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Lecture 8

Lecture 8 - Population Distribution and Abundance

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Biology 2483A
Hugh Henry

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LECTURE 8: POPULATION DISTRIBUTION AND ABUNDANCE  Population – group of interacting individuals of the same species living in a particular area  Interactions within populations include sexual reproduction and competition o Influence each other  Populations are dynamic – distribution and abundance can change over time and space. Understanding the factors that influence these dynamics help us manage populations for harvest or conservation  Distribution – geographic area where individuals of a species occur  Abundance – number of individuals in a given area o Abundance can be reported as a population size (# of individuals), or density (# of individuals per unit area) o Example: On a 20-hectare island there are 2,500 lizards.  Population density = 125/hectare  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 is not fully known, an area is delimited based on best available knowledge of the species.  Species vary in their ability to disperse o In plants, dispersal occurs by seed movement. The distance moved can be very small o Other species, such as whales, can move thousands of kilometers in a year  Some populations exist in isolated patches that are linked by dispersal  This can result from physical features of the environment, or human activities that subdivide populations o Example: Heathlands in England have been fragmented by human development  For some species, it’s hard to determine what an individual is o We see this a lot in plants, but it can be seen in animals  Individuals can be defined as products of a single fertilization: The aspen grove would be a single genetic individual, or genet  If members of a genet are independent physiologically, each member is called a ramet o A ramet is a “subset” of a genet Distribution and Abundance  The distributions and abundances of organisms are limited by habitat suitability, historical factors, and dispersal  Habitat suitability o Abiotic features: moisture, temperature, pH, sunlight, nutrients, etc.  Availability limits the distribution of some species o Some species can tolerate broad ranges of physical conditions, others have narrow ranges o Creosote bush is very tolerant of dry conditions and occurs widely in North American deserts. o Saguaro cactus can tolerate dry conditions, but not cold temperatures and has a more limited distribution o Biotic features: organisms are affected by herbivores, predators, competitors, parasites, and pathogens o In Australia, an introduced cactus became a pest species, spreading over vast areas. o A moth that feeds on cactus was then released, and distribution and abundance of the cactus has been greatly reduced  Abiotic and biotic features can interact to determine distribution and abundance  The range of the barnacle Semibalanus balanoides is restricted by temperature. But competition from other species precludes it from some areas with suitable temperatures.  Some species distributions depend on disturbance—events that kill or damage some individuals, creating opportunities for other individuals to grow and reproduce. o Example: Some species persist only where there are periodic fires  Seeds that are designed to respond to fires, fire-dependent in order to avoid competition with other species  Historical factors o Evolutionary history and geologic events affect modern distribution of species o Example: Polar bears evolved from brown bears in the Arctic. They are not found in Antarctica because of an inability to disperse through tropical regions. o Continental drift explains the distributions of some species. o Wallace (1860) observed very different animal species on the Philippines and New Guinea, even though they are close together  Were not historically close together, separated by expansive body of water  Dispersal o Dispersal limitation can prevent species from reaching areas of suitable habitat. o Example: The Hawaiian Islands have only one native mammal, the hoary bat, which was able to fly there o Dispersal limitation has also been shown in plant species o Dispersal can also affect population density, and vice versa o Many species of aphids produce winged forms (capable of dispersing) in response to crowding  As crowding increases, the number of winged form aphids increases as well o Desert pupfish live in pools that are sometimes connected after heavy rains o Dispersal may result in better chances for survival and reproduction than staying in crowded pools with limited food Geographic Range  Geographic range—the entire geographic region over which a species is found  Many species have a patchy distribution of populations across their geographic range  There is also great variation in species ranges: o Many tropical plants have small ranges. In 1978, 90 new species were discovered, restricted to a single mountain ridge in Ecuador. o Other species, such as they coyote, have very large geographic ranges o Some species are found on several continents o Few species are found on all continents except humans, Norway rats, and the bacterium E. coli  Geographic range includes areas occupied during all life stages  Some species, such as monarch butterflies, migrate long distances between summer and winter habitats  For some species, it is difficult to find all the life stages and the ranges they inhabit  Not all habitats within a range are suitable,
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