CHANGES to ECOSYSTEMS and POPULATIONS

Population Fluctuations Natality (birth) rates Mortality (death) rates Emigration Immigration (Natality + Immigration) – (Mortality + Emigration)

Calculations Birth rate = number of births in a given time Initial population = births per 100 (or as a %) Death rate = number of deaths in given time = deaths per 100 (or as a %)

POPULATION GROWTH CURVES Basic Features J-Curve Overshoot Stable phase maximum number of individuals of a species that can be supported by a particular ecosystem on a long-term basis Exponential growth Lag phase S-Curve Environmental resistance

Population Growth Result of positive feedback mechanisms  change S-curves start with exponential growth (no limiting factors affect growth at this stage) above a certain population size, the growth rate slows down gradually finally result in a constant population size growth is slowed down more in larger populations (pattern is consistent with density dependent limiting factors) Result of negative feedback mechanisms  stability J-curves show ‘boom and bust’ pattern population grows exponentially at first, followed by sudden collapse collapses are called diebacks (population exceeds carrying capacity before collapse occurs i.e. overshoot) does not show the gradual slow down of population growth with increasing population size (therefore likely that density independent limiting factor is causing population decrease) growth curve typical of microbes, invertebrates, fish and small mammals

Typical S and J curves dieback

Carrying Capacity Carrying capacity can be determined by: Examining the requirements of the species Examining the available resources  

Limiting Factors Factors that act as limits on population growth: Density-independent: effect does not vary with population density - lack of suitable habitat - adverse weather (b) Density-dependent: effect varies with population density - regulate or keep populations within a narrow range of change (in equilibrium) - operate as negative feedback mechanisms  stability - Eg: Food Water Shelter Disease

Population Equilibrium Mechanisms that tend to keep population change within a narrow range are: Predator-prey dynamics Parasites Plant-herbivore interaction Keystone species Competition Biotic potential - factors which tend to promote population growth Environmental resistance - factors which tend to inhibit or discourage population growth and keep populations within their carrying capacity

Environmental Resistance Forces or factors which tend to inhibit or discourage population growth and keep populations within their carrying capacity or in equilibrium (population forms an S-curve)