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Investigation 3 – Concept Day








Ecosystems: Investigation 3

Concept Day


Note: In this final Investigation of the Ecosystems CELL, we wish to develop the concept of biomass as the dry weight of the sum total of living organisms in each trophic level.

It would be virtually impossible to count, identify, and weigh each organism in an ecosystem. Therefore, this Investigation will also focus on the simple concept of taking small, representative samples that can be accurately measured and extrapolating to larger systems. You will perform such sampling methods in Investigation 3 lab.



  • This first slide was seen in the previous two Investigations of Ecosystems. It shows a food chain, sometimes called an ecological pyramid, when presented in this format.
  • The red text on the left is most relevant here as it points out that both the number of organisms at each higher trophic level decreases as do their total biomass. In an ecosystem, biomass is the sum totals of all of its organism’s dry weights.
  • Perhaps not surprisingly, the biomass at each trophic level decreases as one moves up toward the top of the food chain. One snake, for example, must eat many mice annually to survive. Importantly, the actual number of snakes of a particular type that can survive in an ecosystem may be entirely limited by the number of mice present in it. A sharp decrease in the mouse population may well be expected to cause a decrease in the snake population as well.
  • Why might the mouse population decline? Perhaps there is a brush fire and vast amounts of primary producer plants are destroyed. Such a decrease in plants may well cause a rapid decrease in the beetle population for lack of food. With fewer beetles to feed on, the mouse population suffers starvation and decreased breeding behavior. This, in turn, will impact the snakes and the birds of prey above them. Thus, a problem anywhere in the ecosystem may well impact organisms across the system at various trophic levels. That is why we refer to a “Food Chain” – each trophic level is linked to the next.



  • This slide defines biomass as the dry weight of the organisms of an ecosystem. As noted, to get the total biomass of an ecosystem, the biomass of each of its trophic levels must be known. Much of the remainder of this presentation will focus on various methods of sampling ecosystems so that such numbers can be obtained.



  • This slide asks the question, “How could anyone possibly count all the living organisms in an ecosystem?”

Note: This is a good question and worth discussing how you might go about solving this problem before introducing some of the sampling techniques included in the following slides.

Note: If you do come up with sampling suggestions, you might extend this question by asking how one might be able to determine if populations change at various times of the year or from year to year. These are questions and problems that ecologists must constantly face.



  • This slide introduces the sampling grid concept. By having a grid of standard size, one can count, identify, weigh, etc. (i.e. sample) the organisms found in a “countable” quantity and extend the results as a representation of a much larger area.
  • To do this, one must be sure that the smaller sample is indeed representative of the larger area. This typically requires multiple samples in a given area with the averaging of results and systematic spot sampling across the larger area under study.



  • Let’s say one uses a square grid 0.5 meter by 0.5 meter as shown in this slide. This represents a total area within the grid of 0.25m2 or a quarter meter squared. Next, let’s say the grid is placed flat on a forest floor of a northern wood at the end of September and the number of fallen leaves is counted that are within the grid. We count 173 leaves.
  • Since our grid is only 0.25m2, we would need to multiply this number by 4 to get the number of leaves is one meter squared, which would be 692 leaves/m2.
  • Now let’s say that we know that one region of the forest extends between a few logging roads and has a surface area of 1 km squared. That would 1,000 meters by 1,000 meters or 1,000,000 m2. Since we know that there were 692 leaves in one m2, we could calculate that this tract of forest floor has 692 times 1,000,000 or about 692,000,000 or 692 million leaves on the floor of the entire tract.
  • One would, of course, sample a number of times in different spots to be sure that our average number of leaves per meter squared is accurate across the entire area. Nonetheless, this sampling method is much preferable to hand-counting each and every leaf on the entire forest floor!
  • If we were then to determine the mass of the original 173 leaves then multiplied by 4 and then 1,000,000, we would have an idea of the mass of all the leaves on the km2 forest floor. As you can see, grid sampling is an indispensable part of ecosystem studies.



  • This slide shows an interesting twist of the grid sampling technique. If you wished to determine how much and what type of material fell from trees to the forest floor per hour, you could first cover all existing material with a cloth and then place a sampling grid on it.
  • At the end of any period of time, one could examine the material in the grid and expand the results to much larger surface areas or longer periods of time.



  • Here is an entirely different way to study the canopy of a forest! Instead of determining what falls to the ground from the canopy, researchers actually go right on up and take samples directly.
  • Sampling platforms and grids like these can be lowered by helicopter. Notice how relatively small an individual researcher looks in this environment. Such platforms can be a hundred meters above the forest or jungle floor. The picture to the upper right demonstrates one of the occupational hazards of hands-one field ecology… bug bites!



  • Aquatic ecosystems have their own unique sampling challenges and solutions. To sample small organisms like plankton that are suspended in the “water column”, a plankton net is used. These nets can be of various sizes from small, hand-held nets not much bigger than a butterfly net, to very large nets towed by boats or ships.
  • The top illustration on the right shows a plankton net being towed behind a boat. Researchers can use boat speed and weights to cause the net to sample at various depths so that they can study which species of planktonic organisms live at the different levels in the water column. As the open net sweeps through the water, organisms are trapped and forced toward the back of the net where a receptacle is located that can be removed and taken to the lab for study.
  • The picture at the lower left shows a fisheries scientist rinsing captured plankton down into the receptacle. The image at the lower right shows a microscopic image of a variety of plankton.



  • Another sampling apparatus used for studying aquatic ecosystems is the benthic grab. In this case, a heavy metal “grab” with its jaws locked open is lowered to the bottom of a lake, river, or ocean. The surface area of the open jaws is known and can be used to calculate the sample size.
  • Once the grab reaches the bottom; a metal messenger weight is dropped down the line, which triggers the grab to snap shut, encasing the benthic sample for its trip back to the surface.
  • Benthic samples can be filtered through screens of various pore sizes allowing different types of organisms to be collected and analyzed. The picture at the lower right shows materials brought up from the bottom. This looks like a marine (ocean) sample, with the long stick-like structures representing sand-encrusted casings of polychaete tubeworms.