FBS- LECTURE NO.6
Diversity in Ecological Communities
Definition of Diversity
Diversity refers to the variety of species within a community, which includes their relative abundance.
A community is considered diverse when it encompasses a large range of different species.
Species diversity measures biological complexity in terms of:
Richness: The number of different species present.
Evenness: How equally the populations of different species are distributed.
Reasons for Varying Species Diversity
Several factors can lead to varying species richness across communities:
Resources: Availability of essential elements like food, water, and light.
Adaptation: A species' ability to adapt to its environment influences diversity.
Niche Overlap: Competition arises when species share similar ecological roles, potentially limiting diversity.
Survival Suitability: Environmental conditions that support a species' survival impact community diversity.
Factors Affecting Diversity
Unique History: Historical events and context influence diversity levels.
Time: Newer communities typically exhibit fewer species, which can gradually increase over time due to adaptation and evolution.
Extreme Habitats: Harsh environments often support a lower number of species.
Resource Diversity: Ecosystems with varied resources tend to have higher diversity; for instance:
Areas with varied topography (hills, valleys, rocky areas) support more species than uniform landscapes.
Forests with multiple layers tend to harbor more bird species than simpler ecosystems like grasslands.
Ecotone: An area where two ecosystems meet often exhibits distinct diversity due to the overlap of species from both ecosystems.
Productivity: The Engine of Ecosystems
Productivity: This refers to the rate at which biomass (matter derived from plant photosynthesis) is generated in an ecosystem; high productivity supports greater species richness.
Primary Production
The rate at which radiant energy is stored by autotrophs (e.g., plants) through photosynthesis in organic matter is termed Primary Production:
Gross Primary Production (GPP): Total energy stored by autotrophs.
Net Primary Production (NPP): Energy remaining after used for respiration by the plants.
Net Community Production (NCP): Energy stored but not utilized by heterotrophs during the growing season.
Secondary Production
Secondary Production: Refers to the energy stored at the consumer (heterotroph) levels:
Gross Secondary Production (GSP): Total energy stored by heterotrophs.
Net Secondary Production (NSP): Energy remaining after heterotrophs have used some for respiration.
Control Mechanisms
Control: Refers to exercising restraint or directing influence, understood through the field of Cybernetics, which examines control in both animate and inanimate systems.
Servomechanism: A mechanical feedback mechanism.
Homeostatic Mechanism: An organic feedback mechanism aimed at maintaining stability (also referred to as homeorhesis).
Categories of Stability
Resistance Stability: The ability of a community to remain stable despite stress.
Resilience Stability: The ability to recover quickly from stress.
Limiting Factors
Limiting Factors: These restrict population growth or distribution. Key laws include:
Liebig’s Law of the Minimum: Growth is controlled by the scarcest resource, not the total amount available.
Shelford’s Law of Tolerance: Species distribution is controlled by the limiting effects of both minimum and maximum physical factors.
Law of Tolerance: Pertains to the ranges of environmental conditions affecting species distribution.
Allelopathy: Negative effects of one plant on another via the release of biochemicals, affecting competition and survival.
Community Interactions (Biological Interactions)
Action: Effects of non-living components on living organisms.
Reaction: Effects of living organisms on non-living components.
Coaction: Effects of living organisms on each other.
Types of Coaction
Predation: One organism hunts, kills, and consumes another.
Parasitism: One organism benefits at the expense of another, harming it without total consumption.
Competition:
Intraspecific Competition: Competition among individuals of the same species.
Interspecific Competition: Competition between individuals of different species.
Commensalism: One organism benefits while the other is unaffected (e.g., an orchid growing on a tree).
Amensalism: One organism is inhibited while the other benefits (e.g., black walnut trees inhibiting nearby plants).
Mutualism: Interaction where both organisms benefit and often depend on each other (e.g., clownfish and sea anemone).
Protocooperation: Both organisms benefit and are intimate but not obligatorily dependent (e.g., capybara and egret).
Neutralism: Organisms remain unaffected by each other.
Keystone Species: A species that has a disproportionate effect on its environment relative to its abundance.
Community Organization
Qualitative Characters
Physiognomy: The study of a community's external appearance characterized by dominance, density, height, and color.
Phenology: Study of climatic effects on periodic biological events (e.g., leafing, flowering).
Stratification: The vertical arrangement of plants within a community:
Emergent Plants: Crowns extending above the canopy.
Canopy Plants: Main upper layer with the most individuals.
Understory Plants: Crowns below the canopy.
Forest Floor Plants: Crowns beneath understory (e.g., shrubs, grasses).
Abundance
Relative distribution of species in a plant community (e.g., rare, frequent, abundant).
Stochastic and Social Patterns
Dispersion: The horizontal distribution of individuals (random, regular, or clumped).
Sociability: The relationships and closeness of individuals.
Vitality: Indicates normal growth and reproductive capacity of species in a community.
Life Form
Related to structured patterns of organisms within a community:
Disseminule Structure
Anemochore: Seeds adapted for wind dispersal (e.g., plumed seeds of a milkweed).
Hydrochore: Seeds adapted for water dispersal (e.g., Rhizophora stylosa propagule).
Zoochore: Seeds adapted for animal dispersal (myrmecochory by ants, ornithochory by birds).
Autochore: Seeds adapted for self-dispersal.
Habitat
The natural environment where an organism lives can be categorized:
Based on Size:
Macrohabitat: Large-scale habitats (e.g., forestland).
Microhabitat: Small-scale habitats (e.g., surfaces of soil, tree holes).
Based on Site: Types such as terrestrial, freshwater, and marine.
Ecological Niche
The functional role of an organism within a community, including total environmental requirements:
Spatial Niche: Defined by structural and instinctive limitations.
Trophic Niche: Based on different energy sources or food habits.
Hypervolume Niche: A multi-dimensional space for survival.
Fundamental Niche: Full range of environmental conditions usable with no competition.
Realized Niche: Actual niche occupied, limited by biotic interactions.
Community Change
Types of Community Change
Directional Change: Continuous change over time (e.g., forest evolving from grass to shrubs to trees).
Non-directional Change: Change interrupted by external forces (e.g., a forest destroyed by typhoon).
Population in Ecological Succession
Succession: The series of changes in species structure of an ecological community over time.
Based on Causes
Autogenic Succession: Change driven internally by community interactions.
Allogenic Succession: Change caused by external forces.
Based on Substrate
Primary Succession: Occurs on newly formed land (e.g., bare rock).
Secondary Succession: Occurs where an existing community has been disturbed but substrate remains (e.g., after fire).
Based on Origin
Hydrach Succession: Begins in aquatic environments.
Xerach Succession: Begins in dry environments.
Population Attributes
Population Density: Number of individuals per unit area or volume:
Crude Density: Total number of individuals in a given total space.
Ecological Density: Actual number of individuals in the habitat space.
Natality: Capability to increase through reproduction:
Physiological Natality: Theoretical maximum potential under ideal conditions.
Ecological Natality: Actual population increase under specific conditions.
Mortality: The rate of death in a population:
Minimum Mortality: Death rate under ideal conditions.
Realized Mortality: Death rate under actual environmental conditions.
Factors Affecting Mortality
Accidents, starvation, adverse weather, predation, parasitism, hunting, and fire.
Population Distribution
Describes individual spacing within habitats.
Population Characteristics
Based on Age Structure:
Rapidly Expanding Population: High proportion of young individuals.
Stable Population: Young and old individuals in roughly equal numbers.
Declining Population: More old individuals than young.
Normal Population: Birth rates equal death rates.
Population Dispersal
Movement of individuals into or out of a population which may affect growth:
Emigration: Outward movement.
Immigration: Inward movement.
Migration: Periodic movements affecting natality and mortality.
Population Growth Types
Population Models
Logistic Population Growth: S-shaped curve;
Rapid initial growth followed by a slowdown as it nears carrying capacity (maximum sustainable population size).
Typical for larger animals with fewer offspring.
Exponential Population Growth: J-shaped curve;
Continuous growth absent resource limitations.
Common in small animals with many offspring or bacteria in ideal conditions.