Principles of Crop Ecology

• 1866 – Ernst Haeckel: Coined the term “Ecology” (from Greek oikos = house, logos = study).
• Ecologyis the scientific study of the interactions that determine the distribution and abundance of organisms.
• Crop Ecologyis a sub-discipline that applies ecological principles to agricultural crops to understand how they interact with their environment (biotic and abiotic factors) to improve management and productivity.

Key Principles:

• Law of the Minimum (Liebig’s Law):Crop growth and yield are limited by the resource (e.g., nutrient, water, light) that is in shortest supply relative to the plant’s need. The limiting factor must be addressed to see improvement. Example: Even with ample N, P, and K, if water is limiting, yield will be low.
• Law of Tolerance (Shelford’s Law):For each environmental factor, a crop has a minimum, maximum, and optimum range. Growth only occurs within this tolerance range. Example: Mango cannot tolerate frost (low minimum temp), while apple requires chilling (low optimum temp).
• Principle of Limiting Factors:The rate of a physiological process is constrained by the pace of its slowest factor. It’s an extension of Liebig’s Law. Example: On a cloudy day, light is the limiting factor for photosynthesis, not CO₂.
• Competition:Crops compete with weeds (interspecific) and among themselves (intraspecific) for resources like light, water, and nutrients. This principle underpins the need for practices like optimum plant population, thinning, and weed control.
• Succession:The process of change in the species structure of an ecological community over time. In agriculture, this is seen as weed succession (e.g., broadleaf weeds are often replaced by grassy weeds under continuous herbicide use).
• Ecological Amplitude:The range of environmental conditions under which a species can survive. Crops with a wide amplitude (e.g., maize) are more widely adapted than those with a narrow amplitude (e.g., cocoa).

Ecosystem Concept

• An ecosystem is a functional unit of nature where living organisms (biotic community) interact among themselves and with their physical (abiotic) environment, leading to energy flow and nutrient cycling.

Components of an Agroecosystem (Agricultural Ecosystem):

Structure of an Ecosystem:

• Species Composition:The types and numbers of species present (e.g., monoculture vs. intercropping).
• Stratification:Vertical layers (e.g., in agroforestry: tree canopy, shrub layer, ground crop layer).
• Trophic Structure:The organization of organisms based on their source of energy (food chain: Producer → Primary Consumer → Secondary Consumer → Decomposer).

Functions of an Ecosystem:

• Energy Flow:Unidirectional flow of energy from the sun through producers to consumers and decomposers. (10% Law of Lindeman).
• Nutrient Cycling (Biogeochemical Cycles):Cyclic movement of nutrients (e.g., C, N, P, S) between biotic and abiotic components.
• Ecological Succession:The process of change in the ecosystem over time.

Productivity of Ecosystem

Productivity is the rate of biomass production per unit area per unit time. It is usually expressed in terms of energy (kcal/m²/yr) or dry organic matter (g/m²/yr).

Key Terms:

• Biomass:The total amount of living or organic matter in an ecosystem at any time. (g/m²)
• Standing Crop:The amount of biomass present at a given time.
• Primary Production:The synthesis of organic compounds from atmospheric or aqueous CO₂ by autotrophs.

Types of Productivity:

Primary Productivity:

• Gross Primary Productivity (GPP):The total rate of organic matter production by photosynthesis, including the organic matter used in respiration. GPP = NPP + R
• Net Primary Productivity (NPP):The rate of storage of organic matter after respiration. It is the biomass available for consumption by heterotrophs. NPP = GPP – R (where R is respiration loss).
  • This is the most important measure for agronomists.
  • World average NPP for agriculture: ~650 g/m²/yr (Much higher than grasslands but lower than forests).

Secondary Productivity: The rate of assimilation of food energy by consumers (herbivores, carnivores).

Factors Influencing Productivity in Agroecosystems:

• Solar Radiation:The primary source of energy. PAR (Photosynthetically Active Radiation, 400-700 nm) is crucial.
• Temperature:Affects all enzymatic processes, including photosynthesis and respiration.
• Water Availability:Critical for photosynthesis, nutrient uptake, and cell turgor.
• Nutrient Availability:Essential elements (N, P, K, etc.) are building blocks for biomass.
• Crop Characteristics:C₃ vs. C₄ plants. C₄ plants (e.g., Maize, Sorghum) have higher NPP than C₃ plants (e.g., Wheat, Rice) under high temperature and light due to lack of photorespiration.

Management Practices:

• Crop Rotation:Maintains soil fertility and breaks pest cycles.
• Intercropping/Multi-tier cropping:Better spatial resource use efficiency.
• Irrigation & Fertilization:Alleviates water and nutrient stress, the most common limiting factors.
• Pest and Weed Management:Reduces competition and losses to consumers.

Comparison of NPP (g/m²/yr) of Different Ecosystems (For Perspective):

• Tropical Rainforest: 2000-2500
• Agricultural Crops: 600-1500
• Temperate Grassland: 500-1500
• Open Ocean: 500-1000
• Deserts: <100

Important One-Liners for Quick Revision

• The study of relation of crops to environment is Crop Ecology.
• Liebig’s Law of Minimumstates that growth is controlled by the scarcest resource.
• Shelford’s Law of Tolerancedefines the minimum, optimum, and maximum range for a factor.
• The functional unit of ecology is the Ecosystem.
• The abiotic components of an agroecosystem are climatic and edaphic
• The ultimate source of energy for any ecosystem is the Sun.
• The flow of energy in an ecosystem is unidirectional.
• The cycling of materials in an ecosystem is cyclic.
• Gross Primary Productivity (GPP)is the total rate of photosynthesis.
• Net Primary Productivity (NPP) = GPP – Respiration losses.
• NPPrepresents the biomass available for harvest.
• C₄ plantshave higher potential productivity than C₃ plants in tropical regions.
• The most productive natural ecosystem is the Tropical Rainforest.
• The 10% energy transfer law between trophic levels was given by Lindeman.
• 1935 – A.G. Tansley: Coined the term Ecosystem → “System formed by interaction of organisms with their environment.
• 1942 – Raymond Lindeman: Introduced Trophic Dynamics Concept (energy flow in food chains).
• 1950s – Odum Brothers (Eugene & Howard): Developed modern ecosystem ecology; Eugene Odum = Father of Modern Ecology.