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Bio 1m03 chapters 52-55 notes.docx

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Susan A Dudley

Chapter 52: Population Ecology Population Ecology: the study of how and why the number of individuals in a population changes over time 52.1 Demography -the number of individuals present in a population depends on 4 factors 1) Birth 2) death 3) immigration and 4) emigration -populations increase due to births and immigration and decrease due to deaths and emigration Demography: the study of factors that determine the size and structure of populations through time Generation: the time between a mothers’ first offspring and her daughter’s 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 -allows researchers to calculate the number of offspring produced that survived and how many offspring each female produced Survivorship: the proportion of offspring produced that survive on average, to a particular age Eg. If 1000 lizards were produced, at 1yr old 424 lizards were left, survivorship for the first year would be 0.424 Fecundity: the number of offspring produced by each female in the population Age Specific Fecundity: the average number of female offspring produced by a female in age class ‘x’ = survivorship x fecundity Age class: a group of individuals of a specific age Fitness Trade-offs -why it’s not possible for species to have both high fecundity and high survivorship -every individual has a specific amount of energy and time at its disposal -if an individual devotes a lot of energy to feeding a large amount of offspring, she won’t be able to take care of herself Life History: how an individual allocates resources to growth, reproduction and survival -as fecundity increases, probability of surviving decreases 52.2 Population Growth ΔN= number of individuals in a population Δt= a unit of time B= birth rate D= death rate -if no immigration or emigration occur, growth rate = number of individuals x (births- deaths) Per capita rate of increase (r): the difference between the birth and death rate Intrinsic rate of increase: (r max): birth rates are as high as possible; death rates are as low as possible ΔN/ Δt = r max -a function of species life history traits - r = high = high fecundity max - max = low = low fecundity Exponential Growth Rate -r doesn’t change over time -growth rate doesn’t depend on number of individuals in the population -it’s not possible for exponential growth rate to continue indefinitely Population Density: number of individuals per unit area -when population density increases, birth rate per capita would be expected to decrease and death rate to increases, causing r to decline Logistic Growth Rate Carrying Capacity (k): the max number of individuals in a population that can be supported in a particular habitat over a period of time -carry capacity depends on food, space, water, soil quality, disease etc -if a population is below k then population should continue to grow -a population’s growth rate is proportional to (K-N)\K ΔN/ Δt = r N((K-N)/K) max What Limits Growth Rates and Population Sizes? -population sizes change as a result of 2 general types of factors 1) Density Independent -alter birth and death rates irrespective of the number of individuals in the population -when an increase in population size doesn’t affect ‘r’ -usually abiotic factors like weather patterns 2) Density Dependent -alters birth and death rates when population becomes too large -usually biotic factors like competition for food and starvation, increases predation if increase in population -density dependent changes in survivorship and fecundity cause logistic growth -survival and fecundity decline at high population density -density dependent factors define a habitat’s carrying capacity -k varies among species and populations -some habitats are better than others due to food availability, predator abundance etc. 52.3 Population Dynamics -changes in population through time -species with a lower rmax put more energy into survivorship and not fecundity so their populations stay fairly constant over time How do metapopulations change through time? Metapopulation: if individuals from a species occupy many small patches of habitat, they form independent populations, or populations of a population -the overall number of individuals stays stable because even if subpopulations go extinct, migration from nearby subpopulations can recolonize empty habitats Why do some populations’ cycle? Population cycle: regular fluctuations in size -many hypotheses explain that population cycles depend on some density-dependent factors -Predation, disease, food shortages – intensify dramatically at high population density and cause population numbers to crash Lynx and Hare example: -hare’s use up all their food and resources at high density so they starve, lynx’s also starve -lynx populations reach high density in response to hare density, at high density lynx’s eat a lot of hare’s so then lynx’s population also crashes How does age structure affect population growth? Age structure: the proportion of individuals that are at each possible age -has a dramatic influence on population growth over time Age structure in woodland herb -roses in high light are a younger population than roses in lower light Age structure in human populations -can be represented by age pyramids -varies by country, developed vs. undeveloped Analyzing change in growth rate of human populations -human growth rate has increased over 250 years -very steeply rising curve over past few decades, however dropped 1.2% every year now since 1970 -when fertility at the replacement rate is sustained for a generation (mother reproduces just enough children to replace her and her mate) that produces a zero population growth rate 52.4 How can population ecology help endangered species? Using life tables -allows biologists to alter the values for survivorship and fecundity at a particular age and assess the consequence Preserving metapopulations -humans are reducing areas of forest and grassland which forces species into metapopulation structure -the best surviving metapopulations are larger ones which occupy more areas and are more genetically diverse Chapter 53: Community Ecology 53.1 Species Interactions Fitness: the ability to survive and produce offspring Positive Interaction: a relationship between 2 species that provides a fitness benefit to members of one species Negative Interaction: a relationship between 2 species that hurts members of one species 0 Interaction: a relationship that has no effect on the members of either species +/+ mutualism +/- consumption or parasitism -/- competition 0/+ commensalism Commensalism: one species receives a benefit from the other, but it has no impact on the other species Eg. Birds follow ants south during the winter 3 main themes 1) Species interactions may affect the distribution and abundance of a particular species 2) Species act as agents of natural selection when they interact (coevolution) 3) The outcome of species interactions is dynamic and conditional Competition (-/-) -occurs when individuals use the same resources and when those resources are limiting Intraspecific Competition: occurs between members of the same species -competition for space, sunlight, food etc -major cause of density dependent growth Interspecific Competition: occurs between members of different species 1. Consumptive Competition: when individuals consume the same resources 2. Preemptive Competition: exists when one species makes spaces unavailable to other species 3. Overgrowth Competition: happens when one species grows over another 4. Chemical Competition: takes place when one species produces toxins that negatively affect another species 5. Terrestrial Competition: arises when a mobile species protects its feeding or breeding territory against other species 6. Encounter Competition: occurs when 2 species interfere physically and directly for access to specific recourses Using a niche concept to analyze competition Niche: the range of resources that a species is able to use or the range of conditions it can tolerate -interspecific competition occurs when 2 species niches overlap Eg. Seed size competition -2 species both eat intermediate sizes seeds, so now there is more competition for intermediate seeds What happens when one species is a better competitor? Competitive Exclusion Principal: it is not possible for species with the same niche to coexist Asymmetric Competition: occurs when one species suffers a much greater fitness decline than the other (one a poorer competitor and one stronger) Symmetric Competition: occurs when each species experiences an equal decrease in fitness (equal competitors) -if asymmetric competition occurs, the stronger competitor is likely to drive the weaker species to extinction Fundamental Niche: the resources it uses or conditions it tolerates in the absence of competitors Realized Niche: the resources it uses or conditions it tolerates when competition occurs -if asymmetric competition occurs and niches don’t fully overlap then organisms will live in their realized niche, not fundamental Experimental studies of competition -2 species of barnacles, one lives in harsh conditions the other in good conditions -researcher placed rock in harsh conditions into good conditions and removed competitor -when competitor was removed, species survived better Mechanisms of Co-existence: fitness trade offs -the ability to compete for a particular resource is only one aspect of an organism’s niche -if individuals are good at competing for resources they are probably less good at enduring drought etc Barnacle example: -1 species was good at competing for space; the other was good at enduring harsh conditions Niche Differentiation -natural selection makes a tendency to avoid competition. This results in a genetic change in traits that allows the amount of niche overlap to decrease -change in resource use Character Displacement: change in species traits Consumption (+/-) -when one organism eats another 3 major types 1) Herbivory: takes place when herbivores consume plant tissue 2) Parasitism: occurs when a parasite consumes relatively small amounts of tissue or nutrients from another individual (host) 3) Predation: occurs when a predator kills and consumes all or most of another individual Parasitoids: like predators because they are lethal and like parasites because they are smaller than the host Eg. Wasps lay eggs on caterpillars, eggs hatch and larvae each caterpillar from the inside How do Prey Defend themselves? -prey have adaptations that reduce their likelihood of becoming victims. These adaptations are responses to natural selection exerted by predators Constitutive/Standing Defenses: defenses that are always present. Including camouflage, schooling, weaponry, mimicry and aposematic colouring Mimicry: the close resemblance of one species to another 1) Müllerian Mimicry: the resemblance of 2 harmful prey species 2) Batesian Mimicry: the resemblance of an innocuous prey to a dangerous prey species Aposematic Colouring: warning for toxicity How do prey defend themselves? Inducible Defenses: defensive traits that are only produced in response to the presence of a predator Eg crab and muscles -muscles found downstream of a crab had thicker shells than those not in the presence of a crab Are animal predators efficient enough to reduce prey populations? -yes, predators play a role in density dependent growth of prey populations Eg in Alaska 55-80% of wolves were removed and moose population tripled Why don’t herbivores eat everything? Why is the world green? -herbivores don’t eat all available food Why? 1) Top down control hypothesis: suggests that predation or disease limits herbivores 2) Poor nutrition hypothesis: proposes that plants are a poor food source in terms of nutrients they provide herbivores, especially nitrogen, this limits herbivore density 3) Plant defense hypothesis: suggests that plants defend themselves effectively enough to limit herbivores Beaver and beetle example -cottonwood trees resprout and release a compound that deters beavers, after being cut down by a beaver -beetles store this compound to deter ants so are unaffected and thrive in its environment -tree attracts beetle and deters beaver Adaptation at arms race -when consumers and prey interact overtime, a coevolutionary arms race begins -consumers evolve traits that increase their efficiency -in response, prey evolve traits that make them unpalatable Eg malaria parasite and humans -Humans with evolved gene HLA –B53 can detect and destroy malaria parasite before it multiples in the liver Can parasites manipulate their hosts? -yes eg. Land snails infected with flukes become attracted to light; in addition, the flukes burrow into the snails tentacles and make them wiggle. The snails are therefore, more easily seen by birds and thus continuing the life cycle of the fluke Mutualism (+/+) -most critical mutualism = bacteria that fix nitrogen providing nitrogen and plants providing sugar -interaction doesn’t involve altruism -benefits received in a mutualism are a byproduct of each individual pursuing its own self- interest by maximizing its ability to survive and reproduce Eg ants and tree hoppers -ants harvest food off of tree hoppers and ants ward away predators for the tree hoppers -only mutualism when there are lots of predators 52.3 Community Structure 2 theories 1) Clements – the climax community concept -biological communities have a tightly prescribed organization and composition 2) Gleason -biological communities are a loose assemblage of species, each distributed to its fundamental or realized niche and chance Mapping Current and Past Species Distributions -if communities are predictable, the same group of species should almost always be found growing together -historical data on plant communities showed that groups of species didn’t change their distributions, in close association, instead, species tend to change their ranges independently of one another -studies of fossil pollen suggest that the composition of most plant communities has been dynamic and contingent on historical events, rather than static Experimental Tests -Sterilized ponds and waited to see if community is predictable – ponds were the same or unpredictable -ponds were independent of one another -each pond had its own species assemblage, this suggests that Clements view is too extreme and Gleason’s is right -chance and history also play a role How do Keystone species structure communities? -the presence or absence of a single species can drastically alter a community Keystone species: a species that has a much greater impact on the surrounding species than its abundance would suggest Eg sea star -when removed from ecosystem, it altered the habitat completely, mainly in the number of species present 53.3 Community Dynamics -how communities change through time Disturbance and Change in Ecologi
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