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Unit D: Population and Community Dynamics Chapter 20: Population Growth and Interactions Section 20.1 β Population Growth A. Density and Distribution of Populations a. All populations can be described in terms of two fundamental characteristics: density and distribution. i. _______________________________ (DP) is defined as the number of individual organisms (N) in a given area (A) or volume (V). π·π = π π ππ π·π = π΄ π ii. There are three theoretical distribution patterns for populations that could affect the accuracy of population density estimates β uniform, _________________________ and clumped. B. Factors That Affect Distribution Patterns a. Distribution patterns are influenced by distribution of resources in a habitat and the interactions among members of a population or of a community. i. Example: Random distribution would occur when resources are abundant and population members do not have to compete with one another. Generally, random distribution in nature is ____________________. ii. Example: Clumped distribution occurs when populations are found in close proximity to each other in various groups within their habitat due to the proximity of food, water, or shelter. Most populations, including humans, exhibit a clumped pattern of distribution. iii. Example: Uniform distribution often occurs in artificial populations such as plants in orchards or agricultural fields as well as some species that behave __________________________ to defend resources and protect their young. b. Most populations do not perfectly fit any one pattern of distribution and often represent different distribution patterns at different times. C. Population Growth a. A populationβs size directly depends on how much and how fast it grows. There are four processes that can change the size (number of individuals) in a population (ΞN): i. Births (b) and immigration (i) cause _____________________ in population size ii. Deaths (d) and emigration (e) cause ____________________ in population size. b. The change in population size can be calculated with the following equation: βπ = [π + π] β [π + π] c. The Rate of Population Growth i. Just as important as the change in the size of a population is the speed at which the change occurs. 1. An increase in the size of a population that spreads before it can be contained is called a ____________________________________. 2. A decrease that occurs very rapidly is referred to as a population crash. ii. The change in the number of individuals in a population (ΞN) over a specific time frame (Ξt) is known as a populationβs growth grate (gr): βπ ππ = βπ‘ iii. To compare populations of the same species that are different sizes or live in different habitats, the change in population size can be expressed as the rate of change per individual. This measurement, the __________________________ __________________ (cgr), can be determined by calculating the change in the number of individuals (ΞN) relative to N (the original number of individuals), and then dividing this change in the number of individuals by the original number in the population (N): ππππππ β π βπ πππ = ππ πππ = π π D. Factors That Affect Population Growth a. Both biotic and abiotic factors limit the growth of a population. i. Biotic Potential 1. Each species has an intrinsic rate of growth that is possible given unlimited resources and ideal living conditions. The highest possible per capita growth rate for a population is called its _________________ (r). Factors that determine this are: a. The number of offspring per reproductive cycle b. The number of offspring that survive long enough to reproduce c. The age of reproductive maturity and number of times individuals reproduce in a life span d. The life span of individuals 2. A population that is growing at its biotic potential would be expected to grow exponentially. The brief lag phase, followed by a steep increase in the growth curve β called an __________________________________ ___________________ β can be described as a J-shaped curve. ii. Carrying Capacity 1. The initial stage is called the lag phase since growth is small because there are only a few individuals to reproduce. 2. The birth rate during the exponential growth phase is much greater than the death rate, so population size will increase rapidly. 3. This rapid growth rate cannot continue indefinitely. Eventually, competition for resources and other limiting factors will slow the rate of growth until it reaches the _____________________________, when the birth rate and death rate are equivalent. 4. This pattern of population growth is illustrated as an S-shaped (sigmoidal) curve known as the logistic growth pattern. The population growth levels off into the habitatβs carrying capacity (K) β the theoretical maximum population size that the environment can sustain over an extended period of time. 5. This carrying capacity may change from season to season or from year to year but overall the population size will fluctuate around the carrying capacity in a stable ___________________________. 6. Factors that limit a habitatβs carrying capacity can be categorized as density-dependent or density-independent: a. Density-dependent factors are biotic and include things like parasites and diseases that spread more easily in dense populations as well as predation. b. Density-independent factors are abiotic and include harsh weather, drought, floods, forest fires, etc. 7. The combined effects of various, interacting limiting factors is described as the ___________________________________ to population growth. iii. Life Strategies 1. Bacteria and other species that reproduce close to their biotic potential, (r), represent one end of the life strategies continuum. They are said to have _______________________________________ which include short life spans, early reproductive ages, large broods of offspring that receive little or no parental care. They take advantage of favourable environmental conditions to reproduce quickly. 2. The other end of the continuum are populations that live close to the carrying capacity (K) of their habitats so they are described as having Kselected strategies. They have few offspring per reproductive cycle, one or both parents care for the offspring when young, take a relatively long time to mature and reach reproductive age, and live a relatively long time. 3. Recognizing the various K-selected and r-selected strategies of a species can be used to predict the success of a population in a particular habitat.