KW

LECTURE NOTES - Week 4

Population Distribution and Abundance

Populations

  • A group of individuals of the same species that live in the same area at the same time and interact with one another


Populations: Some Terminology

  • Abundance - The number of individuals of a species or population (population size)

  • Distribution:

    • The geographic area where the individuals of the species are present (i.e., abundance of species > 0)


So What Is An Individual

  • Population size estimates rely on measuring the number of individuals

  • Clone - a genetically identical copy of an individual


Defining the Individual

  • Genet - the product of a single fertilization event (a single genetic individual)

  • Genets can be physiologically independent, compete for resources, etc.

  • So functionally are individuals

  • Ramet - actually or potentially independent members of a genet


Methods For Estimating Population Size

  • Absolute population size - the actual number of individuals

  • Relative population size - the number of individuals in one time interval or place relative to the number in another

  • Uses a measure that correlates with the absolute population size

    • E.g., # tracks in an area, fish catch per unit effort, bird songs or sightings along a fixed distanced


Area-Based Counts

  • Used to estimate population abundance of immobile (or limited mobility) organisms

  • Quadrat or sample plot-based counts

  • Size of quadrat is determined based on the size of the organism in question

  • Known area of plot sizes allows an estimation of the density within a larger area

  • The more quadrats the more accurate your estimate


Pin Frame Methods


Density Estimation From Area-Based Counts and Estimate Precision


Distance Methods

  • Distances of individuals from random positions along a transect are measured 

  • Can be used to estimate relative or absolute organisms

  • Visibility issues


Example of A Detection Function

  • Density Estimation: D = n/2woL

    • D = estimated density

    • n = number of objects observed

    • wo = distance from the line where detectability is assumed perfect

    • L = length of transect

  • On a line transect the length L = 100 meters, a total of y = 18 birds were detected

    • D = 12/(2(20)(100)) = 0.003 birds/m2 or 30 birds/ha


Example 2: Ordered Distance Method Procedure (Morsita 1957)

  • Use the area of the circle to estimate density at each point then average these to get site level density


(Capture) Mark-Recapture Studies

  • R/M = C/N

  • N = (M*C)/R

    • R = number of individuals Recaptured (those with a mark)

    • M = number of individuals captured and Marked

    • C = total number Captured the second time (with and without a mark)

    • N = estimated Number of individuals in the population


Example: Snail Population Estimation

  • Let's say that your neighborhood has a population of snails, which come out onto the sidewalk when the sprinklers come on, and the rest of the time they hide in the vegetation. If you catch 20, and mark them and then, a week later, after they have had a chance to disperse into the population, you catch 15, and 6 have marks on them, what is the estimated size of the population?

    • N = (M*C) / R

    • N = (20 * 15) / 6

    • N = 50

Capture-Mark-Recapture Assumptions

  • If no marked individuals are recaptured, R = 0 and your result is undefined. Mark more individuals and try again

  • The marking technique must not harm the individual or affect its survival by predation

  • The mark must not wash off or wear away

  • There must be no immigration into or emigration out of the population (i.e., closed population)

  • There must be no morality between the mark and recapture times

  • The marking experience must not make an individual more or less likely to be recaptured


The Culprit: Sea Urchins

  • Tested in two ways:

    • 1) Observational study:

      • Examined relationships between urchin abundance and kelp forest condition

    • 2) Experimental study:

      • Urchin removal and monitoring of kelp recovery

  • But why are there differences in urchin densities


Movement Between Populations

  • Dispersal - the movement of individuals into (immigration) or out of (emigration) an existing population)

  • Metapopulation - a group of geographically isolated populations linked together by dispersal


Bornean Tree Distributions


Geographic Range Size

  • The entire geographic region over which that species is found

  • Includes the areas it occupies during all life stages


Types of Distributions

  • Endemic:

    • A taxa that is native to a certain limited area; special taxa because they are found only in one location and nowhere else on the planet

  • Cosmopolitan:

    • Range extends across all or most of the world in appropriate habitats



Patchy Distributions

  • Not all habitat is suitable throughout a region

  • Naturally patchy or human modified landscapes can both cause this

  • Sometimes individuals can move between patches, but this depends on what is in between



What To Do If You Don’t Know The True Distribution?

  • Characterize how biotic and abiotic factors impact the occurrence of abundance of a species

  • Then we can model this

  • Species distribution models (SDMs) - a tool that predicts a species’ geographic distribution based on environmental conditions at locations the species is known to occupy


Habitat Suitability Determines Distribution and Abundance


Abiotic Factors


Biotic Factors

  • Food resources

  • Vegetation structure and composition

  • Herbivory/predation

  • Competition


Joint Effects of Biotic and Abiotic Factors


Disturbance

  • An abiotic event that kills or damages some individuals and thereby creates opportunities for other individuals to grow and reproduce


Evolutionary and Geologic History




Wallace’s Line


Dispersal and Migration

  • Dispersal:

    • The movement of organisms away from their point of origin

  • Migration:

    • The movement of individuals, and commonly of whole populations from one region to another


The Evolutionary Role of Dispersal

  • Dispersal - from the point of view of the biology of individuals:

    • Dispersal leads to “discovery” of new habitats

    • Dispersal may be necessary to complete life cycle (true for many parasites)

    • Factors to consider:

      • Habitat similarity and quality - and adaptations of the organisms

      • Dispersal as an ESS (evolutionary context)

  • Spreading away from a “birth” place - pros and cons

    • Pros:

      • Reduces competition with parent

      • Reduces competition with siblings

      • There is a chance of finding a similar (and possibly better) environment

    • Cons:

      • New environment could be worse - role of distance

  • Natural selection tends to favour individuals that move their “birth” place


Metapopulations

  • “A population of populations”

  • A set of spatially isolated populations linked to one another by dispersal

  • Sources: 

    • The number of individuals that disperse is larger than the number of individuals that arrive

  • Sinks: 

    • Populations that receive more immigrants than the number of emigrants that they produce

A Blinking On and Off of Populations

  • Although individual populations may be prone to extinction, the collection of populations (the metapopulation) persists because of a balance between extinction  and colonization


Levin’s Representation of Metapopulation Dynamics

  • Levin’s model:

      • t = time

      • p = prop. patches occupied at time t

      • c = colonization rate

      • e = extinction rate

  • Assumptions:

    • There is a very large number of identical habitat patches 

    • All patches have an equal chance of receiving colonists (spatial arrangement doesn’t matter)

    • All patches have an equal chance of extinction

  • For a metapopulation to exist for a long time, e/c < 1


Metapopulations, Dispersal, and Habitat Fragmentation

  • Fragmentation = greater barriers to dispersal

  • This can reduce colonization rates

  • Extinction may also increase as fragments become smaller and more isolated

  • With sufficient fragmentation extinction will exceed colonization leading to metapopulation extinction despite suitable habitat remaining


Isolation by Distance

  • Chance of extinction highest in small patches and those far from the nearest occupied patch

Rescue Effect and Source - Sink

  • In reality not all patches are equally good and not all populations have equal chances of extinction

  • High rates of immigration can help to prevent extinction where a patch is small or lower quality


A Rapidly Changing Distribution


From A “Forest” To A “Desert”


Climate Warming Induced Loss of a Key Predator

  • Warming waters making this predator more susceptible to disease


Reduction in Otters A Critical Factor









And Source-Sink Dynamics Play A Role

  • Slower recovery of kelp with reintroduction of otters on the Aleutian Islands

  • Ocean currents and dispersal