All of the interconnected and overlapping predator-prey relationships in an ecosystem—including producers, consumers and decomposers—make up a food web. As one
organism eats another, a food chain is formed. Each step along a food chain is known as a trophic level or feeding level, and every organism can be categorized by its trophic level. A food web is made up of all of the interconnected food chains in an ecosystem. Through a food chain, organic compounds originally produced by a plant pass through successively higher trophic levels: Food production and consumption in the Bay are rarely as simple or direct as this example. Seldom does one organism feed exclusively on another. How are food webs important?Each organism within a food web is connected to and depends on others for food. Filter feeders such as oysters, clams and menhaden must have enough plankton available to sustain themselves. Striped bass and bluefish, part of a higher trophic level, rely on menhaden and bay anchovies as their primary food source. A healthy ecosystem is one with a balanced food web—not too much production or consumption of any one of the producers or consumers. An ecosystem must be enormously productive to support substantial populations of species at the highest trophic levels; for example, every pound of commercial fish taken from the Bay requires almost 8,000 pounds of underlying producers and consumers. However, an overabundance of algae can be harmful, reducing oxygen in the water and blocking sunlight from reaching underwater grasses. How do chemical contaminants move through food webs?Food webs can also be a pathway for harmful chemical contaminants. Mercury and PCBs can pass to higher trophic levels in a process called bioaccumulation.
The severe
decline of bald eagles and osprey in the 1950s through the 1970s resulted from the bioaccumulation of DDT, a pesticide used to control insects and agricultural pests. DDT caused eagles to lay extremely thin-shelled eggs that would break in the nest. Life on the Food ChainHave you ever wondered why we can't seem to feed the world's hungry? It's a complex issue, but it might surprise you to learn that it's not because there isn't enough food; current agricultural capacity, based on current technology, exists to feed as many as 10 billion people. The Earth's population is "only" about 7 billion. The big question really is: If we want to feed everyone, what would everyone need to eat? To answer that question, download this excel spreadsheet and try plugging in some numbers.
The Food Chain: The answer has to do with trophic levels. As you probably know, the organisms at the base of the food chain are photosynthetic; plants on land and phytoplankton (algae) in the oceans. These organisms are called the producers, and they get their energy directly from sunlight and inorganic nutrients. The organisms that eat the producers are the primary consumers. They tend to be small in size and there are many of them. The primary consumers are herbivores (vegetarians). The organisms that eat the primary consumers are meat eaters (carnivores) and are called the secondary consumers. The secondary consumers tend to be larger and fewer in number. This continues on, all the way up to the top of the food chain. About 50% of the energy (possibly as much as 90%) in food is lost at each trophic level when an organism is eaten, so it is less efficient to be a higher order consumer than a primary consumer. Therefore, the energy transfer from one trophic level to the next, up the food chain, is like a pyramid; wider at the base and narrower at the top. Because of this inefficiency, there is only enough food for a few top level consumers, but there is lots of food for herbivores lower down on the food chain. There are fewer consumers than producers.  Land and aquatic energy pyramids
Food Web: At each trophic level, there may be many more species than indicated in the table above. Food webs can be very complex. Food availability may vary seasonally or by time of day. An organism like a mouse might play two roles, eating insects on occasion (making it a secondary consumer), but also dining directly on plants (making it a primary consumer). A food web of who eats who in the southwest American desert biome might look something like this: image source: http://iqa.evergreenps.org/science/biology/ecosystem_files/food-web.jpg Keystone Species: In some food webs, there is one critical "keystone species" upon which the entire system depends. In the same way that an arch collapses when the keystone is removed, an entire food chain can collapse if there is a decline in a keystone species. Often, the keystone species is a predator that keeps the herbivores in check, and prevents them from overconsuming the plants, leading to a massive die off. When we remove top predators like grizzly bears, orca whales, or wolves, for example, there is evidence that it affects not just the prey species, but even the physical environment. Apex Predators: These species are at the top of the food chain and the healthy adults have no natural predators. The young and old may in some cases be preyed upon, but they typically succumb to disease, hunger, the effects of aging, or some combination of them. The also suffer from competition with humans, who often eliminate the top predators in order to have exclusive access to the prey species, or through habitat destruction, which is an indirect form of competition. Decomposers: When organisms die, they are sometimes eaten by scavengers but the remaining tissues are broken down by fungi and bacteria. In this way, the nutrients that were part of the body are returned to the bottom of the trophic pyramid. Bioaccumulation: In addition to being less energy efficient, eating higher up the food chain has its risks. Pesticides and heavy metals like mercury, arsenic, and lead tend to be consumed in small quantities by the primary consumers. These toxins get stored in the fats of the animal. When this animal is eaten by a secondary consumer, these toxins become more concentrated because secondary consumers eat lots of primary consumers, and often live longer too. Swordfish and tuna are near the top of the aquatic food chain and, when we eat them, we are consuming all of the toxins that they have accumulated over a lifetime. For this reason, pregnant women are advised against eating these foods. Solve the following problems mathematically. 1. Given: 10 billion people can be fed a basic vegetarian diet that is nutritionally complete. How many people could we feed at the American standard-a tertiary level of consumption (3rd order consumers?). 50% of the energy is lost by each higher level. 2. If there are 250 million people in the United States most of them eating at the Tertiary (3rd) level of consumption, how many people could we feed at the Primary level? 3. Some animals like sharks are 5th order consumers! Sharks eat tuna that eat mackerel that eat herring that eat copepods that eat diatoms. If we were to make the reasonable assumption that each of these animals eats 2 of its prey each day, how many organisms died to feed the shark in one day? What is a food chain and what is a food web?A food chain outlines who eats whom. A food web is all of the food chains in an ecosystem. Each organism in an ecosystem occupies a specific trophic level or position in the food chain or web. Producers, who make their own food using photosynthesis or chemosynthesis, make up the bottom of the trophic pyramid.
What term is used to describe an organism that makes its own food?An autotroph is an organism that can produce its own food using light, water, carbon dioxide, or other chemicals. Because autotrophs produce their own food, they are sometimes called producers. Plants are the most familiar type of autotroph, but there are many different kinds of autotrophic organisms.
When organisms feed on other organisms it is known as?A heterotroph is an organism that eats other plants or animals for energy and nutrients. The term stems from the Greek words hetero for “other” and trophe for “nourishment.” Organisms are characterized into two broad categories based upon how they obtain their energy and nutrients: autotrophs and heterotrophs.
Which organism in the food web below is found in the first trophic level of the ecosystem?The first and lowest level contains the producers, green plants. The plants or their products are consumed by the second-level organisms—the herbivores, or plant eaters. At the third level, primary carnivores, or meat eaters, eat the herbivores; and at the fourth level, secondary carnivores eat the primary carnivores.
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Species that utilize the same source of nutrition within a food web can best be described as
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