Biology 2B Ecosystems Population dynamics

Factors affecting population size 1 Births – number of new organisms Deaths – number organisms dying Migration Immigration – organisms entering the population Emigration – organisms leaving the population These are all generally given as rates per 1000. Growth = (births + immigration) – (deaths + emigration)

Factors affecting population size 2 Density independent Physical (abiotic) factors Rainfall Humidity Temperature Acidity Salinity Catastrophic events Flood Fire Drought Volcanic eruptions Earthquakes Tsunamis Density dependent (biotic factors) Food supply (abundance or distribution of prey species) - this becomes more critical as population size and density increases Disease - this is easier to spread as population size and density increases Parasites - these spread more easily as population size and density increases Competition - this becomes more critical as population size and density increases Predation (abundance or distribution of predator species) - this becomes more critical as population size and density decreases The carrying capacity is the number of organisms that can be supported by the environment in which the population exists.

Population growth A population will grow exponentially unless its growth is limited by factors in the environment. A new or colonising population will often show exponential growth initially until it is using all the resources present. If the population numbers are not controlled eg by a predator or disease the population will reach a size that uses all the resources in the ecosystem and the population size drops rapidly – a crash. As ecosystems may be damaged by such large numbers, it may take longer for the population to increase again. Populations controlled by competition in this way show large swings in population numbers – booms and busts.

Population growth 2 Biotic potential is a term that describes how rapidly a population can increase. It is determined by the species ability to survive, breed and colonise new areas. Environmental resistance is a term that describes the forces that act to slow population growth. These include resources available and the interaction of other species with the population. If biotic potential is greater than environmental resistance, populations will grow. If environmental resistance is greater than biotic potential, populations will shrink. Stable populations reach a size where biotic potential matches environmental resistance and population sizes fluctuate around the carrying capacity of the ecosystem.

Sustainable population growth Most populations are controlled by density dependent factors eg predators or disease. This limits their size to the carrying capacity that can be sustained in their particular environment. The population size tends to fluctuate just above and just below the levels of the carrying capacity. Biologists use the terms J-shaped (or J-curve) and S-shaped (or S-curve) to describe the growth patterns of populations. S-shaped populations are more stable than J-shaped populations as they are held in balance by other factors in the ecosystem.

Natural and artificial populations Natural ecosystems are those that exist without human intervention Artificial ecosystems are those that are produced by humans.

Calculating population growth Birth rate = number of births per 1000 head of population number of births ÷ total population x 1000 Death rate = number of births per 1000 head of population number of deaths ÷ total population x 1000 Immigration rate = number of births per 1000 head of population number of individuals entering ÷ total population x 1000 Emigration rate = number of births per 1000 head of population number of individuals leaving ÷ total population x 1000 Growth rate = (birth rate + immigration rate) – (death rate + emigration rate)

Distribution patterns Distribution refers to the region in which members of a population can be found. Distribution patterns can be described as even (regular), clumped or random. Distribution is usually affected by the presence or absence of resources (eg near water) and social interactions between members of the same species (eg juveniles with adults).