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Chapter 44

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Wilfrid Laurier University
Tristan Long

Chapter 44: Population Ecology Chapter 44: Population Ecology Habitat - The specific environment in which a population lives, as characterized by biotic and abiotic features. Age Structure - Astatistical description or graph of the relative numbers of individual in each age class in a population. Generation Time - The average time between the birth of an organism and the birth of its offspring. Sex Ratio - The relative proportions of males and females in a population. Demography - The statistical study of the process that change a population's’size and density through time. Cohort -Agroup of individuals of similar age. Life History - The lifetime pattern of growth, maturation, and reproduction that is characteristic of a population or species. Carrying Capacity - The maximum size of a population that an environment can support indefinitely. Intraspecific Competition - The dependence of two or more individuals in a population on the same limiting resource. R-selected -Ashort-lived species adapted to function well in a rapidly changing environment. K-selected - Long-lived, slow reproducing species that thrive in more stable environment. 44.1 The Science of Ecology ● In basic ecology, major research questions relate to the distribution and abundance of species and how they interact with each other and the physical environment. ● In applied ecology, workers develop conservation plans and amelioration programs to limit, repair, and mitigate ecological damage caused by human activities. ● Organismal ecology is the study of organisms to determine adaptations to the abiotic environment, including morphological, physiological, biochemical, behavioural, and genetic adaptations. ● Population ecologists document changes in size and other characteristics of populations of species over space and time. ● Community ecologists study sympatric populations, the interactions among them, and how these interactions affect the community’s growth. Interactions may include predation and competition. ● Ecosystem ecologists study nutrient cycling and energy flow through the biotic and the abiotic environment. ● Mathematical models express hypotheses about ecological relationships and different variables, allowing researchers to manipulate the model and document resulting changes. In this way, researchers can stimulate natural events before investing in lab work. ● Experimental and control treatments are necessary because they allow ecologists to separate cause and effect. 44.2 Population Characteristics ● Seven characteristics can be described for any population: a. Every population has a Geographic range b. Every population occupies a habitat c. Population size is the number of individuals making up the population at a specified time. d. Population density is the number of individuals per unit area or per unit volume of habitat. e. Populations can vary in their dispersion, the spatial distribution of individuals within the geographic range f. All populations have an age structure, a statistical description of the relative numbers of individuals in each age class. g. Populations growth is also affected by its generation time, the average time between the birth of an organism and the birth of its offspring. h. Populations of sexually reproducing organisms also very in their sex ratio, the relative proportions of male and females. ● Geographic range is the overall spatial boundary around a population. Individuals in the populations often live in a specific habitat within a range. ● Alower population density means that individuals have greater access to resources such as sunlight and water. The capture-mark-recapture technique assumes that: a. a mark has no effect on an individual`s survival b. marked and unmarked individuals mix randomly c. there is no migration throughout the estimation period d. marked and unmarked individuals are equally likely to be caught ● Three types of dispersion are clumped, uniform, and random. ● Clumped dispersion is the most common in nature because suitable conditions are usually patchily disturbed and animals often live in social groups.Asexual reproduction patterns can also lead to clumped aggregations. ● Uniform dispersion occurs when individuals repel one another because resources are in short supply ● Random dispersion occurs when environmental conditions do not vary much within a habitat, and individuals are neither attracted to nor repelled by others of their species. ● Generation time increases with body size. ● The number of males in a population of mammals has little impact on population growth because females bear the costs of reproduction (pregnancy and lactation), thus limiting population growth. 44.3 Demography ● Alife table summarizes the demographic characteristics of a population, using a cohort to collect information. ● Age-specific mortality and age-specific survivorship deal with age intervals. In any one interval, age-specific mortality is the proportion of individuals that died during the time.Age- specific survivorship is the number surviving during the interval. The two values must sum 1. ● Age-specific fecundity is the average number of offspring produced by surviving females during each age interval. ● In a type 1 curve, high survivorship at a young age decreases rapidly later in life. Type 1 curves are common for large animals, including humans. ● In a type 2 curves, the relationship is linear because there is a constant rate of mortality across the lifespan. Songbirds fit in this category. ● Atype 3 curve shows high mortality at a young age that stabilizes as individuals grow older and larger, Insects fall into this category. 44.4 The Evolution of Life Histories ● Life history of an organism includes the lifetime patterns of growth, maturation, and reproduction. There is a tradeoff in the allocation of resources to these three activities. Natural selection adjusts the allocation of resources to maximize an individual's number of surviving offspring. ● Every organism has a finite energy budget, the total amount of energy it can accumulate and use to fuel its activities. ● Maintenance, growth, and reproduction are the three main energy-consuming process. ● Passive care occurs in animals that simply lay eggs and leave them, or in mammals, as nutrients cross the placenta from the mother to the developing baby. ● Active care involves nursing and other care provided after birth. ● Salmon have a short lifespan and devote a great deal of energy to reproduction. Deciduous trees may reproduce more than once and use only some energy in any reproductive more than once and use only some energy in any reproductive event, balancing reproduction and growth. ● Early reproduction is favoured if adult survival rates are low or if, when animal age, they do not increase in size. In this case, fecundity does not increase in size. 44.5 Models of Population Growth. ● An exponential model is used when a population has unlimited growth. ● dN/dt = change in a population’s size during a given time period; b = per capita birth rate; d = per capita death rate; N = number of individuals in the population; b-d = per capita growth rate = r ● When r>0, the birth rate exceeds the death rate, and the population is growing. When r<0, the birth rate is less than the death rate, and the population is decreasing. When r=0, the birth rate is equal to the death rate, and the population is neither growing nor shrinking. The intrinsic rate of increase (r max) is the maximum per capita growth rate. This value usually varies inversely with generation time, so a shorter generation time means a higher r max. ● Alogistic model has the following pattern: when the population growth is low, the population is small. At intermediate population sizes, growth is more rapid because more individuals breed and r is high. When population growth approaches K (carrying capacity), competition increases, r decreases, and the growth of the population is reduced. 44.6 Population Regulation ● Density-dependent factors include intraspecific competition and predation.At high density, fewer resources are available for individuals, which, in turn, use more energy in maintenance needs and less in reproduction. Offspring produced at higher population densities are often smaller in number or size and less likely to survive.At high population levels, adults may be smaller and weaker. ● Density-independent factors, such as fire, earthquakes, storms, floods, and other natural disturbances, reduce a population size regardless of density. ● r-selected species often have large numbers of small young, whereas k-selected species usually have small numbers of larger young. ● Extrinsic control includes interactions between individuals in a population and their food and predators. Once a food supply is exhausted, reproduction will decrease and mortality will increase. ● Intrinsic control can be hormonal changes within dispersal, population that cause increased aggressiveness, faster dispersal, and reduced reproduction.Aggression can cause weaker individuals to be forced to disperse to reduce the population density. 44.7b Population Growth andAge Structure: NotAll Populations Are the Same ● For all long-lived species, differences in age structure are a major determinant of differences in population growth rates. ● Countries with ZPG, there are approximately equal numbers of people of reproductive and pre reproductive ages. The ZPG situation is exacerbated when reproductives have very few offspring, meaning that pre reproductives may not even replace themselves in the population. ● In countries with negative growth (without immigration), post reproductives out-number reproductives, and these populations will not experience a growth spurt when today’s children reach reproductive age. ● Countries with rapid growth rate have a broad-based age structure, with many youngsters born during the previous 15 years. Ecology  Ecologists study levels of organization ranging from individual organisms to the biosphere  Study of the interactions between organisms (biotic) and their environments (abiotic) Testing Hypotheses  Ecologists use experimental or observational data  Field or laboratory studies used to test predictions by manipulating variables  Data collected about “natural experiments” do not require manipulations  Sometimes hypotheses framed in mathematical models  Allows computer models to simulate natural events and large-scale experiments Science of Ecology  Basic ecology  Focuses on undisturbed natural systems  Applied ecology  Considers effects of human disturbance  Overarching goal of ecology is to understand the causes and consequences of changes of population/community structure What characters to study?  Geographic range / Habitat  Population size  Population density  Population dispersion  Age structure  Generation time  Sex Ratio  Proportion of individuals in reproductive condition Geographic Range  Geographic Range: Overall spatial boundaries within which a population lives  Habitat: Specific environment in which a population lives, as characterized by its biotic and abiotic features Estimating Population Size  Often a difficult task  Too many?  Cryptic?  Mark-Recapture methodology is one solution Mark-Recapture Study  Step 1: Capture a Random Sample of Individuals (n ) 1hat are marked  Step 2: Release marked individuals back into population  Step 3: Capture a second random sample of Individuals (n )2 some of whom are marked (n2m  Step 4: Calculate initial population size (x) using the formula1n /x =2m /2 Assumption of Mark-Recapture  Random sample?  Marking has no effect on survival or probability of being resampled later  Marked individuals have time to mix with unmarked individuals before second sample  Marks are not lost  Population is “closed”  No births, deaths, immigration or emmigration Dispersion Patterns  Influenced by:  Distribution of resources in environment/Landscape features  Habitat requirements of each individual  Social structure of species  Dispersal capacity/mode of reproduction of species  Presence/absence of other species Random Distribution  Distance between individuals is “random” with respect to each other’s location  Individuals neither attract nor repel one another.  Rarest type of distribution  Why?  Found in homogeneous environments Uniform Distribution  Maximal distance between any two individuals.  Found in many plants due to competition for resources  Also can result from territorial behavior Clumped Distribution  Most common form of distribution (Why?)  Resources rare  Poor dispersal capacity  Benefits to group living  Foraging, defence, parental care Further Population Characteristics  Population age structure  Relative numbers of individuals of different ages  Sex ratio  Relative proportion of males and females  Proportion Reproducing  Particularly relevant to conservation of any species in which individuals are rare or widely dispersed in habitat  Generation time  Average time between individual’s birth and birth of its offspring  Apopulations’characteristics can have dramatic effects on capacity for future growth (strong target of selection) Demography  Statistical study of processes that change a population’s size and density through time  Population growth factors  Births and immigration  Population decline factors  Deaths and emigration Life Tables  Summarize demographics of a population  Age-specific mortality  Age-specific survivorship  Age-specific fecundity  Cohort  Group of individuals of similar age Survivorship Curves Type I : High survivorship until late in life Type II : Constant rate of mortality in all age classes Type III : High juvenile mortality, followed by low mortality after critical age and size Life Histories  The schedule, frequency and duration of events in an organisms’lifetime: patterns of growth, maturation, and reproduction  Allocations of resources influence evolution of maintenance, growth, and reproductive traits  Usually adjusted to maximize an individual’s number of surviving offspring (shaped by selection) Evolution of Life Histories  Influenced by organism’s physical and ecological environment:  development rate  age of sexual maturity / first reproduction  number of offspring / frequency of breeding  level of parental investment  senescence and death Fecundity vs. Parental Care  Fixed energy budget may result in trade-offs between fecundity and parental care  Passive parental care before offspring born: 10 eggs at 100 units energy each = 1,000 total 1,000 eggs at 1 unit energy each = 1,000 total  Active parental care after offspring born: many young = little care few young = more care How Often to Breed?  Semelparity  One reproductive episode  Devotes all stored energy  Maximum fecundity  Death after reproduction  Iteroparity  Multiple reproductive episodes  Only some energy devoted in each event Age at First Reproduction  Early reproduction favoured  Adult survival rates low  Animals do not grow more fecund with age  Later reproduction favoured  If sexually mature adults likely to become older  If organisms grow older with age  If larger organisms have higher fecundity Life History Differences  Most organisms fall between two extremes:  r-selected species and K-selected species Population Growth • organisms often have the capcity to produce more offspring than are needed to replace themselves in the next generation • study of population growth is central to ecology o change in N/change in T = B-D o N = change in population size o T = time period o B = births o D = deaths Exponential Growth Model  By expressing per capita birth and death rates, equation describes unlimited population growth: dN/dt = rMAXN rMAX = Intrinsic rate of increase; i.e. maximum per capita growth rate (a constant) • When there are no limits on population growth, follows exponential pattern Life History Evolution  Trade-offs reflect differences in strategies to maximize fitness  LH Character 1: Male displays  Probability of being predated increases brightness/number of spots  Attractiveness vs survival trade-off  Trade-offs reflect differences in strategies to maximize fitness  LH Character 2: Body size  Probability of being predated decreases with body size  Effect on time of first reproduction  LH Character 3: Parental Care/Fecundity trade-off  Many small offspring, produced more frequently vs. fewer, better provisioned offspring produced more occasionally Bacterial Population Growth • Bacteria reproduce by binary fission; their populations double in size each generation o Short generation time o Small body size o Large r MAX Life History Traits and Populations  The specifics of a species’life history characteristics can have important consequences for:  Population growth  Population fluctuations  Nature of interactions with other species (including in context of community ecology) Logistic Growth Model  Most population growth is NOT exponential – limited in some fashion  Carrying capacity (K): Maximum population size that environment can sustain*  Per capita population growth rate (r) decreases as N approaches K dN/dt = rmaxN[K-N/K] • Modify exponential rate of growth relative to carrying capacity • Time lags in responses to increased density may cause N to oscillate around K • Crowding decreases individual growth rates, adult size, and survivorship Density-Dependant • Fecundity is density-dependant • Competition within populations or between species • Predator–prey interactions • Parasites • Spread of infectious diseases Density-Independent Factors  Reduce population growth regardless of population size (abiotic environmental factors)  May interact with density-dependent factors  Food shortage caused by high population density (a density-dependent factor) may lead to malnourishment and…  Malnourished individuals may be more likely to succumb to stress of extreme weather (a density-independent factor) Cyclic Fluctuations  Some animal populations exhibit cyclic fluctuations in size
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