Organisms and Populations

Organisms and Populations

This chapter Organisms and Populations, focuses on the interactions between organisms and their environment, exploring key concepts such as abiotic factors, adaptations, population dynamics, and various types of species interactions.

Understanding Our Living World

• Our world is diverse and complex.
• We study it at different levels: macromolecules, cells, tissues, organs, organisms, populations, communities, ecosystems, and biomes.

Ecology:

• Ecology studies the interactions among organisms and between organisms and their physical (abiotic) environment
• Focuses on four levels:
  • organisms,
  • populations,
  • communities, and
  • biomes (Desert, Rainforest & Tundra)
• This chapter explores organisms and populations mainly.

Organism and Its Environment

Physiological Ecology:

• Studies how organisms adapt to their environment for survival and reproduction.

Environmental Factors:

• Planet rotation and axis tilt cause seasonal changes in temperature and precipitation.
• These changes create different biomes like deserts, rainforests, and tundra.
• Variations within biomes lead to various habitats.

Habitats:

• Life exists in extreme and diverse habitats: deserts, forests, oceans, streams, polar regions, mountains, thermal springs, compost pits, and even intestines.
• Key elements (Abiotic Factors) in habitats: temperature, water, light, and soil.
• Habitats include abiotic (physical/chemical) and biotic (living) components like pathogens, parasites, predators, and competitors.

Adaptations:

• Organisms adapt over time through natural selection to survive and reproduce in their habitats.
• Niche – Each organism occupies a niche in a habitat characterized by the following features:
  • A defined range of environmental conditions it can tolerate
  • A variety of resources it utilizes
  • A specific functional role in the ecosystem

Major Abiotic Factors

Temperature

• Importance: Temperature is a crucial environmental factor.
• Variation:
  • Varies seasonally on land.
  • Decreases from the equator to poles and from plains to mountains.
  • Ranges from subzero in polar areas to >50°C in tropical deserts, >100°C in thermal springs and deep-sea vents.
• Impact on Organisms:
  • Affects enzyme kinetics (activity) and metabolism.
  • Some organisms thrive in a wide temperature range (eurythermal).
  • Most organisms thrive in a narrow temperature range (stenothermal).
• Distribution: Thermal tolerance affects geographical distribution of organisms.

Water

• Importance: Essential for all life on Earth.
• Availability varies: scarce in deserts, abundant in oceans, lakes, rivers.
  • Desert Adaptations: Special adaptations allow survival in limited water.
• Aquatic Challenges:
  • Quality of water is crucial (chemical composition, pH).
  • Salinity varies: <5 ppt in inland waters (freshwater) , 30-35 ppt in the sea, >100 ppt in hypersaline lagoons.
• Salinity Tolerance:
  • Euryhaline: Tolerate wide salinity range.
  • Stenohaline: Tolerate narrow salinity range.
• Osmotic Problems: Freshwater animals struggle in sea water and vice versa due to different salinity ranges.

Light

• Importance for Plants:
  • Essential for photosynthesis.
  • Some plants adapt to low light in forests (due to overshadowed by tall trees.).
  • Photoperiod (Sunlight) affects flowering.
• Importance for Animals:
  • Animals use light for foraging, reproduction, and migration.
• Light and Temperature: Availability of light is linked to temperature.
• Deep Ocean: Inhabitants rely on sources other than sunlight.
• Spectral Quality:
  • UV radiation is harmful.
  • Marine plants use different parts of the visible spectrum at various depths.

Soil

• Variation: Nature & properties of soil is differed due to climate, weathering, sedimentation, method of soil development etc.
• Characteristics:
  • Composition, grain size, and aggregation affect water percolation and holding capacity.
  • pH, minerals, and topography influence vegetation.
• Impact on Vegetation: Determines types of plants and animals in an area.
• Aquatic Environment: Sediment characteristics affect benthic animals.

Responses to Abiotic Factors

Organisms face varying abiotic conditions and have developed ways to cope with these stresses. The main strategies include:

Homeostasis

• Definition: Maintaining a stable internal environment (e.g., temperature, osmotic concentration) despite external changes.
• Ensures maximal efficiency of biochemical reactions and physiological functions.
• Example: A person using air conditioners and heaters to keep a constant temperature of 25°C.

Strategies for Coping with Stressful Conditions

1. Regulate
   • Some organisms maintain homeostasis through physiological or behavioral means.
   • Birds, mammals, and a few lower vertebrates and invertebrates can regulate body temperature and osmotic concentration.
   • Example: Humans sweat in summer to cool down and shiver in winter to generate heat.
   • Plants generally lack these mechanisms.
2. Conform
   • Most animals (99%) and nearly all plants cannot maintain a constant internal environment.
   • Body temperature and osmotic concentration change with the environment.
   • Thermoregulation is energetically expensive, especially for small animals because they have a larger body surface area relative to their volume.
   • Example: Shrews and hummingbirds are not found in polar regions due to high energy costs of maintaining body heat.
3. Migrate
   • Temporary movement to more hospitable areas during stressful periods.
   • Example: Birds migrating to warmer regions in winter.
     • Humans moving from hot cities to cooler places during summer.
4. Suspend
   • Temporary escape from stress by reducing metabolic activity or entering a dormant state.
   • Examples in plants:
     • Formation of thick-walled spores in bacteria, fungi, and lower plants.
     • Seeds and vegetative structures in higher plants remain dormant until favorable conditions return.
   • Examples in animals:
     • Bears hibernating in winter.
     • Snails and fish aestivating to avoid summer heat.
     • Zooplankton entering diapause (suspended development) in lakes and ponds.

These strategies help organisms survive and thrive despite challenging environmental conditions.

Adaptations

• Definition: Any attribute (morphological, physiological, behavioral) that helps an organism survive and reproduce in its habitat.
• Morphological Adaptations examples :
  • Desert Plants:
    • Thick Cuticle: Minimizes water loss.
    • Sunken Stomata: Reduces transpiration.
    • CAM Pathway: Stomata closed during the day to save water.
    • Spines instead of Leaves: Photosynthesis done by flattened stems (e.g., Opuntia).
  • Cold Climate Mammals:
    • Shorter Ears and Limbs: Reduce heat loss due to shorter extremities (Allen’s Rule).
    • Blubber: Thick layer of fat in seals to insulate and keep warm.
• Physiological Adaptations examples :
  • Kangaroo Rat: Lives in North American deserts.
    • Water from Fat Oxidation: Meets water needs through internal fat oxidation.
    • Concentrated Urine: Uses minimal water for excretion.
  • High Altitude Adaptations:
    • Increased Red Blood Cells: Compensates for low oxygen.
    • Higher Breathing Rate: Adjusts to low oxygen availability.
    • Example: Tribes in the Himalayas have higher red blood cell counts.
  • Microbial Adaptations:
    • Archaebacteria thrive in hot springs and deep-sea vents above 100°C.
    • Fish survive in Antarctic waters below freezing.
    • Marine invertebrates and fish live under high pressure with special enzymes.
• Behavioral Adaptations examples :
  • Desert Lizards:
    • Basking: Absorb heat from the sun when cold.
    • Seeking Shade: Move into shade when too hot.
    • Burrowing: Hide in soil to escape heat.

These adaptations help organisms thrive in extreme environments, ensuring their survival and reproduction.

Populations

Population Attributes

• Definition of Population:
  • Groups of the same species living in a specific area.
  • Share or compete for similar resources.
  • Can interbreed, even if they reproduce asexually.
  • Examples: bacteria in a petri dish, rats in a house, teak trees in a forest.
• Importance in Ecology:
  • Natural selection operates at the population level to evolve traits..
  • Links ecology to population genetics and evolution.
• Attributes of a Population:
  • Birth and Death Rates:
    • Rates are per capita (per individual).
    • Example: If a pond had 20 lotus plants and 8 new ones grew, the birth rate is 8/20 = 0.4.
    • Example: If 4 out of 40 fruit flies die, the death rate is 4/40 = 0.1.
  • Sex Ratio:
    • Percentage of males and females in a population.
    • Example: 60% females and 40% males.
  • Age Distribution:
    • Population composed of individuals of different ages.
    • Age distribution plotted as an age pyramid.
    • Indicates if the population is growing, stable, or declining.
• Population Size:
  • Indicates status in the habitat.
  • Can vary from very few to millions.
  • Population Density Measures (N):
    • Total Number: Total number of individuals but can be impractical for very large populations.
    • Per Cent Cover or Biomass: Used when total numbers are impractical or misleading (e.g., a large banyan tree vs. many carrot grass plants).
    • Relative Density: Useful for certain ecological studies; examples include Estimating bacteria in a petri dish or number of fish caught per trap.
    • Indirect Estimation: Used when direct counting is impractical, such as using pug marks and fecal pellets for tiger censuses.

Population Growth

• Population Size Changes:
  • Dynamic Nature: Population size is not static, changes over time due to factors like food availability, predation pressure, and weather.
  • Population Density: Helps understand whether a population is increasing or decreasing.
• Factors Affecting Population Density:
  • Population density in a habitat changes due to four basic processes:
    1. Natality (Births):
       • Number of births during a given period.
       • Increases population density.
    2. Mortality (Deaths):
       • Number of deaths during a given period.
       • Decreases population density.
    3. Immigration:
       • Number of individuals entering the habitat from elsewhere.
       • Increases population density.
    4. Emigration:
       • Number of individuals leaving the habitat.
       • Decreases population density.
• Population Density Equation:
  • If N is the population density at time t, then at time t+1:
  • 𝑁_𝑡+1=𝑁_𝑡 + [(𝐵+𝐼)−(𝐷+𝐸)]
    • B = Number of births
    • I = Number of immigrants
    • D = Number of deaths
    • E = Number of emigrants
• Understanding Population Density:
  • Population density increases if births and immigrants (B + I) are more than deaths and emigrants (D + E).
  • Normally, births and deaths are the main factors.
  • Immigration and emigration become important in special conditions, such as when a new habitat is colonized.

Growth Models

Introduction to Population Growth

• Population size changes over time.
• Changes depend on factors like food, predation, and weather.
• We can learn from nature to understand and control population growth.

1. Exponential Growth

• Unlimited Resources: When resources are unlimited, populations grow rapidly.
• Formula:
  • Birth rate (b) and death rate (d) determine growth.
  • Increase in population (dN/dt) = (b – d) × N.
  • Simplified to dN/dt = rN, where r is the intrinsic rate of natural increase.
• Example:
  • Norway rat: r = 0.015
  • Flour beetle: r = 0.12
  • Humans in India (1981): r = 0.0205
• J-shaped Curve: Population grows exponentially, forming a J-shaped curve over time.
• Anecdote (Example): Starting with one grain on a chessboard square, doubling on each subsequent square results in a huge number by the 64th square.
  • Doubling each day, tiny Paramecium can reach huge numbers quickly.

2. Logistic Growth

• Limited Resources: Resources are finite, leading to competition.
• Carrying Capacity (K): Maximum population size that a habitat can support.
• Growth Phases:
  • Lag phase: Slow initial growth.
  • Acceleration phase: Rapid growth.
  • Deceleration phase: Slowing growth as resources become limited.
  • Asymptote: Growth stops at carrying capacity.
• S-shaped Curve: When plotted, this growth forms an S-shaped curve.
• Formula:
  • Logistic growth equation:
• Realistic Model: Logistic growth is more realistic or natural for most populations due to limited resources.

Life History Variation

• Reproductive Fitness: Populations evolve to maximize their reproductive success, known as Darwinian fitness (high r value).
• Reproductive Strategies:
  • Single Breeding Event: Some species, like Pacific salmon fish and bamboo, breed only once.
  • Multiple Breeding Events: Other species, like most birds and mammals, breed many times.
• Offspring Size and Number:
  • Many Small Offspring: Oysters and pelagic fishes produce many small offspring.
  • Few Large Offspring: Birds and mammals produce fewer but larger offspring.
• Maximizing Fitness: The best strategy depends on the environmental pressures and constraints.
• Ecological Research: Understanding life history traits is a key area of ecological research.

Life history traits evolve based on the habitat’s abiotic (non-living) and biotic (living) factors, helping organisms adapt and survive.

Population Interactions

• No Single-Species Habitat:
  • No habitat on Earth is inhabited by just one species.
  • Every species requires interaction with others for survival.
• Biological Community:
  • Animals, plants, and microbes interact to form biological communities.
  • Even plants need soil microbes for nutrient absorption and animal agents for pollination.
• Interspecific Interactions:
  • Arise between populations of different species.
  • Can be beneficial, detrimental, or neutral for one or both species.
• Possible Outcomes:
  • Mutualism (+/+): Both species benefit (e.g., pollination by bees).
  • Competition (-/-): Both species lose due to competition for resources.
  • Parasitism (+/-): One species benefits (parasite) at the expense of the other (host).
  • Predation (+/-): One species benefits (predator) by consuming the other (prey).
  • Commensalism (+/0): One species benefits, and the other is neither benefited nor harmed.
  • Amensalism (-/0): One species is harmed, while the other is unaffected.

Predation

• Energy Transfer:
  • Predation helps transfer energy fixed by plants to higher trophic levels.
  • Example: Tiger eating a deer, sparrow eating seeds.
• Roles of Predators:
  • Control prey populations.
  • Prevent ecosystem instability.
  • Exotic species become invasive due to lack of natural predators.
    • Example: Prickly pear cactus in Australia was controlled by introducing a moth.
• Biological Control:
  • Used in agriculture to control pests by regulating prey population.
• Maintaining Species Diversity:
  • Predators reduce competition among prey species.
  • Example: Removal of starfish Pisaster led to extinction of over 10 species in the American Pacific Coast.
• Prudent Predators:
  • Overexploitation of prey can lead to extinction of both prey and predator.
• Prey Defenses:
  • Insects and frogs use camouflage.
  • Some species are poisonous.
    • Example: Monarch butterfly’s distasteful chemical from feeding on a poisonous weed.
• Plant Defenses:
  • Herbivores act as predators for plants.
  • Plants cannot escape, so they evolve defenses.
  • Morphological Defenses:
    • Thorns in Acacia and Cactus.
  • Chemical Defenses:
    • Chemicals that make herbivores sick, inhibit feeding, disrupt digestion or reproduction.
    • Example: Calotropis produces poisonous cardiac glycosides.
    • Many commercial substances like nicotine and caffeine are plant defenses.

Competition

• Darwin’s Concept:
  • Darwin’s idea of “struggle for existence” and “survival of the fittest” highlights the role of competition in evolution.
  • Competition isn’t just between closely related species; unrelated species can also compete for the same resources.
• Types of Competition:
  • Resource Competition: Occurs when species compete for the same limited resources.
    • Example: Flamingoes and fishes in South American lakes compete for zooplankton.
  • Interference Competition: One species reduces the feeding efficiency of another, even if resources are abundant.
• Definition:
  • Competition is a process where the fitness (measured as the intrinsic rate of increase, ‘r’) of one species is lowered by the presence of another species.
• Evidence and Examples:
  • Laboratory experiments show that the superior competitor eliminates the other when resources are limited.
  • Competitive Exclusion: Gause’s principle states that two closely related species competing for the same resources cannot coexist indefinitely.
  • Examples in Nature:
    • Abingdon tortoise extinction in Galapagos Islands due to introduced goats.
    • Connell’s experiment: Larger barnacle Balanus excludes smaller barnacle Chathamalus on Scotland’s rocky coasts.
• Competitive Release:
  • When a competing species is removed, the restricted species expands its range.
• Resource Partitioning:
  • Species evolve mechanisms to coexist by dividing resources.
  • Example: MacArthur’s study showed five warbler species coexisting on the same tree by having different foraging behaviors.
• Impact on Different Organisms:
  • Herbivores and plants are more affected by competition than carnivores.

Key Takeaways

• Competition shapes the evolution and behavior of species.
• It can lead to exclusion or coexistence depending on the species and circumstances.
• Examples from nature illustrate the complex dynamics of competition and coexistence.

Parasitism

• Definition and Evolution:
  • Parasitism provides free lodging (shelter) and meals for the parasite.
  • Evolved in many groups from plants to vertebrates.
  • Parasites and hosts co-evolve: if a host evolves resistance, the parasite evolves ways to counteract it.
• Adaptations of Parasites:
  • Loss of unnecessary sense organs.
  • Adhesive organs or suckers to cling to the host.
  • Loss of digestive system.
  • High reproductive capacity.
• Complex Life Cycles:
  • Often involve intermediate hosts or vectors.
  • Example: Human liver fluke uses a snail and a fish as intermediate hosts.
  • Example: Malarial parasite spreads through a mosquito vector.
• Impact on Hosts:
  • Parasites harm hosts by reducing survival, growth, reproduction, and population density.
  • Weakened hosts become more vulnerable to predators.
• Types of Parasites:
  1. Ectoparasites:
     • Live on the external surface of the host.
     • Examples: Lice on humans, ticks on dogs.
     • Cuscuta plant loses chlorophyll and leaves, derives nutrition from host plants.
  2. Endoparasites:
     • Live inside the host body (liver, kidney, lungs, etc.).
     • Have complex life cycles and specialized features.
     • High reproductive potential.
• Brood Parasitism:
  • Parasitic birds lay eggs in the nests of other birds.
  • Host birds incubate the parasitic eggs.
  • Parasitic bird eggs evolve to resemble host eggs to avoid detection.
  • Example: Cuckoo (koel) and crow.

Key Examples

• Human Liver Fluke:
  • Uses snail and fish as intermediate hosts.
• Malarial Parasite:
  • Uses mosquitoes to spread.
• Ectoparasites:
  • Lice on humans, ticks on dogs, Cuscuta plant on other plants.
• Brood Parasitism:
  • Cuckoo (koel) lays eggs in crow’s nest.

Interesting Facts

• Female mosquitoes are not considered parasites because they only need blood for reproduction.
• Parasites can make hosts more susceptible to predators by making them weak.
• Observing cuckoos and crows during the breeding season can show brood parasitism in action.

Commensalism

• Definition:
  • Interaction where one species benefits, and the other is neither harmed nor benefited.
• Examples:
  • Orchid on Mango Tree: Orchid benefits, mango tree is unaffected.
  • Barnacles on Whale: Barnacles benefit, whale is unaffected.
  • Cattle Egret and Cattle: Egrets get insects stirred up by grazing cattle.
  • Clown Fish and Sea Anemone: Clown fish gets protection from predators; sea anemone is unaffected.

Mutualism

• Definition:
  • Interaction where both species benefit.
• Examples:
  • Lichens: Partnership between fungus and algae/cyanobacteria.
  • Mycorrhizae: Fungi and plant roots; fungi help in nutrient absorption, plants provide carbohydrates.
• Plant-Animal Relationships:
  • Plants and animals often co-evolve to benefit each other.
• Pollination and Seed Dispersal:
  • Plants offer nectar and fruits as rewards to animals for pollination and seed dispersal.
  • Example: Figs and Wasps
    • Figs pollinated by specific wasp species.
    • Wasps use fig fruit for laying eggs and feeding larvae.
• Orchids and Pollinators:
  • Orchids have diverse floral patterns to attract specific pollinators like bees and bumblebees.
  • Example: Mediterranean Orchid (Ophrys)
    • Resembles female bee to attract male bee.
    • Male bee pseudocopulates, transfers pollen, ensuring pollination.
• Co-Evolution:
  • Mutualistic relationships often involve co-evolution.
  • Example: Orchid and Bee
    • Orchid petal resembles female bee (sexual deceit).
    • Any change in bee’s appearance must be matched by the orchid to maintain pollination success.

Chapter Summary:

• Ecology is the study of how living organisms interact with their environment, including both non-living (abiotic) factors like temperature and living (biotic) factors like other species.
• It focuses on four levels of biological organization: organisms, populations, communities, and biomes.
• Organisms adapt to physical factors like temperature, light, water, and soil in various ways.
• Some organisms can maintain a constant internal environment (homeostasis), while others cannot and either partially regulate or simply conform.
• Some species have adaptations to avoid unfavorable conditions through migration or by entering states like aestivation, hibernation, and diapause.
• Evolutionary changes occur at the population level, making population ecology important.
• A population consists of individuals of the same species sharing or competing for resources in a defined area.
• Populations have characteristics like birth rates, death rates, sex ratio, and age distribution, which can be illustrated using an age pyramid.
• The ecological effects on a population are reflected in its size or population density, which can be measured in different ways like numbers, biomass, or percent cover.
• Populations grow through births and immigration and decline through deaths and emigration, with growth patterns influenced by resource availability and carrying capacity.
• The intrinsic rate of natural increase (r) measures a population’s inherent growth potential.
• Different species in a habitat interact in various ways, including competition, predation, parasitism, commensalism, amensalism, and mutualism.
• Predation facilitates trophic energy transfer and helps control prey populations, while plants have defenses against herbivory.
• Competition can lead to the exclusion of inferior competitors, but some species coexist through various mechanisms.
• Mutualism, like plant-pollinator interactions, benefits both species involved.