Population Dynamics 3/13

• In most cases marine biologists are using estimates for population abundance.

• Mark-recapture

  • Useful when populations aggregate

    • Example: breeding grounds or feeding grounds (but not in between these areas since they are very spread out)

  • Individual specific data

  • Method

    • Sample (N1) - “Captured,” marked, released

      • Ideal to use natural markings

        • Humpback whales: Use fluke markings/patterns. 

        • Bottlenose dolphins: Use dorsal fins markings

    • Sample (N2) - “Captured,” of which (M2) already marked.

      • What proportion are “marked” and “unmarked”

    • Proportion of marked vs. unmarked used to estimate population size

      • N = (N2/M2) x N1

  • Assumptions

    • Marks are unique

    • Marks cannot be lost

      • For dolphins, it’s important to go out frequently to document changes to fins over time

      • For humpback whales and dolphins, their markings increase over time, but they do not disappear

    • All marks are correctly recorded/identified

    • Marking does not affect survival or future catch-ability

      • More invasive marking techniques may change an animal’s behavior (ie. if they are freezebranded, they may avoid the area that they are freezebranded, thus changing their probability of being “captured” again)

    • Equal probability of capture within each sample

      • In SJR, we increase survey frequency in the summer to be sure that we haven’t missed animals that have moved out (more dolphins in the summer in the river)

  • Uses for mark recapture data

    • Movement patterns (home ranges, site fidelity - how often they come to a particular area)

    • Life history and population dynamics

      • Survival and reproductive success

        • Fun story: Nala is a humpback whale in Australia that does headstands in the water when she is pregnant

Population dynamics

• Population dynamics: How and why abundance is changing in a population

• Demographic parameters

  • Natality (similar to a birth rate): how many females are there and how many are reproducing each year

    • Increases population size

  • Mortality (death rate): How many animals are dying each year

  • If birth rate is > death rate, then the population will increase. If birth rate is < death rate, population will decrease

  • Immigration: New individuals coming into the population

  • Emigration: Individuals leaving the population

  • Immigration and emigration tend to cancel each other out and often have a negligible effect on the change in population size.

  • Change in population size (N) = (B-D) + (I-E) *CHECK EQUATION WITH SLIDES

• Population growth (exponential vs logistic growth)

  • Intrinsic growth rate(r): Max rate of growth when no environmental factors limit population increase. (ideal situations for an animal. Unlimited food, no predators, unlimited water, etc.)

  • See slide for graph

    • Exponential growth

      • dN = change in population size

      • dt = change in time

      • r = intrinsic growth rate

      • N = population size

    • Logistic growth

      • Starts out with exponential growth because population size is small meaning there are plentiful resources.

      • Levels off as it reaches K (carrying capacity)

      • K (carrying capacity): The maximum number of individuals that an area can support

    • *Do not memorize equations, but just be able to see the lines on a graph and understand what is happening.

• R strategist vs K strategist theory

  • R strategists have evolved characteristics that allow them to reproduce rapidly,  similarly to exponential growth

  • K strategists have evolved characteristics that allow them to maintain stable populations over time

    • *Exhibit logistic growth pattern

    • Body size: Large

    • Lifespan: Long

    • Population: Stable (at or near carrying capacity)

    • Fecundity: Low

    • Age at sexual maturity: Old

    • Rate of development of offspring: Slow

• Density dependence

  • As a population nears carrying capacity, we expect to see (in this order):

    • 1. Increase in juvenile mortality 

      • Most vulnerable population

        • Less experience

        • Smaller size

        • No longer have support of mom

        • Overall, poor competitors

    • 2. Increase in age at sexual maturation

      • Sexual maturation: The age at which animals are physically capable of reproducing

      • In order for females to be physically capable of reproducing, they need a lot of resources. If there are a lot of animals, it is more difficult to compete for food so they may be less nourished, delaying sexual maturation.

    • 3. Decrease in fecundity

      • Females may need longer gaps between births to increase energy reserves necessary for reproduction

    • 4. Increase in adult mortality

      • Adults/sicker animals will die at an earlier age than they would have otherwise, because their energy reserves are smaller due to increased competition for resources.

Life history (LH) characteristics: Growth, Maintenance, and Reproduction

• Maintenance: Takes energy to maintain bodily functions

• Growth: Takes energy to make bodies larger. 

• Reproduction: Takes energy to reproduce 

• Juveniles are splitting their energy between growth and maintenance

• Adults are splitting their energy between maintenance and reproduction

• Large body size is correlated with

  • Long life-span

  • Slow growth, and delayed sexual maturity

  • Produce few offspring and invest heavily in each

• Reproductive LH traits → Natality (births) and r: 

  • Age of sexual maturity (at what age do they start reproducing)

  • # of offspring per reproductive cycle

    • Marine mammals typically have one. The only exception to this is polar bears!

    • There have been documented twins in cetaceans, but the second twin does not survive to nursing. Probably because females could not sustain their own body condition while feeding multiple calves, since milk is so fatty.

  • Frequency of reproduction (Combination of gestation length and inter-birth interval)

    • Gestation length: Length of pregnancy

    • Inter-birth interval (IBI): Time between births

      • In most marine mammals, there is a longer IBI than gestation length

  • Reproductive lifespan: Age of sexual maturity to the endpoint of reproduction

    • In most mammals, they reproduce throughout their lives

    • Some animals undergo menopause, which ends their reproductive lifespan earlier than their overall lifespan

      • Humans

      • Resident orcas

      • Belugas and Narwhals

        • Menopause is thought to have come from a common ancestor in these species since these are closely related.

      • Short-finned pilot whales

    • Senescence: Reproductive rate slows down as animals age, but animals continue to reproduce up until death.