Population Structure, growth and dynamics

A population is made up of the individuals of a species within a particular area.

• Population structure relates to several characteristics:

  • a)  density and spacing of individuals within suitable habitat

  • b)  proportions of individuals in various age classes

  • c)  genetic structure

 Populations exhibit dynamic behavior, changing through time because of births, deaths, and movements of individuals (change is the norm).

• The distribution of a population is its geographic range. Within this range, areas of suitable and unsuitable habitat may occur.

• Many populations are broken into a large number of isolatedsubpopulations living in patches of suitable habitat.

• Dispersion describes spacing of individuals with respect to one another in a patch (spatial population structure).

Each subpopulation lives in a “Patch” of suitable habitat.

• Patchy distributions of plants are often determined by availability of suitable habitats.

Dispersion describes spacing of individuals with respect to one another in a patch. (spatial population structure)

Three primary patterns are observed:

• clumped(individuals in discrete groups)

• random(individuals distributed independently of others within a homogeneous area)

• evenlyspaced(each individual maintains similar distances from other individuals)

 ​​Dispersion: Evenly spaced

PLANTS

• Often caused by density dependent mortality(maintenance of minimum distance for survival).

• Often seen in deserts with limited water and nutrients in soils.

ANIMALS

• Often caused by antagonistic behavior. (Competition for space, water, food, etc)

• Here in breeding colonies, also among animals maintaining territories for mating or foraging for food

Dispersion: Clumped
May arise from one or more of following:

1) clumped distribution of resources
2) social predisposition to form groups
3) tendency of progeny to remain near parents

Dispersion: Random

• Individuals distributed independently of others within a homogeneous area.

• Random rarely observed and difficult to prove.

• Deviation from random distribution is used as the basis to select between more clumped or more evenly spaced.

Populations are broken into a large number of isolated subpopulations living in patches of suitable habitat.

 Degree to which members of subpopulations are isolated from one another depends on several factors.

1) nature of intervening environment

 2) distances between subpopulations 

3) mobility of the species

limited mobility can result in extreme isolation

Three common models define relationship between subpopulations in space.

Metapopulation model: Key characteristics:

• Views a population as set of subpopulations occupying patches of a particular habitat.

• Densities are not considered in the simplest models (e.g., Levin’s model), only whether patches are occupied or not.

• As long as one patch is occupied, the population persists.

• Intervening habitat is referred to as the habitat matrix, and is also generally not considered.

Basics of the model:

• Extinctions and recolonizations of individual patches occur continuously through time, giving impression of “blinking lights.”

• As long as one patch is occupied, the population persists.

• If percentage of patches occupied drops to zero, the metapopulation collapses (population goes extinct.

Source-Sink model

• Unlike metapopulation model, recognizes species densities and differences in quality of habitat patches explicitly.

Source patch

• Resources abundant.

• Birth rate higher than death rate.

• Surplus offspring disperse to sink patches.

Sink patch

• Resources are scarce.

• Death rate higher than birth rate.

• Population size is maintained by net immigration of individuals from source patches.

landscape model

• Unlike metapopulation and source- sink models, the landscape model also considers effects of differences in quality of habitat matrix.

• Quality of habitat patch is affected by nature of surrounding matrix.

• Some matrix habitats more easily traversed than others:

  • Resources (food)

  • Abiotic stresses

  • Predators and competitors

How do we measure density?

• A total count may be feasible

  • –  suitable for small populations where individuals
    can be distinctively marked

  • –  often employed for endangered species, particularly for larger animals such as mammals and birds.

• Density may also be estimated using plots
  – Determine average density in randomly selected
  plots, then extrapolate to entire area occupied.
  – Common for sessile organisms in large numbers over large area.

Mark-Recapture Method is often used for mobile organisms with large population size.

• 1)  First sample collected and all individuals are distinctively marked.

• 2)  Marked animals then are released into the population and allowed to mix.

• 3)  Second sample collected and the number of marked and unmarked animals is tallied.

Population size tends to vary over space and time, sometimes to a great degree.

Long-term records often reveal fluctuations that might be overlooked in shorter term.

Movement of individuals of a population across space and time is the rule rather than the exception.

• Dispersal is the movement of one or more individuals from one population to another (joining another population or forming a new one).

• Migration is in some uses equivalent to dispersal, though migration more typically refers to a particular directional and synchronized type of movement of many individuals.

One of the most important concepts in demography is that populations grow by multiplication.

A population increases in proportion to its size just like a savings account earns interest on principal:

With a constant 10% annual rate of increase:

• A population of 100 will add 10 individuals per year.

• A population of 1000 will add 100 individuals per year.

• A population of 10 billion add 1 billion individuals per year.
  Allowed to grow unchecked, a population growing at a constant rate would rapidly climb toward infinity.

Comparing exponential and geometric growth

Geometric growth occurs when a population with synchronous reproduction changes in size by a constant proportion from one discrete time period to the next.

Exponential growth occurs when a population with continuous reproduction changes in size by a constant proportion at each instant in time.

• Exponential and geometric growth patterns have the potential to overlap because the equations are closely related to each other.

Limits to exponential and geometric population growth

• Populations experience geometric and exponential growth when environmental resources are unlimiting (superabundant).

• Because all resources eventually becoming limiting, exponential and geometric growth never continue indefinitely.

Declining growth rates occur because population size is regulated by density-dependent factors

• Negative density dependent factors:
• Become worse with increasing density (crowding)
• Increase death rates (d) and/or reduce birth rates (b).

• Examples:
• Food supply shortages
• Competition
• Predation and spread of disease

• Density dependent factors slow and eventually halt population growth (result of extreme crowding).

As density increases, reproduction rates fall.

As density increases, death rates rise

Together, density dependent reproduction and death rates regulate population size.

Trophic interactions

• Most forms of life are both consumers and the consumed.

• Trophic interactions organize biological communities into Food chains.

• Producers are eaten by primary (1st order) consumers, primary consumers by secondary (2nd order) consumers, and so on.

Food chains show only one path of energy flow through more complex food webs.

Food Chain : Producer , Primary Consumer, Secondary Consumer, Tertiary Consumer, Quaternary Consumer 

Several ecological factors are hypothesized to control the length of food chains.

1) Disturbance (populations higher in food chain are more vulnerable to disturbance).

2) Primaryproduction(Ecosystems with higher primary production are believed to support longer food chains).

3) Ecosystem size (larger ecosystems have longer food chains).

Lindeman’s Biomass Pyramid

Only a small amount (~10%) of energy consumed becomes biomass in successively higher levels of food chain.

Remaining 90% of energy consumed is lost to waste or used to support metabolic activity.

Takeaway: The greater the primary production of an ecosystem (i.e., the larger the biomass of producers), the more energy that is available to support successively higher trophic levels in the food chain.

Changes in the structure of food chains can cause trophic cascades. Population densities

are controlled:

...from above by predation pressure:

increasing 2nd order consumer decreases 1st order consumer increases first order producer

...from below by resource availability:

Increases 2nd order consumer increases 1st order consumer Increasing first order producer

Predator Hunting Strategies

Cursorial predators

• Predators that actively move and forage throughout their habitat in search of prey.

• These include animals like sharks, wolves, and hawks.
  Sit-and-wait predators

• Predators that remain in one place and attack prey that move within striking distance.

• These include animals like web-building Crocodiles, spiders, snakes, and some birds.

Many adaptations exist to help predators

locate and capture prey.

• Size

• Speed (rapid capture)

• Superior vision, hearing, and smell

• Teeth, talons, and mandibles

• Camouflage/ambush behavior

• Poison to paralyze or kill prey

Pack (cooperative) hunting

Predators are generally larger than their prey.

• Beyond a certain prey size, a predator cannot successfully subdue and consume the prey.

• Injuries from prey fighting back can be fatal.

• Cooperative hunters are exception.

Many adaptations exist to help species avoid becoming prey.

• Size (the larger the prey, the more difficult it is to kill)

• Speed (rapid flight)

• Superior vision, hearing, and smell

• Physical defenses (shells and spines).

• Fighting back (sometimes accompanied by warning sounds)

• Camouflage (crypsis)

• Chemical defense (Bad taste/smell, poison, and weapon)

• Herding behavior (to confuse predator and/or limit probability of capture).

• Finding refugia (places the predator can’t hunt and kill)

• Batesian mimicry (faking a defense)

Antipredator strategies:

Other defensive behaviors

• Not foraging in open areas (predator-avoidance)

• Alarm calls to warn population of danger

• Defensive aggregations to protect young

Antipredator strategies: Crypsis (camouflage)

Chemical defense: odor

Chemical defense (bad taste)

Chemical defense (as weapon)

Antipredator strategies: Warning coloration

Why aren’t more prey unpalatable? • Chemical defenses may be difficult to evolve.

• Some noxious animals rely on consuming host plants and/or animals for their supply of defensive chemicals.

– Not all food plants contain such chemicals.

– Animals utilizing such chemicals must evolve their own means to avoid toxic effects.

antipredator strategies: Batesian mimicry

Müllerian mimicry occurs among similarly unpalatable species that come to resemble one another through convergent evolution.

Plants have herbivore defenses.

• Mast seeding

• Structural defenses
  • Spines
  • Hairs
  • Tough coatings
  • Compensation
  • Secondary compounds (chemical defenses)

  • low nutritional content of plant tissues

  • toxic compounds synthesized by the plants make unpalatable or toxic.

Phytochemicals and plant secondary metabolites have many human uses.

Predators can drive prey populations to extinction.

Herbivores can have dramatic effects on plant species that they consume.