S7L4 Students will examine the dependence of organisms on one another and their environments. c. Recognize that changes in environmental conditions can affect the survival of both individuals and entire species.

Population Dynamics Population is a group of organisms of the same species living in the same geographic area. Growth rate is how much the population as a whole grows, rather than its individuals. Density is how many organisms per unit of space. Population Dynamics is the study of these dynamics

Growth Growth rate is its change in population size per unit of time. Immigration is when organisms move into a population. Emigration is when organisms move out of a population. Growth rate can be positive (more in the population than before), negative (less in the population than before) or zero (no change). Density of a population is the number of organisms per unit area; the more organisms living in a given amount of space, the denser the population in that space.

Carrying Capacity Carrying capacity is the number of individuals that a given environment can support.

Population Size and Limiting Factors Populations cannot continue to grow without reaching some environmental limits, such as lack of nutrients, energy, disease, living space and other resources. These are called limiting factors because they limit how many members of a population can be sustained in an area. There are two main categories of limiting factors: density-dependent factors and density-independent factors.

Density Dependent Factors Issues like competition, disease and predation that only become limiting when a population in a given area reaches a certain size. When a city grows too fast or has a huge population increase, adequate health care may be difficult to obtain, and so the death rate increases.

Density Independent Factor Issues like unusual weather, natural disasters and seasonal cycles. These factors affect all individuals within an area, regardless of population, size or density. Human modifications to the environment, like damming a river, can affect organisms.

Changed Environmental Conditions Some species can make adaptations or have characteristics that allow them to survive and reproduce when conditions change. If conditions change faster than a species can adapt, the species might become extinct. The more individuals of a species there are, the greater the mathematical possibility that individuals exist that are better suited to survive. The faster the reproductive cycle, the more quickly the adaptation will become a dominant characteristic.