Lecture 10 Populations

‘members of a particular species that inhabit a particular area’ What is a population? ‘members of a particular species that inhabit a particular area’ Various aspects: Range and distribution Size Density Age structure Growth Genetic uniqueness  subpopulations (ecotype)

Population growth rate: Discrete-time Geometric growth Species which have discrete breeding seasons Continuous time Exponential growth

Exponential Population growth logeλ = r Geometric Growth: N(t+1) = N(t) λ : at each interval of time population grows by the multiple λ Exponential Population growth logeλ = r Change over time Intrinsic rate of increase Growth rate = dN/dt = rN No. of individuals in a population The actual rate of population increase is Birthrate Deathrate Net immigration r = (b – d) + (i – e) Net emigration

Geometric Growth – with discrete reproductive seasons Estimate population at same time in each year Mortality of young

Slope (at any point) = dN/dt = rN Log population size increasing exponential against time produces straight line Slope (at any point) = dN/dt = rN

Logistic Population Growth

Carrying Capacity No matter how fast populations grow, they eventually reach a limit This is imposed by shortages of important environmental factors Nutrients, water, space, light The carrying capacity is the maximum number of individuals that an area can support It is symbolized by k

As resources are depleted, population growth rate slows and eventually stops: logistic population growth. Sigmoid (S-shaped) population growth curve.

Growth slows as N approaches value of K or as (1-N/K) approaches 0 dN/dt = rN K – N K ( ) = rN(1-N/K) Growth slows as N approaches value of K or as (1-N/K) approaches 0

Limits to Population Growth Environment limits population growth by altering birth and death rates. Density-dependent factors Disease, Resource competition Density-independent factors Natural disasters

Reproductive success decreases as population size increases Density-dependent effects Competition for resources food Suitable habitat – example: nesting sites Effects that are dependent on population size and act to regulate growth Reproductive success decreases as population size increases Song sparrow These effects have an increasing effect as population size increases

Density-independent effects Effects that are independent of population size but still regulate growth Most are aspects of the external environment Weather Droughts, storms, floods Physical disruptions Fire, road construction

Size of Populations Abundance: number of individuals within a specified area Abundance/area = Density How do we determine how many individuals there are?

Estimation of population sizes Choice of technique depends on motility of target species Nature of habitat Resources Resolution required Generally rely on statistical sampling /various assumptions Examples of techniques Capture-mark-recapture Christmas Bird Counts Aerial surveys Acoustic monitoring http://www.pwrc.usgs.gov/monmanual/approaches/popsize.htm

Survivorship and Age structure Age structure: Proportion of individuals in various age classes Survivorship is the percentage of an original population that survives to a given age Involves study development of life table Cohort Example: Cactus finch Static Example: Dall sheep

Age Structure Diagrams: Visualization of future population growth

Where is a species found? Range: Geographical boundaries a species occupies Determined by basic ecological parameters No indication of distribution or abundance Fundamental niche: Indication of parts of habitat in which a species may be found Typically patchy locally aggregated) w/i range Realized niche: Portion of fundamental niche in which species is actually found Factors which impact range: Physiological adaptations Available food, nesting sites, etc. – factors which define suitable habitat Predators Competition – competitive exclusion principle – to be discussed later Chance – past climatic and physiological events Species could/does survive elsewhere, has not been introduced Current and past climate influences all these things

Factors which impact range: Physiological adaptations Available food, nesting sites, etc. – factors which define suitable habitat Predators Competition – competitive exclusion principle – to be discussed later Chance – past climatic and physiological events Species could/does survive elsewhere, has not been introduced Current and past climate influences all these things

Example: Range of Canyon Wren Distribution: ‘confined to areas with rock faces’, canyons, bluffs

Fundamental niche: Realized niche: Indication of parts of habitat in which a species may be found Typically patchy locally aggregated) w/i range Realized niche: Portion of fundamental niche in which species is actually found Factors which impact range: Physiological adaptations Available food, nesting sites, etc. – factors which define suitable habitat Predators Competition – competitive exclusion principle – to be discussed later Chance – past climatic and physiological events Species could/does survive elsewhere, has not been introduced Current and past climate influences all these things

Patchiness and Subpopulations Metapopulations – Local Populations (demes) in suitable habitat isolated in matrix of unsuitable habitat Source/Sink Populations – source population over-reproduces, sink absorbs population Landscape – Metapopulations linked in matrix of varied quality

Marmots on Vancouver Island Unique species – isolated populations in cleared areas – impacted by fire/forestry practices Loss of local populations results in fewer ‘stepping stones’ – genetically isolated metapopulations Loss in genetic diversity Movement between populations maintains variability within species Important to continued viability of species From: http://www.marmots.org/notes_vim.html

Sub-populations adapted to particular local environments Ecotypes Sub-populations adapted to particular local environments Unique genetic make-up? Same species Common Garden Experiment Seed collected from plants of same species growing in different environments grow in same location(s) (p 282) Isolation may lead to differentiation into different species – uniquely adapted to specific environments –( see p 200)  restricted range