What this chapter is about
This chapter introduces ecology at the level of individual organisms and groups of individuals of the same species (populations). It explores how organisms respond to their physical environment and how populations grow, change, and interact with each other over time. You will learn about the abiotic factors that influence where organisms can live, the physiological and behavioural responses organisms show to environmental changes, and the mathematical models that describe population growth.
Understanding ecology at this level is essential because it forms the foundation for studying larger ecological systems like communities and ecosystems in later chapters. A Class 12 student studying this chapter will be able to explain how temperature, water, light, and soil affect organisms, describe different types of interactions between species, and apply population growth equations to real biological situations.
After studying this chapter, you should be able to analyse how organisms adapt to their environment, calculate population density and growth rates, and distinguish between different types of species interactions such as competition, predation, parasitism, and mutualism.
Key ideas
- Ecology operates at four levels of biological organisation: organisms, populations, communities, and ecosystems; this chapter focuses on the first two levels.
- Abiotic factors such as temperature, water, light, and soil determine where organisms can survive and reproduce; each species has a specific range of tolerance for each factor.
- Organisms respond to environmental stress through three main strategies: regulation (maintaining internal constancy), conformation (allowing internal conditions to change with external environment), and migration or suspension of activities (dormancy, hibernation, aestivation).
- A population has attributes like density, natality (birth rate), mortality (death rate), age structure, and sex ratio that individuals do not possess.
- Population growth can be described by two models: exponential growth (dN/dt = rN) when resources are unlimited, and logistic growth (dN/dt = rN[(K - N)/K]) when resources are limited.
- Carrying capacity (K) is the maximum population size an environment can sustain indefinitely given available resources.
- Interspecific interactions include competition (both species harmed), predation and parasitism (one benefits, one harmed), commensalism (one benefits, other neutral), and mutualism (both benefit).
- Organisms have evolved various adaptations — morphological, physiological, and behavioural — that help them survive and reproduce in their specific habitats.
Formulas and facts to remember
- Population density (N) = Total number of individuals / Area or volume of habitat. It measures how many organisms occupy a unit space.
- Exponential growth equation: dN/dt = rN, where N is population size, t is time, and r is intrinsic rate of natural increase. This produces a J-shaped curve.
- Intrinsic rate of natural increase (r) = (Births – Deaths) / N. A positive r means population is growing.
- Logistic growth equation: dN/dt = rN[(K - N)/K], where K is carrying capacity. This produces an S-shaped (sigmoid) curve.
- Allen's rule: Mammals in colder climates tend to have shorter ears and limbs to reduce heat loss (smaller surface area to volume ratio).
- Bergmann's rule: Mammals in colder regions tend to be larger in body size than related species in warmer regions.
- Types of species interactions: Competition (– –), Predation (+–), Parasitism (+–), Commensalism (+0), Mutualism (++), Amensalism (–0).
- Gause's competitive exclusion principle: Two species competing for the same limited resource cannot coexist indefinitely; one will be eliminated.
Worked examples
Example 1: Calculating population density
In a forest patch of 5 hectares, a researcher counts 250 sal trees.
Solution: Population density = Total number of individuals / Area Population density = 250 trees / 5 hectares Population density = 50 trees per hectare
This tells us there are, on average, 50 sal trees in every hectare of this forest patch.
Example 2: Applying the exponential growth model
A bacterial culture starts with 1000 cells. The intrinsic rate of increase (r) is 0.5 per hour. Calculate the population size after 4 hours using the integrated form Nt = N₀ × e^(rt).
Solution: Given: N₀ = 1000 cells, r = 0.5 per hour, t = 4 hours Using Nt = N₀ × e^(rt) Nt = 1000 × e^(0.5 × 4) Nt = 1000 × e^2 Nt = 1000 × 7.389 Nt = 7389 cells (approximately)
The population grows from 1000 to about 7389 cells in 4 hours under unlimited resources.
Example 3: Understanding logistic growth
A pond can support a maximum of 500 fish (K = 500). Currently, there are 100 fish (N = 100). If r = 0.1 per month, what is the population growth rate?
Solution: Using dN/dt = rN[(K - N)/K] dN/dt = 0.1 × 100 × [(500 - 100)/500] dN/dt = 10 × [400/500] dN/dt = 10 × 0.8 dN/dt = 8 fish per month
When the population is well below carrying capacity, it grows relatively fast. As N approaches K, growth rate will slow to zero.
Common mistakes
- Confusing exponential and logistic growth → Exponential (J-curve) assumes unlimited resources; logistic (S-curve) includes carrying capacity limiting growth.
- Thinking carrying capacity (K) is fixed forever → K can change with environmental conditions such as drought or habitat destruction.
- Mixing up parasitism and predation → In predation, the prey is usually killed; in parasitism, the host is harmed but typically kept alive.
- Believing competition always means fighting → Competition often involves indirect resource use (exploitative competition), not direct aggression.
- Assuming all organisms regulate their body temperature → Only homeotherms (birds and mammals) regulate; most animals are conformers whose body temperature varies with the environment.
Quick revision
- Ecology studies organism-environment interactions at four levels: organism, population, community, ecosystem.
- Organisms cope with stress by regulating, conforming, migrating, or entering dormancy (hibernation in winter, aestivation in summer).
- Exponential growth (dN/dt = rN) gives J-curve; logistic growth with carrying capacity (dN/dt = rN[(K-N)/K]) gives S-curve.
- Population attributes include density, natality, mortality, age distribution, and sex ratio.
- Mutualism benefits both species; parasitism and predation benefit one at the cost of another; competition harms both.
- Adaptations can be morphological (thick cuticle in desert plants), physiological (producing antifreeze proteins), or behavioural (migration).