Biological Communities and Species Interaction Chapter 4

Outline: Critical Environmental Factors Adaptation Natural Selection Speciation Ecological Niche Population Dynamics Community Properties Succession Introduced Species

Critical Environmental Factors Von Liebig proposed the single factor in shortest supply relative to demand is the critical determinant in species distribution. Shelford later expanded by stating each environmental factor has both minimum and maximum levels, tolerance limits, beyond which a particular species cannot survive.

Tolerance Limits

Critical Environmental Factors For many species, the interaction of several factors, rather than a single limiting factor, determines biogeographical distribution. For some organisms, there may be a specific critical factor that mostly determines abundance and distribution. Species requirements and tolerances can also be used as useful indicators. Environmental indicators

Adaptation Adapt is used in two ways: Range of physiological modifications available to individual organisms. Inheritance of specific genetic traits allowing a species to live in a particular environment. (Population level) Explained by process of evolution

Natural Selection Natural Selection - Members of a population best suited for a particular set of environmental conditions survive and produce offspring more successfully than their competitors. Acts on pre-existing genetic diversity. Limited resources place selective pressures on a population.

Speciation Given enough geographical isolation or selective pressure, members of a population become so different from their ancestors that they may be considered an entirely new species. Alternatively, isolation of population subsets, preventing genetic exchange, can result in branching off of new species that coexist with the parental line.

Divergent vs. Convergent Evolution Divergent Evolution - Mutations and different selective pressures cause populations to evolve along dissimilar paths. Convergent Evolution - Unrelated organisms evolve separately to cope with environmental conditions in the same fashion. (Look alike - Act alike)

Ecological Niche Habitat - Place or set of environmental conditions where a particular organism lives. Ecological Niche - Description of the role a species plays in a biological community, or the total set of environmental factors that determines species distribution. Generalists - Broad niche Specialists - Narrow niche

Competition

Resource Partitioning Law of Competitive Exclusion - No two species will occupy the same niche and compete for exactly the same resources for an extended period of time. One will either migrate, become extinct, or partition the resource and utilize a sub-set of the same resource. Given resource can only be partitioned a finite number of times.

Resource Partitioning

POPULATION DYNAMICS Predation - A predator is an organism that feeds directly upon another living organism, whether or not it kills the prey in doing so. Prey most successfully on slowest, weakest, least fit members of target population. Reduce competition, population overgrowth, and stimulate natural selection. Co-evolution

Keystone Species Keystone Species - A species or group of species whose impact on its community or ecosystem is much larger and more influential than would be expected from mere abundance. Often, many species are intricately interconnected so that it is difficult to tell which is the essential component.

Competition Interspecific - Competition between members of different species. Intraspecific - Competition among members of the same species. Often intense due to same space and nutritional requirements. Territoriality - Organisms defend specific area containing resources, primarily against members of own species. Resource Allocation and Spacing

Symbiosis Symbiosis - Intimate living together of members of two or more species. Commensalism - One member benefits while other is neither benefited nor harmed. Cattle and Cattle Egrets Mutualism - Both members benefit. Lichens (Fungus and cyanobacterium) Parasitism - One member benefits at the expense of other. Humans and Tapeworms

Defensive Mechanisms Batesian Mimicry - Harmless species evolve characteristics that mimic unpalatable or poisonous species. Mullerian Mimicry - Two unpalatable species evolve to look alike.

COMMUNITY PROPERTIES Primary Productivity - Rate of biomass production. Used as an indication of the rate of solar energy conversion to chemical energy. Net Primary Productivity - Energy left after respiration.

Abundance and Diversity Abundance -Total number of organisms in a community. Diversity - Number of different species, ecological niches, or genetic variation. Abundance of a particular species often inversely related to community diversity. As general rule, diversity decreases and abundance within species increases when moving from the equator to the poles.

Complexity and Connectedness Complexity - Number of species at each trophic level, and the number of trophic levels, in a community. Diverse community may not be complex if all species are clustered in a few trophic levels. Highly interconnected community may have many trophic levels, some of which can be compartmentalized.

Resilience and Stability Constancy (Lack of fluctuation) Inertia (Resistance to pertubation) Renewal (Ability to repair damage) MacArthur proposed complex, interconnected communities would be more stable and resilient in the face of disturbance. Controversial

Edges and Boundaries Edge Effects - Important aspect of community structure is the boundary between one habitat and others. Ecotones - Boundaries between adjacent communities. Sharp boundaries - Closed communities Indistinct boundaries - Open communities

COMMUNITIES IN TRANSITION Ecological Succession Primary Succession - A community begins to develop on a site previously unoccupied by living organisms. Pioneer Species Secondary Succession - An existing community is disrupted and a new one subsequently develops at the site.

Terrestrial Primary Succession

Ecological Succession Ecological Development - Process of environmental modification (facilitation) by organisms. Climax Community - Community that develops and seemingly resists further change. Equilibrium Communities (Disclimax Communities) - Never reach stable climax because they are adapted to periodic disruption.

Introduced Species If introduced species prey upon or compete more successfully than native populations, the nature of the community may be altered. Human history littered with examples of introducing exotic species to solve problems caused by previous introductions. Mongoose and Rats in Caribbean

Summary: Critical Environmental Factors Adaptation Natural Selection Speciation Ecological Niche Population Dynamics Community Properties Succession Introduced Species