Organisms and PopulationsClass 12 Biology Notes

Introduction to Ecology

Ecology is the branch of biology that gives us a holistic view of the living world. It is the study of how organisms interact with each other and with their physical (abiotic) environment. The essence of ecology is understanding how individual organisms form organized groups like populations, communities, and ecosystems, all the way up to the entire biosphere.

Ecology is concerned with four main levels of biological organization:

1. Organisms: Individual living beings.
2. Populations: Groups of individuals of the same species.
3. Communities: Groups of different populations interacting in an area.
4. Biomes: Large-scale ecosystems with specific climate conditions.

This chapter will focus on ecology at the population level.

Father of Ecology in India: Ramdeo Misra

Ramdeo Misra (1908-1998) is known as the Father of Ecology in India.

• He earned his Ph.D. in Ecology in 1937 from Leeds University, UK.
• He established ecology research and teaching at the Banaras Hindu University (BHU), Varanasi.
• His research was foundational for understanding tropical ecosystems, including community succession, plant population responses, and nutrient cycling.
• He created the first postgraduate course in ecology in India.
• His efforts led to the establishment of the National Committee for Environmental Planning and Coordination (1972), which later became the Ministry of Environment and Forests (1984).

Populations

In nature, individuals of a species rarely live in isolation. They live in groups within a defined geographical area, where they share or compete for resources and can interbreed. This group of individuals is called a population.

Examples of a population include:

• All the cormorants in a wetland.
• Rats in an abandoned building.
• Teakwood trees in a forest.
• Bacteria in a culture plate.
• Lotus plants in a pond.

While an individual organism must adapt to its environment, it is at the population level that natural selection occurs, leading to the evolution of desired traits. Therefore, population ecology is a crucial field that connects ecology with population genetics and evolution.

Population Attributes

A population has certain characteristics, or attributes, that an individual organism does not.

• Birth Rates and Death Rates: An individual is born and dies, but a population has a birth rate (natality) and a death rate (mortality). These rates are expressed as per capita (per individual) changes.

  • Example: If a pond with 20 lotus plants produces 8 new plants in a year, the birth rate is $8/20 = 0.4$ offspring per lotus per year.
  • Example: If 4 fruit flies die in a lab population of 40 over a week, the death rate is $4/40 = 0.1$ individuals per fruit fly per week.

• Sex Ratio: An individual is either male or female, but a population has a sex ratio, which is the percentage of males and females in the population (e.g., 60% females and 40% males).

• Age Distribution and Age Pyramids: A population is composed of individuals of different ages. Plotting the percentage of individuals in different age groups creates a structure called an age pyramid. The shape of the pyramid indicates the growth status of the population:

  • (a) Expanding (Growing): A pyramid with a broad base, indicating a high proportion of young individuals.
  • (b) Stable: A more even distribution across age groups, resulting in a bell-shaped structure.
  • (c) Declining: A pyramid with a narrow base, indicating fewer young individuals than older ones.

• Population Size (Population Density): The size of a population, technically called population density (N), tells us about its status in the habitat. It's a key parameter for studying ecological processes like competition or predation.

  • Measurement: While total number is the most common measure, it's not always the best.
    • Percent Cover or Biomass: In cases like a forest with one huge banyan tree and 200 small carrot grass plants, simply counting numbers underestimates the banyan's role. Here, measuring the area covered (percent cover) or the total weight (biomass) is more meaningful.
    • Relative Density: When a population is huge and direct counting is impossible, relative density is used. For example, the number of fish caught per trap can estimate the total fish population in a lake. Similarly, tiger populations are often estimated using pug marks and fecal pellets.

Population Growth

The size of a population is not static; it changes over time due to factors like food availability, predation, and weather. These fluctuations are caused by four basic processes:

1. Natality (B): The number of births in the population during a given period. This increases population density.
2. Immigration (I): The number of individuals of the same species that move into the habitat from elsewhere. This also increases population density.
3. Mortality (D): The number of deaths in the population during a given period. This decreases population density.
4. Emigration (E): The number of individuals that leave the habitat and move elsewhere. This also decreases population density.

The relationship between these factors can be expressed in an equation. If $N_t$ is the population density at time $t$, then the density at time $t+1$ is: $N_{t+1} = N_t + [(B+I) - (D+E)]$

Population density increases if the sum of births and immigration $(B+I)$ is greater than the sum of deaths and emigration $(D+E)$.

Growth Models

Populations can exhibit specific patterns of growth over time.

Exponential Growth

When resources like food and space are unlimited, a population can grow at its full potential. This is called exponential growth.

The rate of change in population size ($dN/dt$) is given by: $dN/dt = (b-d) \times N$ where $b$ is the per capita birth rate and $d$ is the per capita death rate.

If we let $(b-d) = r$, where r is the intrinsic rate of natural increase, the equation becomes: $dN/dt = rN$

The value of 'r' is a key parameter for assessing the impact of any factor on population growth. For example, for the Norway rat, $r$ is 0.015, while for the flour beetle, it is 0.12.

When population size (N) is plotted against time (t), exponential growth produces a J-shaped curve.

The integral form of the exponential growth equation is: $N_t = N_0 e^{rt}$ Where:

• $N_t$ = Population density after time t
• $N_0$ = Population density at time zero
• $r$ = Intrinsic rate of natural increase
• $e$ = The base of natural logarithms (approx. 2.71828)

Logistic Growth

In reality, no population has unlimited resources. As a population grows, resources become limited, leading to competition among individuals. This slows down the growth rate. A given habitat can only support a maximum number of individuals, a limit known as the carrying capacity (K).

This more realistic growth pattern is called logistic growth. It includes several phases:

1. Lag Phase: Initial slow growth.
2. Acceleration Phase: Rapid growth.
3. Deceleration Phase: Growth slows down as it approaches the carrying capacity.
4. Asymptote: Growth stops, and the population size stabilizes around the carrying capacity (K).

When population size (N) is plotted against time (t), logistic growth produces a sigmoid or S-shaped curve.

This type of growth is described by the Verhulst-Pearl Logistic Growth equation: $dN/dt = rN \left(\frac{K-N}{K}\right)$ Where:

• $N$ = Population density at time t
• $r$ = Intrinsic rate of natural increase
• $K$ = Carrying capacity

Since resources are almost always finite, the logistic growth model is considered more realistic for most animal populations.

Life History Variation

Populations evolve to maximize their reproductive fitness, also called Darwinian fitness (a high 'r' value), in their specific habitat. This leads to different reproductive strategies.

• Some organisms, like the Pacific salmon fish and bamboo, breed only once in their lifetime.
• Others, like most birds and mammals, breed many times.
• Some produce a large number of small-sized offspring (e.g., oysters, pelagic fishes).
• Others produce a small number of large-sized offspring (e.g., birds, mammals).

These life history traits evolve based on the constraints imposed by the abiotic (non-living) and biotic (living) components of the habitat.

Population Interactions

In nature, no species can live in complete isolation. Organisms must interact with other species to form a biological community. These interactions between populations of two different species are called interspecific interactions. They can be beneficial (+), detrimental (-), or neutral (0) for the species involved.

Predation (+, -)

In this interaction, one species (the predator) kills and eats another (the prey). Herbivores eating plants are also considered predators in a broad ecological sense.

Roles of Predators:

1. Energy Transfer: Predation is nature's way of transferring energy fixed by plants to higher trophic levels.
2. Prey Population Control: Predators keep prey populations in check, preventing them from reaching densities that could cause ecosystem instability.
   • Example: The invasive prickly pear cactus in Australia was controlled only after introducing a cactus-feeding moth (a predator) from its natural habitat. This is the principle behind biological control of agricultural pests.
3. Maintaining Species Diversity: By reducing the intensity of competition among prey species, predators can help maintain diversity.
   • Example: In an experiment on the American Pacific Coast, removing the predator starfish Pisaster caused more than 10 species of invertebrates to go extinct due to increased interspecific competition.

Prey Defenses:

• Camouflage (Cryptic Coloration): Some insects and frogs blend in with their surroundings to avoid detection.
• Chemical Defense: Some animals are poisonous. The Monarch butterfly is distasteful to birds because it accumulates a toxic chemical from a poisonous weed it eats as a caterpillar.
• Plant Defenses:
  • Morphological: Thorns on plants like Acacia and Cactus.
  • Chemical: Plants produce chemicals that make herbivores sick, inhibit their digestion, or even kill them. Examples include the poisonous cardiac glycosides in Calotropis, as well as nicotine, caffeine, quinine, and opium.

Competition (-, -)

Competition is an interaction where both species are harmed. It occurs when species compete for the same limited resources.

• Competition can occur between closely related species or totally unrelated species. For example, in some South American lakes, visiting flamingoes and resident fish compete for the same food: zooplankton.
• Interference competition can occur even when resources are abundant. In this case, the presence of one species reduces the feeding efficiency of another.

Competition is best defined as a process where the fitness of one species (measured by its 'r' value) is significantly lower in the presence of another species.

Gause's Competitive Exclusion Principle: This principle states that two closely related species competing for the same limiting resources cannot coexist indefinitely; the competitively inferior one will eventually be eliminated.

• Example: The Abingdon tortoise in the Galapagos Islands became extinct within a decade after goats were introduced, likely due to the goats' superior browsing efficiency.

Mechanisms for Coexistence: Species facing competition may evolve ways to coexist rather than being eliminated.

• Competitive Release: A species whose distribution is restricted by a superior competitor can expand its range dramatically if the competitor is removed.
  • Example: On the rocky sea coasts of Scotland, the larger barnacle Balanus excludes the smaller barnacle Chathamalus from the intertidal zone. When Balanus is removed, Chathamalus expands its range.
• Resource Partitioning: If two species compete for the same resource, they can avoid competition by using it at different times or in different ways.
  • Example: MacArthur showed that five closely related species of warblers living on the same tree could coexist by having different foraging patterns and feeding times.

Parasitism (+, -)

In this interaction, one organism (the parasite) derives nutrition from another organism (the host), which is harmed. Parasitism has evolved in many groups, from plants to vertebrates.

• Co-evolution: Many parasites are host-specific and tend to co-evolve with their host. If the host develops a defense, the parasite must evolve a way to counteract it.
• Adaptations of Parasites:
  • Loss of unnecessary sense organs.
  • Presence of adhesive organs or suckers to cling to the host.
  • Loss of a digestive system.
  • High reproductive capacity.
• Types of Parasites:
  • Ectoparasites: Live on the external surface of the host. Examples include lice on humans, ticks on dogs, and the parasitic plant Cuscuta, which has lost its chlorophyll and gets nutrition from hedge plants.
  • Endoparasites: Live inside the host's body (e.g., liver, kidneys, red blood cells). Their life cycles are often more complex, sometimes involving intermediate hosts (e.g., the human liver fluke uses a snail and a fish).
• Brood Parasitism: A fascinating example where a parasitic bird (like the cuckoo or koel) lays its eggs in the nest of a host bird (like the crow), letting the host incubate them. The parasite's eggs have evolved to resemble the host's eggs to avoid detection and rejection.

Commensalism (+, 0)

This is an interaction where one species benefits, and the other is neither harmed nor benefited.

• Example: An orchid growing as an epiphyte on a mango branch gets a place to grow but does not harm or benefit the mango tree.
• Example: Barnacles growing on the back of a whale get a home and transportation, while the whale is unaffected.
• Example: The cattle egret forages near grazing cattle. As the cattle move, they stir up insects from the vegetation, making them easier for the egret to catch. The cattle are unaffected.
• Example: The clown fish lives among the stinging tentacles of a sea anemone. The fish gets protection from predators, while the anemone appears to derive no benefit.

Mutualism (+, +)

This is an interaction that benefits both interacting species.

• Example: Lichens are a mutualistic relationship between a fungus and photosynthesizing algae or cyanobacteria. The fungus provides shelter and absorbs nutrients, while the alga provides food through photosynthesis.
• Example: Mycorrhizae are associations between fungi and the roots of higher plants. The fungi help the plant absorb essential nutrients, and the plant provides the fungi with energy-yielding carbohydrates.
• Plant-Animal Mutualism: These are some of the most fascinating examples.
  • Pollination: Animals pollinate flowers in exchange for rewards like nectar and pollen.
  • Seed Dispersal: Animals eat juicy fruits and disperse the seeds.
  • Co-evolution: These relationships often involve co-evolution. The evolution of a flower is tightly linked to the evolution of its pollinator.
  • Example: Fig trees have a one-to-one relationship with their partner wasp species. The female wasp uses the fig fruit to lay her eggs and uses the developing seeds to nourish her larvae. In the process, she pollinates the fig's inflorescence.
  • Sexual Deceit: The Mediterranean orchid Ophrys has a petal that mimics the female of a bee species in size, color, and markings. The male bee attempts to 'pseudocopulate' with the flower, picking up pollen and transferring it to the next flower it visits. This is an example of how co-evolution works: if the female bee's appearance changes, the orchid must also co-evolve to maintain the resemblance.