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Interactions How Can You Tell If Organisms Are Influencing Each

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Interactions How Can You Tell If Organisms Are Influencing Each Effect of interaction on growth rate of : Interactions How can you tell if organisms are influencing each other? Population/ Population/ • Intra-specific = interactions among members of the same Name of Interaction Species A Species B species/population. Neutralism (is this really 0 0 • Study the relationship between population growth rate and interaction?) • Inter-specific = interactions among members of different population density. Commensalism (table scraps) 0 + species. • If there is a relationship (either + or -), then assume that Mutualism or synergism + + the organisms are influencing each other, if not, there is no 0 - interaction (neutralism). Ammensalism + - Parasitism, predation - - Competition Competition Classic studies of resource competition by Gause (1934, 1935) A. Two major types: Resource competition can only occur when population growth rates of both individuals/populations/species are limited by the same resource. 1. Interference competition = Competition between two individuals/populations/species in which one physically or Paramecium chemically excludes the other from a habitat. If no resources are in limiting supply, then competition does caudatum not occur. 2. Resource competition = Occurs when use of a resource by Paramecium aurelia one individual/population/species reduces the availability of that resource to other individuals/populations. Paramecium bursaria 1 Gause found that interactions between Paramecium aurelia and P. caudatum always ended in competitive exclusion. In contrast, Paramecium bursia and P. caudatum could coexist. Because they inhabited different regions of the flask and ate different food. P. bursia fed on the bottom of the flask, and P. caudatum ate the bacteria in suspension. Within 14 days Paramecium aurelia In contrast, both P. aurelia and P. caudatum ate the bacteria WON! Paramecium bursaria in suspension. WHY? Paramecium caudatum Gause’s Principle (Law) Gause’s Principle (Law) Competition is central to both evolutionary theory and ecological theory: When the niches* of two species overlap, there will be When the niches* of two species overlap, there will be • Competition can be a powerful selection pressure. competition; and, if the overlap is extreme, there will be competition; and, if the overlap is extreme, there will be competitive exclusion. competitive exclusion. • Competition structures of communities. *Niche = the ranges of conditions and resources within which an organism or species persists, often conceived as a multidimensional space. This is also called the Competitive Exclusion Principle 2 µ = µmax S/(Km + S) µ = the maximum per capita growth rate under conditions max Km for different types of organisms of resource saturation S = the concentration of a growth-limiting resource heterotrophs u K = resource concentration at which growth occurs at half Asteronella formosa m intermediate the maximum rate. oligotrophs Ko Ki Kh Cyclotella meneghiniana [S] µ = µmax S/(Ks + S) Which species is the better competitor for PO4 ? Which species is the better competitor for SiO2 ? Tilman, 1976: Asterionella formosa Ks for PO4 = 0.04 Asterionella formosa Ks for SiO2 = 3.9 Asterionella formosa Ks for PO4 = 0.04 Ks for SiO2 = 3.9 Cyclotella meneghiniana Ks for PO4 = 0.25 Cyclotella meneghiniana Ks for SiO2 = 1.4 Cyclotella meneghiniana Ks for PO4 = 0.25 Ks for SiO2 = 1.4 3 When more than one nutrient is limiting, the outcome of competition is also influenced by the ratio of the two limiting nutrients. Tilman’s microcosm data: Stable Stable co-existence co-existence Stable Asterionella Cyclotella Asterionella Asterionella Cyclotella Asterionella Intra-specific Cyclotella wins wins wins wins wins competition wins co-existence limited by SiO 2 > Cyclotella Inter-specific limited by competition Flow rate day) (volumes per Flow rate day) (volumes per Flow rate day) (volumes per PO4 1,000 97 10 5.6 1.0 1,000 97 10 5.6 1.0 1,000 97 10 5.6 1.0 Nutrient ratio [SiO2]/[PO4] Nutrient ratio [SiO2]/[PO4] Nutrient ratio [SiO2]/[PO4] Interactions between microbes and other Survival depends on symbiotic relationship with bacteria that convert hydrothermal vent chemicals to worm food. The trophosome contains types of organisms: Cool mutualisms specialized compartments called “bacteriocytes”, where Tube worm (Riftia pachyptila) grows up to 9 feet tall. Lives The tubeworm has no mouth – how do the bacteria get in? the bacteria grow. on Pacific Ocean floor near hydrothermal vents. Fish and crab feed off of red plume of tubeworm. The bacteria fix CO2 and oxidize H2S. They then provide the worm with fixed C. Nitrogen metabolism is also linked between these organisms, but details are scarce. 4 5.
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