Organisms and Populations

Ecology: The Study of Interactions

• Ecology is the study of interactions among organisms and between the organisms and their physical (abiotic) environment.
• It offers a holistic perspective on biology, connecting different areas of biological information into a unifying principle.
• Biological understanding focuses on how individual organisms interact with other organisms and physical habitats as organized wholes, such as populations, communities, ecosystems, or even the entire biosphere.
• Ecology addresses issues like anthropogenic environmental degradation and related socio-political concerns.
• Ecology is primarily concerned with four levels of biological organisation:
  • Organisms
  • Populations
  • Communities
  • Biomes
• When observing nature, we can ask two types of questions about biological processes:
  • ‘How-type’ questions: Seek the mechanism behind a process (e.g., How does a bird sing? The answer relates to its voice box and vibrating bone).
  • ‘Why-type’ questions: Seek the significance of a process (e.g., Why does a bird sing? The answer relates to communicating with a mate during breeding season).

Father of Ecology in India: Ramdeo Misra

• Ramdeo Misra (1908-1998) is widely recognised as the Father of Ecology in India.
• He earned his Ph.D. in Ecology in 1937 from Leeds University, UK.
• He pioneered teaching and research in ecology at the Department of Botany, Banaras Hindu University, Varanasi.
• His research established foundational understanding of:
  • Tropical communities and their succession
  • Environmental responses of plant populations
  • Productivity and nutrient cycling in tropical forest and grassland ecosystems
• Misra developed the first postgraduate course in ecology in India.
• Over 50 scholars completed their Ph.D. under his guidance, spreading ecology teaching and research across the country.
• He received prestigious honours, including fellowships from the Indian National Science Academy and World Academy of Arts and Science, and the Sanjay Gandhi Award in Environment and Ecology.
• His efforts led to the formation of the National Committee for Environmental Planning and Coordination (1972), which later paved the way for the Ministry of Environment and Forests (1984).

Organisms and Populations

11.1 Populations

• In natural environments, individual organisms of a species rarely live in isolation; they typically exist in groups within a defined geographical area.
• These groups share or compete for similar resources, have the potential to interbreed, and collectively form a population.
• Even groups resulting from asexual reproduction are generally considered populations for ecological studies.
• Examples of populations include all cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate, or lotus plants in a pond.
• While individual organisms adapt to environmental changes, natural selection operates at the population level to facilitate the evolution of desired traits.
• Population ecology is a critical field as it forms a link between ecology, population genetics, and evolution.

11.1.1 Population Attributes

Populations possess collective characteristics that are not exhibited by individual organisms.

• Birth Rates and Death Rates:

  • While an individual experiences birth and death, a population has measurable birth rates and death rates.
  • These are expressed as per capita rates, indicating changes in numbers (increase or decrease) relative to the existing population size.
  • Birth Rate Example: If a pond initially has 20 lotus plants and 8 new plants are added through reproduction, the birth rate is 8/20 = 0.4 offspring per lotus per year.
  • Death Rate Example: If 4 fruitflies die in a laboratory population of 40 during a week, the death rate is 4/40 = 0.1 individuals per fruitfly per week.

• Sex Ratio:

  • An individual is either male or female, but a population exhibits a sex ratio.
  • Example: A population might have a sex ratio of 60% females and 40% males.

• Age Distribution (Age Pyramid):

  • At any given time, a population consists of individuals across various age groups.
  • Age distribution refers to the percentage of individuals within specific age groups.
  • Plotting this distribution for a population creates an age pyramid, which for human populations typically illustrates the age distribution of both males and females.
  • The shape of the pyramid provides insight into the population’s growth status: indicating whether it is (a) growing, (b) stable, or (c) declining.

• Population Size (Population Density - N):

  • The size of a population (N) reveals significant information about its status within a habitat.
  • Population size can vary widely, from very low (e.g., <10 Siberian cranes at Bharatpur wetlands) to millions (e.g., Chlamydomonas in a pond).
  • Measurement Methods:
    • While total number is often the most appropriate measure, it can be meaningless or difficult to ascertain in certain cases.
    • For example, counting 200 carrot grass plants versus a single large banyan tree by number alone can underestimate the banyan’s ecological role; in such cases, percent cover or biomass are more meaningful measures.
    • Counting can be impossible or time-consuming for huge populations (e.g., dense bacterial cultures).
    • Sometimes, relative densities are sufficient for ecological investigations (e.g., number of fish caught per trap as a measure of total population density in a lake).
    • Indirect estimation is frequently employed, such as using pug marks and fecal pellets for tiger censuses in national parks.

11.1.2 Population Growth

The size of a population is a dynamic parameter, constantly changing based on factors like food availability, predation pressure, and adverse weather conditions. These fluctuations in population density offer insights into whether a population is flourishing or declining.

Population density in a given habitat over a period is influenced by four basic processes:

• Natality (B): The number of births that add to the initial population density during a specific period.
• Immigration (I): The number of individuals of the same species that enter the habitat from elsewhere during the specified time.
• Mortality (D): The number of deaths occurring in the population during a given period.
• Emigration (E): The number of individuals who leave the habitat and move elsewhere during the specified time.

The population density at time t+1 (N t+1) can be calculated using the equation: N t+1 = N t + [(B + I) – (D + E)]

• Population density will increase if the sum of births and immigration is greater than the sum of deaths and emigration.
• Births and deaths are typically the most significant factors affecting population density. However, immigration can become particularly important when a new habitat is being colonised.

Growth Models

Population growth over time often exhibits specific and predictable patterns.

• Exponential Growth (J-shaped curve):

  • This growth pattern occurs when resources (food and space) in the habitat are unlimited.
  • Under such ideal conditions, each species can fully realise its innate potential to grow exponentially or geometrically, as observed by Darwin in his theory of natural selection.
  • The equation describing exponential growth is: dN/dt = rN
    • dN/dt: Represents the change in population size (N) over a unit time period (t).
    • b: Per capita birth rates.
    • d: Per capita death rates.
    • r: The intrinsic rate of natural increase, calculated as (b – d). This is a crucial parameter for evaluating the impact of any biotic or abiotic factor on population growth.
  • The integral form of the exponential growth equation is: N t = N 0 ert
    • N t: Population density after time t.
    • N 0: Population density at time zero.
    • e: The base of natural logarithms (approximately 2.71828).
  • When population density (N) is plotted against time, exponential growth results in a J-shaped curve.
  • Species growing exponentially in unlimited resource conditions can achieve enormous population densities very quickly.
  • Examples of ‘r’ values: For the Norway rat, r is 0.015; for the flour beetle, it is 0.12. In 1981, the r value for the human population in India was 0.0205.
  • Anecdote: The story of a king and a minister playing chess, where grains of wheat are doubled on each square, dramatically illustrates how rapidly a huge population can build up under exponential growth.
  • Paramecium Example: A single Paramecium doubling daily through binary fission would reach a mind-boggling population size in 64 days, assuming unlimited food and space.

• Logistic Growth (Sigmoid or S-shaped curve):

  • This model is considered more realistic because in nature, no species has unlimited resources at its disposal, and resources eventually become limiting.
  • Limited resources lead to competition among individuals.
  • Carrying Capacity (K): In a given habitat, there are enough resources to support a maximum possible number of individuals for a species. This limit is called nature’s carrying capacity (K), beyond which no further population growth is possible.
  • A population growing with limited resources typically shows distinct phases:
    1. An initial lag phase.
    2. Followed by phases of acceleration.
    3. Then deceleration.
    4. And finally, an asymptote, where the population density stabilises at the carrying capacity (K).
  • When population density (N) is plotted against time (t), this type of growth results in a sigmoid (S-shaped) curve.
  • This growth pattern is known as Verhulst-Pearl Logistic Growth.
  • The equation for logistic growth is: dN/dt = rN [(K - N)/K]
    • N: Population density at time t.
    • r: Intrinsic rate of natural increase.
    • K: Carrying capacity.

11.1.3 Life History Variation

• Populations evolve to maximise their reproductive fitness, also known as Darwinian fitness (achieving a high ‘r’ value), within their specific habitat.
• Under particular selection pressures, organisms develop the most efficient reproductive strategies.
• Variations in life history traits include:
  • Breeding Frequency: Some organisms breed only once in their lifetime (e.g., Pacific salmon fish, bamboo), while others breed many times (e.g., most birds and mammals).
  • Offspring Size and Number: Some produce a large number of small-sized offspring (e.g., oysters, pelagic fishes), whereas others produce a small number of large-sized offspring (e.g., birds, mammals).
• Ecologists propose that the life history traits of organisms have evolved in response to the constraints imposed by the abiotic and biotic components of their environment.
• The evolution of these life history traits across different species is currently a significant area of ecological research.

11.1.4 Population Interactions

No natural habitat on Earth is inhabited by a single species alone; such a situation is inconceivable. For any species, at least one other species is minimally required for survival, even if it’s just for food or nutrient cycling. Therefore, animals, plants, and microbes do not and cannot live in isolation but interact in diverse ways to form a biological community.

Interspecific interactions occur between populations of two different species. Their outcomes can be classified as beneficial (+), detrimental (-), or neutral (0) for one or both species.

Predation

• Predation is often considered nature’s mechanism for transferring energy from autotrophic organisms (plants) to higher trophic levels.
• While a tiger eating a deer is a classic example, a sparrow eating a seed is also a form of predation. Herbivores (animals eating plants) are broadly categorised as predators in an ecological context.
• Important Roles of Predators:
  • Control Prey Populations: Predators prevent prey species from achieving excessively high population densities, which could otherwise lead to ecosystem instability.
    • Example: The invasive prickly pear cactus, introduced to Australia in the early 1920s, spread rapidly due to the absence of natural predators. It was brought under control only after the introduction of a cactus-feeding moth from its native habitat. Biological control methods in agricultural pest management are based on this principle.
  • Maintain Species Diversity: Predators help maintain species diversity within a community by reducing the intensity of competition among different prey species.
    • Example: In the rocky intertidal communities of the American Pacific Coast, the starfish Pisaster is a crucial predator. When all starfish were experimentally removed from an enclosed area, more than 10 invertebrate species became extinct within a year due to intense interspecific competition.
• Prudent Predators: Predators in nature are typically “prudent”; if a predator becomes too efficient and overexploits its prey, the prey might go extinct, leading to the predator’s own extinction due to lack of food.
• Prey Defenses against Predation: Prey species have evolved various mechanisms to lessen the impact of predation.
  • Cryptic Coloration (Camouflage): Some insects and frogs are camouflaged to avoid easy detection by predators.
  • Chemical Defenses: Some prey are poisonous and are thus avoided by predators.
    • Example: The Monarch butterfly is highly distasteful to its bird predators because it acquires a special chemical by feeding on a poisonous weed during its caterpillar stage.
  • Plant Defenses against Herbivores: Plants, unlike animals, cannot run away from predators (herbivores). They have evolved an astonishing array of defenses.
    • Morphological Defenses: Thorns (e.g., Acacia, Cactus) are common examples.
    • Chemical Defenses: Many plants produce and store chemicals that, when ingested by herbivores, can cause sickness, inhibit feeding or digestion, disrupt reproduction, or even be lethal.
      • Examples: The weed Calotropis produces highly poisonous cardiac glycosides, which deters cattle and goats from browsing on it. Various commercially extracted chemical substances (e.g., nicotine, caffeine, quinine, strychnine, opium) are naturally produced by plants as defenses against grazers and browsers.

Competition

• Darwin believed that interspecific competition is a potent force in organic evolution, leading to the “struggle for existence” and “survival of the fittest”.
• Definition: Competition is best defined as a process in which the fitness of one species (measured by its ‘r’ or intrinsic rate of increase) is significantly lower when another species is present.
• Misconceptions about Competition:
  • Not limited to closely related species: Unrelated species can also compete for the same resource. For instance, flamingoes and resident fish in some South American lakes compete for zooplankton.
  • Resources need not be limiting: In interference competition, the feeding efficiency of one species can be reduced simply by the interfering or inhibitory presence of another species, even if resources (food and space) are abundant.
• Evidence for Competition:
  • Laboratory Experiments: Ecologists like Gause demonstrated that under limited resource conditions, the competitively superior species eventually eliminates the other.
  • Circumstantial Evidence in Nature:
    • Competitive Exclusion: The Abingdon tortoise in the Galapagos Islands went extinct within a decade after goats were introduced, apparently due to the goats’ greater browsing efficiency.
    • Competitive Release: A species whose distribution is restricted to a small geographical area due to the presence of a superior competitor often expands its distributional range dramatically when the competing species is experimentally removed.
      • Example: Connell’s field experiments on the rocky sea coasts of Scotland showed that the larger, competitively superior barnacle Balanus dominates the intertidal zone, excluding the smaller barnacle Chathamalus.
• Impact: Herbivores and plants generally appear to be more adversely affected by competition than carnivores.
• Gause’s ‘Competitive Exclusion Principle’: States that two closely related species competing for the same limiting resources cannot co-exist indefinitely, and the competitively inferior one will ultimately be eliminated.
  • However, more recent studies suggest that while interspecific competition occurs, species facing competition may evolve mechanisms to promote co-existence rather than exclusion.
• Mechanism for Co-existence:
  • Resource Partitioning: If two species compete for the same resource, they can avoid direct competition by choosing different times for feeding or adopting different foraging patterns.
    • Example: MacArthur observed that five closely related species of warblers living on the same tree were able to co-exist due to behavioral differences in their foraging activities.

Parasitism

• The parasitic mode of life, offering “free lodging and meals,” has evolved across many taxonomic groups, from plants to higher vertebrates.
• Many parasites are host-specific, meaning they can parasitise only a single host species. This often leads to co-evolution between the host and the parasite, where evolutionary changes in one trigger counter-adaptations in the other.
• Parasite Adaptations: In accordance with their lifestyles, parasites have evolved special adaptations:
  • Loss of unnecessary sense organs.
  • Presence of adhesive organs or suckers to cling to the host.
  • Loss of their own digestive system.
  • High reproductive capacity.
• Complex Life Cycles: Parasite life cycles are frequently complex, involving one or two intermediate hosts or vectors to facilitate their transmission to the primary host.
  • Examples: The human liver fluke (a trematode) depends on two intermediate hosts (a snail and a fish) to complete its life cycle. The malarial parasite requires a mosquito as a vector to spread to other hosts.
• Harm to Host: Most parasites harm their hosts, leading to reduced survival, growth, and reproduction. They can also decrease the host’s population density and make the host more vulnerable to predation by weakening it physically.
• Types of Parasites:
  • Ectoparasites: Live on the external surface of the host organism.
    • Examples: Lice on humans, ticks on dogs, ectoparasitic copepods on marine fish. Cuscuta, a parasitic plant, has lost its chlorophyll and leaves and derives nutrition from its host plant.
  • Endoparasites: Live inside the host body at various sites (e.g., liver, kidney, lungs, red blood cells).
    • Their life cycles are more complex due to extreme specialization.
    • Their morphological and anatomical features are greatly simplified, with a strong emphasis on their reproductive potential.
• Brood Parasitism in Birds: A fascinating example where a parasitic bird lays its eggs in the nest of its host bird, and the host incubates them.
  • Through evolution, the eggs of the parasitic bird have come to resemble the host’s eggs in size and color, reducing the likelihood of the host detecting and ejecting the foreign eggs.
  • Example: The cuckoo (koel) laying eggs in the crow’s nest.

Commensalism

• This is an interaction where one species benefits, while the other is neither harmed nor benefited.
• Examples:
  • Orchid growing as an epiphyte on a mango branch: The orchid benefits by having a place to grow, while the mango tree is unaffected.
  • Barnacles growing on the back of a whale: Barnacles benefit from transport and access to food particles, while the whale receives no apparent benefit or harm.
  • Cattle egret and grazing cattle: Egrets forage close to cattle because the cattle, as they move, stir up insects from the vegetation, making them easier for the egrets to find and catch. The cattle remain unaffected.
  • Sea anemone and clown fish: The clown fish lives among the sea anemone’s stinging tentacles, gaining protection from predators that avoid the stings. The anemone does not appear to benefit from hosting the clown fish.

Mutualism

• This interaction involves benefits for both interacting species.
• Examples:
  • Lichens: Represent an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria.
  • Mycorrhizae: Associations between fungi and the roots of higher plants. The fungi aid the plant in absorbing essential soil nutrients, and in return, the plant provides the fungi with energy-yielding carbohydrates.
  • Plant-Animal Relationships (Pollination and Seed Dispersal): These are some of the most spectacular and evolutionarily fascinating examples of mutualism.
    • Plants require animals for pollinating their flowers and dispersing their seeds.
    • Animals are “paid fees” by plants for these services, typically in the form of pollen and nectar for pollinators, and juicy, nutritious fruits for seed dispersers.
    • Mutually beneficial systems must also be safeguarded against “cheaters” (e.g., animals that steal nectar without aiding pollination).
    • Plant-animal interactions often involve co-evolution, where the evolution of the flower and its pollinator species are tightly linked.
    • Fig Tree and Wasp: Many fig species have a tight, one-to-one relationship with a specific pollinator wasp species; no other wasp species can pollinate that particular fig. The female wasp uses the fig fruit not only as an oviposition (egg-laying) site but also for nourishing its larvae with developing seeds. The wasp pollinates the fig inflorescence while searching for suitable egg-laying sites, and in return, the fig provides some of its developing seeds as food for the wasp larvae.
    • Orchids and Pollinator Insects (Bees and Bumblebees): Orchids display astonishing diversity in floral patterns, many of which have evolved to attract specific pollinator insects and ensure guaranteed pollination.
      • Not all orchids offer rewards. The Mediterranean orchid Ophrys uses “sexual deceit” to achieve pollination. One petal of its flower remarkably resembles the female of a bee species in size, color, and markings. The male bee is attracted to what it perceives as a female, “pseudocopulates” with the flower, and during this process, gets dusted with pollen. When the same bee “pseudocopulates” with another Ophrys flower, it transfers the pollen, thus pollinating the flower. This illustrates how co-evolution works: if the female bee’s color patterns change, the orchid flower must co-evolve to maintain its resemblance to ensure pollination success.