Birth, Death, and Movement: Exploring Population Dynamics

1. Quiz over. Welcome back

2. CHAPTER 5 - BIRTH, DEATH, AND MOVEMENT Chapter 5 (selections)

3. From the start… • Populations • Impacted by birth, death, and movement into and out of that population • What causes these changes? • An individual • Unitary organisms – whole form is predictable and determinate • Modular organisms – grow by repeated production of modules and almost always form a branching structure; structure and development are indeterminate • Genet: individual that starts life as a single-celled zygote • Module: starts life as a multicellular outgrowth from another module • Examples: coral reefs, peat-forming mosses…

4. Counting … • How to count individuals (even unitary ones)? • Estimate rather than count • Representative samples • Mark-recapture • The Lincoln–Petersen method-- can be used to estimate population size if only two visits are made to the study area. This method assumes that the study population is "closed" and assumes that no marks fall off animals between visits to the field site by the researcher, and that the researcher correctly records all marks. • Given those conditions, estimated population size is: • N = Estimate of total population size • M = Total number of animals captured and marked on the first visit • C = Total number of animals captured on the second visit • R = Number of animals captured on the first visit that were then recaptured on the second visit

5. Life cycles • Life cycle and reproduction • Birth  pre-reproductive period  reproductive period  post-reproductive period  death due to senescence • Annuals, perennials  fit this cycle into one year or extend it over many years • For all: growth before any reproduction; growth slows down (or stops) when reproduction starts. Conflict between the two. • Iteroparous species • multiple reproductive episodes. Examples? • Semelparous species • Single terminal reproductive episode. Examples – salmon and bamboo

6. Life tables - To monitor and quantify survival • Cohort life table  records survivorship of members of cohort over time • follow the fate of individuals from the same cohort within a population (i.e. all individuals born within a particular period) • Static life table  records the numbers of survivors of different ages • Assesses whole population during a single period and notes the numbers of survivors of different ages in the population

7. Cohort life tables • Most straightforward: one for annuals. • Why? – non-overlapping generations • Typically containing for plants • Age interval (days? Months? Years?) • Number surviving to day X • Proportion of original cohort surviving to day X • Seeds produced in each stage • Seeds produced per surviving individual in each stage • Seeds produced per original individual in each stage

8. Cohort life tables • Typically containing for animals • Age interval (days? Months? Years?) • Number surviving to day X • Proportion of original cohort surviving to day X • Number of female young produced by each age class • Number of female young produced per surviving individual in each stage • Number of female young produced per original individual in each stage

9. Cohort life tables • Interested in survivorship • And fecundity • An age class contributes most to the next generation when a large proportion of individuals have survived and are highly fecund • Basic reproductive rate (R) the average number of offspring produced over the lifetime of an individual (under ideal conditions). • Can be calculated, provided an ‘death rate’ is given • [fyi: In epidemiology the basic reproductive rate of an infection is the mean number of secondary cases a typical single infected case will cause in a population with no immunity to the disease in the absence of interventions to control the infection.]

10. Static life table • Repeated breeding seasons • A population snapshot; • Difficulty: individuals of different ages living together, reproducing.. • only be treated and interpreted in the same way [as a cohort life table] if patterns of birth and survival in the population remained the same since birth of oldest – a rarity]

11. Survivorship curves

12. Dispersal and migration • Dispersal: the way individuals spread away from each other • Migration: Mass directional movement of large numbers of a species • Monarch butterflies of North America: travel 3,000 miles (almost 5000 km) • Fly en masse – often to the same trees • The monarch is the only butterfly that migrates both north and south as the birds do on a regular basis. But no single individual makes the entire round trip. Female monarchs deposit eggs for the next generation during these migrations

13. Dispersal and migration • Average density: total number of individuals dived by total size of habitat • How do we define habitat? • Crowding: real measures of crowding as experienced by individuals are likely to be more important forces driving dispersal and migration than some average value of population density

14. Dispersal determining abundance • Quite important • Example: 57% of breeding birds of great tits in the UK were immigrants, not born in population • Colorado potato beetle: average emigration rate of newly emerged adults – 97% • Factors impacting/provoking dispersal • Ability to get there • Intense competition suffered by crowded individuals • Emigration dispersal typically density-dispersal

15. Inverse density dependence… • Dispersal at lowest densities? • Why? – to avoid inbreeding • Typically juvenile males

16. Role of migration • Planktonic plants • Migrate to lower, nutrient-rich depths at night • Crabs • Migrate alone shore w/ tides • Transhumance • Moving livestock • Many others…

17. Role of migration

18. Impact of intraspecific competition • Intensity of such competition typically dependent on resource availability – and density/crowding • Over a sufficiently large density range, as density increases, competition between individuals reduces per capita birth rate and increases death rate

19. Carrying capacity (K) Population regulation: by many factors, thus broad lines

20. Life history patterns • Some simple, useful – but not perfect – patterns linking different types of life history and different types of habitat • Remember this point? -> in any life history, there is a limited total amount of energy (or some other resource) available to an organism for growth and reproduction. • Trade-off may be necessary • How? • Semelparous? • Early reproduction? • Has its trade-offs too

21. Life history patterns • r species • Potential of a species to multiply rapidly is advantageous in environments that are short-lived or highly variable • Typical of terrestrial organisms that invade disturbed land or colonize newly opened habitats, and of aquatic inhabitants of temporary puddles and ponds • Production of large numbers of progeny, early in the life cycle • Opportunistic • K-species • Live in populations that are at or near equilibrium conditions for long periods of time; usually live near env. carrying capacity • Competitive for limited resources • Long maturation rate, breeding relatively late, long lifespan, produce relatively few offspring, large newborn offspring, low mortality rates of young, extensive parental care

22. r/K – trade-off • Trade-off between number of offspring produced in a clutch and individual fitness of those offspring • Negative relationship between size of offspring (measured by snout-vent length) and number of offspring in a litter

23. Trees in relatively K-selecting woodland habitats have • Relatively high probability of being iteroparous and relatively small reproductive allocation • Relatively large seeds • Relatively long-lived with relatively delayed reproduction

24. r/K species Both r and K are comparative terms. Most animals are somewhere along the endpoints of this continuum. Cats are r-selected compared to humans, and K-selected compared to cockroaches.

25. End of chapter 5