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## Ecology of Populations

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**Essential knowledge 2.D.1:**• All biological systems from cells and organisms to populations, communities and ecosystems are affected by complex biotic and abiotic interactions involving exchange of matter and free energy. • a. Cell activities are affected by interactions with biotic and abiotic factors. (Temperature, Water availability, Sunlight) • b. Organism activities are affected by interactions with biotic and abiotic factors. {Symbiosis (mutualism, commensalism, parasitism),Predator–prey relationships, Water and nutrient availability, temperature, salinity, pH}**Essential knowledge 2.D.1:**• c. The stability of populations, communities and ecosystems is affected by interactions with biotic and abiotic factors. (Water and nutrient availability, Availability of nesting materials and sites, Food chains and food webs, Species diversity, Population density, Algal blooms)**Essential knowledge 2.D.3:**• Biological systems are affected by disruptions to their dynamic homeostasis. • a. Disruptions at the molecular and cellular levels affect the health of the organism. (Physiological responses to toxic substances,Dehydration) • b. Disruptions to ecosystems impact the dynamic homeostasis or balance of the ecosystem. (Invasive and/or eruptive species, Human impact, Hurricanes, floods, earthquakes, volcanoes, fires, Water limitation, Salination)**Essential knowledge 4.A.5:**• Communities are composed of populations of organisms that interact in complex ways. • a. The structure of a community is measured and described in terms of species composition and species diversity. • b. Mathematical or computer models are used to illustrate and investigate population interactions within and environmental impacts on a community. (Predator/prey relationships spreadsheet model, Symbiotic relationship, Graphical representation of field data, Introduction of species, Global climate change models)**Essential knowledge 4.A.5:**• c. Mathematical models and graphical representations are used to illustrate population growth patterns and interactions. • 1. Reproduction without constraints results in the exponential growth of a population. • 2. A population can produce a density of individuals that exceeds the system’s resource availability. • 3. As limits to growth due to density-dependent and density independent factors are imposed, a logistic growth model generally ensues. • 4. Demographics data with respect to age distributions and fecundity can be used to study human populations.**Essential knowledge 4.B.3:**• Interactions between and within populations influence patterns of species distribution and abundance. • a. Interactions between populations affect the distributions and abundance of populations. • 1. Competition, parasitism, predation, mutualism and commensalism can affect population dynamics. • 2. Relationships among interacting populations can be characterized by positive and negative effects, and can be modeled mathematically (predator/prey, epidemiological models, invasive species). • 3. Many complex symbiotic relationships exist in an ecosystem, and feedback control systems play a role in the functioning of these ecosystems.**Essential knowledge 4.B.3:**• b. A population of organisms has properties that are different from those of the individuals that make up the population. The cooperation and competition between individuals contributes to these different properties. • c. Species-specific and environmental catastrophes, geological events, the sudden influx/depletion of abiotic resources or increased human activities affect species distribution and abundance. (Loss of keystone species, Kudzu,Dutch elm disease)**Population ecology is the study of populations in relation**to environment**Scope of Ecology**• Ecology • The study of the interactions of organisms with • other organisms, and • The physical environment • Population - All the individuals of a species within a particular space • Community – Specified populations interacting with each other • Ecosystem - Community interacting with environment**Ecologists studying populations might study the factors that**affect the growth and regulation of population size • Ecologists studying communities want to know how interactions such as predation and competition affect the organization of a community**Dynamic biological processes influence population density,**dispersion, and demography**Demographics of Populations**• Demography is the statistical study of a population, which includes its density, distribution, rate of growth**Density and Distribution of Populations**• Population Density - Number of individuals per unit area or volume • Population Distribution - Pattern of dispersal of individuals within a space of interest • Ecologists analyze what causes the spatial and temporal “patchiness” of organisms • Affected by the availability of resources**Density: A Dynamic Perspective**• Determining the density of natural populations • Is possible, but difficult to accomplish • In most cases**Patterns of Dispersion**• Environmental and social factors • Influence the spacing of individuals in a population**A clumped dispersion**• Is one in which individuals aggregate in patches**A uniform dispersion**• Is one in which individuals are evenly distributed**A random dispersion**• Is one in which the position of each individual is independent of other individuals**Population Growth**• Exponential Growth • Rate of increase increases as the total number of females increases • Biotic Potential • Maximum population growth that can possibly occur under ideal circumstances • Environmental Resistance • All environmental conditions that prevent populations from achieving biotic potential**Mortality Patterns**• A cohort • Survivorship • The probability that newborn individuals of a cohort will survive to a particular age • Survivorship Curves**Survivorship Curves**• A survivorship curve is a graphic way of representing the data in a life table that shows the differential mortality rates in relation to age**The survivorship curve for Belding’s ground squirrels**• Shows that the death rate is relatively constant**Survivorship curves can be classified into three general**types • Type I, Type II, and Type III**Type I curve shows low infant mortality**• These species produce few offspring but provide them with a high degree of parental care. This increases the likelihood that they will survive to maturity • Show a low rmax**Type III curve have high death rates for very young. Death**rates drop as individuals survive to increased ages • Species produce very large numbers of offspring but provide little or no care for them • Have a high rmax**An organisms life history influences the growth rate of a**population**Population Growth Models**• Discrete breeding - members of population have only one single reproductive event in their lifetime • Many insects, annual plants • Continuous breeding - members of population experience many reproductive events throughout their lifetime**The exponential model describes population growth in an**idealized, unlimited environment • It is useful to study population growth in an idealized situation**Per Capita Rate of Increase**• If immigration and emigration are ignored**dN** rN dt • Zero population growth • The population growth equation can be expressed as**dN** rN dt G = rN G = N = population size r = intrinsic rate of increase If r is constant, then the rate at which population grows depends on the number of individuals already in the population (N), value of r depends on the kind of organism G = dN/dt (∆N/∆t) = change in # of individuals over a given time r = (b-d) = birth rate - death rate G = rN or ∆N/∆t = (b-d)N**Exponential Growth**• Exponential population growth • Is population increase under idealized conditions (unlimited resources) • Under these conditions**dN** rmaxN dt • The equation of exponential population growth is**Exponential population growth**• Results in a J-shaped curve**The J-shaped curve of exponential growth**• Is characteristic of some populations that are rebounding**Environmental factors that restrict population growth are**called population-limiting factors • Environmental factors limit the growth of populations, preventing exponential growth • Logistic growth model - idealized population growth that is slowed by limiting factors**The logistic growth model includes the concept of carrying**capacity • Exponential growth • Cannot be sustained for long in any population • A more realistic population model**Carrying capacity (K)**• Is the maximum population size the environment can support with no net increase or decrease • The value of K varies, depending on species and habitat**(K N)**dN rmax N dt K • The logistic growth equation • Includes K, the carrying capacity**The Logistic Model and Real Populations**• The growth of laboratory populations of paramecia • Fits an S-shaped curve**Some populations overshoot K**• Before settling down to a relatively stable density**The logistic model fits few real populations**• But is useful for estimating possible growth**Population Dynamics**• The study of population dynamics • Focuses on the complex interactions between biotic and abiotic factors that cause variation in population size**Population Change and Population Density**• In density-independent populations • In density-dependent populations • Birth rates fall and death rates rise with population density**Density-dependent factors**• Population limiting factors whose effects depend on population density • As the number of individuals increases, so does the percentage of individuals affected