Population Ecology The study of the growth, abundance, and distribution of populations (dynamics of populations) Population Dynamics - the study of changes in populations

Population Growth Population growth is the increase in population size over time. Certain populations can show exponential growth, but only for brief periods. Growth is controlled by limiting factors that determine the carrying capacity of the environment.

When a population regulated by environmental limits is graphed over longer periods, the curve has a characteristic S shape. Note the J shape at the beginning when there are few environmental limits followed by a leveling out as the carrying capacity of the environment is approached

Populations of organisms do not experience linear growth. Rather, the graph of a growing population first resembles a J-shaped curve. As the number of reproducing organisms increases, the population growth increases rapidly. This represents the exponential nature of a population growth. As a population gets larger, it also grows faster, and can result in a population explosion.

Exponential Growth Certain populations may exhibit exponential growth – but only for brief periods of time.

Limiting Factors A population cannot grow indefinitely. It will usually reach an environmental limit such as a lack of nutrients, energy, living space or other resources. These factors will cause the population size to stabilize and logistic growth will begin as the population size levels out. This is called an S-shaped curve. Limiting factors include any biotic or abiotic factor that restricts the existence, numbers, reproduction or distribution of organisms.

Two types of limiting factors: a. Density dependent - disease, competition, parasites, and food; as population increases these have an increasing effect on the population ( ex. a disease spreads more quickly in a dense population than in a less dense one) b. Density independent – affect populations regardless of their density and include temperature, storms, floods, droughts and habitat disruption ( no matter how many mosquitoes there are, a cold winter will kill them)

Factors that limit one population can also have an indirect effect on another population. Example – A drought can cause a decrease in grass growth which can affect on the number of grass seeds available for a mouse population. The change in the mouse population can then affect the number of hawks in a habitat. Example – A drought can cause a decrease in grass growth which can affect on the number of grass seeds available for a mouse population. The change in the mouse population can then affect the number of hawks in a habitat.Tolerance Tolerance is the ability of an organism to withstand fluctuations in biotic and abiotic environmental factors. Some species can tolerate conditions that another cannot. Example – Trout and bass cannot tolerate the same range of water temperature and dissolved oxygen content. Trout prefer colder water with more dissolved oxygen than the bass. Example – Trout and bass cannot tolerate the same range of water temperature and dissolved oxygen content. Trout prefer colder water with more dissolved oxygen than the bass.

Logistic Growth

Carrying Capacity The number of organisms of one species that an environment can support is its carrying capacity. When populations are under the carrying capacity, births exceed deaths until the c.c. is reached. If the population temporarily overshoots the carrying capacity, deaths exceed births until the population levels are once again below carrying capacity.

Dynamic Equilibrium Wildlife populations are never truly static…… they tend to fluctuate as part of an overall “balance of nature” in response to a variety of stimulating and limiting factors. Nature is never really in balance or equilibrium. Ecological systems are involved in a process of continual change.

Consider this illustration representing muskrats in a marsh. The barrel represents the marsh habitat-the amount of food, water, and cover for a fixed number of muskrats. The water in the barrel is the number of muskrats the habitat can support. The pipe pouring water into the barrel represents the new muskrats that are born in the marsh or wander in from other habitats. The water spilling out is the number of muskrats that die each year due to starvation, disease, predators or other factors. The barrel can only hold so much water. That is, there is a limit to the number of muskrats that can survive here from year to year unless the habitat (the size of the barrel) is changed in some way. Every habitat has a different carrying capacity for every kind of wildlife that lives there. A pristine cattail marsh would be a deep barrel for muskrats, while a dune forest would hold few, if any.

Organism Interactions Limit Population Size Predation – predators affect population size of the prey Competition – when population numbers are low, the resources are plentiful and when numbers are high, the resources become scarce and competition increases –Intraspecific – between two of same species –Interspecific – between two of different species Parasitism – as the number of parasites increases, the population of the host can decrease and as the number of parasites decreases, the population of the host can increase

Demography The study of human population growth characteristics Describe and give examples of demographic characteristics of populations. a. Growth rate – difference between birth rate and death rate; change in population size/unit of time (hours, years, decades); organisms can enter a population by birth or immigration (moving in) and can exit a population by death or emigration ( moving out) b. Fertility rate – number of offspring a female produces during her reproductive years c. Age structure – the proportions of a population that are at different age levels; helps determine if a population is growing rapidly, slowly, or not at all; rapid growth countries have an age structure with a wide base because a large % of the population is teenagers and children who will likely mature into adults and have their own children

d. Geographic distribution – pattern of where the organisms live; Social behavior and location of resources determine the pattern Social behavior and location of resources determine the pattern of distribution. of distribution. 1. random – no set pattern within the ecosystem 1. random – no set pattern within the ecosystem 2. clumped – individuals found in groups usually near a resource 2. clumped – individuals found in groups usually near a resource 3. even – set pattern of even spacing is seen often with highly 3. even – set pattern of even spacing is seen often with highly territorial animals territorial animals (random and even are rarely seen in populations) (random and even are rarely seen in populations) e. Sex ratio – ratio of males to females Factors which affect human population growth Factors which affect human population growth Density dependent limiting factors such as disease, war, immigration, emigration, birth rate, death rate, food availability and water availability Density-independent limiting factors might include pollution of water, soil and air

Patterns of Population Distribution