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Levels of Ecological Organization in Freshwater Systems Population Community Ecosystem Population Biology in Freshwater Systems Lecture Goals • To discuss basic controls on population size and population dynamics in freshwater systems. • To use the primary literature to explore specific mechanisms regulating population size and population dynamics in freshwater systems. What is a population? • A group of interacting individuals of the same species in a particular place, at a particular time. Population regulation: What determines the size of a population? Population dynamics: How does population size change over time? • A group of interacting individuals of the same species in a particular place, at a particular time. • A group of interacting individuals of the same species in a particular place, at a particular time. The Fundamentals Nt+1 = Nt + B – D + I – E “local” “spatial” I and E Along streams and rivers… Among ponds and lakes… Lecture structure • Life history and Reproduction (B) • Mortality (D) Lecture structure • Life history and Reproduction (B) Life History: Changes experienced by an individual between birth and death that determine habitat requirements, ecology, and reproductive output. Life History: Changes experienced by an individual between birth and death that determine habitat requirements, ecology, and reproductive output. Intrinsic differences in life history Extrinsic ecological factors acting on stages Variation in population size over space and time Lecture structure • Life history and Reproduction (B) > Reproductive strategies > Variation in vital rates with life history - Density dependence > Abiotic controls on life history Lecture structure • Life history and Reproduction (B) > Reproductive strategies Reproductive Strategies • Semelparity: Reproduce once in lifetime, then die. • Iteroparity: Reproduce multiple times in lifetime. • Semelparity: Reproduce once in lifetime, then die. Implications of Semelparity • To contribute to B, just need to survive to reproduce. • Females can invest everything they have in reproduction once they reach some “threshold”. • If reproduce in bad year, then fitness can go to 0 (i.e., all eggs in one basket). ***Dead bodies go right back into food web*** Implications of Semelparity • Iteroparity: Reproduce multiple times in lifetime. Implications of Iteroparity • To have a significant impact on B, need to survive to reproduce multiple times. • Current investment in reproduction may reduce future reproductive potential. • If reproduce in a bad year, then can still have high fitness over lifetime (i.e., eggs are in multiple baskets). Cladoceran Life Cycle Prop. of offspring over lifetime Fine-tuning Iteroparity (Dubycha 2001) Lecture structure • Life history and Reproduction (B) > Reproductive strategies > Variation in vital rates with life history Variation in vital rates with life history • Births Stage (i.e., juvenile vs. adult) Size B and Stage VS. VS. log Egg Number B and Size (Bruce 1978) log Snout-Vent Length Variation in vital rates over life history • Births • Deaths Age (i.e., senescence) Stage / Size Stage-specific effects on D Larvae Brook Trout Embeddedness Adults Gyrinophilus Adults (Lowe et al. 2004) Variation in vital rates over life history • Births • Deaths • Dispersal Stage Size The colonization cycle of freshwater insects What is the demographic importance of drifters? > “Excess” individuals > Low-fitness individuals If drifters ARE demographically important… If drifters ARE NOT demographically important… How are we quantifying dispersal? Dispersal and Drift 15N (MacNeale et al. 2005) 15N (MacNeale et al. 2005) Sticky Traps (MacNeale et al. 2005) Lecture structure • Life history and Reproduction (B) > Reproductive strategies > Variation in vital rates with life history - Density dependence Density Dependent Recruitment • Brown trout (Salmo trutta) in two streams in UK • Egg density ≈ Density of reproductive adults • May depend on range of observations (Elliott 1987) Lecture structure • Life history and Reproduction (B) > Reproductive strategies > Variation in vital rates with life history - Density dependence > Abiotic controls on life history Abiotic controls on life history Abiotic controls on life history Broader implications Mediates exposure to other factors (e.g., predators) Regulates how closely a population can track resources Affects the rate at which populations can respond to natural selection Lecture structure • Life history and Reproduction (B) • Mortality (D) Important controls on mortality in freshwater systems • Drying of ephemeral pools and streams • Flooding and bed movement • Rapid changes in chemical or physical conditions • Predation • Others… Predation in freshwater systems Prey mortality Prey mortality The “rules”: Predator density Prey density …but there are important and interesting exceptions to this rule that have been shown in studies of freshwater organisms. Predation in freshwater systems • Predator functional response • Interactions among predators • Prey refuges Predation in freshwater systems • Predator functional response Predator functional response Prey mortality How does predation rate (or prey mortality) change with prey density? Prey density Predator functional response How does predation rate (or prey mortality) change with prey density? Time spent searching for prey Predation Rate Time spent “handling” prey Predator functional response (Begon et al. 1990) Predator functional response Broader implications Even at high predator densities, prey mortality is limited by handling time. There will always be a maximum predation rate that prey can offset with reproduction. Creates the opportunity for predator swamping. Individual predation risk Predator swamping: Reduction in individual predation risk by aggregating. Prey density Assumptions: • Predator density is fixed • Search time is low and independent of prey density (i.e., aggregations no more likely to be found that individuals) Predator swamping: Reduction in individual predation risk by aggregating. Examples: • Fish schools in lakes • Synchronous emergence in aquatic insects • Zooplankton patches in lakes Predation in freshwater systems • Predator functional response • Interactions among predators Interactions among Predators: Interference With Interference Prey mortality Prey mortality Without Interference Predator density Predator density Interactions among Predators: Feeding Frenzy!! With Frenzy Prey mortality Prey mortality Without Frenzy Predator density Predator density Predation in freshwater systems • Predator functional response • Interactions among predators • Prey refuges Refuge in size Refuge in protection Refuge in space Refuge in time Prey mortality Prey mortality Prey refuges Predator density Prey density Refuge in Protection (Boyero et al. 2006) Potamophylax latipennis Refuge in Protection (Boyero et al. 2006) Refuge in Space Ambystoma barbouri (Sih et al. 1992) Refuge in Space (Sih et al. 1992) Refuge in Time STRESS (e.g., fish predation) Refuge in Time Family Taeniopterygidae and Capniidae (Winter Stoneflies)