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Chapter 52

Unit 9 - Chapter 52 Bio 1M03 .docx

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James S Quinn

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Bio 1M03 Unit Nine: Ecology Chapter 52: Population Ecology  Population – a group of individuals of the same species that live in the same area at the same time o Basic unit of analysis in ecology and evolutionary biology  Population Ecology – the study pof how and why the number of individuals in a population changes over time 52.1 Demography  The number of individuals that are present in a population depends on o Birth – causes population growth o Death –causes population to decline o Immigration – individuals enter a population by moving from another population; causes population growth o Emigration – individuals leave a population to join another population causes population to decline  Demography – the study of factors that determine the size and structure of populations through time  Generation – the average time between a mothers first offspring and her daughters first offspring Life Tables  Life Table – summarizes the probability that an individual will survive and reproduce in any given time interval over the course of its lifetime  Survivorship – the proportion of offspring produced that survive, on average, to a particular age o Cohort – a group of the same age that can be followed through time o Survivorship Curve – the logarithm of the number of survivors versus age; used to recognize general patterns in survivorship and make comparisons among populations or species  Type I  Eg/ Humans  Throughout life – survivorship is high  Most individuals approach the species maximum life span  Type II  Eg/ Songbirds  Throughout life – constant mortality  Type III  Eg/ Many plants  High mortality when young  Fecundity – the number of offspring produced by each female in the population o Age-specific fecundity – the average number of female offspring produced by a female in age class x o Age class – group of individuals of a specific age The Role of Life History  Fitness Trade-Offs o Occur because every individual has a restricted amount of time and energy at its disposal – it’s resources are limited greatly o A female can maximize fecundity, maximize survival or strike a balance between the two  Life History – consists of how an individual allocates resources to growth, reproduction and activities or structures that are related to survival  Organisms with high fecundity – tend to grow quickly, reach sexual maturity at a young age and produce many small eggs or seeds  Organisms with high survivorship – tend to grow slowly and invest resources in traits that reduce damage from enemies and increase their own ability to compete for water, sunlight or food  L. vivipara Bio 1M03 o Females who live a long time but mature late and have few offspring – higher fitness in cold, high- elevation habitats o Females who have short lives but mature early and have many offspring – higher fitness in warm, low elevation habitats 52.2 Population Growth  Populations Growth Rate – the change in the number of individuals in the population (ΔN) per unit time (Δt)  If no emigration or immigration --- population growth rate = number of individuals (N) in the population x the different between the birth rate per individual (b) and the death rate per individual (d)  Per Capita Rate of Increase (r) – difference between birth rate and death rate per individual  Intrinsic Rate of Increasmax)– when r reaches maximal value (many births, few deaths) o Population Growth Rate is Expressed as: ΔN/Δt =max o maxis a function of a species life-history traits o Each species has a characteristic that does not change max  At any specific time, each population of a species has an instantaneous growth rate, or per-capital rate of increase (r), which is less than or equmaxto r Exponential Growth  Exponential Population Growth – occurs when r does not change over time; growth rate does not depend on the number of individuals in the population  Density independent  Adds an increasing number of individuals as the total number of individuals, N, gets larger  Eg/ r = 0.02  population of 1 billion adds 20 million per year, population of 100 adds 2 per year  Not possible for exponential growth to continue indefinitely  Situations exponential growth occurs o New population in a new habitat o Population has been devastated by a catastrophe and beings to recover, starting with a few survivors  Population Density – the number of individuals per unit area  When population density gets very high – expect birth rate to decrease and death rate to increase  r declines Logistic Growth  Growth in the natural world is more often density dependent  Carrying Capacity (K) – the maximum number of individuals in a population that can be supported in a particular habitat over a sustained period of time o Depends on food, space, water, soil quality, resting or nesting sites, and the intensity of disease and predation o Can change from year to year, depending on conditions  If N < K (population below carrying capacity)  population should continue to grow  Populations growth rate is proportional to (K-N)/K o Numerator defines the number of additional individuals that can be accommodated in a habitat with carrying capacity K o Describes the proportion of “unused resrouces and space” in the habitat o When N is small – (K-N)/K is close to 1  growth rate should be high o When N gets larger – (K-N)/K gets smaller o When N = K (N reaches carrying capacity) – (K-N)/K = 0  growth stops  Growth rate slows as population approaches carrying capacity  Logistic Growth Equation o ΔN = maxN(K – N) Δt K Bio 1M03 o Describes logistic population growth – changes in growth rate that occur as a function of population size Discrete Growth  N = N λt t 0 o Nt= population size; subscript indicates the year/generation o N0= initial population size o λ = finite rate of increase  Parameter – a variable or constant term that affects the shape of a function but does not affect its general nature  λ > 1  population is growing  λ < 1  population is declining  λ =1  population stable  λ = 0 /g o R = net reproductive rate o g = generation time o Dividing the net reproductive rate by the generation time  get discrete rate Continuous Growth r  Relationship between λ and r  λ = e o λ = finite rate of increase; expresses a populations growth rate over a discrete interval of time o r = instantaneous rate of increase; gives populations per-capital growth rate at any particular instant o e = base of the natural logarithm  Populations that breed continuously, not at defined intervals  When λ = r  differences between discrete and continuous growth are negligible  Initially – growth rate is exponential – r is constant  N increases to the point where competition for resources or other density-dependent factors beings to occur – growth rate begins to decline  Population reaches carrying capacity – growth rate eventually reaches 0 (graph is flat)  Two Paramecium species; 20 individuals of one species in 5 mL solution  Conditions the same – food, washing solution and pH 8  Carrying capacity differed between species  P. aurelia – maximum density of 448 individuals/mL  P. caudatum – maximum density of 128 individuals/mL What Limits Growth Rates and Population Sizes?  Population sizes change as a result of two general types of factors 1. Density-Independent Factors o Alter birth and death rates, irrespective of the number of individuals in the population o Usually triggered by changes in the abiotic environment  Variation in weather patterns Bio 1M03  Catastrophic events (cold snaps, hurricanes, volcanic eruptions, drought etc) 2. Density Dependent Factors o Usually biotic o Change in intensity as a function of population size – death rates may increase and birth rates may decrease when populations are at high density  Eg/ When trees are crowded, they have less water, nutrients and sunlight at their disposal and make fewer seeds a) Proportion of surviving individuals is higher when initial density is lower b) Larger average clutch size when density of females is lower o Density-dependent changes in survivorship and fecundity cause logistic population growth – density- dependent factors define a particular habitats carrying capacity o K varies among species and populations – this variation affects growth rates and population sizes o K varies because  Some habitats are better than other habitats due to  Differences in food availability  Predator abundance  Other density-dependent factors  Time – as conditions in some years are better than in others  Same region may have a different carrying capacity for different species  Same area able to support more small bodied species than large bodied species 52.3 Population Dynamics  Population Dynamics – changes in populations through time o Has uncovered a wide array of patterns in natural populations I addition to exponential and logistic growth How Do Metapopulations Change through Time?  Metapopulation – individuals from a species that occupy many small patches of habitat, so that they form many independent populations  Balance between extinction and recolonization o Population within metapopulation might go extinct  Due to catastrophe, influx of predators or disease outbreak o Migration from nearby population can reestablish population in empty habitat fragments o Subpopulations blink on and off – overall population is maintained at a stable number of individuals Why Do Some Populations Cycle?  Population Cycles – regular fluctuations in size that some animal populations exhibit  Eg/ Snowshoe Hare and Lynx – Two hypotheses for fluctuations 1. Hares use up all their food when their populations reach high density and starve; in response, lynx also starve  Hares control lynx population 2
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