Biotic and Abiotic Factors in Ecosystems: Population Dynamics
Measuring Biotic Components of the Ecosystem
Abundance (relative representation of a species in an ecosystem) for motile organisms:
Direct Methods
- Pitfall traps: Pots buried in the soil in which animals walk into and cannot escape from.
- Homemade Pooter: Plastic straws are attached to a pot. One tube is put in the mouth: suction creates a negative pressure in the pot so that animals are drawn into it.
Indirect Methods
- Lincoln Index: The “capture-mark-release-recapture” technique. It involves:
- Collecting a sample from the population and marking the organisms.
Species, Habitat, and Niche
Species: A group of organisms sharing common characteristics that interbreed and produce fertile offspring. Example: Snow Leopard (Panthera uncia).
Hybrid: Sterile offspring resulting from the breeding of two species. Example: Horse (Equus caballus) + Donkey (Equus asinus) = Mule.
Species interact with their environment (the external surroundings that act on a species, influencing its survival and development). It contains biotic and abiotic elements.
Habitat: The environment in which a species normally lives.
Niche: Describes the set of abiotic and biotic conditions and resources to which an organism or population responds.
The fundamental niche describes the full range of conditions and resources in which a species could survive and reproduce.
The realized niche describes the actual conditions and resources in which a species exists due to biotic interactions.
No two species can occupy the same niche, so the degree to which niches overlap determines the degree of interspecific competition. If two species occupying similar niches are grown together, the poorer competitor will be eliminated.
Abiotic and Biotic Factors
Abiotic factors: Non-living, physical factors that influence the organisms and ecosystem, such as temperature, sunlight, pH, salinity, and precipitation.
Biotic factors: Interactions between the organisms, such as predation, herbivory, parasitism, mutualism, disease, and competition.
Population Growth
Sigmoidal curve/S-curve: Population growth curve which shows an initial rapid growth (exponential growth) and then slows down as the carrying capacity is reached. Population size fluctuates around a set point (carrying capacity).
Productivity
Gross Primary Productivity (GPP): Is equivalent to the mass of glucose created by photosynthesis per unit area per unit time in primary producers.
Net Primary Productivity (NPP): Is the gain by producers in energy or biomass per unit area per unit time remaining after allowing for respiratory losses (R).
NPP = GPP – R
Gross Secondary Productivity (GSP): The total energy gained through absorption in consumers.
GSP = Food Eaten – Faecal Loss
Net Secondary Productivity (NSP): Is the gain by consumers in energy or biomass per unit area per unit time remaining after allowing for respiratory losses (R).
NSP = GSP – R
Sustainable Yield
Sustainable yield: Rate of increase in natural capital (natural income) that can be exploited without depleting the original stock or its potential for replenishment.
Maximum Sustainable Yield: Maximum flow of a given resource such that the stock does not decline over time. It is equivalent to the net primary or net secondary productivity of a system.
Species Diversity
Species richness: Number of species in a community.
Species diversity: Number of species and their relative abundance and can be compared using an index such as the Simpson diversity index.
A high value of D suggests a stable and mature site. A low value of D suggests pollution or agricultural management. D =
Population Dynamics
Crude Birth Rate (CBR) =
Total Fertility Rate (TFR) = Average number of births per woman of child-bearing age
Changes in fertility rates are caused by socio-cultural and economic factors:
- Education level: The higher the level of parental education, the fewer the children.
Crude Death Rate (CDR) =
Natural Increase Rate (NIR) = CBR – CDR = %
Doubling Time (DT): Number of years needed by a population to double its size =
= year
Renewable Natural Capital
Renewable natural capital can be generated and/or replaced as fast as it is being used. It includes living species as well as non-living items. Examples: Groundwater and the ozone layer.