Population Concepts & Impacts on the Environment Ecology Westerville Central High School

As we know from our studies in previous units, species evolve to be more successful in their environment; they adapt to occupy a niche. Yet the biotic and abiotic factors which influence their niches change over time as the environment changes, affecting the number of individuals that a niche can support. Fluctuations in populations (both up and down) can significantly affect the stability of ecosystems.

I. Population dynamics A population is a group of individuals of the same species occupying a particular area at the same time. The number of individuals in a given area is known as ecological density. There are many factors which influence the size and density of a population, which is why populations in nature are rarely stable.

I. Population dynamics Organisms naturally strive to maintain the existence of their species by producing offspring. If allowed, organisms will reproduce at the fastest rate possible to ensure the survival of their species.

I. Population dynamics This type of growth (called exponential growth) -2, 4, 8, 16, etc), rarely occurs in nature because it requires ideal conditions that are not found in the environment (resources on earth, such as space, are limited). Many other factors also act upon populations, forcing them to fluctuate naturally (populations rarely remain stable).

Increases in Populations Growth of populations may occur due to factors such as: Abundant resources - food, water, space, etc. Characteristics that lend themselves to a high growth rate, such as: a high number of offspring per mating (a high birth rate, or natality), many matings per year, and a low age of sexual maturity; A long life span; Low infant mortality rate (deaths per 1000 individuals born); More individuals migrating into an area (immigration) than out of an area (emigration)

Increases in Populations “Total growth rate” of a population is measured as: (births - deaths) + (immigration - emigration).

Increases in Populations As populations grow, they utilize resources in their habitats. These resources, such as shelter, food, water, living space, etc. are limited and can therefore only support a certain number of individuals. The maximum number of individuals that an ecosystem can support is known as it carrying capacity (ex 1 deer per 10 acres). As populations grow towards their ecosystem’s carrying capacity, there are 2 types of curves demonstrated:

S-Shaped growth curves represent organisms whose populations: Start small, adjusting to their environment ( called lag phase - a period of little growth where births equal deaths) Increase rapidly (exponential growth - births outnumber deaths) Slow down as carrying capacity is approached, finally leveling out just below carrying capacity where births equal deaths and the population may remain indefinitely stable (equilibrium).

Draw and label an “S” shaped curve:

Draw and label an “S” shaped curve:

J-Shaped growth curves represent organisms whose populations: Start small and slow (lag phase) increase so quickly (exponential growth) that they surpass the carrying capacity of their ecosystem Drop in number (crash) as a result of a lack of sufficient resources above the carrying capacity.

Draw and label a ‘J’ shaped growth curve:

B. Decreases in populations There are many factors which may act to slow the growth of a population (called limiting factors). Some of these factors influencing decreases in population are: Factors influenced by the population size itself (density dependent factors), such as *predation, diseases, *lack of food, water, space, shelter, accumulation of wastes, etc. (the larger the population, the more these factors influence the population);

B. Decreases in populations There are many factors which may act to slow the growth of a population (called limiting factors). Some of these factors influencing decreases in population are: Factors influencing the population size regardless of how big to small the population is (density independent limiting factors), such as weather, natural disasters, human disturbance - pollution, habitat destruction, hunting, etc.;

Short life span; High infant mortality rate (number of deaths per 1000 individuals born; More individuals migrating out of an area (emigration) than into an area (immigration)

There are two general “life strategies” demonstrated by organisms as they survive the environments’ many pressures:

“R” selected species, the most common (plants, fish, insects, amphibians, etc.), usually have small body sizes, many offspring, little or no infant care (relatively speaking), and high infant mortality.

“K” selected species (large mammals, typically) have large body sizes, few offspring, high parental care, and low infant mortality.

Both “R” and “K” selected species exhibit a specific survivorship curve as a result of their “strategies for life”:

Generally speaking, most organisms are subject to a higher mortality rate (death rate) at a younger stage of life (infancy) Those that have a high infant mortality rate generally produce the most offspring (ex. Cod- 6 million, seeds in plants, quail - 15 eggs, deer - 1 or 2 fawns, etc).