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(De)stabilizing Factors • Models assume no competition among predators, type I functional response. • Predator/prey population may be stabilized or destabilized by: – Mutual interference (reduce predator effectiveness) – Different predator functional response (reduce predator impact on prey) – Density dependence or other specific predator/prey population dynamics • Ratio dependent predation – Predation rate (and therefore predator population rate of increase) is a function of the predator:prey ratio. (De)stabilizing Factors • Allee Effect – recruitment disproportionately low at low density, producing a hump shaped prey isocline – High efficiency type II predator destabilizes – Lower efficicncy type II predator stabalizes • A predator isocline to the left of the hump destabilizes oscillations. (De)stabilizing Factors • (a) and (b) Similar to Lotka-Volterra, isoclines curved to reflect density dependent effects. • (c) ratio dependent predation • (i) – moderate predator efficiency, least stable • (ii) – less efficient, more stable • (iii) – Predator self limitation, no oscillations, low P. (De)stabilizing Factors • A type III predator switching among several potential prey – (i) More efficient predator is stabilizing while less efficient predator (ii) is less stabilizing • may have a stable population independent of individual prey populations. 1 Multiple Equilibrium Points Patch Dynamics and Predator-Prey Interactions • Adding patchiness increases complexity, we know patchiness is inherent in all biological systems. • What does patchiness do to predator-prey systems? (a) habitat (b) habitat Detritivores and Decomposers • Typical consumer – consumer takes from resource from prey, prey fitness negatively effected (+/-) • Donor controlled system – waste products donated to consumer (detritivore or decomposer). • Decomposition has no adverse effect on donor (+/0) and there may often be an indirect mutualistic effect (+/+). Tertiary Consumer • Allee effect combined with type III functional response produces prey isocline with hump and vertical section at low N • X, Y and Z are all stable equilibrium points, possible explanation for outbreaks of particular species. Detritivores and Decomposers • Nutrient Source – Parts of modular organisms – Parts of unitary organisms – Carcasses – Feces • Carnivore feces less desirable than herbivores • Nutrient fate – Immobilization Secondary Consumer – Mineralization Primary Consumer Primary Production 2 Detritivores and Decomposers • • • • Cellulose and Chitin Mostly bacteria and fungi Food for decomposers is extremely patchy (feces, carcass etc.) Good colonization, succession and competition model – Understudied, historical taxonomic difficulties • Energy tied up in cellulose or chitin not usually physiologically digestible. • Cellulase and chitinase enzymes restricted almost entirely to microbial decomposers. Good dispersers/colonizers utilize easy nutrients first, later colonizers outcompete for more difficult nutrients • Two sided debate – Organisms use cellulose and chitin to be less digestible (poorer prey quality, OFT) – No selection to produce chitinase or cellulase when you can rely on microbial symbionts. Symbiosis and decomposition • Considerable energy not physiologically digestible by animals – Internal and/or external rumen – compensation for lack of cellulase? – Coprophagy Microbivores • Microbivores – protozoan consumers of bacteria and yeasts. • Decomposing material supports a diverse community. • Different taxa play different roles in decomposition. Greater diversity=greater rate of decomposition though facilitation. 3 Decomposition in major types of ecosystems • Streams, small lakes, terrestrial – input is large, heavy in cellulose, shredder types necessary. – River Continuum Concept Coprophagy and Autocoprophagy • • Feed on others feces – plant material is mechanically digested, has microbial community Feed on your own feces – pass material through your own digestive system, process twice Large bodied detritivores • Most nutrients come from meiofauna, microbivore fauna • Probably best described as omnivores, diet composed mostly of decomposers, not actual “detritus”. • Detritivore dominated communities – low/no productivity compared to outside organic contribution. • Allochthonous vs autochthynous organic input Carion Specialists • Large non-plant carcasses are a high quality meal • Sucession similar as in plant detritus, except that energy is more readily available (no cellulose), but tends to be patchier. • Eg. American burying beetle, hagfish, maggots 4