Biol 301 1nd Edition Lecture 18 Outline of Last Lecture I. Greenhouse EffectII. Atmospheric Air CurrentsIII. Heating of Earth by SunOutline of Current Lecture II. Community BoundariesIII. Community DiversityIV. Species DiversityV. Community food websVI. Community StabilityCurrent LectureCommunity Boundaries- A community is an assemblage of species living together in an area. Species composition changes across a landscape; with changes in environmental conditions, some species are better able to survive. The zone in which each species grows reflects different tolerance ranges for various environmental conditions, and the ability to compete with other species. Community zonation also occurs in aquatic communities.- Communities are often categorized by their dominant organisms or by physical conditions that affect the distribution of species. Aquatic systems are often categorized by physical characteristics (e.g., stream or lake communities) or by dominant organisms (e.g., coral reef communities). Ecologists rarely study every species in a community; rather, they focus on a subset of species that live in an area.- Ecotone: a boundary created by sharp changes in environmental conditions over a relatively short distance, accompanied by a major change in the composition of species. Some speciesmove between adjacent communities; most species live in one of the communities and spread into the ecotone. Ecotones support a large number of species, including those from adjoining habitats, and species specifically adapted to the ecotone. Line-transect surveys can demonstrate the existence of an ecotone. Sharp changes in the distribution of species should occur across the ecotone.- Interdependent (open) communities: communities in which species depend on each other to exist.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Independent (closed) communities: communities in which species do not depend on each other to exist. If species distributions are independent, they should depend only on individual habitat requirements; there should be gradual changes in species along a line transect. However, abrupt changes in species distributions may be the result of abrupt changes in abiotic conditions (e.g., moving from lake to land), and do not necessarily suggestthat species are interdependent.Community Diversity- Species richness: the number of species in a community.- Relative abundance: the proportion of individuals in a community represented by each species. In a typical community, only a few species have low or high abundance; most species have intermediate abundance; they follow a log-normal distribution.- Log-normal distribution: a normal, or bell-shaped, distribution that uses a logarithmic scale on the x-axis.Species Diversity- The species richness of a community can be affected by the amount of available resources. To understand the influence of resources, ecologists have examined the relationship between productivity and species richness. Across aquatic and terrestrial environments, the correlation between richness and productivity exhibits a wide range of patterns. Observed patterns include U-shaped, negative, neutral, positive, and hump-shaped correlations. Across studies of vertebrates and invertebrates in aquatic and terrestrial systems, a hump-shaped curve is the most commonly observed relationship between diversity and productivity. This indicates that a site with medium productivity has higher species richness than sites with either low or high productivity. Experiments have manipulated productivity by adding nutrients (e.g., nitrogen) to an ecosystem. Added fertility commonly causes a decline in the species richness of producers (e.g., plants and algae).- Communities with a higher diversity of habitats should offer more potential niches (e.g., places to feed and breed) and a higher diversity of species.- Keystone species: a species that substantially affects the structure of communities, althoughspecies might not be particularly numerous. Removal of a keystone species can cause a community to collapse. Ecosystem engineers are keystones species that affect communities by influencing the structure of a habitat.- Intermediate disturbance hypothesis: the hypothesis that more species are present in a community that experiences occasional disturbances than in a community with either frequent or rare disturbances. When disturbances are rare, populations grow until resourcesare scarce, and competitively superior species become dominant. When disturbances are frequent, habitats typically support a small number of species that are adapted todisturbances. When disturbances occur at an intermediate frequency, both types of species can persist.Community Food Webs- Food chain: a linear representation of how different species in a community feed on each other.- Food web: a complex and realistic representation of how species feed on each other in a community. Arrows in a food web indicate consumption and the movement of energy and nutrients.- Trophic level: a level in a food chain or food web of an ecosystem. Producers are the autotrophs that convert light energy and CO2 into carbohydrates through photosynthesis. Producers form the first trophic level of a food web.- Primary consumer: a species that eats producers.- Secondary consumer: a species that eats primary consumers.- Tertiary consumer: a species that eats secondary consumers.- Omnivore: a species that feeds at several trophic levels.- Guild: within a given trophic level, a group of species that feeds on similar items (e.g., guilds of leaf eaters); members of the group are not necessarily related.- Bottom-up control: when the abundances of trophic groups are determined by the amount of energy available from producers.- Top-down control: when the abundances of trophic groups are determined by the existenceof predators at the top of the food web.Community Stability- Community stability: the ability of a community to maintain a particular structure.- Community resistance: the amount a community changes when acted upon by a disturbance (e.g., addition or removal of a species).- Community resilience: the time it takes after a disturbance for a community to return to its original state. Diversity may promote community