of talking about numbers/populations at different trophic levels in the Energy Pyramid, scientists often talk about the amount of biomass available or sustainable.


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    The diagram above demonstrates the same point, but with a different food chain.

    FOOD WEBS, ECOSYSTEMS AND NICHES

    An ecosystem is a community of living organisms (plants, animals and microbes) in conjunction with the non-living components of their environment (things like air, water and mineral soil), interacting as a system. Scientists studying these communities and their inter-relationships are often called ecologists.


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    The diagram above is a simplified food web or ecosystem. Reality is much more complex with far more organisms and more connections between different organisms! If you examine the food web you will notice many different food chains interacting with each other to make a food web. There are tens of thousands of different food webs on our planet depending upon the locality.

    There are different ecosystems for woodlands, estuaries, fresh water lakes, tidal zones along sea coasts, savannahs, rainforests etc. Each living organism has its preferred locality or habitat. A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant, or other type of organism. It is the natural environment in which an organism lives, or the physical environment that surrounds a species population.

    In ecology, a niche is a term describing the relational position of a species or population in an ecosystem. The description of a niche may include descriptions of the organism's life history, habitat, and place in the food chain. According to the competitive exclusion principle, no two species can occupy the same niche in the same environment for a long time. Competition between them will cause one species to move or become extinct.

    A reduction in the population of any one (or more) organisms due to dramatic weather changes, pollution or other human activity, can have serious consequences for many other organisms in an ecosystem or the food web – and ultimately impact on humans. A good example of this was the impact of using sheep-dips (to kill insect pests) on the population of golden eagles in Scotland. The insecticide caused problems because it entered the body of the eagles, and affected their metabolism. The egg shells became thinner and prone to breaking before the chicks could hatch. Similar things happened to other birds of prey, and the practice of sheep dipping was changed and the population of birds of prey recovered. This experience also reinforced the important fact that a poison which cannot be readily broken down in the environment, is concentrated each time it moves from one trophic level to the next. So while only low concentrations of poisons might be initially used, they are concentrated by organisms in a food chin/web until they reach a toxic level.

    When a species is introduced deliberately or accidentally into a new environment in which it does not belong, these invasive alien species can have serious consequences. Invasive species can be plants or animals. That is one reason why custom officials at airports are so rigorous about what living organisms can or cannot be brought into a country. To better understand the dangers posed by invasive alien species watch the short video below:-

    Invasive Alien Species



    At times the elimination of an organism might initially appear harmless but can have severe unintended repercussions. Europeans in the early days of the U.S. eliminated mountain lions in certain localities to protect deer. A few years later this lead to a population crash of deer. The deer because of the lack of their natural predator, swelled in numbers. This led to overgrazing of pastures and deer died of starvation. There have been many other examples too of this type of good intentions on the part of humans leading to negative consequences. Many natural top predators like wolves, bears and large wild cats have been eliminated by humans. In recent years as our understanding of ecosystems has grown, ecologists are advocating the re-introduction of wolves and bears into certain localities for the health of ecosystems as a whole.

    At times humans have for their own purposes introduced organisms into new habitats where they had been absent e.g. rabbits were introduced to Australia, initially for sport. Since there were no natural predators against the rabbits, the consequences were severe for the sheep farmers who witnessed the destruction of a great deal of grassland. Myxomatosis is a disease that affects rabbits and is caused by the myxoma virus. It was deliberately introduced into Australia in 1950 in an attempt to control the rabbit population.

    There are numerous other examples of such ‘mistakes’ including the introduction of African killer bees into South America to improve the honey yields. There are numerous documentaries on the internet about the history and spread of killer bees across the Americas.

    Dangerous animals like wolves and bears have been widely eliminated across many parts of the world. While people may think that this is not particularly important, in reality these animals play a very important role in ecosystems and often in ways that we never suspected. Watch the video on the re-introduction of wolves into Yellowstone National Park in the U.S. and the beneficial effects that followed.




    While we can all understand the consequences of a sharp drop in the population of honeybees for agriculture, we may be less sympathetic towards wasps. However wasps also play an important role in the ecosystems upon which humans rely. Just because we cannot see a direct human benefit to retaining certain organisms does not mean that we can eliminate them without consequences. The elimination of species is not desirable for ecosystems and the general health of the planet.

    Gaia Hypothesis

    The Gaia hypothesis, also known as Gaia theory or Gaia principle, proposes that organisms interact with their inorganic surroundings on Earth to form a self-regulating, complex system that contributes to maintaining the conditions for life on the planet. Topics of interest include how the biosphere and the evolution of life forms affect the stability of global temperature, ocean salinity, oxygen in the atmosphere and other environmental variables that affect the habitability of Earth.

    The hypothesis was formulated by the chemist James Lovelock and co-developed by the microbiologist Lynn Margulis in the 1970s. The hypothesis was initially criticized for being teleological and contradicting principles of natural selection, but later refinements resulted in ideas framed by the Gaia hypothesis being used in fields such as Earth system science, biogeochemistry, systems ecology, and the emerging subject of geophysiology. Nevertheless, the Gaia hypothesis continues to attract criticism, and today many scientists consider it to be only weakly supported by, or at odds with, the available evidence. In 2006, the Geological Society of London awarded Lovelock the Wollaston Medal largely for his work on the Gaia theory.

    Recent Criticism
    Aspects of the Gaia hypothesis continue to be sceptically received by relevant scientists. For instance, arguments both for and against it were laid out in the journal Climatic Change in 2002 and 2003. A main reason for doubting it, it was suggested, are the many examples where life has detrimental and/or destabilising effects on the environment. Several recent books have criticised the Gaia hypothesis, with comments ranging from “... the Gaia hypothesis lacks unambiguous observational support and has significant theoretical difficulties” to “Suspended uncomfortably between tainted metaphor, fact, and false science, I prefer to leave Gaia firmly in the background” to “The Gaia hypothesis is supported neither by evolutionary theory nor by the empirical evidence of the geological record”. The CLAW hypothesis, previously held up as confirmation of the success of Gaia, has subsequently been discredited. In 2009 the direct opposite hypothesis to Gaia was proposed: that life has highly detrimental (biocidal) impacts on planetary conditions. In a recent book-length evaluation of the Gaia hypothesis considering modern evidence from across the various relevant disciplines (hailed by the publishers as the first of its kind) the author, Toby Tyrrell of the National Oceanography Centre (UK), concluded that: “I believe Gaia is a dead end. Its study has, however, generated many new and thought provoking questions. While rejecting Gaia, we can at the same time appreciate Lovelock's originality and breadth of vision, and recognise that his audacious concept has helped to stimulate many new ideas about the Earth, and to champion a holistic approach to studying it.” Elsewhere he presents his conclusion “The Gaia hypothesis is not an accurate picture of how our world works”. (This statement need to be understood as referring to the "strong" and "moderate" forms of Gaia—that the biota obeys a principle that works to make Earth optimal (strength) or favourable for life (strength) or that it works as a homeostatic mechanism (strength). The latter is the "weakest" form of Gaia that Lovelock has advocated. Tyrrell rejects it. However, he finds that the two weaker forms of Gaia – Co-evolutionary Gaia and Influential Gaia, which assert that there are close links between the evolution of life and the environment and that biology affects the physical and chemical environment—are both credible, but that it is not useful to use the term "Gaia" in this sense.

    The Gaia idea has caught on with mostly non-scientists in particular. To such people it has become a form of new age religion.
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    Re: Science education: Food webs, pyramids & ecosystems

    [MENTION=2]Aryan_B[/MENTION]

    Brother, you have placed the science education post under the Announcements Section. It normally goes under the Science & Education Section. The other mention of the food webs was simply the announcement giving fore-warning that the science material was was being posted and what it would cover.
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    Re: Science education: Food webs, pyramids & ecosystems

    Quote Originally Posted by SHAMAS View Post
    [MENTION=2]Aryan_B[/MENTION]

    Brother, you have placed the science education post under the Announcements Section. It normally goes under the Science & Education Section. The other mention of the food webs was simply the announcement giving fore-warning that the science material was was being posted and what it would cover.
    now moved to science section

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