BIOL208 Lecture Notes - Lecture 23: Detritivore, Nitrogen Cycle, Nitrogen Fixation
1 May 2018
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Nutrient and Organisms
• Organisms can modify the distribution and cycling of nutrients in ecosystems
• Aquatic systems tend to have greater proportion of autotrophs consumed by
herbivores rather than detritivores compared to the terrestrial system
• Why does the proportion of herbivores: Detritivores matter?
o Aquatic: More herbivores = less detritus (since they will be eating the live
material rather than having them die and rot)
o Terrestrial = more detritus and slower cycling
• Nutrient spiraling: Nutrient cycling doesn’t occur in one, stationary location
o Spiraling length: Length of stream required for an atom of a nutrient to
complete a cycle from release into the water column to re-entry into the benthic
environment (river bottoms)
o Nutrient Retentiveness: Inverse of spiraling length, or the tendency of a stream
to retain nutrients
• Nutrient Cycling in Aquatic Ecosystems: Streams
o Presence of large abundance of macroinvertebrates can speed up the nutrient
cycling in streams
▪ Faster nutrient cycling = greater primary production
o Most herbivores and detritivores must overcome large differences between the
low nutrient content of their food and their own elemental requirements
▪ Therefore, differences in the ratio of N: P for different species could
influence the ratio of N:P recycled into the environment
▪ Low N:P ratio in your body = will excrete more N because your
requirements are met and nutrient cycle will be faster
• Nutrient Cycling in Terrestrial Ecosystems: Grasslands
o Pocket Gopher
o Richardson Ground Squirrel
▪ 60% of grass growing there goes to prairie dogs – biomass is reduced by
30%
▪ This increases turnover = increase overcompensation
▪ Organic matter is young, young grass have more N than older grass
• Nutrient Cycling in Terrestrial Ecosystems: Temperate Forests
o Population of deer increases = more cycling
• Humans and the Nitrogen Cycle
o Haber Process
▪ Artificial nitrogen fixation. Combines N gas and H gas using a metal
catalyst under high heat and pressure to form NH3
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