Ecology is the study of the interactions among organisms and between organisms and their physical environment. This chapter covers two levels of ecological organisation: organisms (their adaptations to the environment) and populations (groups of individuals of the same species living in an area).
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The Abiotic Environment
Organisms live in a wide variety of environments characterised by several major abiotic factors:
1. Temperature: Most organisms are adapted to a narrow temperature range. Organisms that can tolerate a wide temperature range are eurythermal; those restricted to a narrow range are stenothermal. Most biochemical reactions and enzyme activity are temperature-sensitive.
2. Water: Water availability determines the distribution of life on land. Water is critical for all metabolic processes. Aquatic organisms face problems of osmotic balance — marine fish continuously lose water by osmosis and must drink sea water; freshwater fish gain water by osmosis.
3. Light: Sunlight is the ultimate energy source for autotrophs (photosynthesis). Light intensity, quality, and duration (photoperiod) influence plant flowering, animal reproduction, and migration.
4. Soil: Soil type (texture, composition, pH, minerals) determines plant distribution and agricultural productivity.
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Adaptations of Organisms
- Desert Adaptations (Kangaroo rat):
- Never drinks water — metabolic water from oxidation of fat in food suffices
- Very concentrated urine to conserve water
- Temperature Adaptations:
- Thermoregulation: Mammals and birds maintain constant internal body temperature despite external variation (endotherms / homoiotherms).
- Allen's Rule: Animals in cold climates have shorter extremities (ears, limbs) to reduce heat loss. Example: Arctic fox has smaller ears than desert fox.
- Hibernation: deep sleep in winter (bears, bats) to conserve energy when food is scarce.
- Aestivation: summer dormancy in snails and fish like lungfish.
- Aquatic Adaptations:
- Deep-sea fish can withstand enormous pressure.
- Many aquatic invertebrates are conformers (internal osmolarity changes with environment); mammals and birds are regulators (maintain constant internal environment).
- Altitude Adaptations (high-altitude sickness):
- At high altitudes, atmospheric oxygen is low. Body responds by increasing breathing rate, increasing RBC count, and producing more haemoglobin (acclimatisation).
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Population Attributes
A population is a group of individuals of the same species living in a defined area at a given time.
Key attributes of populations:
- 1. Birth Rate (Natality): Number of births per individual per unit time.
- Example: A pond population of water hyacinth increases from 10 to 300 in a year: birth rate = (300 - 10) / 10 = 29 per individual per year.
2. Death Rate (Mortality): Number of deaths per individual per unit time.
3. Sex Ratio: Proportion of males to females in a population.
- 4. Age Distribution (Age Pyramid):
- Expanding population: broad base (many young individuals)
- Stable population: bell-shaped
- Declining population: narrow base (few young individuals)
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Population Growth
Exponential Growth (J-curve):
When resources are unlimited, a population grows exponentially:
dN/dt = rN
Where: N = population size, t = time, r = intrinsic rate of natural increase (r = b - d, where b = birth rate per capita, d = death rate per capita)
Integrated form: Nt = N0 × ert
Under exponential growth, the population J-shaped growth curve is produced.
Logistic Growth (S-curve / Sigmoid curve):
In nature, resources are finite. As population grows, it experiences competition, disease, and predation. Growth slows as population approaches the Carrying Capacity (K) — maximum population the environment can sustainably support.
dN/dt = rN × (K - N)/K
The term (K - N)/K is the unused growth capacity. When N is small, (K - N)/K approaches 1 (exponential growth). When N approaches K, (K - N)/K approaches 0 (no growth).
The sigmoid (S-shaped) growth curve has an inflection point at N = K/2, where growth rate is maximum.
Darwin's fitness: Species with logistic growth that best allocate resources to reproduction and survival are most fit.
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Population Interactions
Interactions between two species designated (+), (-), or (0):
| Interaction | Species A | Species B |
|---|---|---|
| Mutualism | + | + |
| Competition | - | - |
| Predation | + | - |
| Parasitism | + | - |
| Commensalism | + | 0 |
| Amensalism | - | 0 |
- Predation:
- Controls prey populations; keeps ecosystems in balance
- Camouflage (stick insect), warning colouration (Monarch butterfly — toxic, bright orange/black)
- Batesian mimicry: harmless species mimics a harmful species (viceroy mimics monarch)
- Mullerian mimicry: two harmful species mimic each other
Competition (Gause's Principle / Competitive Exclusion Principle):
Two species competing for the same limited resource cannot coexist stably — one will outcompete the other. Gause demonstrated this with Paramecium. However, resource partitioning allows related species to coexist by using different aspects of the same resource.
- Parasitism:
- Parasite benefits; host is harmed.
- Co-evolution: host and parasite evolve together — hosts develop immune defences; parasites evolve to evade them.
- Brood parasitism: cuckoo lays eggs in crow's nest — cuckoo chick outcompetes crow chicks.
- Mutualism (Symbiosis):
- Both benefit: mycorrhiza (fungi + plant roots), lichens (fungi + algae), Rhizobium + legumes.
- Fig and fig wasp: obligate mutualism — the fig provides habitat for the wasp; the wasp pollinates the fig. Neither can survive without the other.
- Commensalism:
- One benefits; other unaffected: barnacles on whale body; orchids growing on tree branches (use the tree for support, no harm to tree).
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A population of 500 rabbits has an intrinsic rate of increase (r) of 0.2 per year. What is the rate of increase after one year if resources are unlimited?
Using dN/dt = rN: dN/dt = 0.2 × 500 = 100 rabbits per year.
Define carrying capacity (K) and explain what happens when N > K.
K is the maximum population size an environment can support given available resources. When N exceeds K, resources become insufficient — death rate exceeds birth rate, and the population declines back toward K.
Explain Gause's Competitive Exclusion Principle with an example.
Gause showed that when two Paramecium species (P. caudatum and P. aurelia) competed for the same food resource in a single medium, P. caudatum was always eliminated; P. aurelia outcompeted it. The principle states: two species competing for identical resources cannot coexist indefinitely.
How does the Monarch butterfly avoid predation without physical defences?
Monarch butterflies accumulate toxins (cardiac glycosides) from milkweed plants they eat as larvae. Their bright orange-black colouration is a warning (aposematic) signal to predators. Birds that eat them become ill and learn to avoid them. The Viceroy butterfly (non-toxic) mimics this appearance — a form of Batesian mimicry.
What is brood parasitism? Give an example.
Brood parasitism is when a parasite lays its eggs in the nest of a host species, which then raises the parasite's young. The Asian Koel (cuckoo) lays eggs in crow nests; the koel egg often mimics crow eggs. The koel chick may push out crow eggs/chicks and receives all parental care.
Calculate r (intrinsic rate of natural increase) if b = 0.06 and d = 0.02 per individual per year.
r = b - d = 0.06 - 0.02 = 0.04 per individual per year.
In a logistic growth model with K = 1000 and N = 500, what fraction of growth potential is being realised?
Unused growth capacity = (K - N)/K = (1000 - 500)/1000 = 500/1000 = 0.5 or 50%. Growth rate is at half its maximum potential — this is also the inflection point of the S-curve.
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- Key Formulas
- Exponential growth: dN/dt = rN; Nt = N0 × ert
- Logistic growth: dN/dt = rN × (K - N)/K
- r = b - d (intrinsic rate of natural increase)
- Maximum growth rate in logistic model: when N = K/2
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Common mistakes
Students often confuse exponential growth (unlimited resources, J-curve) with logistic growth (limited resources, S-curve). K is the UPPER LIMIT (carrying capacity), not the point of maximum growth. Maximum growth rate occurs at N = K/2, not at N = K. Mutualism (+/+) differs from commensalism (+/0) — in commensalism, one species is completely unaffected.
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Summary
Organisms adapt to their abiotic environment through structural, behavioural, and physiological mechanisms. Populations grow exponentially under ideal conditions but logistically in nature due to limited resources (carrying capacity K). Species interactions — predation, competition, parasitism, mutualism — shape community structure and evolution.