Ecosystems and Conservation Biology

1. Energy flow and productivity

Energy enters ecosystems as sunlight captured by producers (plants, algae, cyanobacteria) through photosynthesis and moves through consumers (herbivores, carnivores) and decomposers (bacteria and fungi). Energy flow is one-way and inefficient: only about 10% of the energy at one trophic level is stored as biomass in the next, because most is used for metabolism and lost as heat. This "10% rule" explains why energy and biomass pyramids are broad at the base and narrow at the top.

Primary productivity measures how fast producers convert energy to biomass. Gross primary productivity (GPP) is the total energy captured; net primary productivity (NPP) is what remains after producers' own respiration (NPP = GPP − respiration). NPP is the energy available to consumers and is highest in warm, wet, nutrient-rich environments such as tropical rainforests and estuaries, and lowest in deserts and the open ocean.

2. Biogeochemical cycles

Biogeochemical cycles move matter between living (biotic) and nonliving (abiotic) reservoirs:

• Water cycle — evaporation, transpiration, condensation, precipitation, and runoff move water among ocean, atmosphere, land, and organisms.
• Carbon cycle — photosynthesis fixes CO₂ into organic compounds; respiration and decomposition return it. Burning fossil fuels and deforestation release stored carbon, raising atmospheric CO₂. The ocean and forests are major carbon sinks.
• Nitrogen cycle — nitrogen fixation (by bacteria and cyanobacteria, including Rhizobium in legume root nodules) converts atmospheric N₂ into ammonia; nitrification converts ammonia to nitrite then nitrate, which plants absorb; ammonification returns nitrogen from dead organisms to ammonia; denitrification returns nitrogen gas to the atmosphere.
• Phosphorus cycle — the slowest cycle and, unlike the others, essentially atmospheric-free. Phosphorus weathers from rocks and moves through soil, water, and organisms, often limiting in fresh water. Excess phosphorus from fertilizer causes eutrophication (algal blooms that deplete oxygen).

3. Biodiversity and conservation

Biodiversity has three levels: genetic diversity (variation within species), species diversity (number and abundance of species), and ecosystem diversity (variety of habitats and communities). Biodiversity provides ecosystem services — pollination, water purification, nutrient cycling, pest control, and climate regulation — on which human well-being depends.

The major threats are often summarized as HIPPO: Habitat loss (the leading cause), Invasive species, Pollution, Population (human) growth, and Overexploitation — with climate change increasingly added. Conservation biology is the applied science of protecting and restoring biodiversity. Strategies include protected areas and wildlife corridors, habitat restoration, invasive-species control, pollution reduction, sustainable resource use, and prioritizing biodiversity hotspots (small areas with many endemic species under high threat).

How it works

The nitrogen cycle, step by step:

1. Nitrogen fixation — symbiotic bacteria (e.g., Rhizobium in legumes) and free-living cyanobacteria convert atmospheric N₂ into ammonia (NH₃), which becomes ammonium (NH₄⁺).
2. Nitrification — soil bacteria oxidize ammonium to nitrite (NO₂⁻), then to nitrate (NO₃⁻), the form most plants absorb.
3. Assimilation — plants take up nitrate and ammonium to build proteins and nucleic acids; consumers obtain nitrogen by eating plants or other animals.
4. Ammonification — decomposers break down dead organisms and wastes, returning nitrogen to the soil as ammonium.
5. Denitrification — under low-oxygen conditions, bacteria convert nitrate back to nitrogen gas (N₂), completing the cycle.

Common confusions

Memory aids

"Energy flows, matter cycles." Four cycles to remember — W-C-N-P (Water, Carbon, Nitrogen, Phosphorus). For threats, remember HIPPO: Habitat loss, Invasive species, Pollution, Population growth, Overexploitation. For nitrogen, chain the steps: Fix → Nitrify → Assimilate → Ammonify → Denitrify ("FNAAD").

Quick review

Topic Recap

• Ecosystems transfer energy one-way (producers → consumers → decomposers) and recycle matter through biogeochemical cycles.
• About 10% of energy transfers between trophic levels; NPP is the energy available to consumers.
• The water, carbon, nitrogen, and phosphorus cycles connect biotic and abiotic reservoirs through specific processes.
• Biodiversity (genetic, species, ecosystem) provides essential ecosystem services and faces threats summarized as HIPPO.
• Conservation biology protects and restores biodiversity through protected areas, restoration, and hotspot prioritization.

Knowledge Check

1. Why does energy flow one-way through an ecosystem while matter cycles?
2. What is the relationship between gross and net primary productivity?
3. Name the process that converts atmospheric nitrogen into a form plants can use.
4. Why is the phosphorus cycle considered essentially atmospheric-free, and what problem does excess phosphorus cause?
5. List the HIPPO threats to biodiversity and identify the leading one.

Answers and Rationales

1. Answer: Energy enters as sunlight and is lost as heat at every transfer, so it cannot be reused, whereas the chemical elements of matter are continuously recycled by biogeochemical processes. Why: Energy transfers are lossy, but atoms can be reused indefinitely.
2. Answer: NPP = GPP − the energy producers use in respiration; NPP is the energy available to the rest of the ecosystem. Why: Producers must pay their own metabolic costs before anything is left for consumers.
3. Answer: Nitrogen fixation, carried out by certain bacteria and cyanobacteria (including Rhizobium in legumes). Why: Only these organisms can break the strong triple bond of atmospheric N₂.
4. Answer: Phosphorus has no significant gaseous phase, so it cycles through rocks, soil, water, and organisms; excess phosphorus runoff causes eutrophication (algal blooms that deplete oxygen). Why: Its lack of an atmospheric reservoir makes it a common limiting nutrient.
5. Answer: Habitat loss, Invasive species, Pollution, Population growth, and Overexploitation — with habitat loss the leading threat. Why: Habitat loss destroys the foundation that all other species depend on.