Bottom-Up Control in Ecosystems

• Chapter 18 discusses primary and secondary production in ecosystems and focuses on the factors influencing these processes in terrestrial and aquatic environments.

Introduction

• Ecosystem ecologists study 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 measures the total primary production by all primary producers.
• Net primary production is gross primary production minus respiration, representing the biomass accessible to consumers.
• Secondary production is the biomass production by consumers over some time.
• Trophic level is a species' position in a food web.
• The trophic level depends on the amount of energy transfers from primary producers to that level.
• Primary producers are at the first trophic level.
• Primary consumers occupy the second level and are herbivores and detritivores.
• Secondary consumers are on the third level and are carnivores that eat herbivores and detritivores.
• Tertiary consumers are at the fourth trophic level.

Patterns of Terrestrial Primary Production

• Terrestrial primary production is mainly limited by temperature, moisture, and available nutrients.
• Temperature and moisture significantly correlate with primary production, with warm, wet conditions fostering the highest rates.
• Rosenzweig (1968) tied moisture and temperature influence to primary production rates.
• Annual net primary production and actual evapotranspiration (AET) have a clear relationship.
• Actual Evapotranspiration (AET) is the yearly amount of water that evaporates and transpires from a area.
• Ecosystems in cold and dry conditions have low AET levels.
• Sala et al. (1988) researched primary production controls in central USA grasslands.
• The research found highest production in the east and lowest in the west, aligning with rainfall levels
• Variations in soil fertility can explain the significant differences in terrestrial primary production.
• In arctic tundra, net primary production almost doubled in fertilized plots compared to unfertilized ones.
• Adding nutrients has been shown to increase primary production in both dry and wet meadows.
• Nitrogen is a limiting factor in dry meadows, while both nitrogen and phosphorus limit wet meadows.

Patterns of Aquatic Primary Production

• Aquatic primary production is usually limited by how many nutrients are available.
• There is a quantitative relationship between phosphorus and phytoplankton biomass.
• Nutrients, phosphorus, control phytoplankton biomass in freshwater ecosystems.
• The Experimental Lakes Area in Canada separated a lake into two 500,000 m³ basins.
• Adding sucrose and nitrate increased phytoplankton biomass by 2–4 times in one basin.
• Adding carbon, nitrate, and phosphorus to a freshwater basin resulted in a 4–8x increase in phytoplankton biomass.
• The highest marine phytoplankton primary production happens in areas with higher nutrient availability.
• These areas include continental margins and areas of upwelling, influenced by nutrient runoff and sediment disturbance.
• Open oceans are nutrient-poor.
• Vertical mixing serves as the main nutrient source, but is blocked by the thermocline in the tropics.
• Granéli et al. (1990) found that primary production in the Baltic Sea is nutrient limited.
• Nitrate increased chlorophyll concentrations, but phosphates did not.
• Nitrogen is the main limiting nutrient in the Baltic Sea and Black Sea.

Primary Producer Diversity

• The diversity of primary producers contributes to increased primary production within ecosystems.
• Both physical and chemical factors and biological factors affect primary production rates.
• Plant functional groups are plants with similar physiological and anatomical characteristics according to Tilman et al. (2001).
• Plant species richness correlates with primary production.
• Higher productivity happens with N-fixing legume and C4 grass functional groups.

Consumer Influences

• Consumers affect the rates of primary production in aquatic and terrestrial ecosystems through trophic cascades.
• Bottom-up controls are the physical and chemical factors on ecosystems.
• Top-down controls are the influences consumers have on the ecosystem.
• Carpenter et al. (1985) found that piscivorous and planktivorous fish cause primary productivity deviations.
• The influence consumers have on a lake's primary productivity spreads through food webs.
• Trophic cascades' predator effects alter more than one trophic level.
• Trophic cascades involve indirect interactions.
• A decline in planktivorous fish led to reduced pressure on zooplankton, resulting in large-bodied zooplankton.
• There was decreased phytoplankton biomass and a reduced rate of primary production.
• A negative correlation exists between zooplankton body size and primary production.
• Serengeti grazers consume about 66% of annual primary production, according to McNaughton (1985).
• Primary production in the Serengeti positively correlates with rainfall quantity.
• Grazing can increase the rate of primary production by increasing the growth rate of grasses and through compensatory growths
• Compensatory growth increases due to reduced respiration and self-shading and improved water balance.
• Compensatory growth maximizes at intermediate grazing intensities.
• Light grazing is not effective for compensatory growth.
• Heavy grazing lowers a plant's capacity to recover.

Secondary Production

• Greater primary production in ecosystems generally supports higher secondary production levels.
• Energy is lost during each energy transfer from one trophic level to another.
• Ingested food is used for respiration, excretion, and biomass production (growth and reproduction), or secondary production.
• Lindeman (1942) says the ecosystem is key to studying energy transfer within an ecosystem (also known as trophic dynamics).
• It is suggested to group organisms within an ecosystem into trophic levels.
• Each energy trophic level feeds on the level immediately below.
• Ecological efficiency is the percentage of energy transferred from one trophic level to the next.
• Ecological efficiency varies from about 5% to 20%.
• Pyramid-shaped distribution of energy results from these energy transfers among trophic levels
• Secondary production focuses on understanding energy flow in ecosystems.
• Benke studied dragonfly larvae for 10 months to estimate secondary production by following a cohort over time.
• Secondary production is expressed as biomass per unit area per unit time.
• The prey-dependent model predicts that increased primary production will increase secondary consumer abundance but not primary consumers.
• The ratio-dependent model says increased primary production increases both primary and secondary consumers.

Bottom-up controls in ecosystems are driven by resource availability. Primary producers, such as plants, form the base of the food web and influence higher trophic levels. The abundance and quality of these resources ultimately shape the structure of the entire ecosystem.