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Feeding Relationships in Ecosystems All living things need a continuous supply of energy in order to survive, and they need extra energy in order to grow and reproduce. They use whatever abilities they have to get the energy they need to survive and thrive (thrive – grow and reproduce!) Some organisms have the ability to produce their own food by harnessing the energy from the sun. They are called producers because they PRODUCE their own food rather than CONSUMING it by eating something else (consumers). As stated previously, energy from the sun flows into the biotic components of an ecosystem (living things) through the process of photosynthesis. Producers are responsible for this unique and essential process, where the energy from sunlight and abiotic components (nutrients) are combined to produce sugar (oxygen is a byproduct of this process). The producers use the sugar to survive just as all other living things do, but the extra sugar is stored as starch or is used to grow or to reproduce. The excess sugar production, whether it is used to grow or reproduce forms a net biomass (mass of living tissue). Ecologists study how different living things obtain their food and the relationships that organisms have between each other as a result of this primal need for energy. The way in which an organism interacts with other species well as with their environment is called a species’ niche in an ecosystem. This is sort of the role or job that species plays in the very complex set of interactions that exist in an ecosystem. We’ll start by focussing on the interaction that organisms have with other species by focussing on how they obtain their food. Food Chains and Food Webs Feeding relationships often form a succession of steps or levels where one organism is consumed by another, and that organism is consumed by another … and so on forming a long chain called a food chain. The feeding ALWAYS begins with the producers, organisms that produce food by using the sun’s energy and the nutrients in the soil and the air to produce their own food. They feed themselves, and as such they are called autotrophs (‘self feeders’). Other organisms that cannot produce their own food (but still need nutrients) get their food by feeding on producers, called herbivores. These organisms are called heterotrophs (‘other feeders’). Next in the chain are a group of heterotrophs that cannot easily digest the bodies of the producers and therefore need to get their energy by consuming the herbivores (or any other animal), called carnivores. There are also animals that perform both options, where they eat both producers and other consumers (still heterotrophs) called omnivores. Each feeding level in a food chain is called a trophic level. The first trophic level is ALWAYS occupied by the producers. The second trophic level can be occupied by either herbivores or omnivores (as they consume the producers) and the later trophic levels are occupied by the carnivores and the omnivores (herbivores cannot occupy theses levels as they would be eating other animals rather than plants). The diagram below illustrates these levels: Fifth Trophic Level Fourth Trophic Level Third Trophic Level Second Trophic Level First Trophic Level We also label these organisms as primary, secondary, tertiary consumers (suggesting a levels or ordering in the food chain) with the producers always called the primary producers. Feeding relationships in ecosystems are not always so simple. A consumer may have multiple food sources and as such feed on many different organisms. In addition, many consumers may feed on one specific species. This reality causes the many food chains to cross over and intertwine, forming a food web. A food web is the sum total of all the feeding relationships between all species in an ecosystem. A salt marsh food web In a food web, the arrows point toward the organism that is consuming or eating the other … arrows away means they are eaten, arrows toward mean that organism does the eating. eg – the arrow from the rat to the snake means that the snake eats the rat (fairly simple!) An aquatic food web A food web is very versatile and adaptive. If certain species decline in number due to disease or over-consumption, the web adjusts and flexes to accommodate the change. For example … if the food web above if the crab population declines due to overfishing by humans, everything it feeds on will increase in number (no predator), everything it eats will decrease in number (less available food). The food web then adapts … the predators find an alternate food source, the prey are eaten by other predators, and the food web survives and thrives. Some food webs have very important species called indicator species. These species are essential to normally functioning food webs and must be present. If they disappear or change in number then the food web is definitely at risk of collapse. In this aquatic food web … the indicator species is the Krill. The food web will have great difficulty adjusting with the loss of this species and can in fact collapse as a result of this loss. Food Pyramids In an ecosystem, feeding relationships exist so that organisms can get energy to live, grow, and reproduce. Unfortunately, the organisms do not keep all of the energy they obtain from the food and add it to their bodies as biomass. Most of the energy is lost as heat, to maintain life, and to collect and hunt food. As a result approximately 10% of what an animal consumes is actually added to its biomass for the next animal in the food chain to obtain. The diagram below shows the energy transfer in a simple food chain. As a result of this relationship, those at the top of a food chain require a great deal or organisms to support it. We can first look at this idea using a pyramid of numbers … In this figure we see that due to the energy loss at each level 1,500,000 producers are necessary to support the 200,000 herbivores that support the 90,000 primary carnivores that support a single top carnivore (btw - this is for a carnivore’s entire lifetime) These pyramids aren’t always direct measures of the nature of feeding relationships. If one oak tree is given the same value as a small weed the numbers are an inaccurate way of analyzing feeding relationships. This is due to the fact that an oak tree could support a much larger number of organisms than a single dandilion. To see feeding relationships in an ecosystem more clearly, we focus on the biomass (in kg) of the prey and how it supports a given biomass of a predator in a food chain. This pyramid of biomass shows a more accurate version of energy transfer: This shows a comparison of the benefits of a pyramid of numbers and a pyramid of biomass when observing the energy transfer in feeding relationships The most accurate vision of energy transfer (in J) is a pyramid of energy. This pyramid shows directly the energy necessary to support a top consumer: These pyramids are evidence of a few important factors. An ecosystem requires a great deal of numbers/biomass/energy to support a single top consumer. This is simply why top consumers are almost always small in number in comparison to primary carnivores or herbivores in any ecosystem. Most food chains cannot be sustained if they are to contain more than three to four members. THIS … is a compelling advertisement for a vegetarian diet for the human population of the future … think about the pure energetics of it!!!! Homework Pg 47 #1 – 8 + Food Web Assignment