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Life History Strategies: Trade-offs with reproduction and survival Sources: Case (2000), UCSD website; Ricklefs and Miller (1999), Stearns (1992), Reznick et al. (1997, 2002) Some slides presented here modified from Case’s website Motivation: diversity of life history strategies displayed in nature and their role on the dynamics of populations Evolutionary and conservation implications Just as adaptation in functional form occurs, so does “demographic adaptation”– life history strategies (will later link these ideas to elasticity analyses of matrix pop. models) 1 Types of survival curves Pearl’s Type I, II, and III idealized survivorship curves Type I: Low mortality early on– investment for long life span Type III: High early (e.g., juvenile) mortality Types of reproductive strategies •Semelparity (death after reproduction) •Iteroparous (repeated bouts of reproduction) Reproductive parameters: Age at maturity, size/fecundity relationship, number of eggs/litter size, frequency, egg size, egg physiology, etc. Variation at all levels: individuals, populations, species, and higher taxa Proximate and ultimate factors Broad range of genetic heritabilities of reproductive effort (0-90% based on empirical studies) 2 Life-History as Adaptations to the Environment “… it will be regarded as axiomatic that the reproductive potentials of existing species are related to their requirements for survival; that any life history features affecting reproductive potential are subject to natural selection [and thus must be heritable] and that such features observed in existing species Should be considered adaptations, just as purely morphological or behavioral patterns are so considered” From Lamont Cole (1954): The population consequences Of life history phenomena. Quart. Rev. Biol. 29:103-137 Background: terms and concepts useful for understanding readings • r = exponential growth rate • r-selection = selection on traits that determine fecundity and survival to favor exp growth at low density • k = carrying capacity • k-selection = selection on traits that determine fecundity and survival to favor competitive ability at densities near carrying capacity (k) Pop. Density Logistic Equation k r Time 3 Background: terms and concepts useful for understanding readings Sx= probability of survival from age-class x to x + 1 lx = probability of survival to age x bx = fecundity, number of offspring Ro = lifetime (net) fecundity 8 = Population growth (er) • Fitness = the genetic contribution by an individual’s descendants to future generations • measured as 8 or r in Case (Ch. 7), and thus a function of survival and fecundity • selecting a proper metric difficult in the case of densitydependence (Ro sometimes used) • “measured only by a genotype’s rate of increase relative to other genotypes” (Futuyma 1986) What is life history? • The life history is the schedule of an organism’s life, including: – age/size at maturity – number of reproductive events – allocation of energy to reproduction – number and size of offspring – age and size specific pattern of growth – life span (patterns of survival) 4 What is a life history? A life-history is a set of environment and condition-dependent decision rules governing an organisms scheduling of allocation of resources towards growth, survival and reproduction. Life Histories • Consider the following remarkable differences in life history between two birds of similar size: – thrushes • reproduce when 1 year old • produce several broods of 3-4 young per year • rarely live beyond 3 or 4 years – storm petrels • do not reproduce until they are 4 to 5 years old • produce at most a single young per year • may live to be 30 to 40 years old Ok, a fish example: some flounders mature in first year, other species at 10 yr • Similarly, intra-specific and individual variation is often vary high Ok, a fish example: a species of flounders mature in 3 yr, 20 cm in Scotland but takes 15 yr, 40 cm in Newfoundland 5 What influences life histories? • Life histories are influenced by: – body plan and life style of the organism – evolutionary responses to many factors, including: • physical conditions • food supply • predators • other biotic factors, such as competition • temporal variation of these factors A Classic Study • David Lack of Oxford University first placed life histories in an evolutionary context Review details in Case (2000) The “Lack” Clutch: the clutch size that produces the greatest number of (reproducing) offspring Reproductive effort model: a “theoretical tradition” that aims to optimize reproductive effort taking the costs of reproduction into account 6 Lack’s Proposal • Lack made 3 key points, suggesting that life histories are shaped by natural selection: – because life history traits (such as number of eggs per clutch) contribute to reproductive success they also influence evolutionary fitness – life histories vary in a consistent way with respect to factors in the environment – hypotheses about life histories are subject to experimental tests Optimal Reproductive Effort The Lack Model and Beyond What is Lack’s Model? What does it predict? Most birds produce fewer young than predicted by Lack’s model 1. Probably most studies (incorrectly) equate number of fledglings and the number young breeding in future population 2. Other costs are involved besides juvenile survival 3. Temporal variability of environment Bet-hedging: reduce reproductive effort to live longer and have better chance of reproducing in “good” years 7 Cost of reproduction in Kestrels. Predictions on the Effects of Different Life History Strategies On Population Growth Rate Empirical support? 8 Life History Studies of the Trinidad Guppy Trinidadian guppies (Poecilia reticulata) • D. Reznick et al. 1990. Nature 345:357-9, 1997. Science 275:1934-1937 • Live in streams in Trinidad • Exposed to different predation pressures in different streams and parts of streams. • Waterfalls often separate populations exposed to a given predator from those that are not. 9 Aripo river, Trinidad. Waterfalls like this often separate populations exposed to different predators. Predators and prey Crenicichla alta Rivulus hartii 10 Predators impose different mortality pressures. • Crenicichla alta (a cichlid) feeds mainly on adult guppies, lowering adult survival. • Rivulus hartii (a killifish) preys mainly on juvenile guppies, lowering juvenile survival. Life-history predictions: • Rivulus predation • Juvenile survival low, adult survival high • Increase size of offspring at the expense of offspring number. • Lower reproductive effort. • Crenicichla predation • Adult survival low, juvenile survival high. • Many small offspring. • High reproductive effort 11 Comparative approach using common garden experiments • Find populations exposed to different predators. • Collect individuals from each population and bring to lab. • Rear fish under common environmental conditions. • Measure life history traits and their heritabilities. Fish room (common garden) 12 Life history traits for two populations of guppies with Rivulus and two populations with Crenicichla. Life history trait Age of females at 1st brood (days) Rivulus 81.9 Crenicichla 71.5 Size of females, 1st brood (mg wet) 270.0 218.0 Size of 1st litter 3.2 5.2 Offspring size (mg dry), litter 1 Reproductive effort (%) 0.99 19.2 0.84 25.1 Comparative approach: correlation or causation. • Differences seen are clearly genetic (common garden experiment). • Differences seen correlated with identity of predator. • Is the predator the cause of differences? • Need experimental approach. 13 Experiment • Transplant guppies from a habitat with Crenicichla (preys on adults) to one with Rivulus (preys on juveniles). • Predictions: Guppies should evolve toward fewer, larger offspring, lower reproductive effort, longer time to first reproduction and longer interval between broods. 14 Results of transplant experiment (traits measured in common environment) Life history trait Age of females at 1st brood (days) Size of females, 1st brood (mg wet) Size of 1st litter Offspring size (mg dry), litter 1 Rivulus (introduction) 92.3 185.6 3.3 0.95 Crenicichla (control) 85.7 161.5 4.5 0.87 Experimental results in guppies generally confirm comparative findings. • Exposure to high juvenile vs. adult mortality caused the evolution of delayed reproduction, larger size at reproduction, fewer larger offspring (particularly for first brood), and reduced reproductive effort. 15 Principal Findings and Relation to Life-History Theory Multiple Factors Density-dependent? r- and k-selection theory useful? “…it became clear that the predictions of r- and K-selection Were not always upheld….This dose of reality helped the field develop a more rigorous theory… The predictions of more heavily derived models are often dependent on…factors hard to measure in natural populations. It is this disparity between theoretical concepts and empirical realities that continue to create a “muddle in life-history thinking” Reznick et al. 2002 16