Ecosystem Ecology: Primary & Secondary Production

• Ecosystem ecologists focus on the flows of energy, water, and nutrients within ecosystems.
• Primary production is the creation of new organic matter per unit area in an ecosystem over time.
• Gross primary production is the total primary production by all producers while net primary production is gross primary production minus respiration.
• Secondary production is the biomass production by consumers over a period.
• A trophic level signifies a position in a food web.
• It's determined by the number of energy transfers from primary producers.
• Primary producers occupy the first trophic level.
• Primary consumers, which are herbivores and detritivores, occupy the second trophic level.
• Secondary consumers, carnivores that consume herbivores and detritivores, are in the third trophic level.
• Tertiary consumers are in the fourth trophic level.

Patterns of Terrestrial Primary Production

• Terrestrial primary production is generally limited by temperature, moisture and nutrients.
• Temperature and moisture correlate strongly with primary production.
• Highest rates occur in warm, moist conditions.
• Rosenzweig (1968) studied how temperature and moisture affect primary production rates.
• Actual evapotranspiration (AET) is the annual amount of water that evaporates and transpires.
• Cold, dry ecosystems tend to have low AET.
• Soil fertility variations can explain terrestrial primary production differences.
• Shaver and Chapin (1986) discovered net primary production nearly doubled in arctic tundra on fertilized plots.
• Bowman et al. (1993) found that adding nutrients increased production in dry and wet meadows.
• Nitrogen is limiting in dry meadows, while nitrogen and phosphorus are limiting in wet meadows.

Patterns of Aquatic Primary Production

• Aquatic primary production is generally limited by nutrient availability.
• A quantitative relationship exists between phosphorus and phytoplankton biomass.
• Nutrients, especially phosphorus, control phytoplankton biomass in freshwater ecosystems.
• Experimental Lakes Area conducted whole-lake experiments in Canada.
• Dividing a lake into two 500,000 m³ basins revealed that sucrose and nitrate additions increased phytoplankton biomass 2 to 4x.
• Carbon, nitrate, and phosphorus additions increased phytoplankton biomass 4 to 8x.
• Marine phytoplankton's primary production concentrates in areas with high nutrient availability.
• The highest rates are found along continental margins and in upwelling areas due to nutrient runoff and sediment disturbance.
• Open oceans tend to be nutrient-poor; vertical mixing serves as the main nutrient source.
• This mixing is blocked in the tropics by the thermocline.
• Granéli et al. (1990) suggested that primary production in the Baltic Sea is nutrient-limited.
• Increased nitrate led to an increase in chlorophyll concentrations, while increased phosphates did not.
• Nitrogen appears to be the limiting nutrient, not phosphorus, in these areas.

Primary Producer Diversity

• Primary producer diversity contributes to increased primary production.
• Rates of primary production are affected by physical, chemical, and biological factors.
• Aspects of biodiversity are important to primary production and other ecological processes.
• Tilman et al. (2001) studied plant functional groups with similar physiological and anatomical traits.
• Manipulating the number of species and functional groups in 168 plots showed that plant species richness correlated with primary production.
• N-fixing legume and C4 grass functional groups had higher productivity.
• Biological influences are comparably important to chemical and physical factors.
• Cardinale (2011) found increased nitrate uptake and biomass with increasing algal species richness.
• A study of 1,157 lakes in the USA reported a correlation between nitrogen and phosphorus availability, algal diversity, and primary production.

Consumer Influences

• Consumers can influence primary production rates in aquatic and terrestrial ecosystems through trophic cascades.
• Bottom-up controls are influences of physical and chemical factors on ecosystems.
• Top-down controls are influences of consumers on ecosystems.
• Carpenter et al. (1985) proposed that piscivorous and planktivorous fish can significantly deviate primary productivity.
• Trophic cascade describes how predator effects on prey can alter more than trophic level.
• Trophic cascades involve indirect interactions.
• Carpenter and Kitchell (1993) found that a reduction in planktivorous fish populations led to reduced feeding pressure on zooplankton.
• Large-bodied zooplankton soon dominate.
• This results in reduced phytoplankton biomass and rate of primary production, consistent with the negative correlation.
• McNaughton (1985) determined that the Serengeti grazers consume an average of 66% of annual primary production.
• Primary production rate in the Serengeti positively correlates with rainfall quantity.
• Grazing can increase primary production.

Grazing & Compensatory Growth

• Grazing increases the growth rate of many grasses.
• Compensatory growth sees lower respiration, reduced self-shading, and improves water balance.
• McNaughton found that compensatory growth is highest at intermediate grazing intensities.
• Light grazing is insufficient to produce growth.
• Heavy grazing reduces a plant's capacity to recover.

Secondary Production

• Ecosystems with high levels of primary production generally support higher levels of secondary production.
• Energy is lost with each energy transfer from one to another level.
• Food ingested is used for respiration, excretion, and production of biomass (growth and reproduction), known as secondary production.
• Lindeman (1942) concluded that the ecosystem concept is fundamental to studying energy ecosystems (trophic dynamics).
• Grouping organisms within an ecosystem is suggested.
• Every level feeds immediately below.
• Ecological efficiency is the percentage of energy transferred.
• There's a variance of approximately 5% to 20%.
• The energy distribution leads to a pyramids-shaped pattern of energy among levels.
• Secondary production is key to understanding energy flow in ecosystems.
• Benke's approach estimated secondary production by tracking cohorts.
• This approach involved a study of dragonfly larvae over 10 months.
• Secondary production is often represented as biomass per unit area per unit time.

Linking Primary to Secondary Production

• Predictions of the relationship depends on which depends on which predetor-prey is used
• The prey-dependent models predicts; increased primary production increases abundance of secondary (predetors) not change
• The ratio-dependent models predicts to increases abundace of primary and secondary production

Ecosystem ecology focuses on energy, water, and nutrient flows. Primary production is new organic matter creation. Trophic levels indicate positions in a food web, determined by energy transfers from primary producers.