Global Patterns of Primary Production and Limiting Factors in Ecosystems

Global Climate Patterns and Sunlight

Sunlight and Climate

Global climate patterns, including temperature, moisture, and wind, are primarily determined by the amount of sunlight absorbed by the planet. Geographic differences in sunlight establish these patterns, while local conditions can further modify them.

• Sunlight is the fundamental driver of climate and ecosystem energy flow.

• Local factors (e.g., topography, water bodies) can alter global climate effects.

Greenhouse Effect

The greenhouse effect describes how solar radiation warms the planet, causing it to emit infrared (IR) radiation. Certain atmospheric gases absorb this IR radiation, warming the atmosphere and the planet.

• CO2, H2O, and other gases absorb IR radiation but not solar radiation.

• Increasing atmospheric CO2 leads to more IR absorption and greater warming.

Biome Changes Due to Climate Change

Climate change alters temperature and precipitation patterns, leading to shifts in biome distribution and vulnerability.

• Maps show changes in temperature and biome types over time.

• Regions may become more or less suitable for certain biomes due to climate change.

Photosynthesis and Energy Entry into Ecosystems

Sunlight as the Entry Point

Sunlight striking Earth is the primary entry point of energy into ecological communities. This energy is captured by photosynthetic organisms.

• Photosynthesis is the process by which light energy is converted into chemical energy.

• Key event: Carbon fixation (conversion of CO2 into organic molecules).

Efficiency of Light Use in Photosynthesis

Only a small fraction of sunlight is used in photosynthesis.

• Light striking Earth: 600–650 joules/(m2 × sec)

• ~70% of Earth's surface is ocean; photic zone is the top ~100 m.

• Less than 1% of light striking Earth is used in photosynthesis.

Leaf Area Index (LAI)

LAI measures the leaf area of plants per unit of ground area, indicating the potential for photosynthetic activity.

• LAI range: 0.1–12, depending on habitat (e.g., tundra vs. rainforest).

Primary Production in Ecosystems

Gross Primary Production (GPP)

GPP is the total amount of carbon fixed by all autotrophs in an ecosystem.

• Some energy captured is used for cellular respiration.

Net Primary Production (NPP)

NPP is the amount of energy captured by autotrophs that results in an increase in living plant matter (biomass).

• $\text{NPP} = \text{GPP} - \text{Respiration}$

• ~0.1% of light striking Earth is used to make plant tissue.

Global Patterns of NPP

NPP varies across ecosystems and is influenced by environmental factors.

• Total global NPP: 105 Gt carbon fixed/year

• Terrestrial NPP: 426 g carbon/m2/yr

• Marine NPP: 140 g carbon/m2/yr

• 46% of NPP from marine, 54% from terrestrial ecosystems

Factors Influencing NPP

• Sunlight

• Temperature

• Precipitation

• Nutrients

Effects of Rainfall and Temperature on NPP

• Higher rainfall and temperature generally increase NPP.

• Graphs show positive correlation between these factors and NPP.

NPP Variation with Latitude

• NPP is highest near the equator and decreases toward the poles.

Average NPP by Ecosystem Type

Different ecosystems have varying average NPP values.

• Swamps, marshes, and tropical rainforests have highest NPP.

• Deserts and open ocean have lowest NPP.

Terrestrial Biome NPP Table

Biome	NPP (g C/m2/yr)	% of Terrestrial NPP
Tropical forest	2,500	35
Tropical savanna	1,080	24
Temperate forest	1,550	13
Temperate grassland	750	9
Boreal forest	390	4
Temperate shrubland	500	2
Tundra	180	1
Desert	250	5
Crops	610	6

Climate Change Effects on NPP

Recent Trends

• NPP increased in terrestrial ecosystems (1982–1999) due to decreased cloud cover over tropical forests.

• Since 2000, gains in NPP have been erased by major droughts in the southern hemisphere.

Net Ecosystem Production (NEP)

Definition and Significance

NEP is the difference between GPP and total ecosystem respiration (Rt).

• $\text{NEP} = \text{GPP} - R_t$

• If NEP > 0, the ecosystem is a carbon sink; if NEP < 0, it is a carbon source.

• Long-term release of CO2 from Arctic tundra (permafrost) is an example of NEP dynamics.

Limiting Nutrients in Ecosystems

Definition

A limiting nutrient is one that is in short supply and restricts further growth in an ecosystem or organism.

• Typical limiting nutrients: Nitrogen, Phosphorous, Iron, Calcium, Potassium

Terrestrial Habitats

• Nitrogen is often the limiting nutrient.

• Farm fields lose productivity due to nitrogen depletion.

• Solutions: Nitrogen-based fertilizers, leaving fields fallow, crop rotation

Nitrogen Fixation and Nitrification

• Nitrogen fixation: Conversion of atmospheric N2 to NH3 by bacteria (e.g., Azotobacter, Rhizobium).

• Nitrification: Conversion of NH3 to NO2- (nitrite) and then to NO3- (nitrate) by nitrifying bacteria.

• NO3- is absorbed by plant roots.

Coastal and Open Ocean Habitats

• Coastal habitats are often limited by nitrates.

• Open ocean is typically limited by iron.

Experimental Evidence for Limiting Nutrients

Nutrients Added to Experimental Culture	Relative Uptake of 14C by Cultures
None (controls)	1.00
Nitrogen (N) + phosphorus (P) only	1.10
N + P + metals, excluding iron (Fe)	1.08
N + P + metals, including Fe	12.90
N + P + Fe	12.00

• Iron supplements can cause a ~4x increase in NPP in open ocean regions.

Example

Legumes such as soybeans host nitrogen-fixing bacteria in root nodules, enhancing soil nitrogen and productivity.

Additional info: These notes cover key concepts in ecosystem ecology, including energy flow, primary production, limiting nutrients, and the impact of climate change on productivity. They are suitable for exam preparation in General Biology.