How to Measure Biomass

Measurement of the weight of biological material in an ecosystem can be hard to do.  Dry weight is used to measure the biomass in an ecosystem. Let’s take a tree for example. To measure the biomass of the tree you would have to chop the tree down, deplete its water content (because the water content of organisms [even of the same species] varies), and then measure the weight of what is left. The size of the organism in question is also taken into account. This is the most unethical way to measure biomass, but it is one way to get the job done.


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Biomass is defined as the amount of organic matter in a given habitat (usually expressed as weight).  As trophic levels increase, the amount of biomass decreases because the amount of organisms per tropic level decreases as the trophic level increases. However, if the organisms are large (like a large tree), then it is possible for there to be more biomass with fewer numbers at the bottom of the graph:

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Principal of Competitive Exclusion

The Principal of Competitive Exclusion is the inability of two species with the same described niche to coexist in the same location. Example: The American Grey Squirrel and the Red Squirrel in Britain.  The Red Squirrel was edged out of parts of its fundamental niche after the introduction of the Grey Squirrel (and after that squirrel began to take over some of the Red Squirrel’s resources).

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Interactions Between Species

Competition – Occurs when 2 or more species compete over resources. Dominated by the largest, most aggressive species. Examples: when hyenas compete for a carcass with vultures and marabou starks, and when two species of wolves compete over territory for food resources. One species benefits/is negatively affected and the other is negatively affected.

Herbivory – Occurs when a species exploits plant resources for food/consumption. Examples: Caterpillars consuming leaves, and rabbits consuming short grass. One species benefits and the other is negatively affected.

Predation – Occurs when one species kills and eats another. Examples: Lions killing and eating caribou, and wolves killing and eating elk. One species benefits and the other is negatively affected.

Symbiosis is a close association/physical proximital relationship between organisms.

  • Mutualism – is an association between species where both species benefit. Examples: Termites and the cellulose digesting bacteria in their guts, and ruminants and their gut microflora of bacteria and cilliates. Both species benefit.
  • Parasitism – Occurs when a parasite exploits the resources of its host. Examples: Tapeworms and humans, and cat fleas and its host. One species benefits and the other species is negatively affected.
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Fundamental v. Realized Niche

A fundamental niche are the niches that a species can potentially exploit, whereas a realized niche is narrower (contained withing the fundamental niche of the organism) and is the niche for which an organism is best adapted (which is sometimes the result of competition).

See earlier post for more on this topic

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Niches are the functional position of a species in its ecosystem, how it responds to the distribution of its resources, and how it, in turn, alters those resources for other species.

Fundamental Niche is the total tolerance range of a species.

Realized Niche is the niche for which an organism is best adapted. This is narrower than the organisms fundamental niche due to competition for resources and other limiting factors.

It is possible (and not at all rare) for two or more species’ niches to overlap

However, a overlap in two or more niches’ of species fundamental niches results in competition causing species to adapt to a narrower, realized niche over time.

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Transects provide information on the distribution of species in a given community. This is valuable when environmental factors change over the sample distance. Looking at the change in abiotic environmental factors with living organisms, transects are able to find correlations between plant and animal distribution and abiotic factors.

Also, transects provide information on community composition. Thus, transects do not only show the distribution of species over a given area, but the distribution of abiotic resources as well.

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