Question

3 - biomass in food webs

1. what patterns have scientists identified while observing the amount of biomass at each level of a food chain?
2. what happens to the matter that is lost between each level of a food chain?
3. why does a bison that eats 8 kg of grass in a day not gain 8 kg of mass by the end of the day?

Explanation:

Question 1

Scientists study biomass in food chains. Biomass at each level (producers, consumers) shows a pyramid - like pattern (pyramid of biomass). Producers (plants) have the most biomass as they make energy via photosynthesis. Each successive consumer level (herbivores, carnivores) has less biomass. This is because energy is lost (as heat, in waste, for life processes) at each trophic level, so less energy (and thus less biomass) is available for the next level. The pattern is a decreasing biomass with each higher trophic level.

In a food chain, matter (like carbon, nitrogen, nutrients, and the organic matter that makes up biomass) is not “lost” but transformed or transferred. Some matter is used by organisms for respiration (converted to CO₂ and energy), some is excreted as waste (which is decomposed by detritivores and recyclers back into the ecosystem), and some is used for growth and repair (becomes part of the organism's biomass). Also, energy (tied to matter in chemical bonds) is lost as heat, but matter cycles. For example, a herbivore eats plant matter: some is respired (CO₂ released), some is pooped (decomposed), and some becomes the herbivore's body mass (which may be eaten by a carnivore, or the herbivore dies and is decomposed).

A bison eats 8 kg of grass but doesn't gain 8 kg mass. First, respiration: the bison uses energy from the grass for life processes (moving, digesting, maintaining body temperature). This energy comes from breaking down organic matter in the grass, and some of that matter is converted to CO₂ and water (released into the environment). Second, waste: the bison excretes undigested or unassimilated matter (feces, urine). Third, not all ingested grass is assimilated: only a portion of the grass's nutrients are absorbed into the bison's body to build new tissue (muscle, fat, etc.). Most of the grass's mass is either used for energy (and lost as heat/CO₂) or excreted, so the bison's mass gain is much less than 8 kg.

Answer:

Scientists identify a pyramid - shaped pattern (pyramid of biomass) where biomass decreases at each successive trophic level (from producers to primary consumers to secondary consumers, etc.) in a food chain. This is due to energy loss (via respiration, waste, heat) at each level, limiting the biomass that can be supported at higher trophic levels.

Question 2