Chapter 11 Population Distributions PDF

Rick Relyea · Robert Ricklefs The Economy of Nature 7th edition Lecture PowerPoint Chapter 11 Population Distributions © 2014 by W. H. Freeman and Company  Chapter 11 concepts The distribution of populations is limited to ecologi...

Rick Relyea · Robert Ricklefs The Economy of Nature 7th edition Lecture PowerPoint Chapter 11 Population Distributions © 2014 by W. H. Freeman and Company  Chapter 11 concepts The distribution of populations is limited to ecologically 1 suitable habitats. Population distributions have five important 2 characteristics. The distribution properties of populations can be 3 estimated. Population abundance and density are related to 4 geographic range and adult body size. 5 Dispersal is essential to colonizing new areas. 6 Many populations live in distinct patches of habitat.  Chapter 11 concepts The distribution of populations is limited to ecologically 1 suitable habitats. Population distributions have five important 2 characteristics. The distribution properties of populations can be 3 estimated. Population abundance and density are related to 4 geographic range and adult body size. 5 Dispersal is essential to colonizing new areas. 6 Many populations live in distinct patches of habitat. 1 Distributions of populations Spatial structure: the pattern of density and spacing of individuals in a population. Fundamental niche: the range of abiotic conditions (e.g., temperature, humidity, salinity) under which a species can persist. Competitors, predators, and pathogens may prevent a population from persisting in an area. Realized niche: the range of abiotic and biotic conditions under which a species does persist. Geographic range: a measure of the total area covered by a population (e.g., temperature and drought define the range of sugar maple). 1 Distributions of populations Small-scale variation in the environment creates geographic ranges that are composed of small patches of suitable habitat. Example: The geographic range of Fremont’s leather flower is just three counties in Missouri. Within those counties, plants are restricted to dry, rocky soils on limestone outcroppings (i.e., limestone glades). Plants are further restricted by variation in glade soil structure and quality. 1 Effects of global climate change During the past century, the average temperature of the Earth has increased by 0.9°C. Temperature change can cause a shift in the geographic range of species. Example: Average temperatures in the North Sea have increased 2°C from 1977–2003. Fish species richness in the North Sea has increased steadily over this time and is positively correlated with ocean temperature. Warmer northern temperatures have caused southern fish species to expand their ranges northward.  Chapter 11 concepts The distribution of populations is limited to ecologically 1 suitable habitats. Population distributions have five important 2 characteristics. The distribution properties of populations can be 3 estimated. Population abundance and density are related to 4 geographic range and adult body size. 5 Dispersal is essential to colonizing new areas. 6 Many populations live in distinct patches of habitat. 2 Population characteristics 1 Geographic range Endemic: species that live in a single, often isolated, location. Cosmopolitan: species with very large geographic ranges that can span several continents. 2 Abundance: the total number of individuals in a population that exist within a defined area (e.g., total number of lizards on a mountain). The total abundance of a population provides a measure of whether a population is thriving or on the brink of extinction. 2 Population density 3 Density: in a population, the number of individuals per unit area or volume; calculated by dividing abundance by area. If population density is greater than what the habitat can support, some Dickcissel individuals must leave or the population will experience lower growth and survival. The largest density of individuals typically occurs near the center of a population’s geographic range. Near the edges of the range conditions become less ideal, and population densities decrease. 2 4 Population dispersion Dispersion: the spacing of individuals with respect to one another within the geographic range of a population. Clustered dispersion: when individuals are aggregated in discrete groups (e.g., social groups or clustering around resources). Evenly spaced (Uniform) dispersion: when each individual maintains a uniform distance between itself and its neighbors (e.g., defended territories, croplands). Random dispersion: when the position of each individual is independent of other individuals; not common due to non- random environmental heterogeneity. 2 Population dispersal 5 Dispersal: the movement of individuals from one area to another. Dispersal is distinct from migration, which is the seasonal movement of individuals back and forth between habitats. It is the mechanism by which individuals can move between suitable habitats. Dispersal allows species to colonize areas outside of their geographic ranges. Dispersal can be a way to avoid areas of high competition or high predation risk.  Chapter 11 concepts The distribution of populations is limited to ecologically 1 suitable habitats. Population distributions have five important 2 characteristics. The distribution properties of populations can be 3 estimated. Population abundance and density are related to 4 geographic range and adult body size. 5 Dispersal is essential to colonizing new areas. 6 Many populations live in distinct patches of habitat. 3 Quantifying individuals Census: counting every individual in a population. Conducting a census is not feasible for most species, so scientists conduct a survey, in which they count a subset of the population. Area- and volume-based surveys: surveys that define the boundaries of an area or volume and then count all of the individuals in the space. The size of the defined space is related to the abundance and density of the population. By taking multiple samples, it is possible to determine how many individuals are in an average sample. 3 Quantifying individuals Line-transect surveys: surveys that count the number of individuals observed as one moves along a line. This data can be converted into area estimates of a population. Examples: Counting the number of small plants along a string tied between two fixed points. Counting all individuals observed within a fixed distance of a line. During the annual Christmas bird count, thousands of volunteers follow a predetermined path that covers a 24-km circle and count the number of bird species they can see or hear. 3 Quantifying individuals Many animals are sensitive to the presence of researchers and will leave the area when surveyed. Other species are camouflaged and may be difficult to find. Mark-recapture survey: a method of population estimation in which researchers capture and mark a subset of a population from an area, return it to the area, and capture a second sample of the population after time has passed. Population size is estimated by assuming that: 3 Quantifying dispersal Dispersal can be quantified by measuring how far individuals travel from a single source location. Alternatively, individuals can be marked and recaptured (e.g., with ear tags, radio transmitters, leg bands) to determine how far they moved from the location they were marked. Lifetime dispersal distance: the average distance an individual moves from where it was born to where it reproduces. This provides an estimate for how fast a population can increase its geographic range.  Chapter 11 concepts The distribution of populations is limited to ecologically 1 suitable habitats. Population distributions have five important 2 characteristics. The distribution properties of populations can be 3 estimated. Population abundance and density are related to 4 geographic range and adult body size. 5 Dispersal is essential to colonizing new areas. 6 Many populations live in distinct patches of habitat. 4 Population abundance and range Populations with high abundance also have large geographic ranges. This pattern has been observed for many organisms (e.g., birds). This may be caused by resource availability; species are likely to cover the area that contains the resources that they require. The large amount of variation in the abundance-range size regression may be caused by fluctuations within a geographic range. This suggests that reducing the range of a population (e.g., by converting land to agriculture) will reduce the size of that population. 4 Population density and body size The density of a population is negatively correlated to the body size of the species. Examples: In herbivorous animals, the smallest- bodied species (e.g., mice) live at the highest densities, whereas the largest- bodied animals (e.g., elephants) live at the lowest densities. From this relationship, we expect a given plot of habitat to support fewer large individuals than it does small individuals.  Chapter 11 concepts The distribution of populations is limited to ecologically 1 suitable habitats. Population distributions have five important 2 characteristics. The distribution properties of populations can be 3 estimated. Population abundance and density are related to 4 geographic range and adult body size. 5 Dispersal is essential to colonizing new areas. 6 Many populations live in distinct patches of habitat. 5 Dispersal limitation Dispersal limitation: the absence of a population from suitable habitat because of barriers to dispersal. Barriers often include large expanses of inhospitable habitat that an organism cannot cross (e.g., oceans, deserts). Occasionally, organisms cross these barriers, such as the many species of plants and animals that colonized the Hawaiian Islands. Humans have created barriers for dispersal (e.g., roads, forest clearings) and have assisted (and inhibited) the dispersal of many species. Examples: Aboriginal peoples brought dogs to Australia; pine trees have been transplanted all over the world from California; aquatic species are transported in ballast water. Most dispersed species do not establish viable populations. Habitat Fragmentation http://www.forestry.gov.uk/fr/INFD-673ER6 5 Habitat corridors Habitat corridor: a strip of favorable habitat located between two large patches of habitat that facilitates dispersal (e.g., a narrow band of trees that connects forests). Example: Cleared areas in pine forests provide habitat for many species of animals. Researchers cleared four plots of pine forest around a central cleared plot. One plot had a corridor connecting to the central plot; the other plots (“winged” and “rectangular”) were unconnected. Researchers tracked dispersal of butterflies from the central plot and found that more individuals dispersed to the connected plot. 5 Habitat corridors Conservation efforts have increasingly considered the preservation of corridor habitats. Example: Biologists have pushed to protect riverside habitats along the Rio Grande that would allow species to move easily among large patches of protected land.  Chapter 11 concepts The distribution of populations is limited to ecologically 1 suitable habitats. Population distributions have five important 2 characteristics. The distribution properties of populations can be 3 estimated. Population abundance and density are related to 4 geographic range and adult body size. 5 Dispersal is essential to colonizing new areas. 6 Many populations live in distinct patches of habitat. 6 The ideal free distribution Whenever possible, individuals choose habitats that provide the most energy. As individuals move to a high-quality habitat, resources must be divided among more individuals (i.e., reduced per capita benefit). Per capita benefit can fall so low that an individual would benefit by moving to the low-quality habitat. Ideal free distribution: when individuals distribute themselves among different habitats in a way that allows them to have the same per capita benefit.  IDEAL FREE DISTRIBUTIONS Assumes perfect knowledge of patches Fitness maximizers Individuals in patch realize same fitness Resource depletion lowers fitness Where should an individual go? 6 The ideal free distribution Example: Stickleback fish were distributed evenly throughout an aquarium. Researchers manipulated the abundance of prey (i.e., water fleas) on each side of the aquarium such that one side had one-fifth the abundance of water fleas as the other side. Within five minutes of starting the experiment, the fish distributed themselves across the aquarium in a ratio that was approximately four to one. When the abundance of food on each side was changed, the fish quickly adjusted their distribution ratio. 6 The ideal free distribution Individuals in nature rarely meet the expectations required by the ideal free distribution. Individuals may not be aware that other habitats exist. Fitness is not solely determined by maximizing resources; other factors may influence distribution such as the presence of predators or territory owners.