Chapter 9 Populations & Communities ST EST 1. Studying Populations In ecological terms, a population is a group of individuals of the same species, living in a shared space at a specific point in time. 3 main characteristics are used to describe a population: (1) Size (2) Density (3) Distribution ST EST 1.1 Population Size Population size refers to the number of individuals in a population. Why study populations?? Population studies provide scientists with important information on the factors causing the increase, decrease and stability of individual numbers over time. For example, a population study of two caribou herds in the Gaspesie and Riviere aux Feuilles regions of Quebec in 2001 revealed an alarming decline in the Gaspesie population (just 250 individuals). As a result of this study, the Gaspesie herd was designated as vulnerable and a conservation strategy adopted to deal with their declining numbers. ST EST 4 key factors influencing changes in population size over time: FactorDescription Births Births of individuals within a population. Deaths Deaths of individuals within a population. Immigration Arrival among the population of individuals from other regions. Emigration Departure of individuals to other regions Table 9.5 Comparing factors affecting population size over time (Observatory p. 294). 3 METHODS FOR MEASURING THE SIZE OF A POPULATION 1.COUNTING INDIVIDUALS E.g. Aerial photos of zebras can be used to count the total number of animals in a certain region. 2. COUNTING BY SAMPLE AREA Average number of individuals per section = Population size Area of a section Total study area Therefore: Population size = Average number of individuals x Total study area Area of a section This method consists of counting the individuals in randomly selected sections of the study area and then estimating the total population size with the following calculations. Example of counting by sample area method. 3. MARK AND RECAPTURE The mark and recapture method is most often used by scientists to estimate the population of mobile species including, birds, fish, mammals, that are widely scattered in their habitat Estimating the size of the population includes the following calculation: The # of marked animals recaptured = # of marked animals Total number of animals captured the second time Population size Therefore: Population size=# of marked animals X total # of animals captured the second time Number of marked animals recaptured In the following example, 100 Canada geese have been captured, marked and released. If, on the second capture, 50 out of 200 geese are tagged, then the population size is estimated to be 400 individuals: Population size = 100 x 200 = 400 individuals 50 1.2 POPULATION DENSITY Population density refers to the number of individuals per unit area or volume. E.g. 56 black bear/Km2 The following calculation can be used to determine the density of a population: Population density = Number of individuals Space (area or volume) occupied E.g. An island has an area of approximately 400 Km2. On the same island is a population of 136 white-tailed deer. The density of the deer population on that island would be: Population density = 136 individuals = 0.34 deer/Km2 400 Km2 ST EST 1.3 POPULATION DISTRIBUTION Population distribution is the way in which individuals are dispersed within their habitat. 1.Clumped distribution is the most common distribution and involves individuals forming groups in areas of preferred habitat. E.g. Caterpillars congregating at preferred food source (leaf). ST EST 2. Uniform distribution involves patterns of distribution whereby individuals are dispersed evenly throughout the populations habitat. E.g. Seabirds space their nests at regular intervals to allow each bird access to limited territory. 3. Random distribution is rare in nature and is characterized by individuals randomly dispersing themselves in a particular habitat type. The photo below illustrates the random distribution of diseased yellow- pine seedlings in a Canadian Forestry Department test plot. Source: 1.4 ECOLOGICAL FACTORS An ecological factor is an aspect of a habitat that can affect the organisms living there. There are two types of ecological factors: 1. Abiotic factors are physical or chemical aspects of the environment. 2. Biotic factors are related to the actions of living organisms in a habitat. Abiotic factorsBiotic factors Amount of lightBirth rate Soil or water pHDisease TerrainAmount of food Depth of snowPredation TemperatureCompetition Air humidityHuman activity ST EST A limiting factor is an ecological factor that causes the density of a population to decrease. The following scenario illustrates the influence of a limiting factor on the presence or absence of individuals in a particular habitat. Insectivorous bats rely heavily on insects as a source of food. When the air temperature at night falls below 5 C, insect activity falls dramatically. In this case, air temperature is a limiting factor affecting the availability of food for bats. If nocturnal temperatures remain low on consecutive evenings, some bats will resort to short-term hibernation or torpor in order to conserve energy. 1.5 BIOLOGICAL CYCLES IN POPULATIONS The biological cycle of a population is composed of alternating periods of rise and fall in its size. These periods are of fixed duration and are repeated continually. The biological cycle of the snowshoe hare (prey) and the Canadian Lynx (predator), illustrates the connection between their two populations over a ten year period. ST EST ST EST 2 STUDYING COMMUNITIES A community is a set of populations of different species sharing the same habitat. When a number of species share the same habitat, the set of populations they form makes up a community. Gatineau Park is home to a wide variety of both forest and wetland communities. ST EST 2.1 BIODIVERSITY Biodiversity describes the variety of species living in a community. To measure biodiversity, two components must be considered: 1. Species richness the number of species in the community. 2. Relative abundance the number of individuals of a particular species in relation to the total number of individuals in the community. Therefore, the biodiversity of a community is high when: (a) The number of species is high. (b) The relative abundance of different species is similar. The two communities illustrated below, share the same species richness, but the relative abundance of each species is different Global Biodiversity Hotspots ST EST 2.2 INTERACTION BETWEEN INDIVIDUALS IN A COMMUNITY The 4 main types of interactions that occur between individuals in a community include competition, predation, mutualism and commensalism. ST EST Competition is the interaction between organisms that seek access to the same resource in the their habitat. There are two types of competition: (a) Intraspecific competition occurs between individuals of the same species. E.g. Deer compete among themselves for limited forage in winter. (b) Interspecific competition occurs between individuals of different species. E.g. Passerine (songbirds) compete for similar food resources (insects). ST EST Predation is the interaction between two living organisms in which one feeds on the other. In the example below, an eastern garter snake is feeding on a yellow-legged frog. Parasitism is another form of predation that involves a parasite feeding on a host either internally or externally. The parasite gains something from the interaction, but the host is harmed in some way. Blowfly parasitism on a 6-day old purple martin nestling (Source: Purple Martin Conservation Society) Mutualism is the interaction between two living organisms that benefits both organisms. A good example of mutualism involves the pollination and seed dispersal of many fruit crops by bats in the tropics. ST EST ST EST Commensalism is the interaction between two living organisms in which one organisms in which one organism benefits from the relationship, while the other remains unaffected. The anemonefish lives among the tentacles of an anemone and is protected from potential predators not immune to its sting. ST EST THE EFFECT OF INTERACTION BETWEEN POPULATIONS ON THEIR DENSITIES The table below presents the 4 main types of interaction that can exist between the individuals of a community. It also illustrates the effect of these interactions on the densities of the populations concerned. CHECKUP Observatory pp ST/EST Questions 1-19