Evolution of Population Characteristics - Life History

Transcription

1 Population Biology Although most trees and perennial tropical plants reproduce about once a year, the "Suicide Tree", Tachygalia versicolor is a long-lived canopy species that reproduces only once and then dies. This species and other monocarpic plants are described in Augspurger, C. K. & R. B. Foster (1979). Tropical Monocarpic Plants. Amer. J. Bot. 66, 54. Evolution of Population Characteristics - Life History I. Introduction: Life History Theory - General Application of Physiology to Demography - Based on Resource Allocation Theory. > goal of good physiological ecologist should be to predict reproductive output and survivorship in a specified environment > identify those physiological processes which limit growth and reproduction in a specific environment. Liebigs Law. The Principal of Allocation and Carbon as Currency: Reproductive Effort. Mooney 1970, defined carbon as the limiting "currency" in plants : suggested that each plant function has its cost in terms of carbon and that these functions compete for this currency. Thus there are two ideas here that embody a trade-off function between vegetative and sexual reproductive activities: 1. future vs. present reproduction, i.e., is there a cost to sexual reproduction in terms of present vegetative growth? --> only way to measure this is indirectly : i.e., what would growth have been

2 2. direct cost of reproduction in terms of carbon (or any other substance that is considered limiting) - i.e., how much carbon is put into a fruit and associated reproductive structures and where did this carbon come from. EVIDENCE THAT REPRODUCTION IS LIMITED BY RESOURCE AVAILABILITY: 1. monocarpic species (Agave; Tachygalia; Salmon; Bamboos) 2. biennial species (Pat Werner) 3. Mayapple - reproduction is correlated with leaf surface area. usually requires two leaves (Sohn and Policansky). 4. King and Roughgarden (1983) - are able to predict when annuals will reproduce on the basis of rates of carbon gain. 5. trade off between fruit number and fruit size within a genus and habitat (Primack) 6. Dendrosenecio - cost of reproduction (Smith and Young) 7. Jurik - carbon budgets of strawberries. 8. Chabot and Hicks, Reich, Mulkey - Leaf Life Spans.

3 Point: Leaves with high-energy throughput (i.e. sun leaves), have higher nitrogen content and live shorter lifespans because they can pay for their construction and yield a carbon profit very quickly. The opposite is true for shade leaves, which can live to extreme ages. Which type of life history would you expect to have long-lived leaves, and which should have short-lived leaves? 9. Lobelia on Mt. Kenya (T. P. Young)

4 Most of these infer a cost to reproduction, rather than measure it. Life History Theory. Encompasses the study of the selective factors which determine the schedule of reproduction and the number of babies which a female can have. Terms: annual perennial semelparous iteroparous polycarpic monocarpic II. Perennial Life History Favored by high and relatively constant adult survival. Cole's paradox: What is the relative advantage of being perennial vs being annual? Cole's paradox is that the difference between the number of babies left by an annual and a perennial would need only be 1 per female for the two strategies to be equal. Annual 10 (3babies) => 30 (3) => 90 Perennial 10 (2) => 20(2) + 10 (2) => = 90 The solution to the prolem rests in the trade off between age-specific survival probability and age-specific fecundity. To survive the nonreproductive period, a perennial must allocate considerable resources to storage of materials in roots and formation of freeze-resistant buds (in the temperate zone), all at the expense of reproduction. When do the advantages of being perennial outweigh the cost in lost annual fecundity? We know from our study of resource allocation that a trade-off does exist. Charnov and Schaffer (1973) produced an algebraic solution to Cole's paradox which shows that the annual life history is theoretically favored if the number of offspring produced by the annual exceeds the fecundity of the perennial by ratio of survivorship of offspring during the first year vs. subsequent years S >1 / S 1. III. Trade-off function: Selection on growth or fecundity Above - Relationship between age at sexual maturity and annual survivial rate for various groups of birds.

5 Optimization of the tradeoff between growth and reproduction. Hirshfield and Tinkle 1975 showed this tradeoff for lizards. Study of eastern fence lizard (Sceloporus undulatus) in three locatities -> Texas, S. Carolina, and Ohio where adult mortality was high, intermediate, and low. Feature Texas South Carolina Ohio growth rapid intermed slow adult mortality high intermed low ave. size female ave. clutch size broods per year total offspring produced per year egg mass per egg Proportion of births Texas South Carolina Ohio by age of female Fecundity is a function of (1) body size in Lizards, which is related to (2) age-specific mortality of adults. Thus to allocate energy and resources to eggs rather than to growth is to reduce next years fecundity. If the probability of surviving from one breeding season to the next were small, little would be lost by sacrificing growth to increased fecundity in terms of fitness. POINT: Here is the first evidence that selection has acted on growth and fecundity directly as determined by the opportunity for reproduction early in life relative to that later in life. In other words, if there is a low risk to the parent early in life, then it pays to accumulate carbon and have more babies later in life. This is expecially true if carbon acquired in one season can be "stored" (i.e., larger body size) for reproduction later in life. IV. Correlation between Life History Patterns and growth rate of population. "r and K selection" - Based on the intrinsic rate of population increase (r) and the carrying capacity (K). Pianka: Trade-off between adaptations favored under conditions of high population growth rate and those faovred under conditions of crowding and low resources. r-selected traits: rapid development early reproduction small body size

6 semelparity K-selected traits slow development reduced resource requirements delayed reproduction large body size iteroparity Many researchers have suggested that these are merely a list of traits that tend to be associated, and not an explanation for how those traits may have evolved. Population Ecology Above - Swan Lake National Waterfowl Refuge in North Central Missouri. Thousands of Blue / Snow Geese, Canada Geese, and Bald Eagles overwinter at this location. Because the resource base is maintained artificially high (eg. corn is planted specifically to feed these birds), most do not move further south to traditional wintering grounds along the Gulf Coast. The result is called "shortstopping" which greatly irritates Louisiana hunters. Studies are ongoing to understand the age and sex structure of these populations. I. Introduction Natural selection acts on the individual, but it is the population that is the vehicle for a species to survive. Below some minimum number, extinction is assured. Note the examples of California Condor and the Florida Panther. Each require a minimum critical breeding population to survive. Population biology is the study of births, deaths, and the dynamic forces which regulate the number of individuals in a population. II. Population Structures A. Patterns in space random clumped regular B. Age and Sex Structure

7 type I - typical of humans - highest probability of death late in life type II - birds - equal probability throughout life type III - oysters - highest probability early in life Note the comparison of developing countries and developed countries in your text book. III. Elementary Population Growth A. Geometric and Exponential growth. dn/dt = rn, where r = births-deaths at a time when the population is small and growing rapidly, or N t = N (t - 1) l or N t = N (0) l t These equations are a geometric interval solution (which is best for populations with non-overlapping generations and discrete reproductive periods),.

8 Populations are: decreasing when l is between 0 and 1. constant when l is = 1. Increasing when l is > 1. dn/dt = rn is a continuous exponential function (most populations), which gives the solution: The solution for this is: log e N (t) = log e N (0) + log e e rt but log e e rt = rt, because the natural log of an exponential number with base e is the exponent itself, thus, log e N (t) = log e N (0) + rt We can now solve for r. The yellow line corresponds to r>0, green is r=0, and white is r<0.

9 B. Population DATA: Survivorship tables and life tables Life Table Components: Age (x) ; lx = probability of surviving to each age, x; bx = number of female offspring born to mother of age x. calculated parameters are: lx bx ; x lx bx Example. Consider a sheep population which is censused once a year immediately after breeding season. An abbreviated survivorship and fecundity table might look like this. Note that lx is 1.0 and bx is 0 by definition at the instant of birth (x=0). Age, years (x) Probability of surviving to age x (lx) No. of female offspring born to a mother of age x (bx) Only females are considered in this life-table. However, there is no problem to include male populations in the life table. Then, survival rates should be specified separately for males and females, and the sex ratio of offspring should be taken into account. C. Population Parameters 1. Net Reproductive Rate: the expected number of female offspring to which a newborn female will give birth in her liftime. R o = S lx bx - i.e., the average reproductive value of a female 2. Mean Generation Time (T) = (S x lx bx ) / R o i.e., the average age of reproduction of a female. 3. Derivation of r from Euler's Equation (not shown) leads to T = (log e R o ) / r -> Thus, we can now solve for r from life table data if we assume constant growth rate under conditions of a Stable Age Distribution, i.e., the proportion of individuals in each age class remains constant. Note that this is an approximation for r. D. Example Life Table Calculation: Life Table Raw Data - an example Age No. dying each x No. surv. at x lx

10 Life Table Probabilities - a different example (not derived from the above table) Age X lx bx lx bx x lx bx Ro = S lx bx = 3.1 This is the net reproductive value for a female TWA = Total Weighted Age (a purely theoretical construct) = S x lx bx = 6.9 T =TWA / Ro = 2.23 = Mean Generation Time T = (log e R o ) / r Thus, r =? IV. Population Regulation A. The Equilibrium Model -> - K is when birth rates and death rates are balanced. B. Carrying Capacity of The Environment: The Logistic Model C. Mathematical Description of Population Regulation: dn/dt = r N o, but for Logistic growth -> dn/dt = r((k-nt)/k)n o There are three possible outcomes: where the population will approach a carrying capacity, K, where the bottom axis is time, and the vertical axis is population size. D. Reality - Fluctuations in Natural Populations

11 1. Density Dependence -> influence of these factors changes as the density changes. Many of these are frequently biological in origen. Examples include: a. starvation b. disease c. social facilitation in breeding 2. Density Independence -> Independent of density Largely abiotic factors -> weather (killing frost), etc. V. Examples A. Spruce Sawfly: see your text. B. Population Cycles 1. Mammal populations of the Nearctic. a. Lynx - 10 yr cycle - pelts of trappers to the Hudson Bay company. 2. Causes: b. snowshoe hare - cycles just ahead of lynx a. predator-prey relationship -> cycles much too regularly to be caused by simple variation in the environment and not necessarily linked to each other. b. quality of the food supply: Lemmings - 5 year cycle seems to be tied to food quality variation. VI. Time Lags A. Density Dependent regulation is the only way that the logistic model can be obtained Accordingly, time lags in generation time and population growth rates can cause populations to oscillate around K. Thus density dependent regulation is rarely instantaneous. You are responsible for the example in your text on pages regarding oscillations in Sheep Blowfly adults and larvae.

12 VII. Social Factors A. Territoriality and Dominance Frequently tied to resource availability. Examples come from Polyandry, Polygynous, Monogamous organisms B. Social Pathology General Adaptive Syndrome: term from psychology to describe curtailment of growth and reproduction under conditions of crowding. VIII. Metapopulations and Microsite Adaptations. The classic example of this is Clemantis freemontii given on page 304 of your text (note that Ricklefs does not use this in the metapopulation context, but in fact its is an example of one.)

13 Online Population Ecology Reference Database.