Organisms and Populations Biology Notes - Ecology, Abiotic Factors, Adaptations, Population Growth and Interactions

• Ecology is the study of interactions among organisms and between organisms and their physical (abiotic) environment.
• Ecology is concerned with four levels of biological organization: Organisms, Populations, Communities, and Biomes.

Organism and Its Environment

• Physiological ecology (ecology at the organismic level) is the study of an organism’s adaptation to environments in terms of survival and reproduction.
• The rotation of Earth and the tilt of its axis cause annual variations in temperature and seasons. Major biomes (desert, rainforest, tundra, etc.) are formed due to these variations and precipitation (rain and snow).

• Life exists even in extreme and harsh habitats, such as Rajasthan desert, rain-soaked Meghalaya forests, deep ocean trenches, torrential streams, permafrost polar regions, high mountain tops, thermal springs, and compost pits.
• Our intestine is a habitat for many microbes.
• The physico-chemical (abiotic) components (water, light, temperature, soil, etc.) and biotic components (pathogens, parasites, predators, competitors, etc.) lead to variation in different habitats.
• The distinct role and position of an organism in its environment is called its niche. By this, each organism tolerates various conditions, utilizes various resources, etc.

Abiotic Factors

Temperature

• Temperature is the most ecologically relevant environmental factor.
• Temperature on land varies seasonally, gradually decreasing from the equator to the poles and from plains to mountain tops. It ranges from subzero levels (in polar areas and high altitudes) to over 500°C (in tropical deserts).
• Average temperature in thermal springs and deep-sea hydrothermal vents is above 100°C.
• Mango trees cannot grow in temperate countries (e.g., Canada, Germany). Snow leopards are not found in Kerala forests. Tuna fish are rare beyond tropical latitudes in the ocean.
• Temperature affects the kinetics of enzymes, basal metabolism, and other physiological functions of organisms.
• Based on range of thermal tolerance, organisms are 2 types:
• Eurythermal: Organisms that can tolerate a wide range of temperatures.
• Stenothermal: Organisms that can tolerate only a narrow range of temperatures.

Water

• Water is the second most important factor.
• Desert organisms have special adaptations to limited water.
• Productivity and distribution of plants depend on water.
• For aquatic organisms, water quality (pH, chemical composition) is important. Salt concentration (salinity in parts per thousand) is less than 5 in inland waters, 30–35 in the sea, and over 100 in some hypersaline lagoons.
• Euryhaline: Organisms that tolerate a wide range of salinities.
• Stenohaline: Organisms that tolerate only a narrow range of salinity.
• Many freshwater animals cannot live for long in seawater, and vice versa, due to osmotic problems.

Light

• Plants need sunlight for photosynthesis.
• Small forest plants (herbs and shrubs) are adapted to photosynthesize optimally under very low light because they are overshadowed by tall, canopied trees.
• Many plants depend on sunlight for photoperiodism (e.g., flowering). Many animals use diurnal and seasonal variations in light intensity and photoperiod for timing their foraging, reproductive, and migratory activities.
• The sun is the ultimate source of light and temperature on land. In deep oceans (>500m), the environment is dark, and no energy is available from the sun.
• The spectral quality of solar radiation is important for life. The UV spectrum is harmful to many organisms. Not all color components of the visible spectrum are available for marine plants.

Soil

• The nature and properties of soil differ due to climate, weathering, sedimentation, and soil development methods.
• Soil composition, grain size, and aggregation determine the percolation and water-holding capacity of soils.
• These characteristics, along with parameters like pH, mineral composition, and topography, determine the vegetation and animals in an area.
• In aquatic environments, sediment characteristics determine the type of benthic animals.

Responses to Abiotic Factors

Organisms maintain a stable internal environment (homeostasis) despite varying external environmental conditions through various processes.

a. Regulate

• Regulation is the maintenance of homeostasis by physiological and behavioral means, ensuring constant body temperature (thermoregulation) and osmotic concentration (osmoregulation).
• Examples include all birds, mammals, and very few lower vertebrates and invertebrates.
• Thermoregulation in mammals: The success of mammals is mainly due to their ability to maintain a constant body temperature. In summer, when the outside temperature exceeds body temperature (37°C), sweating occurs, resulting in evaporative cooling to lower body temperature. In winter, when the temperature is below 37°C, shivering produces heat to raise body temperature.
• Most organisms are not regulators or are partial regulators because thermoregulation is energetically expensive, especially for small animals (e.g., shrews, hummingbirds). These animals have a larger surface area relative to their volume, losing body heat quickly in cold conditions and expending significant energy to generate heat. Consequently, very small animals are rare in polar regions.

b. Conform

• Approximately 99% of animals and nearly all plants cannot maintain a constant internal environment. Their body temperature or osmotic concentration changes with surrounding conditions. These are called conformers.

• In aquatic animals, the osmotic concentration of body fluids changes with that of the ambient osmotic concentration.

c. Migrate

• Many animals, such as birds, temporarily move from stressful habitats to more hospitable areas and return when the stressful period is over.
• For example, during winter, Keoladeo National Park (Bharatpur, Rajasthan) hosts migratory birds from Siberia and other extremely cold northern regions.

d. Suspend

• In bacteria, fungi, and lower plants, thick-walled spores help survive unfavorable conditions, germinating under suitable conditions.
• In higher plants, seeds and some vegetative reproductive structures reduce metabolic activity to tide over stress periods, germinating under favorable moisture and temperature.

In animals:

• Hibernation of bears during winter.
• Aestivation of some snails and fishes during summer.
• Diapause, a stage of suspended development in many zooplanktons in lakes and ponds.

Adaptations

Adaptation is the morphological, physiological, and behavioral attribute that enables an organism to survive and reproduce in its habitat. Many adaptations have evolved over long evolutionary time and are genetically fixed.

Adaptations of kangaroo rat in North American deserts:

• Internal fat oxidation produces water as a byproduct when no external water source is available.
• Ability to concentrate urine, minimizing water use for excretory products.

Adaptations of desert plants:

• Presence of a thick cuticle on leaf surfaces.
• Sunken stomata minimize water loss due to transpiration.
• CAM photosynthetic pathway
• enables stomata to remain closed during the day.
• Desert plants like Opuntia have no leaves (reduced to spines), with photosynthesis occurring in stems.

Adaptations of Mammals:

• Mammals in colder climates have shorter ears and limbs to reduce heat loss, known as Allen’s Rule.
• Aquatic mammals like seals, have a thick layer of fat (blubber) below their skin that acts as an insulator and reduces loss of body heat.

Physiological and Biochemical adaptations:

• Archaebacteria thrive in hot springs and deep-sea hydrothermal vents where temperatures exceed 100°C. Many fish survive in Antarctic waters at temperatures below 0°C.
• Many marine invertebrates and fish live at great ocean depths where pressure exceeds 100 times normal atmospheric pressure.
• At high altitudes (>3,500m), low atmospheric pressure causes altitude sickness (symptoms: nausea, heart palpitations, fatigue) due to insufficient oxygen. The body acclimatizes by increasing red blood cell production, breathing rate, and decreasing hemoglobin’s oxygen-binding capacity.

Behavioural adaptations:

• Desert lizards bask in the sun to absorb heat when their body temperature is low but move into shade when ambient temperatures rise.
• Some species burrow into the soil to hide and escape from above-ground heat.

POPULATIONS

A population is a group of individuals of same species that live in a given geographical area, share or compete for similar resources and potentially reproduce.

E.g. All the cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate and lotus plants in a pond etc.

Population ecology is an important area of ecology as it links ecology to population genetics & evolution.

Population Attributes

• Birth rates: Refer to per capita births.

E.g. In a pond, there are 20 lotus plants last year and through reproduction 8 new plants are added.

Hence, the current population = 28

The birth rate = 8/20 = 0.4 offspring per lotus per year.

• Death rates: Refer to per capita deaths.

  E.g. 4 individuals in a laboratory population of 40 fruit flies died during a week.

  Hence, the death rate = 4/40 = 0.1 individuals per fruit fly per week.

• Sex ratio: A population has a sex ratio.

  E.g. 60% of the population is females and 40% males.

• Age pyramid: It is the structure obtained when the age distribution (% individuals of a given age or age group) is plotted for the population.

For human population, age pyramids generally show age distribution of males and females in a combined diagram.

• Population size or population density (N): It is the number of individuals of a species per unit area or volume. E.g. population density of Siberian cranes at Bharatpur wetlands in any year is less than 10. It is millions for Chlamydomonas in a pond.

Population size is also measured in % cover or biomass. E.g. In an area, 200 Parthenium plants and a huge banyan tree are seen. In such cases, measuring % cover or biomass is meaningful to show importance of banyan tree.

Total number is a difficult measure for a huge population. In such cases, relative population density (without knowing absolute population density) is used. E.g. Number of fish caught per trap indicates its total population density in the lake.

In some cases, indirect estimation of population sizes is performed. E.g. Tiger census in national parks & tiger reserves based on pug marks & fecal pellets.

POPULATION GROWTH

The population size changes depending on factors like food availability, predation pressure & weather.

Changes in population density give some idea about the population – whether it is flourishing or declining.

4 basic processes that fluctuate the population density

1. Natality (B): It is the number of births in a population during a given period.
2. Mortality (D): It is the number of deaths in a population during a given period.
3. Immigration (I): It is the number of individuals of the same species that have come into the habitat from elsewhere during a given time period.
4. Emigration (E): It is the number of individuals of the population who left the habitat and gone elsewhere during a given time period.

Natality & immigration increase the population density and mortality & emigration decrease the population density.

If N is the population density at time t, then its density at time t +1 is

N_t+1 = N_t + [(B + I) – (D + E)]

Population density increases if B+I is more than D+E. Otherwise it will decrease.

Under normal conditions, births & deaths are important factors influencing population density. Other 2 factors have importance only under special conditions. E.g. for a new colonizing habitat, immigration may be more significant to population growth than birth rates.

Growth Models

Exponential growth

Resources (food & space) are essential for the unimpeded population growth.

If resources are unlimited, each species shows its full innate potential to grow in number. Then the population grows in an exponential or geometric fashion.

If population size = N, birth rates (per capita births) = b and death rates (per capita deaths) = d, then the increase or decrease in N during a unit time period t (dN/dt) will be

dN/dt = (b – d) × N

Let (b–d) = r, then

dN/dt = rN

The r (‘intrinsic rate of natural increase’) is an important parameter for assessing impacts of any biotic or abiotic factor on population growth.

r value for the Norway rat = 0.015

r value for the flour beetle = 0.12

r value for human population in India (1981) = 0.0205

The integral form of the exponential growth equation is

N_t = N₀e^rt

Where,

N_t = Population density after time t

N₀ = Population density at time zero

r = intrinsic rate of natural increase

e = the base of natural logarithms (2.71828)

Population growth curves

a = exponential growth (J-shaped curve)

b = logistic growth (Sigmoid curve)

Logistic growth

There is no population in nature having unlimited resources for exponential growth. This leads to competition among individuals for limited resources.

Eventually, the ‘fittest’ individuals survive and reproduce.

In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. It is called carrying capacity (K).

A population with limited resources shows initially a lag phase, phases of acceleration & deceleration and finally an asymptote. This type of population growth is called Verhulst-Pearl Logistic Growth. It is described by following equation:

Where N = Population density at time t

r = Intrinsic rate of natural increase

K = Carrying capacity

Since resources for growth for most animal populations are limited, the logistic growth model is more realistic.

Life History Variation

Populations evolve to maximise their reproductive fitness or Darwinian fitness (high r value). Under a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy.

Some organisms breed only once in their lifetime (Pacific salmon fish, bamboo) while others breed many times (most birds and mammals).

Some produce a large number of small-sized offspring (Oysters, pelagic fishes) while others produce a small number of large-sized offspring (birds, mammals).

These facts indicate that life history traits of organisms have evolved due to limited abiotic and biotic components of the habitat.

Population Interactions

Organisms interact in various ways to form a biological community. Interaction between two species is called Interspecific interactions. They include

In predation, parasitism & commensalisms, the interacting species live closely together.

Predation

• In a broad ecological context, all carnivores, herbivores etc. are predators. About 25 % insects are phytophagous.
• If a predator overexploits its prey, then the prey might become extinct. It results in the extinction of predator. Therefore, predators in nature are ‘prudent’.

Importance of predators

• Predators control prey populations.

When certain exotic species are introduced into a geographical area, they spread fast due to the absence its natural predators. E.g. Prickly pear cactus introduced into Australia (1920’s) caused havoc by spreading. Later, it was controlled by introducing a cactus-feeding predator moth.

• Predators are used in Biological control methods.
• Predators maintain species diversity in a community by reducing competition among prey species.

E.g. the predator starfish Pisaster in the rocky intertidal communities of American Pacific Coast. In an experiment, all these starfishes were removed from an enclosed intertidal area. It caused extinction of over 10 invertebrate species within a year, due to interspecific competition.

Defenses of prey species to lessen impact of predation

• Camouflage (cryptic colouration) of some insects & frogs.
• Some are poisonous and so avoided by the predators.
• Monarch butterfly is highly distasteful to its predator bird. It is due to a special chemical in its body. It is acquired during its caterpillar stage by feeding on a poisonous weed.
• Thorns (Acacia, Cactus etc.) are the most common morphological means of defense of plants.
• Many plants produce chemicals that make the herbivore sick, inhibit feeding or digestion, disrupt its reproduction or kill it. E.g. Calotropis produce highly poisonous cardiac glycosides. Therefore cattle or goats do not eat it. Nicotine, caffeine, quinine, strychnine, opium, etc. are defenses against grazers and browsers.

Competition

• It is a process in which fitness of one species (‘r’ value) is significantly lower in presence of another species.
• Interspecific competition is a potent force in organic evolution.
• Competition occurs when closely related species compete for the same limited resources.
• Unrelated species can also compete for the resource. E.g. Flamingoes & fishes in some shallow South American lakes compete for zooplankton.
• Competition occurs in abundant resources also. E.g. In interference competition, the feeding efficiency of one species is reduced due to the interfering and inhibitory presence of other species, even if resources are abundant.

Evidences for competition

• The Abingdon tortoise in Galapagos Islands became extinct within a decade after goats were introduced on the island, due to greater browsing efficiency of the goats.
• Competitive release: It is the expansion of distributional range of a species when the competing species is removed.
• Connell’s field experiments: On the rocky sea coasts of Scotland, there are 2 barnacle species: Balanus (larger & competitively superior) & Chthamalus (smaller). Balanus dominates intertidal area and excludes Chthamalus.

When Connell experimentally removed Balanus, Chthamalus colonized the intertidal zone.

Gause’s ‘Competitive Exclusion Principle’

• It states that two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eliminated eventually.
• This may be true in limited resources, but not otherwise.
• Species facing competition may evolve mechanisms for co-existence rather than exclusion. E.g. resource partitioning.
• Resource partitioning: It is the division of limited resources by species to avoid competition. For this, they choose different feeding times or different foraging patterns. E.g. MacArthur showed that five closely related species of warblers living on a tree could avoid competition and co-exist due to behavioural differences in their foraging activities.

Parasitism

• Many parasites are host-specific (they can parasitize only a single host species). They tend to co-evolve. i.e., if the host evolves special mechanisms against the parasite, the parasite also evolves mechanisms to counteract them to remain with the same host species.
• Adaptations of parasites: Loss of sense organs, presence of adhesive organs or suckers to cling on to the host, loss of digestive system, high reproductive capacity etc.
• Life cycles of parasites are often complex. E.g.
• Human liver fluke depends on 2 intermediate hosts (a snail & a fish) to complete its life cycle.
• Malarial parasite needs mosquito to spread to other hosts.

Parasites harm the host. They may reduce the survival, population density, growth and reproduction of the host. They may make the host physically weak and more vulnerable to predation.

Types of parasites

a. Ectoparasites

Parasites that feed on the external surface of host. E.g.

• Lice on humans.
• Ticks on dogs.
• Ectoparasitic Copepods on many marine fishes.
• Cuscuta plant on hedge plants.

Cuscuta has no chlorophyll and leaves. It derives its nutrition from the host plant.

Female mosquito is not considered a parasite, because it needs our blood only for reproduction, not as food.

b. Endoparasites

• Parasites that live inside the host body at different sites (liver, kidney, lungs, RBC etc).
• The life cycles of endoparasites are more complex.
• They have simple morphological & anatomical features and high reproductive potential.
• Brood parasitism in birds: Here, the parasitic birds lay eggs in the nest of its host and lets the host incubate them. During evolution, eggs of the parasitic bird have evolved to resemble the host’s egg in size and colour. So the host bird cannot detect and eject the foreign eggs easily. E.g. Brood parasitism between cuckoo and crow.

Commensalism

Examples:

• Orchid (+) growing as epiphyte on a mango branch (0).
• Barnacles (+) growing on the back of a whale (0).
• Cattle egret (+) & grazing cattle (0). The egrets forage close to where the cattle are grazing. As the cattle move, the vegetation insects come out. Otherwise it is difficult for the egrets to find and catch the insects.
• Sea anemone (0) & clown fish (+). Stinging tentacles of sea anemone gives protection to fish from predators.

Mutualism

Examples:

• Lichen: It is a mutualistic relationship between a fungus & photosynthesizing algae or cyanobacteria.
• Mycorrhizae: Associations between fungi & the roots of higher plants. The fungi help the plant in the absorption of essential nutrients from the soil while the plant provides the fungi with carbohydrates.
• Mutualism b/w plant & animal through pollination and seed dispersion:

Examples:

1. Fig trees & wasps. The fig species is pollinated only by its ‘partner’ wasp species. Female wasp pollinates the fig inflorescence while searching for suitable egg-laying sites in fruits. The fig offers the wasp some developing seeds, as food for the wasp larvae.
2. Orchids show diversity of floral patterns. They can attract the right pollinator insect (bees & bumblebees) to ensure pollination. Not all orchids offer rewards.
3. ‘Sexual deceit’ of Ophrys (Mediterranean orchid). One petal of its flower resembles female bee in size, colour & markings. So male bee ‘pseudocopulates’ with the flower and is dusted with pollen. When this bee ‘pseudocopulates’ with another flower, it transfers pollen to it.

If the female bee’s colour patterns change slightly during evolution, pollination success will be reduced unless the orchid flower co-evolves to maintain the resemblance of its petal to the female bee.