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Ecology 8.docx

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Western University
Biology 2483A
Mark Moscicki

Ecology-Lecture 8 Oct 8 2013 Population  Group of interacting individuals of the same species living in a particular area  Interactions include sexual reproduction and competition (because some organisms are asexual and do not undergo sexual reproduction)  Ecologists wish to understand factors that influence distribution and abundance. However, this is difficult because populations are not static. They are dynamic and continuously changing. The distribution and abundance of organisms can change over time and space. Ecologists like to understand the factors that influence these dynamics because it helps us manage populations for harvest (plants) or conservation (of species) Distribution/Abundance  Distribution: Geographic area where individuals of a species occur. Some parts may have higher abundance than others. Distribution can be viewed as a map of all areas where the abundance of a species is greater than 0  Abundance: Number of individuals in a given area. If the area is known, abundance may be reported as population size (# individuals) or density (# individuals per unit area)  On a 20 hectare island, there are 2,500 lizards. Population density=2500 lizards/20 hectares=125/hectare. The population size would just be 2500  It can be difficult to determine abundance because sometimes the total area occupied by a population is not known. It is often difficult to know how far organisms or their gametes can travel. When the area isn't fully known, ecologists use the best available information about the biology of the species to delimit an area within which the size and density of the population can be estimated  Number of individuals in a population changes over time. This is true whether measured on a small spatial scale (dandelions in a restricted area along the river) or large scale (cod in Atlantic ocean)  Abundance can change over time and space. An example is with insects that feed on Goldenrod. A study looked at the abundance of Goldenrod in 3 areas. All three of these populations have the same climate, so we expect the populations to fluctuate in sync. However, they do not! Abundances varied from place to place as well as year to year. This proves that climate is not the only determining factor. Montezuma and Maple Island are within 20km (nearby populations). Hector is very far away (60km). Yet Montezuma and Maple Island differ much more than Hector and Maple Island... Dispersal  Species vary in their ability to disperse Figure 8.4 Fragmentation of Dorset Heathlands  In plants, dispersal occurs via seed and pollen movement. The distance moved is typically very small. (low dispersion)  Other species, such as whales can move thousands of km a year (all around the world) Mobile species typically have very high dispersion  For species whose dispersal is not well understood, it can be challenging to determine the social extent of a population  Land use change/physical environmental features can cause populations to have isolated patches. Populations may exist in these isolated patches. These isolated patches may be linked by dispersal.  An example is the Heathlands in England. They have been fragmented by human development  Fragmentation can result in patches that are so isolated that little dispersal can occur among them. This breaks a large population into a series of much smaller populations. What is an Individual?  For some species, it is difficult to Figure 8.6 Plants and Animals That Form Clones determine what an individual is..  An example is with a population of Aspen trees. You have 20 trees. Is this one individual (all 20 trees growing up from a single root system?) or 20 individuals, each growing up from a seed  Genet: Individuals described as products of a single fertilization. The aspen grove would be a single genetic individual or genet.  Ramet: Members of a genet that are independent physiologically. Even though they grew up from a single fertilization event, they may compete for resources! An example is strawberries. Even though they are clones of a parent, they can survive even if not connected to the rest of its genet.  Whether we look at genets or ramets depends on what we are interested in. If competition is what we are interested in, maybe we should look at ramets. If evolutionary change is important, maybe we should look at genets. Limitations of Distribution/Abundance  Several or all of these factors will operate at the same time to influence where a species will be found  Habitat Suitability  Small differences in how well a species survives/reproduces can cause a species abundance to be high in some places and low in others  Many abiotic factors limit distribution and abundance. (moisture, temperature, pH, sunlight, nutrients, etc) These factors determine whether a species will be able to live and thrive in a certain area  Some species can tolerate broad ranges of physical conditions while others have narrow ranges.  An example of a species that can tolerate a broad range of physical conditions is the Creosote bush. It is very tolerant of dry conditions and it occurs widely in N.A deserts. It is also tolerant in cold areas, even further expanding its distribution. Saguara cactus on the other hand has a narrow range of tolerable conditions. It can tolerate dry conditions, but not cold conditions. It has much more limited distribution. Figure 8.7 The Distributions of Two Drought-Tolerant Plants  Biotic factors also affect the distribution and abundance of an organism. Organisms are affected by herbivores, predators, competitors, parasites, and pathogens. Species that depend on another species for their survival/reproduction/growth cannot survive in an area in which that species is absent. Organisms can also be excluded by predatory species. (or anything limiting its survival)  In Australia, a cactus was introduced. This became a pest species, spreading over vast areas  A moth that feeds on this cactus was then introduced. Distribution and abundance of this cactus became greatly reduced. (biological control)  Abiotic and biotic factors can also interact to affect population abundance and distribution.  The range of the Barnacle SemiBalanus Belanoides is restricted by temperature. However, competition from predators restricts it from some areas with suitable temperatures.  Some species depend on disturbance (events that kill or damage some individuals creating opportunities for other individuals to grow and reproduce), creating opportunities for other individuals to grow and reproduce  Some species only exist when there are periodic fires (Seratonous pine cones- Stay closed up and open in response to fire. Other plants clear and these become reproductively active)  Historical Factors  Evolutionary history and geologic events affect modern species distribution  Polar bears evolved from brown bears in the Arctic. They are in the Arctic because they originated there. They are not found in Antarctica because of an inability to disperse through tropical regions (even though arctic has suitable climate and food sources)  Continental drift (gradual movement of continents over time) explains the dispersion of some species.  The animals of the Phillipines are more similar to those in Africa than New Guinea despite the fact that Africa is very far away and the Phillipines and New Guineau are relatively close together  Dispersal  Dispersal Limitation can prevent species from reaching areas of suitable habitat. This is when a species has limited capacity for dispersal. This was shown with Polar bears who refuse to cross the Tropics into Antarctica  Hawaii Islands have only one Native mammal, the Hoary bat which was able to fly there. Other mammals have no means of getting to this island. The only reason we have other mammals there today is because we introduced them to the area (cats, dogs, pigs, goats, etc)  Dispersal limitation has also been shown on smaller scales like in plant species. An experiment was done in which 27 populations of 7-10 individuals were established in apparently suitable habitats located near source populations. Forty five years later, it was seen that they had barely moved. These habitats they were placed in became less suitable due to physical disturbances, yet they did not disperse causing 16 of the experimental populations to go extinct  Dispersion can also affect population density, and vice versa. When individuals disperse from an area, the density of the population they leave decreases and the density of the population they are going to increases. Density also affects dispersion. If you have a high population, crowding is likely to cause a spec
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