Density Dependence #9/10: Density Dependence, Density Independence, Population Regulation

Population Growth Limitations: - Reason why populations don’t increase indefinitely -All populations can increase under favourable conditions, but conditions not always favourable Density Dependent: - Resource Limitation (competition involved too) - Predation (parasitism; pathogen) Density Independent: - Weather/Disturbances

Density-Independent Factors: Factors whose effects on birth and death rates independent of population density (N) - Can have large effects on population size but do not regulate population size Example: - Weather conditions (temperature, rainfall) - Catastrophes (storms, fires, floods) - Geometric growth - Salinity effects - TBT effects Note: Population growth rates = NOT function of density

Density-Dependent Factors: Factors that cause birth, death, immigration or emigration rates to change as population density changes - Regulate population size - Population growth rates change as population density changes Example: - Logistic growth Note: Can be tested by modifying limiting factors

Population Regulation: Density-dependent factors that cause population to: - Increase when density low - Decrease when density high

Population Density: Average number of individuals per unit of area or volume

Carrying Capacity (K): Max population size of species that can be sustained by specific environment, given food, habitat, water, and other resources available - X-intercept (y = 0) on population growth rate vs density graph - Where birth and death line cross (b = d) on rate vs population size (n) graph

Logistic Growth: Population level pattern in which abundance rapidly increases at first and then stabilizes at carrying capacity - Density dependent - Population approaches EQ

Logistic Growth Equation: Modified exponential growth equation that incorporates growth limits and shows how population may stabilize at carrying capacity - Growth rate (r) ↓ as density (N) ↑ - Better equation for mimicking real population behaviour - dN/dt = rN(1-N/K) - If N > K, dN/dt = negative Note: Exponential growth equation does not apply when resources start to run out

r_max: per capita intrinsic (natural) rate of increase under ideal conditions

r_obs: per capita intrinsic rate of inc under actual conditions - Depends on birth and death rates (+ immigration and emigration rates) - r_obs = b - d

Density Dependence Graph: - X-axis = density - Y-axis = population growth rate (r) Note: Not enough information to tell whether births and/or deaths are density-dependent

Birth & Death Rate vs Population Size Graph: - K = at centre intersection and referring to population size - As population size smaller, births bring size forwards to K - As population size bigger, deaths bring size backwards to K Note: Most common relationship between population density and death rate = population size inc and death rate inc (resources scarce) (At K, B = D; assume no immigration/emigration)

Variable vs Fixed Birth & Death Rates: - As K changes over time (function of fluctuating birth and death rates), population size will also fluctuate

Population Fluctuation: Reason that no population growth looks same - All populations fluctuate in size - Fluctions can be erratic in either increases or decreases

Population Extinctions: Fluctuation in population growth rate can increase risk of extinction (especially small populations)

Allee Effects: When population grow rate decreases as population density decreases - Population growth rate increases as population density increases - Density dependent Example: - Important in animals such as bluefin tuna (Thunnus thynnus), which form schools whose protective systems function poorly at small population sizes - Important in species where individuals have difficulty finding mates at low population densities (ex. kakapos; Strigops habroptilus)