Presentation on theme: "Reading: Freeman, Chapter 52"— Presentation transcript:
1Reading: Freeman, Chapter 52 Population EcologyReading: Freeman, Chapter 52
2Every species has a geographic range A geographic range describes where individuals of a species might potentially be located.In the United states, most species have a range of 4-24 states.Cosmopolitan species are an extreme, they are worldwide in distribution.Endemic species are found in only a small, restricted area, they represent the other side of the spectrum.
3This is the geographic range of a species of damselfly. campus.greenmtn.edu/dept/NS/Dragonfly/Vermont
4Factors Determining the Geographic Range of a Species HistoryBiological TolerancesOther SpeciesA combination of the above
5Historical Factors Determining Range: Example Many species have what is called a “Gondwanan” distribution. They occur in the Southern continents of Australia, South Africa, South America, and sometimes India.These places are far away from each other now, but 150 million years ago, they were all linked together in a massive continent.Examples number in the thousands, across many different types of species, including the rattite birds, the bee family colettidae, and the southern beech tree, Nothofagus sp.
9No species are truly ubiquitous, in the sense that all species are restricted to a particular habitat.Suitable habitats tend to be clustered within the geographic range of a population, therefore, most species are composed of discontinuous groups called populations.Clearly, the boundaries between populations can be somewhat subjective.What constitutes a population depends upon the species in question, but in general, members of a population interact, mate, and compete with each other much more frequently than members of different populations.
10Populations are groups of individuals of the same species living in the same place Individuals within a population occupy the same general area, rely on the same resources, and are influenced by the same general environmental conditions.Most of the interaction, including sexual reproduction, between individuals of a species is among members of the same population.
11Some populations moveA single population of salmon may spawn upstream in a river of the Pacific Northwest, and return to the ocean to feed and grow.Most of the interaction between individuals and exchange of alleles, occurs among individuals nesting in the same stream.Likewise, a population of monarch butterflies may migrate, en masse, to over-wintering grounds in Mexico for the winter, returning to the Midwest to reproduce during the summer (although no single individual makes the whole trip).
13Populations have certain emergent properties These properties are consequences of the way an organism interacts with the environment, and with other organisms, and influence its evolution.SizeDensityPatterns of DispersionAge StructureSpatial StructureSex RatioVariability
14SizeSimply the number of individuals in the population at any given time. Sometimes called abundance.
15DensityThe number of individuals in the population per unit area or unit volume.For many organisms, it is the density of a population rather than its actual numbers, that exerts a real effect on the organism.
16Example ProblemThere are 10,400 mice living in a 1000m x 1000m field. What is the density of this population?
17Answer The area of the field is 1,000,000 square meters (m2). The density of mice is therefore 10,400 mice/1,000,000m2=.0104/m2.
18Patterns of Dispersion Populations follow into three different patterns of dispersion, generally.ClumpedRegularRandom
19Clumping is probably the most common situation Clumping is probably the most common situation. Sometimes clumping occurs because some areas of habitat are more suitable than othersi.e., Plethodon sp. salamanders are found clumped under fallen logs in the forestthe night lizard Xantusia sp. is found clumped within fallen Joshua trees in the Mojave desert
20Sometimes species clump for other reasons. Plants often clump because their seeds fall close to the parent plant or because their seeds only germinate in certain environments. Impatiens capensis seeds are heavy and usually fall close to the parent plant-this species grows in dense stands.Species may clump for safety, or social reasons. Ground nesting bees Halictus sp. prefer to nest in the presence of other bees, forming aggregations of solitary nests
21Random distributionThis pattern occurs in the absence of strong attraction or repulsion among individuals.It is uncommon.The trees of some forest species are randomly distributed within areas of suitable habitat.For example, fig trees in the amazon rain forest. This random distribution might be due to seed dispersal by bats.
22Regular DistributionThis generally happens because of interactions between individuals in the population.Competition: Creosote bushes in the Mojave desert are uniformly distributed because competition for water among the root systems of different plants prohibits the establishment of individuals that are too close to others.Territoriality: The desert lizard Uta sp. maintains somewhat regular distribution via fighting and territorial behaviorHuman Intervention: I.e., the spacing of crops.
23Spatial StructureThe scale matters a great deal in describing the spatial distribution of a species.A species may be clumped on the large scale, but evenly distributed on a finer scale.Example: Ground nesting wasps, Sphex sp. are clustered in areas of suitable nesting substrate (packed sand). Within these areas, their nests are evenly distributed because of aggressive interactions.
24Age structureThis is the relative number of individuals at different ages.
25Sex ratioSex ratio is the proportion of individuals of each sex. The number of females is more important in the overall growth rate of populationsExamples: elk; fewer males of reproductive age than females; males breed with more than one female.Wasps: Melittobia sp. may have as many as a hundred females per male. These males never leave the nest and mate with their sisters. Population growth is essentially independent of the number of males.
26Variability is differences among individuals in the population. Most populations show differences among individuals.Some variation has a genetic basis.Other variation is largely environmental.In many cases, variability is caused by both genes and the environment.Sexual Dimorphism is when the two sexes differ greatly in appearance.Metamorphosis is when individuals differ in appearance because of a dramatic transformation as they age.
27It is a fundamental characteristic of living things that all organisms reproduce. Every species is capable of population growth under some set of possible conditions
29Exponential GrowthThis is probably the best, simple, model of population growth…it predicts the rate of growth, or decay, of any population where the per capita rates of growth and death are constant over time.In exponential growth models, births deaths, emigration and immigration take place continuouslyThis is a good approximation for the growth of most biological populationsi.e., human populations grow exponentially when resources are plentiful
30DN/DT=bN-dN Where: b is the per capita birth rate d is the per capita death rateignoring immigration and emmigration.dN/dT=rN (define r as the instantaneous population growth rate; r=b-d)you can integrate this to get the exponential growth formula that follows…Note that we are assuming that there is no emigration or immigration.
31Exponential growth formula N(t)=N0ertwhere r is the exponential growth parameterN0 is the starting populationt is the time elapsedr=0 if the population is constant, r>0 if population is increasing, r<0 if the population is decreasing.
32Example problemThe human population of the earth is growing at approximately 1.8%per year.The population at the start of 2001 was approximately 6 billion.If nothing were to slow the rate of population growth, what would the population be in the year 2101?
34Limits to Growth No population can continue growing forever. Even organism that reproduce very slowly, such as elephants, rhinos, whales, and humans, would outstrip their resources if they reproduced indefinitely.
35Carrying CapacityPopulations grow until one or several limiting resources become rare enough to inhibit reproduction so that the population no longer grows.The limiting resource can be light, water, nesting sites, prey, nutrients or other factors.Eventually, every population reaches its carrying capacity, this is the maximum number of individuals a given environment can sustain.
36Logistic Growth Model dN/dT=rmaxN(K-N)/K The logistic growth model accounts for carrying capacity.K= Carrying Capacity, this is the maximum number of individuals that the population can sustain.N=The Number of individuals in the population at a given timermaxis the maximum population growth ratedN/dT=rmaxN(K-N)/KQuestion: What is dN/dT when N=K?
37Answer When N=K, dN/dT=0 Likewise, when N is small, dN/dT =approximately rmaxWhen N>K the population declines.
39How Well Does the Logistic Model Fit Actual Populations? For laboratory populations of Paramecia, crustaceans, etc.., the logistic provides a pretty good fit.For actual populations, the logistic does not provide such a good fit.There are usually other factors involved.One factor: lag time is the time it takes between reaching carrying capacity and the slowdown in reproduction.
40DemographyDemography is the study of the age structure and growth of populations, especially as it relates to the births and deaths of individuals.The study of human mortality goes back a long way, to the Middle Ages and the early Renaissance. Thomas Malthus was a rather famous early demographer/economist. In his Essay on the Principal of population, 1798, he was the first to reach the conclusion that human populations tended to grow until they outstripped their available food supply.
41Counting IndividualsComplete enumeration: count every individual in the populationSample the Population:count individuals in many small portions of the area (e.g., quadrats) then calculate densitymark and recaptureindex of relative abundance (e.g., pheremone baited insect traps)
42Mark-Recapture Example A population of bats lives under a bridge in Texas. On Monday, 50 bats are netted and marked with ear tags. On Tuesday, the same researchers return and net 100 bats. Of these, 13 are marked.How many bats live under the bridge?
43Answer: First Sample = Marked Recaptures TOTAL POP Total Recaptures 50/T=13/100 (where T is the population of bats under the bridge)multiply both sides by 100T5000=13TT=385
44Life TablesA person’s chance of death is of tremendous interest to a person selling life insurance. Life tables are based on the actuarial tables insurance companies keep.By tabulating deaths, causes of death, and ages of death, it is obvious that a person’s chance of dying is not constant over time. For humans, it increases as we get past a certain age.Biologists have adopted Life Tables and applied them to other species.
45Constructing a Life Table A Cohort is a group of organisms born at the same time.S(0) is the number of individuals born into the cohort. S(x) is the number of individuals alive at the start of interval x.D(x) is the number of individuals dying during interval x.l(x) is the proportion of individuals alive at the start of interval x. l(x)=S(x)/S(0)m(x) is the expected number of female offspring born to a female during interval x.
46Life Table for a Field Grasshopper X s(x) d(x) l(x) m(x) l(x)*m(x)eggsInstar IInstar IIInstar IIIInstar IVImagoR0=.51adapted from Richards and Waloff, 1954
47Survivorship CurvesA survivorship curve traces the decline of a group of newborns over time.Survivorship curves plot the probability of surviving to a certain age for a representative member of the population
48Types of Survivorship Curves Type I: A convex curve. Most individuals live to adulthood with most mortality occurring during old age. I.e., humans, red deer, elephants.Type II: A straight line. An individual’s chance of dying is independent of its age. I.e., small birds and mammals.Type III: A concave curve, few individuals live to adulthood, with the chance of dying decreasing with age. I.e., oysters, redwood trees, snapping turtles.
51Age DistributionsThe age distribution of a population is the proportion of individuals at different ages.It has a significant impact on future population growth.Populations that have remained constant for a long time have stable age distributions, which reflect an individual’s chance of living a given amount of time.Rapidly growing populations will have a disproportionate number of young individuals.
52Age Structure and Human Population -Europeans – many old people, few young people – populations of some countries will decrease in the future.-Africa – many young people, few old people populations will increase in the future.m(x), age specific fecundity, is highest in 20 year olds.for humans, death rates are highest in the first year (babies) and in old age.Per capita death rates are actually higher in the U.S. than Guatemala, Mexico, etc..
54Life History Strategies Life history is the timing of an organisms reproduction and death.An organism’s fitness is how many offspring it produces that are ultimately able to produce their own offspring.There is often a trade-off between survivorship and reproduction.This is because reproduction can be dangerous, and involves the expenditure of resources the organism could use for growth or maintenance.
55For most organisms, life histories are timed to maximize an organism’s expected fitness.
56Factors Limiting Populations Density-Dependent factors intensify as the size of a population increases.Examples: Suitable nesting sites for cliff nesting birds such as gannets, competition for light and water among prairie grasses.Density-Independent factors are independent of population size.Examples: Winter temperatures greatly affect the populations of yellowjackets, sawflies and Melanopus grasshoppers.
57Both Density-Dependent and Density-Independent Factors Affect Populations of Most Species Examples: For Neodiprion sawflies, winter surviorship is greatly affected by the weather, which is density-independent.During the summer, however, parasitic wasps impose very high density-dependent mortality.Pacific mussels, Mytillus sp., are largely limited by density-dependent competition for space on rocky outcrops. Occasionally, density -independent disturbance by floating logs decimates populations.
58An Organism’s Life History is Adapted to its Environment Example: Pacific salmon hatch in the headwaters of streams and migrate to the open ocean. They eventually return to freshwater streams to reproduce.Steelhead Trout are genetic variants of the same species that stay in estuaries rather than migrating to the open ocean.Life history is polymorphic, in these species. Each morph exploits a life history where it avoids competition with the other morph.
59Reproductive Episodes Per Lifetime Semelparity is one large reproductive effort (most insects, annual plants). Examples, grasshoppers, mayflies, octupi and squids Agave.Iteroparity is fewer offspring and many reproductive episodes. Example, perennial plants, most large mammals, sharks, most birds such as gulls and terns.
60Clutch SizeClutch size (or seed set in plants) is the number of offspring produced per reproductive episode.Clutch size varies depending on resource availability.Semelparous organisms expend all their resources on a single clutch.Iteroparous organisms must save some for growth and survival.
61Age at First Reproduction For growing populations, the timing of first reproduction greatly affects fitness.In a growing population, a few offspring early can increase an individual’s fitness more than many offspring later. This is because of the “compound interest effect”.Dying before reproduction also reduces an organism’s fitness to zero.But organisms often have more resources available to them as they age.There is therefore a tradeoff: reproduce early with a few offspring or reproduce later with more offspring.
62A Trade-Off: Generation Time is Related to Body Size Larger organisms are slower to reach reproductive age.E. coli, about 20 minutes.Paramecium, about 24 hours.Elephant or human, years.
63Parental CareParental care, in its many manifestations, is an adaptation by which an organism may potentially increase the survivorship of its offspring.It comes at a cost to the parents, however.Time and resources spent investing in parental care mean that the resources that are available must be spread over fewer offspring.
64Parental Care and Development in Birds Birds (vertebrate class Aves) provide parental care, but different birds provide different amounts of parental care.Superprecocial birds, such as the megapodes, hatch and are able to fend for themselves immediately. Eggs are often laid in warm environments, and parents do not incubate them. In theory, this means more time to forage .On the other end of the spectrum are the superaltrical birds, such as passerines and parrots. Young are born blind and require constant parental care, after a period of egg incubation.