Organisms And Populations
Organism And Its Environment
Ecology is the study of the interactions among organisms and between the organism and its physical (abiotic) environment. The unit of study in ecology can be the individual organism, populations, communities, or biomes.
At the level of an individual organism, ecology deals with how the organism adapts to its environment in terms of survival and reproduction. The environment of an organism includes both abiotic (non-living) and biotic (living) factors.
Major Abiotic Factors
The key physical (non-living) factors that influence organisms and their distribution are:
- Temperature: The most ecologically relevant environmental factor. Organisms can tolerate only a certain range of temperatures. It affects enzyme kinetics, metabolic activity, and physiological functions.
- Eurythermal: Organisms that can tolerate a wide range of temperatures (e.g., many mammals).
- Stenothermal: Organisms that can tolerate only a narrow range of temperatures (e.g., corals, polar bears).
- Water: Essential for all life processes. Its availability is a major limiting factor, especially in terrestrial environments.
- Euryhaline: Organisms that can tolerate a wide range of salinity in water (e.g., salmon).
- Stenohaline: Organisms that can tolerate only a narrow range of salinity (e.g., freshwater fish, marine fish).
- Light: Essential for photosynthesis in plants. Light intensity, duration (photoperiod), and quality (wavelength) affect plant growth, flowering, and distribution. For animals, light is important for vision, photoperiodism (timing of activities), and navigation.
- Soil: The composition, texture, pH, mineral content, and water-holding capacity of soil influence the type of vegetation that can grow in a region. Soil is formed through weathering of rocks and decomposition of organic matter (pedogenesis). Edaphic factors refer to soil-related factors.
Other important abiotic factors include air (wind), humidity, precipitation (rainfall), topography, and pressure.
Responses To Abiotic Factors
Organisms have evolved various ways to cope with environmental conditions that might be stressful or unfavourable.
- Regulate: Some organisms are able to maintain a constant internal environment (homeostasis) despite changes in the external environment. This involves physiological and behavioural mechanisms. Example: Birds and mammals (warm-blooded) regulate their body temperature. They can survive and function optimally over a wider range of external conditions.
- Conform: The internal environment of some organisms changes with the external environment. Their body temperature or osmotic concentration fluctuates with that of the surroundings. Example: Most animals and plants (cold-blooded/poikilothermic). They are limited by the range of environmental fluctuations.
- Migrate: Moving temporarily from a stressful habitat to a more favourable area. Example: Birds migrating to warmer regions during winter, zooplankton moving to deeper water at night.
- Suspend: Entering a temporary state of inactivity or dormancy during unfavourable conditions.
- Hibernation: Winter sleep (e.g., bears).
- Aestivation: Summer sleep, avoiding heat and desiccation (e.g., snails, some fish).
- Spore formation: Bacteria, fungi, some algae can form resistant spores.
- Seed dormancy: Plants enter a state of dormancy in seeds during unfavourable conditions.
- Diapause: A stage of suspended development in many zooplankton and insects under unfavourable conditions.
*(Image shows graphs illustrating temperature regulation (regulator) vs. conformity (conformer), and possibly simple illustrations of migration, hibernation, or aestivation)*
Adaptations
Adaptation is any morphological, physiological, or behavioural attribute of an organism that enables it to survive and reproduce in its habitat.
- Adaptations are the result of evolution by natural selection. Over generations, individuals with traits better suited to the environment have higher survival and reproduction rates, leading to the prevalence of those advantageous traits in the population.
- Adaptations can be short-term (acclimatisation) or long-term (evolutionary).
Examples of Adaptations:
- Morphological adaptations: Structural features.
- Desert plants (xerophytes): Have thick cuticle, sunken stomata, reduced leaves (spines), presence of phylloclades (photosynthetic stems) to reduce water loss. Roots are often extensive. Example: *Opuntia*.
- Desert animals: Kangaroo rat does not drink water, meets its water requirement from internal fat oxidation. Many desert animals are nocturnal to avoid daytime heat.
- Polar animals: Presence of thick fur, layers of subcutaneous fat (blubber in seals) for insulation (Allen's Rule: mammals from colder climates have shorter ears and limbs to minimise heat loss).
- Physiological adaptations: Internal functional adjustments.
- Acclimatisation: Short-term physiological adjustments to new environmental conditions (e.g., increasing RBC production at high altitudes).
- Producing antifreeze proteins in animals living in polar regions to prevent freezing.
- Maintaining constant internal temperature in birds and mammals (homeothermy).
- Behavioural adaptations: Actions or behaviours that help survival.
- Migration (birds moving to warmer areas).
- Seeking shade or burrowing to avoid heat (desert animals).
- Basking in the sun to warm up (some reptiles).
Example 1. Why are mammals from colder climates generally shorter and have shorter extremities (ears, limbs)?
- Desert plants (xerophytes): Have thick cuticle, sunken stomata, reduced leaves (spines), presence of phylloclades (photosynthetic stems) to reduce water loss. Roots are often extensive. Example: *Opuntia*.
- Desert animals: Kangaroo rat does not drink water, meets its water requirement from internal fat oxidation. Many desert animals are nocturnal to avoid daytime heat.
- Polar animals: Presence of thick fur, layers of subcutaneous fat (blubber in seals) for insulation (Allen's Rule: mammals from colder climates have shorter ears and limbs to minimise heat loss).
- Acclimatisation: Short-term physiological adjustments to new environmental conditions (e.g., increasing RBC production at high altitudes).
- Producing antifreeze proteins in animals living in polar regions to prevent freezing.
- Maintaining constant internal temperature in birds and mammals (homeothermy).
- Migration (birds moving to warmer areas).
- Seeking shade or burrowing to avoid heat (desert animals).
- Basking in the sun to warm up (some reptiles).
Example 1. Why are mammals from colder climates generally shorter and have shorter extremities (ears, limbs)?
Answer:
Allen's rule states that mammals living in colder climates tend to have shorter limbs and body appendages (like ears, tails) compared to similar mammals living in warmer climates.
The reason for this is related to thermoregulation (maintaining body temperature). Surface area to volume ratio is important for heat exchange with the environment.
Longer and larger extremities increase the surface area of the body, which leads to greater heat loss. In colder climates, it is advantageous to minimise heat loss to conserve body heat.
Therefore, natural selection favors individuals with shorter limbs and appendages in colder environments, as a lower surface area to volume ratio helps in reducing heat loss. This is a physiological adaptation to cold climates, resulting in morphological differences.
Adaptations enable organisms to thrive in specific ecological niches within their environment.
Populations
In ecology, a population is a group of individuals of the same species living in a defined geographical area at a given time.
Populations are dynamic entities, changing in size, density, and distribution over time due to factors like birth, death, immigration, and emigration.
Population Attributes
Populations have characteristics that are not found in individual organisms. These are collective properties of the population.
- Birth rate (Natality): The number of births per unit population per unit time.
- Death rate (Mortality): The number of deaths per unit population per unit time.
- Sex ratio: The ratio of females to males in a population.
- Age distribution: The proportion of individuals of different age groups in a population. Represented by age pyramids (pre-reproductive, reproductive, post-reproductive).
- Growing population: Larger proportion of young individuals (pyramid shaped).
- Stable population: Similar proportions of young and reproductive individuals (bell-shaped).
- Declining population: Larger proportion of older individuals (urn-shaped).
*(Image shows three age pyramids: one with a broad base tapering upwards (growing), one with a relatively straight middle section and tapering top (stable), and one with a narrower base and wider top sections (declining))*
- Population density: The number of individuals per unit area or volume.
$ \text{Population Density} = \frac{\text{Number of individuals}}{\text{Area or Volume}} $
Sometimes, density is measured in terms of biomass or other relative measures, depending on the organism.
- Population size: The total number of individuals in a population.
Population Growth
The size of a population changes over time due to changes in birth rate, death rate, immigration (individuals entering the population), and emigration (individuals leaving the population).
Change in population size ($N$) at time $t+1$ relative to size at time $t$:
$ N_{t+1} = N_t + [(B+I) - (D+E)] $
Where B = Births, I = Immigration, D = Deaths, E = Emigration during time interval t.
Population Growth Models:
Two main models describe population growth:
- Exponential Growth: Occurs when resources are unlimited. The population grows at an accelerating rate.
- Equation: $ \frac{dN}{dt} = rN $
- $dN/dt$ = rate of change in population size
- $r$ = intrinsic rate of natural increase (birth rate - death rate)
- $N$ = population size
- If plotted over time, it gives a J-shaped curve.
- Equation for population size at time t: $ N_t = N_0 e^{rt} $
- This model is applicable to populations growing in new environments or after a disturbance, where resources are temporarily abundant.
*(Image shows a J-shaped curve with population size on Y-axis and time on X-axis)*
- Logistic Growth: Occurs when resources are limited. The population initially grows exponentially, then the growth rate slows down as it approaches the carrying capacity, and eventually, the population size stabilises around the carrying capacity. This is a more realistic model for most natural populations.
- Carrying capacity (K): The maximum population size that the environment can sustain indefinitely, given the available resources.
- Equation: $ \frac{dN}{dt} = rN \left(\frac{K-N}{K}\right) $
- $(K-N)/K$ represents the environmental resistance, which increases as N approaches K.
- If plotted over time, it gives an S-shaped (Sigmoid) curve.
- Phases: Lag phase, Log (exponential) phase, Deceleration phase, Stationary phase (at K).
*(Image shows an S-shaped curve with population size on Y-axis and time on X-axis, indicating the curve approaching and levelling off at the carrying capacity K)*
Life History Variation
Different organisms have evolved diverse life history strategies in response to their environment and resource availability. These strategies relate to how organisms allocate resources to growth, reproduction, and survival.
- Some organisms breed only once in their lifetime (semelparous) and produce a large number of offspring (e.g., Pacific salmon, bamboo).
- Others breed multiple times during their lifetime (iteroparous) and produce a smaller number of offspring in each breeding event (e.g., birds, mammals).
Selection favors life histories that maximise fitness (reproductive success) in a particular environment. For example, in unpredictable environments, producing many small offspring might be favored, while in stable environments, producing fewer, larger offspring might be favored.
Population Interactions
Individuals of different species living in the same area interact with each other in various ways. These interactions can be beneficial, harmful, or neutral for the species involved. Different types of population interactions:
| Interaction | Species 1 | Species 2 | Description | Example |
|---|---|---|---|---|
| Mutualism | + | + | Both species benefit | Lichens (algae + fungi), Mycorrhiza (fungi + plant roots), Pollination (plant + pollinator) |
| Competition | - | - | Both species are harmed by using the same limited resource | Competition between different plant species for light/water/nutrients, Competition between animals for food/territory |
| Predation | + | - | Predator benefits by feeding on prey; Prey is harmed | Lion ($+$) and Deer ($-$), Bird ($+$) and Insect ($-$) |
| Parasitism | + | - | Parasite benefits by living on or inside host; Host is harmed | Tapeworm ($+$) in human intestine ($-$), Ticks ($+$) on dog ($-$), *Cuscuta* ($+$) on host plant ($-$) |
| Commensalism | + | 0 | One species benefits; the other is neither harmed nor benefited | Epiphytic plant (e.g., orchid) ($+$) growing on a tree (0), Barnacles ($+$) growing on a whale (0) |
| Amensalism | - | 0 | One species is inhibited; the other is unaffected | Penicillium fungus (produces penicillin, inhibiting bacteria) ($-$) and bacteria (0 - fungus not affected positively) |
*(Note: + indicates benefit, - indicates harm, 0 indicates no effect)*
These interactions shape the structure and dynamics of ecological communities and play a crucial role in evolution.