Pyramids of biomass: constructing and interpreting
Ecology • Trophic levels in an ecosystem (biology only)
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Definition of biomass
Biomass is the mass of living or recently dead organisms and is expressed in units such as grams or kilograms. Scientists usually measure biomass as dry mass because water content varies between organisms and over time, which would otherwise introduce large inaccuracies in comparisons. Measurement of dry mass normally requires drying samples before weighing, although many field studies use estimates to avoid excessive killing of organisms.
Pyramid of biomass
A pyramid of biomass is a graphical representation of the total biomass at each trophic level in a feeding relationship. The base represents producers and higher levels represent successive consumers. A correctly drawn pyramid shows decreasing biomass at higher trophic levels because energy and biomass transfer between levels is inefficient. The pyramid shape therefore mirrors energy flow in ecosystems.
Reasons for decreasing biomass between trophic levels
Most energy consumed by organisms enters maintenance and life processes and is lost as heat, carbon dioxide and water during respiration, or passes out as undigested waste; only a small fraction becomes new tissue. Approximately 10% of the biomass (or energy) from one trophic level becomes the biomass of the next level, so successive levels contain progressively less biomass. This loss explains why food chains are usually short and why top-level organisms occur in smaller total mass.
Calculating total biomass from data
Total biomass for a species equals the number of individuals multiplied by the dry mass of one individual; units must match before multiplication. Biomass per trophic level equals the sum of total biomass values for all species within that level. Field data sets often present numbers and dry mass per organism, which require conversion and summation before plotting. Example classroom tasks use organism counts and dry mass to compute total biomass and then group values by trophic level prior to drawing a scaled pyramid.
Constructing a scaled pyramid of biomass
A scaled pyramid represents each trophic level as a horizontal bar whose area or width is proportional to the total biomass at that level. The usual procedure is: convert all biomass values to the same unit, calculate total biomass per trophic level, choose a scale (for example 10 g represented by 1 cm), and draw bars stacked vertically from producers at the base to apex predators at the top. Labels for trophic level names and units must appear on the diagram. Accurate scaling and consistent units ensure the pyramid correctly reflects relative biomass.
Limitations and sources of error
Biomass estimates contain sources of error including sampling bias, seasonal variation, mobile organisms leaving or entering sampled areas, and difficulty measuring small or numerous organisms such as plankton. Drying organisms to obtain dry mass can be destructive, so many surveys use estimates or samples that may not represent the whole population. Sampling strategies and sufficient sample size reduce bias and improve reliability of biomass pyramids.
Key notes
Important points to keep in mind