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Chapter 6 Population and Community Ecology
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Nature exists at several levels of complexity Individual (the unit of natural selection) Population (the unit of evolution, composed of all individuals that belong to the same species and lives in a given area) Community (interactions amongst species within a given area) Ecosystem (flow of energy and matter, both biotic and abiotic factors in a particular location) Biosphere (global processes that incorporates all of the Earth’s ecosystems.
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The factors that regulate population abundance and distribution The study of factors that cause populations to increase or decrease is the science of population ecology. Inputs that increase population size= immigration and births. Outputs that decrease population size= emigration and deaths Population characteristics include: size, density, distribution, sex ratio and age structure.
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Population Size N is the total number of individuals within a defined area at a given time Density dependent factors influence an individual’s probability of survival and reproduction in a manner that depend on the size of the population. Ex: amount of available food (is a limiting resource) K is the carrying capacity of the environment or the limit that the supply can sustain. Density-independent factors is the individuals probability of survival and reproduction at any population size. Ex: tornadoes, hurricanes, fires, floods.
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Population Density-Is the number of individuals per unit of area at any given time. Important to wildlife managers who set up hunting and fishing limits on species Population Distribution- is a description of how individuals are distributed with respect to one another. Ex: Random (no pattern), uniform (evenly spaced out), or clumped (living in large groups enhance feeding opportunities or protection from predators
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Distribution patterns-populations in nature distribute themselves in three ways:
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Population Sex Ratio- ratio of males to females (knowing this helps to figure out the number of offspring a population will produce in the next generation. Population Age structure-is the description of how many individuals fit into particular age categories. If they are all old, they will not reproduce.
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Growth models and population changes Population Growth Models are mathematical equations that an be used to predict population size at any moment in time. Growth Rate=the number of offspring an individual can produce in a given time period, minus the death rates of the individual or its offspring during the same period of time. Intrinsic growth rate= a populations particular maximum potential for growth (r) under ideal conditions
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Exponential Growth Model As time goes by the population increases rapidly, creating a J-shaped curve
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Exponential Growth and Mathematics N t = N o e rt N t = The populations future size N o = The current size of the population r= the intrinsic rate of the population t= the amount of time the population grows The value grows by the same percentage every year, but cannot grow like this indefinitely. Eventually the carrying capacity levels off the population
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The Logistic Growth Model Describes a population whose growth is initially exponential, but slows as the population approaches the carrying capacity of the environment (K). This S- shaped curve shows that as populations reach carrying capacity the populations stop growing and remain at a constant rate. This model is used to predict population growth that is subject to density- dependent constraints
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Population Oscillations When the population is larger than the carrying capacity there is an overshoot. Usually this leads to a die off, or population crash (below carrying capacity) and then oscillates again. Over time, the population will reach carrying capacity
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Population Oscillations in Lynx and Hares Predation is another factor in limiting population growth. This graph indicates that populations of both species cycle over time.
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Reproductive Strategies and Survivorship curves K- selected species(abundance of specie is determined by carrying capacity and have small fluctuations)have certain traits in common: large organisms, produce few large offspring, provide substantial parental care (large mammals and birds). Their populations grow slowly and cannot respond quickly to efforts to save them from extinction.
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r-Selected species= have a high intrinsic growth rate, reproduce often, have large numbers of offspring, and show little or no parental care. These populations do not remain near their carrying capacity but exhibit offshoots and die-offs. Ex: fish, insects, mice, dandelions and other weeds. Most species fall somewhere between these 2 extremes of reproductive strategies.
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Survivorship Curves Type I (man, whales, elephants) have high survival rates throughout most of their life span. As they age, they die out in large numbers. Type II (squirrels, coral) experience a constant decline throughout their life span Type III (mosquitoes, weeds) low early survivorship, few reach adulthood
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Metapopulations contribute to the preservation of biodiversity Metapopulations = separate and distinct populations that have been connected by occasional movements of individuals between them. This provides s species with some protection against threats such as disease. Increases the little genetic variation that occurs in smaller populations
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Community Ecology For a specie to be distributed in an area it must : be able to live in a range of abiotic factors, it must be able to disperse to that area, and it must be able to interact with other species that live there. 4 categories of interactions: competition, predation, mutualism, commensalism Community Ecology studies these interactions which determine the survival of a species in a habitat.
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Competition-the struggle of individuals to obtain a limiting resource Competitive Exclusion Principle-states that two species that compete for the exact same resources cannot stably coexist. In his experiments, Gause found that three species of Paramecium grew well alone in culture tubes. But Paramecium caudatum would decline to extinction when grown with P. aurelia because they shared the same realized niche, and P. aurelia outcompeted P.caudatum for food resources. However, P. caudatum and P. bursaria were able to coexist because the two have different realized niches and thus avoided competition.
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G.F. Gause ’ s Experiment Russian biologist, in lab Both Paramecia feed on bacteria – P. aurelia consistantly outcompetes P. caudatum
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Resource Partitioning Two species divide a limited resource based on differences in the specie’s behavior or morphology. Hunt at different times of day or flowering at different times a year ((temporal resource partitioning). Use different habitats (spatial resource partitioning) Evolution of differences in body size or shape(morphological resource partitioning)
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Predation –the use of one species as a resource by another species True predators: kill their prey and consume most of what they kill. Ex: Lions Herbivores: consume plant as prey, typically eating only a small part of a plant w/o killing it. Ex: deer Parasites: live on or in a host and consume a small portion of a host and rarely does a single individual cause the death of host. Ex: tapeworms Parasitoids: organisms that lay eggs inside other organisms. When they hatch, they consume host from inside to out causing death of host. Ex: wasps and flies
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Parasitism (tick on dog)
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Mutualism- benefits 2 interacting species (by each assisting the other to benefit itself Ex: The acacia tree and ants. Tree supplies food and shelter for ants, ants supply protection for trees by stinging other insects that might eat it. Ex: Lichen- Fungus provides nutrients for the alga and the alga provides carbohydrates to fungus via photosynthesis
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Commensalism-one specie benefits but the other is neither harmed or helped A symbiotic relationship is a relationship of two species that live in close association with each other. Commensalism is one example.
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Keystone Species Is a species that plays a role in its community that is far more important than its relative abundance might suggest. They usually exist in low numbers. They may be predators, sources of food, mutualistic species, or providers of some other essential service. Keystone predator-mediated competition Is an example of how a keystone specie plays such an important role.
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Sea Stars (in the intertidal community) although not very numerous, prey on mussels which normally take over rock space. This cleared space allows other species to attach to rock. Thus, the predatory sea star reduced the abundance of a superior competitor and allowed inferior competitors to persist. Predator-mediated competition
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Ecosystem Engineers A keystone species may create or maintain a habitat for other species. Beavers have a vital role in the forest community. They build dams that convert narrow streams into large ponds, creating new habitats for plants and animals.
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Ecological Succession- the predictable replacement of one group of species by another over time. Primary Succession-occurs on surfaces that are devoid of soil. Ex: rocks, cooled lava etc. Rocks lichens and mosses annual weeds perennial weeds and grasses Shrubs Aspen, cherry, young pine Forest(pioneering species) Beech and maple broadleaf forest (climax forest)
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Primary Succession- starts with a surface devoid of soil
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Secondary Succession Occurs in areas that have been disturbed but have not lost their soil. It usually follows an event like fire or hurricanes.
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Aquatic Succession
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Species richness of a community is influenced by 4 factors Latitude= As you move from the equator to the poles, the number of species decline. This pattern is observed in plants, birds, reptiles and amphibians and insects. Time= The longer a habitat exists, the more colonization, speciation and extinction can occur there. Habitat Size and Distance= Specie richness increases as habitat size increases. Distance matters because many species can disperse only short distances.
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