Organisms and Populations — Study Notes

1. Introduction

Ecology is the study of the interactions between organisms and their environment. The chapter Organisms and Populations explores how individual organisms interact with their abiotic and biotic environment, and how populations of organisms change and adapt. The study of ecology begins with the organism, the basic unit of ecology, and proceeds to populations, which are groups of individuals of the same species living in a given area. The chapter covers the concepts of habitat and niche, the abiotic and biotic components of the environment, and the responses of organisms to environmental factors.

The chapter also introduces population ecology, which deals with the properties of populations, including population size, density, growth, age structure and interactions between populations such as predation, competition, parasitism and mutualism. Understanding the relationships between organisms and their environment is essential for appreciating the dynamics of ecosystems, biodiversity and environmental conservation covered in the subsequent chapters. Population ecology also has practical applications in resource management, agriculture and conservation biology.

2. Organisms and Their Environment

The environment of an organism can be divided into two main components: the abiotic (non-living) and the biotic (living) components. The abiotic components include temperature, water, light and soil, while the biotic components include other organisms with which an organism interacts.

2.1 Major Abiotic Factors

• Temperature: The most ecologically relevant environmental factor. Temperature affects the kinetics of enzymes and the metabolic activity of organisms. The average temperature on land varies seasonally, and organisms may be cold adapted or warm adapted. Some organisms are stenothermal (able to tolerate only a narrow range of temperature), while others are eurythermal (able to tolerate a wide range).
• Water: Essential for life. The distribution of plants and animals is greatly affected by the availability of water. Aquatic organisms face problems of water balance and concentration of solutes. Organisms are classified as stenohaline (tolerate narrow salinity range) or euryhaline (tolerate wide salinity range).
• Light: The source of energy for photosynthesis. Light affects the distribution of plants and the behaviour of animals, such as diurnal and nocturnal rhythms.
• Soil: The physical and chemical characteristics of soil determine the type of vegetation. The pH, mineral content and water holding capacity of soil affect plant distribution.

2.2 Responses of Organisms to Abiotic Factors

Organisms respond to abiotic factors in three ways: - Regulate: Some organisms can maintain constant body temperature and osmotic concentration despite changes in the environment. Birds and mammals regulate their body temperature, while some freshwater organisms regulate their osmotic concentration. This regulatory ability uses energy. - Conform: Most organisms cannot maintain a constant internal environment and conform to the external conditions. For example, most fish and many aquatic animals conform to the temperature of the water. Conforming organisms save energy but cannot tolerate wide fluctuations. - Migrate: Some organisms temporarily move away from stressful conditions. For example, migratory birds move to warmer regions during winter, and many marine animals migrate to avoid unfavourable conditions.

In addition, many organisms have evolved adaptations, which are special features that help them survive in their environment. For example, the kangaroo rat in the desert produces very concentrated urine and does not drink water, surviving on the water from its metabolic processes.

3. Habitat and Niche

• Habitat: The habitat is the physical environment where an organism lives. For example, the pond is the habitat of a fish.
• Niche: The niche is the functional role or position of an organism in its ecosystem, including its activities, its interactions with other organisms and its use of resources. The niche is not just the place where an organism lives but its way of life. For example, two fish species may live in the same pond (same habitat) but occupy different niches by feeding at different times or on different food.

4. Populations and Population Attributes

A population is a group of individuals of the same species living in a given area at a given time. Population ecology is the study of the interactions between a population and its environment. The main attributes of a population are: - Birth rate (natality): The number of births per individual per unit time. - Death rate (mortality): The number of deaths per individual per unit time. - Sex ratio: The ratio of males to females in the population. - Age distribution: The proportion of individuals of different ages in the population. The population is often divided into pre-reproductive, reproductive and post-reproductive stages. - Population density: The number of individuals per unit area or volume.

4.1 Population Growth

Population size changes over time through births, deaths, immigration and emigration. The two main models of population growth are: - Exponential growth: When resources are unlimited, the population grows exponentially, described by the equation dN/dt = rN, where N is the population size and r is the intrinsic rate of natural increase. This results in a J-shaped growth curve. - Logistic growth: In nature, resources are limited, and the population growth slows down as it approaches the carrying capacity (K), the maximum population size that the environment can sustain. The equation is dN/dt = rN((K-N)/K). This results in an S-shaped (sigmoid) growth curve. The population initially grows exponentially but slows down as it approaches K.

4.2 Life History Variation

Organisms show different life history strategies. Some species (r-selected species) produce many small offspring with little parental care, grow rapidly and have a high population growth rate. Others (K-selected species) produce few large offspring with much parental care, grow slowly and maintain populations near the carrying capacity.

5. Population Interactions

The interactions between populations of different species can be classified based on their effect on each other. The important interactions are:

5.1 Predation

Predation is the interaction in which one organism (predator) kills and eats another organism (prey). Predation is an important natural selection pressure that drives the evolution of prey species, such as the evolution of colouration (cryptic colouration), speed and camouflage. Predators also help in regulating the prey population. The concept of predator-prey relationships is important in controlling pest populations, e.g., biological control.

5.2 Competition

Competition is the interaction between organisms for limited resources such as food, space and mates. Competition occurs between individuals of the same species (intraspecific) or between individuals of different species (interspecific). In interspecific competition, one species may be eliminated if the competing species is superior. The competitive exclusion principle states that two species with identical requirements cannot coexist in the same ecological niche.

5.3 Parasitism

Parasitism is the interaction in which one organism (parasite) lives on or in another organism (host) and derives nutrients from it, harming the host. Parasites may be ectoparasites (living on the surface, e.g., lice on humans, ticks on cattle) or endoparasites (living inside the body, e.g., tapeworm in the intestine). Parasites have evolved special adaptations, such as the loss of unnecessary organs and high reproductive capacity.

5.4 Commensalism

Commensalism is the interaction in which one species benefits while the other is neither harmed nor benefited. For example, the orchid growing on a mango tree (the orchid gets support, the mango is unaffected), barnacles on the back of a whale, and the cattle egret feeding on insects disturbed by grazing cattle.

5.5 Mutualism

Mutualism is the interaction in which both species benefit. Examples include: - Lichens, which are a mutualistic association of an alga and a fungus. - Mycorrhiza, the association between a fungus and plant roots. - The mutualistic association between nitrogen fixing bacteria (Rhizobium) and leguminous plants. - Pollination mutualisms between plants and their insect pollinators, e.g., the fig and the fig wasp. - The coral reef association between corals and algae.

5.6 Amensalism

Amensalism is the interaction in which one species is harmed while the other is unaffected. For example, the release of antibiotics by one species (e.g., Penicillium) inhibits the growth of other species.

Quick Revision Tables

Table 1: Abiotic Factors and Responses

Table 2: Population Interactions

Table 3: Population Growth Models

Mind Map

Important Diagrams (SVG)

Common Mistakes

1. Students confuse habitat with niche; the habitat is where an organism lives, while the niche is its functional role and way of life.
2. The exponential growth model is said to apply in nature; it applies only when resources are unlimited, while logistic growth applies in real, resource-limited conditions.
3. In logistic growth, the population is said to grow beyond the carrying capacity indefinitely; it levels off at the carrying capacity K.
4. Conforming is said to be used by birds and mammals; birds and mammals regulate their temperature, while most other organisms conform.
5. Predation is said to always reduce prey to extinction; predators help regulate prey populations, and predation drives prey evolution, but they do not generally eliminate prey.
6. Commensalism is confused with mutualism; in commensalism one species benefits and the other is unaffected, while in mutualism both benefit.
7. Lichens are said to be a parasitic association; lichens are a mutualistic association of an alga and a fungus.
8. The sex ratio and age distribution are said to be individual attributes; they are population attributes, not properties of individuals.

Exam Tips

1. Memorise the four major abiotic factors: temperature, water, light and soil, with one effect of each.
2. Learn the three responses of organisms to the environment: regulate (birds, mammals), conform (most others), migrate (birds), plus adaptation (kangaroo rat).
3. The exponential (dN/dt = rN, J-shaped) and logistic (dN/dt = rN((K-N)/K), S-shaped) growth equations and curves are high-frequency questions.
4. List the population attributes: birth rate, death rate, sex ratio, age distribution, population density.
5. The population interaction table with symbols (+/-/0) and one example of each is guaranteed scoring: predation, competition, parasitism, commensalism, mutualism, amensalism.
6. Learn one example per interaction: predation (lion-deer), competition (two species), parasitism (tapeworm), commensalism (orchid on mango), mutualism (lichens), amensalism (Penicillium).
7. Distinguish stenothermal/eurythermal and stenohaline/euryhaline organisms.

Conclusion

Organisms and populations form the foundational level of ecology, connecting individual survival with the dynamics of species in nature. The chapter examines how organisms interact with the abiotic environment of temperature, water, light and soil, and how they respond through regulation, conformity, migration and adaptation. The distinction between habitat and niche clarifies that an organism's environment includes not only where it lives but how it lives. Population ecology then describes the attributes that define populations and the two models of growth, exponential and logistic, that explain how populations change under different conditions. Finally, the web of interactions between populations, from predation and competition to mutualism and amensalism, reveals the complex balance that sustains ecological communities. This understanding of organisms and populations prepares the student for the study of ecosystems, where these interactions are integrated into the flow of energy and matter.