IB 153 Fall 2006 Life histories and population regulation 9/21/2006. Life history classification schemes. r/k selection (MacArthur and Wilson 1967)

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1 IB 153 Fall 26 Life histories and 9/21/26 Today's lecture: 1. Finish discussion on life histories 2. Review exponential and logistic growth equations 3. Effects of density on vital rates and consequences for dynamics 4. Population limitation versus regulation 5. Population regulation debate: the relative influence of densitydependent versus density-independent factors 6. Density-vague dynamics In a growing population, an individual present early makes a greater proportionate contribution than an individual present later 7 Descendants of one individual.4 present at t= Descendants of one individual present at t= Reproductive value of a life history varies with population density In a declining population, an individual present later makes a greater proportionate contribution than an individual present early 12 Descendants of one individual present at t= Descendants of one individual present at t= What conditions favor iteroparity vs. semelparity? Iteroparity favored when the likelihood of survival is much greater for the parents than their offspring trees may not reproduce for many years, so seedlings are reproductively worthless until established, which most seedlings will never accomplish Iteroparity favored when greater parental care will increase the survivorship of offspring parental care is only possible if the adults haven't exhausted themselves producing eggs vertebrate parents may be able to communicate information to offspring and increase their chances of survival Semelparity is favored when the likelihood of adults surviving until the next reproductive season is low Semelparous females commit all resources to reproduction and save nothing in reserve for surviving the reproductive period and so produce more offspring, often at an early age. Consider salmon salmon are born in freshwater lakes and streams, migrate to the sea, and return to the same stream years later as adults to mate and lay eggs, after which they die. the chance of making it from ocean to breeding lake or stream is low as predators and mischance kill many adults adults would have to make the perilous journey twice more (back to the ocean and upstream once again) to breed again instead, females lay tremendous numbers of eggs and die exhausted after the first upstream journey Life history classification schemes r/k selection (MacArthur and Wilson 1967) r- and K- selection is based on the idea that habitats can be separated into two general types: those that have relatively constant environments characterized by persistent dense populations and intense competition versus those that have variable environments and usually uncrowded populations. 3-cornered continuum model (Grime 1977) Grime has separated habitat variability and physical stress to give three different environmental influences on life history (stable environments, variable environments, and stressful environments), which leads to three different general life history strategies. Climate Population size Competition Lifespan Size Reproduction Leads to R and K selection characteristics r-strategist K-strategist variable constant or predictable variable, recolonization at equilibrium variable, lax keen short, often <1yr long, slower development small larger high investment of energy, early delayed reproduction, reproduction, large number of relatively fewer, larger relatively small offspring offspring high productivity efficient resource use 1

2 IB 153 Fall 26 Life histories and 9/21/26 3-cornered continuum model (Grime 1977) Intensity of Stress Intensity of Disturbance Low High Low Competitive Ruderal High Stress-tolerant No viable 2

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6 IB 153 Fall 26 Life histories and 9/21/26 Definitions Population limitation: process that limits (i.e. reduces) population growth and may determine (but not maintain) an equilibrium density, if one exists. Population regulation: density-dependent process that helps maintain population density within certain limits ( bounded variance around a consistent mean value, in some cases, the carrying capacity, K). If a population is moved away from this mean, a regulating factor will cause the density to return towards its equilibrium density (or range of densities). Population dynamics: changes in population density resulting from the effects of both limiting and regulating factors. Density-Vague Population Change Don Strong suggested the concept of density vagueness was a realistic alternative to the mathematical convenience of explicit density dependence. Density-Vague Population Change Lack of change in growth rate at medium densities where populations spend most of. Emphasis on variance around relationship. 6