Ecosystems and Biogeochemical Cycles Notes

Ecosystems

• Ecosystem: Includes all organisms in a particular place plus the abiotic environment.
• Biogeochemical cycles: Chemicals cycling within ecosystems, affected by biotic and abiotic processes.
• Energy Flow: Energy converted to biological energy (usually carbon fixation by photosynthesis) flows through an ecosystem.

Water Cycling

• Water determines the composition of communities.
• Water in the atmosphere exists as a gas.
• Terrestrial ecosystems: 90% of evaporation is through plants via transpiration (gaseous water).
• Water cools and falls as precipitation.
• Water flows to the ocean or is trapped as groundwater.
• Aquifers: 95% of fresh water used in the United States.

Carbon Cycle

• Earth's reserves of coal and fossil fuels built up over geological time.
• Human burning of fossil fuels creates imbalances in the carbon cycle.
• The concentration of CO_2 in the atmosphere is increasing rapidly.

Geologic Cycling of CO_2

• Early atmosphere had high CO_2 levels.
• CO2 + water in the air -> carbonic acid (H2CO_3).
• Carbonic acid + rocks removes CO_2 from the atmosphere.
• Carbon flows to the bottom of the ocean to form rock.
• Volcanism releases CO_2 and other greenhouse gasses back into the atmosphere.
• CO2 combines with H2O to form carbonic acid: CO2 + H2O \rightarrow H2CO3
• Carbonic acid reacts with rocks, carrying calcium and bicarbonate by rivers.
• Calcium and bicarbonate form calcium carbonate: Ca^{2+} + HCO3 \rightarrow CaCO3

Nutrients

• Living things need more than water and carbon.
• Limiting nutrients: shortest supply relative to the needs of organisms.
• Nitrogen and phosphorus are most common limiting nutrients for terrestrial and aquatic ecosystems.
• Iron is the limiting nutrient for algal populations in about 1/3 of the world's oceans.

Nitrogen

• Nitrogen is a component of all proteins and nucleic acids.
• The atmosphere is 78% nitrogen, but most plants and animals cannot use N_2 (gas).
• Organisms get nitrogen from ammonia or nitrates.
• Microbes facilitate nitrification: Nitrogen (N2) -> Ammonia (NH3) -> Nitrates (NH_3).
• Denitrification recycles nitrogen.

Nitrogen and Agriculture

• Nitrogen is removed when crops are harvested.
• Legumes (peas, soybeans, peanuts) have symbiotic relationships with nitrogen-fixing bacteria in their roots.
• Crop rotation can naturally reintroduce Nitrogen.
• Nitrogenous fertilizers are produced using natural gas, which is a source of greenhouse gas.
• Humans have doubled the rate of transfer of N_2 in usable forms into soils and water.

Runoff

• Fertilizer overuse and runoff -> extra nutrition -> too much marine algae -> not enough Oxygen for other life.
• The Gulf of Mexico dead zone is caused by Mississippi runoff.

Phosphorus

• Phosphorus is required by all organisms.
• It occurs in nucleic acids, membranes, and ATP.
• Phosphorus has no significant gas form in ecosystems; it exists as phosphate.
• Plants and algae use free inorganic phosphorus.
• Water flow often affects its availability.
• Animals eat plants to obtain phosphorus.

Iron as a Limiting Nutrient

• When wind brings in iron-rich dust, algal populations proliferate if the iron is in a usable chemical form.
• Sand storms in the Sahara Desert can increase algal productivity in Pacific waters.

First Law of Thermodynamics

• Energy is neither created nor destroyed; it changes forms (light, chemical-bond energy, motion, heat).

Second Law of Thermodynamics

• Whenever organisms use chemical-bond or light energy, some is converted to heat (entropy).
• Earth functions as an open system for energy.
• The sun is our major source of energy.

Trophism

• Autotrophs (“self-feeders”) synthesize organic compounds from inorganic precursors.
• Photoautotrophs use light as an energy source.
• Chemoautotrophs derive energy from inorganic oxidation reactions (prokaryotic).
• Heterotrophs cannot synthesize organic compounds from inorganic precursors.
• Includes animals that eat plants and other animals, as well as bacteria and fungi that decompose.

Trophic Cascade

• Trophic Level 1: Primary producers.
• Trophic Level 2: Herbivores.
• Trophic Level 3: Primary carnivores.
• Trophic Level 4: Secondary carnivores.
• Detritivores.

Productivity

• Productivity: Rate at which organisms in a trophic level collectively synthesize organic matter for the next trophic level.
• Primary productivity: Productivity of primary producers.
• Sets the energy budget for an ecosystem; all organisms rely on this source of energy.

GPP and NPP

• Gross primary productivity (GPP): Raw rate at which primary producers synthesize new organic matter (around 1% of all solar energy).
• Net primary productivity (NPP): GPP less the respiration of the primary producers.
• Secondary productivity: Productivity of a heterotroph's trophic level.

10% Rule

• The amount of chemical-bond energy decreases as energy is passed from one trophic level to the next.
• Rule of thumb: About 10% of energy at one level is made available to the next level.
• Example: 17% of ingested energy is converted into insect biomass (17% growth, 33% cellular respiration, 50% feces).

Limits on Top Carnivores

• The number of trophic levels is limited by energy availability.
• The exponential decline of chemical-bond energy limits the lengths of trophic chains.
• Only about 1/1000 of the energy captured by photosynthesis passes all the way through to secondary carnivores.

10% Rule and Diet

• Humans are omnivores that get food from many trophic levels.
• Warm-blooded animals use up lots of respiration to regulate body temperature.
• Microbes: ~40%
• Insects: 10-40%
• Fish & Reptiles: ~10%
• Endotherms (birds and mammals): 1-3%

Ecological Pyramids

• Trophic relationships often depicted as pyramids.

• Energy flow/productivity must decrease per level and look like a pyramid.

• Energy Measurement: "First-level carnivore (48 kcal/m^2/year) Herbivore (596 kcal/m^2/year) Photosynthetic plankton (36,380 kcal/m^2/year)"

• Ecological Pyramids: Energy hard to measure - easier to estimate biomass (total dry weight of organisms) or count the number of organisms. Often are similar to energy pyramids.

• Biomass Measurement: "First-level carnivore (11 g/m^2) Herbivore (37 g/m^2) Photosynthetic plankton (807 g/m^2)"

• Number of Organisms: $$"Carnivore:11 Herbivore:4,000,000,000 Photosynthetic plankton:500,000"

Inverted Pyramids

• Sometimes biomass or numbers pyramids can be inverted, with lower levels being smaller.
• Numbers – very large organisms have low numbers. Trees - Primary producers in old growth forest have much smaller numbers than herbivore insects.
• Marine biomass pyramids often inverted because resources are constrained, so phytoplankton are often consumed immediately, quickly converting to primary consumers.
• Example: Herbivorous zooplankton and bottom fauna (21 g/m²) and Phytoplankton (4 g/m²).

Trophic-Level Interactions

• Trophic cascade: Effects exerted at one level affect two or more nearby levels.
• Top-down effects: when effects flow down.
• Bottom-up effects: when effects flow up.

Top-Down Effects: Simple

• Stream enclosures with large carnivorous fish have fewer primary carnivores, more herbivorous insects, and a lower level of algae.

Top-Down Effects: Four-Level

• Primary producers - Algae.
• Primary consumer - herbivorous insects.
• Secondary consumers - carnivorous damselfly nymph.
• Tertiary consumers - fish.

Trophic Cascade: Two States

• Along the West Coast, the sea otter/sea urchin/kelp system exists with low or high sea otter populations.
• Orca predation drives sea otter population down.

Tropical Regions

• Tropical regions have the highest diversity.
• Species diversity cline: biogeographic gradient in the number of species correlated with latitude; reported for plants and animals.

Island Biogeography

• Larger islands have more species.
• MacArthur-Wilson equilibrium model (Robert MacArthur and Edward O. Wilson).
• Islands tend to accumulate more species through dispersion from the mainland.
• The pool of potential colonizing species becomes depleted over time.
• More species on an island mean more will go extinct.
• At some point, extinctions and colonizations should be equal.

MacArthur and Wilson Equilibrium Model

• Further distance to mainland -> less dispersion -> less species diversity.
• Greater Island size -> larger populations -> harder to drive to extinction -> more species at equilibrium.
• True more diversity on larger islands. But also more speciation after colonization.
• Habitat diversity may also play a role.