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Population Ecology 2. 1- Population Ecology: Life History Patterns [Cpt 13] (Reproductive Strategies)

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1- Population Ecology: Life History Patterns [Cpt 13]

  • (Reproductive Strategies)
  • Organisms reproduce either by sexual or asexual means. Sexual reproduction is the more expensive mechanism, but has the benefit of genetic recombination resulting in new genotypes.
  • There can also be other behavioral factors which positively effect fitness e.g.
    • mate choice & sexual selection (discussed later)
    • parental investment in reproduction
1 topics
1- Topics
  • Mating systems
  • Sexual selection
  • Reproductive Effort (Parental care)
  • Gender allocation
  • R-selection and k-selection
  • Plant selection

Mating systems- A mating system includes such aspects as:

  • the number of mates males and females acquire,
  • the manner in which they are acquired,
  • the nature of the pair bond, and
  • the pattern of parental care provided by each sex.
  • The structure of mating systems ranges from monogamy through many variations of polygamy.
  • Mating systems may even vary within a species, involving different strengths of pair bonds.

Monogamy is the formation of a pair bond between one male and one female.

  • It occurs mostly among those species in which cooperation by parents is needed to rear young successfully.

Polygamy means acquiring two or more mates.

  • It can involve one male and several females or one female and several males.
  • Polygyny - an individual male gains control of or access to two or more females.
  • Polyandry - an individual female gains control of or access to two or more males.
  • Promiscuity - males and females mate with one or many of the opposite sex and form no pair bonds.

Sexual selection

    • Intra-sexual selection (competition) - the outcome of intense male rivalry for female attention and may give the victor access to females.
    • Inter-sexual selection - female selection of the fittest mate among competing males, based on a specific characteristic during courtship.

Inter-Sexual selection

    • Resource-based selection – more common in monogamous species – the male controls a territory containing resources, or
    • demonstrates his resource (food) gaining ability
    • “Good genes” selection – the females choose males that will likely provide good genes offspring (and increase their fitness)
    • common in polygamous species

Honest signals – the females choose males according to displays that give an honest indication of the male’s fitness

  • Direct indicators– the females choose males according to a display that directly reflects male “fitness” e.g. roaring of deer
  • Indirect indicators (handicaps) – the females choose males due to an exaggerated feature that is costly to maintain
  • i.e. only fit males can maintain the feature
  • e.g. elongated tail length in birds of paradise

Leks – males hold small territories with no resources and display for females

  • The large number of males in the lek mean that there is lots of competition
  • But because of the large number of males in the lek – many females may be attracted to the area (hotshot model)
  • Or males may cluster in the area because females are commonly found nearby (hotspot model)
  • Or females may prefer the males to assemble in a courtship area as it’s the safest place to mate or means there’s more males to choose from (female choice)

Sperm competition – common in promiscuous species

  • Males and females mate with multiple mates
  • Males have large testes and produce copious amounts of sperm (sometimes specialized varieties of sperm)
  • The last mating male may “displace” the sperm of a previous mating male
  • Males compete by their ability to produce more sperm, and multiple matings (increasing the likelihood of fathering offspring)

Reproductive effort - the nature and amount of resource and time allocations to reproduction over a period of time.

    • When to reproduce, how often to reproduce, and how much effort to place into parental care are key questions in reproductive biology.
    • There are different costs and benefits for different strategies
      • iteroparity (organisms that reproduce often)
      • semelparity (organisms that reproduce once)
      • precocial offspring (needing little care)
      • altricial offspring (helpless offspring)

Precocial (needing little care) is common in species when juvenile mortality is high, i.e. via predation.

Although the animals may not be mature for a long time they can move at, or shortly after birth.

The immediate energetic cost for the parent may be high in producing such a developed child

Altrical the offspring is helpless – costs of rearing may be high, but spread out over a longer period


Iteroparity (reproduce often) is advantageous if juvenile mortality is high, adult mortality is low, and the population growth rate is slow.

Semelparity (reproduce once) is better if population growth rate is high, juvenile mortality is low, and adult survival and the probability of surviving to reproduce a second time are low.

e.g. salmon – one major, suicidal reproductive bout


Energy budgets put constraints on how much energy a parent can afford to put into producing and caring for offspring;

  • There is a tradeoff between energy expenditure on current offspring versus the value of potential future offspring.
    • Reproductive costs also reduce energy for growth and maintenance of the parents, thus energy allocations must be made between these different expenditures.

Gender allocation – if females and males were equally costly to produce, sex ratios would be likely 1:1

But one sex may cost more resources to rear than another – there would be selection for the cheapest sex

Also if the there is a sex ratio bias – the less frequent sex would have an advantage (more mates)


Gender allocation

Also it is suggested that in polygamous species – a good condition high status female should produce more (larger) sons

As the good condition males will attract more mates

A poor condition female should produce more daughters


Gender allocation

But in species which where females do not disperse (stay with the mother)

In poor conditions (few of resources nearby) the mother should produce males

which will disperse to other areas and not compete

In good environmental conditions mothers should produce females (especially if high ranking)

e.g. W pacific Gray whales – current male bias


Gender allocation

Percentage of male offspring born to individual red deer hinds differing in social rank. High-ranking females tend to have sons.


r- and k- selection strategies

    • r-strategists are typically short-lived where selection favors genotypes that confer:
    • high reproductive rate at low population densities,
    • early and single-stage reproduction,
    • rapid development,
    • small body size,
    • large number of offspring (but with low survival), and
    • minimal parental care.
    • They tend to inhabit unstable or unpredictable environments and are good colonizers.

k-strategists are competitive species with stable populations of long-lived individuals where selection favors genotypes that:

  • confer a slower growth rate at low populations, but
  • the ability to maintain that growth rate at high population densities.

In plants – the Grimes three-endpoint system:

  • R-strategist (ruderal) - typically weedy species that
  • occupy uncertain or disturbed habitats,
  • have a short growth form,
  • reproduce early in life,
  • posses high fecundity,
  • experience one lethal reproduction, and
  • have well-dispersed seeds.

C-strategist (competitive)

  • occupy more stable habitats
  • are long-lived, often drastically reducing the opportunity of seedling establishment,
  • high juvenile mortality, but live in a competitive and productive environment,
  • reproduce early,
  • attain maximum vegetative growth, and
  • repeatedly use an annual expenditure of energy stored prior to seed production.
  • Example: grasses in an ungrazed grassland.

S-strategist (stress-tolerant)-occupy more stable habitats, and are

  • long-lived, often drastically reducing the opportunity of seedling establishment,
  • high juvenile mortality,
  • but live in stressed environments,
  • have delayed maturity,
  • intermittent reproductive activity, and
  • long-term energy storage.
  • Example: highly disturbed sites or forest understory
2 topics
2- Topics
  • Species interactions
  • Interspecific competition
  • Competitive exclusion
  • Resource partitioning
  • The niche
    • Niche overlap
    • Niche width
    • Niche responses


Type of Interaction

Organism A

Organism B






















Two-Species Interactions


Types of Interspecific Competition

  • Exploitative - the species use the same resource, such as food and use by one reduces the availability for the other.
  • The outcome is determined by how efficiently each of the competitors uses the resource.
  • Interference - a direct interaction between competitors in which one interferes with or denies access to the resource by another.
  • In animals, interference usually involves aggressive behavior.

The Lotka-Volterra model of interspecific competition is an extension of the logistic growth model with coefficients to account for inter- and intraspecific competition.

    • The presence of species 1 decreases the carrying capacity (K) for species 2 at a certain rate.
    • And, the presence of species 2 decreases the carrying capacity for species 1.
    • This is because both species must share limited resources with the other.

Competition should select either for individuals that can:

  • dominate resources (superior competitors) or
  • that are able to avoid competition and its negative effects sufficiently.
    • The model applies well to animal populations (numbers), but not to plants (biomass).

There are four outcomes of competition between two species as predicted by the Lotka-Volterra model:

    • Species 1 inhibits further increase in species 2 while continuing to increase itself - species 2 is driven to extinction.
    • Species 2 inhibits further increase in species 1 while continuing to increase itself - species 1 is driven to extinction.

Each species when abundant inhibits the growth of other species more than it inhibits its own growth.

The outcome depends on which species is the most abundant.

The two species coexist for some time, but eventually one species wins.

  • The two species coexist, but neither can achieve a density capable of eliminating the other.

Each species inhibits its own population growth more than it inhibits the population growth of the other species.


Assumptions behind the Lotka-Volterra model:

    • The environment is homogeneous and stable, without any fluctuations.
    • Migration is unimportant.
    • The effect of competition is instantaneous.
    • Coexistence requires a stable equilibrium point.
    • Competition is the only important biological interaction.

So not really likely in nature


There are four kinds response to competition:

Coexistence - organisms using identical but limited resources coexist because of different responses to a fluctuating environment and differing life history traits.

Competitive exclusion or Gause’s principle –

complete competitors cannot coexist or two species with identical ecological requirements cannot occupy the same environment.

Two species enter …one species leaves


Species can coexist only if there is a partitioning of available resources to reduce or eliminate competition for one or more limited resources.

  • Conditions needed for competitive exclusion to work:
    • Resources must be in short supply
    • Competitors must remain genetically unchanged for a sufficiently long period of time for one species to exclude the other.
    • Immigrants from areas with different conditions cannot move into the population of the losing species.
    • Environmental conditions must remain constant.
    • Competition must continue long enough for equilibrium to be reached.
    • The absence of these may allow co-existence

allelopathy- chemical inhibition of one species by another (occurs in plants).

diffuse competition- combined effects of minimal competition by several species can be equivalent to strong competition for one resource by a single species.


Resource partitioning – if two organisms occupy the same area and exploit the same type of resource – competition results

But if there is a range of resource (e.g. seed size) organisms may restrict the resource range (e.g. seed size) that they feed on.

i.e. on species feeds on small seeds, the second species feeds on large seeds


Resource partitioning: Theoretical considerations

  • A is the only species occupying area
  • Species B invades the area and partially competes
  • with A.



(c) both A and B narrow their range of resource use.


An important concept in interspecies competition is an organism’s niche

The niche = an organism’s place and function in the environment.

Hutchinson visualized a niche in terms of a multi-dimensional space or hypervolume

e.g. the range of food item size might be one dimension

a temperature range might be another

a height at which the animal shelters in a tree might be another


Fundamental niche - the full range of environmental conditions, biological and physical, under which an organism can exist (absent competition).

  • Realized niche - under the influence of pressure from superior competitors -
  • that portion of the fundamental niche to which the individual is most highly adapted;
  • the actual portion it occupies.

Two niches could theoretically be adjacent, and there would be no competition between the species

  • Niche overlap – when two niches overlap there will be intense competition
  • There will be exclusive competition – the superior competitor will eliminate its competition in the overlapping zone
  • - but the two species will coexist side by side

If the niche of one species (A) lies completely within a larger niche of another species (B)

  • – if A is the superior competitor, it will exclude B from its niche area – and the species will co-exist
  • But if B is the superior competitor, species A will go extinct

But niches are very complex, and multi dimensional

  • – while some factors may overlap (e.g. territory use, food source)
  • Other factors may not (e.g. temperature when active)
  • So actual niche overlap may be reduced when introducing other factors
  • - Resource partitioning also helps to avoid niche overlap when two species occur in the same area

Niche width- the sum total of thedifferent resources exploited by an organism. Measurements of a niche usually involve the measure of some ecological variable such as food size or habitat space. Niche widths are usually described as narrow or broad.

Hypothetical distribution of a species with a broad niche (A) and a species with a narrow niche (B) on a response gradient. The niches overlap (shaded area). Species A overlaps a greater proportion of species B than B overlaps A.


The wider the niche, the more generalized the species is considered to be.

    • Most species have broad niches and sacrifice efficiency in the use of a narrow range of resources for the ability to use a wide range of resources.
    • As competitors, wide niche species are superior to specialists if resources are somewhat undependable (and may be reduced).
    • But - generalist species are subject to invasion and close packing with other species during periods of resource abundance.

The narrower the niche, the more specialized the species is.

    • Specialists are equipped to exploit a specific set of resources.
    • As competitors, they are superior to generalists if resources are dependable and renewable.
    • A dependable resource is closely partitioned among specialists with low interspecific overlap.
    • NB – environmental changes can severely impact narrow niche species, as their niche may vanish

If an area becomes invaded by competitors your may get niche responses:

  • Niche compression - competition that results in the contraction of habitat rather than a change in type of food or resources utilized.
  • e.g. species confine their activities to patches of habitat where they get maximum resources
  • Ecological release - niche expansion in response to reduced interspecific competition
  • (a species using habitat or a food resource it did not previously utilize)
  • e.g. a species invades and island an occupies habitat areas which it did not occupy on the mainland

Niche responses

Niche shift - the adoption of changed behavioral and feeding patterns by two or more competing populations to reduce interspecific competition.

Character displacement - the gradual separation of two species in morphology or physiology as an outcome of competition for a resource.


Apparent character

  • displacement in
  • beak size in
  • populations of the
  • Galapagos finches
  • Geospiza fortis,
  • the medium
  • ground finch, and
  • G. fuligenosa, the
  • small ground
  • finch.
  • G. fortis occurs alone
  • G. fuliginosa occurs alone
  • Both occur together




3 topics
3- Topics
  • Predator prey models (brief overview)
  • Predator response to increasing prey:
    • Functional
    • Numerical
  • Optimal foraging
    • Optimal diet
    • Foraging efficiency
  • Risk-sensitive foraging

Predator-Prey Models

There are four models of predator-prey relations that predict changes in predator and prey population size in response to their interaction:

1. The Lotka-Volterra model predicts regular oscillations of predator and prey population densities, but is based on overly simplistic assumptions.


Densities per orange area of the prey, Eotetranychus sexmaculatus, and the predator, Typhlodromus occidentalis

Predators cannot survive when the prey population is low for a prolonged period relative to the longevity of the predator.

But a self-sustaining predator-prey relationship cannot be maintained without immigration of prey.


2. The Nicholson-Bailey model was developed for parasitism and also predicts oscillating populations of predator and prey, but the assumptions are quite different

(prey uniformly distributed versus randomly distributed, discrete time units versus continuous, delayed conversion of energy into offspring versus immediate conversion).


The Rosenzweig-MacArthur model allows nonlinear growth responses of prey and predator populations. This model allows stable population cycles, unstable cycles, or damped changes in density.

  • (see pages 268 for details)
  • 4. Two versions of a ratio-dependent model.
  • In the first, prey consumption is related to prey density;
  • the second relies on the ratio of prey to predator.
  • This model predicts that both predator and prey equilibrium points will vary with prey productivity.

The central component of each of these models is that predator and prey density patterns are potentially cyclic in nature.

    • Cyclic density patterns arise only when the habitat is heterogeneous,
    • there is prey immigration, and
    • the prey can disperse.
    • Also prey populations must be able to recover from predation or face extinction and predators cannot withstand prolonged periods of low prey density.

Predators respond to increasing prey density in two ways: functional or numerical

  • A functional response in which each predator either consumes more prey or consumes them earlier.
    • Type I model - assumes that consumption rate increases linearly with increasing prey density until predator satiation is reached.
    • Type II model - assumes that the number of prey taken per unit time decreases to a plateau (a maximum number that can be taken (curvilinear) due to a response to “handling time” – i.e. there are only so many moose a wolf pack can deal with

Type I linear functional response of a pair of European kestrels to Micotus vole densities during the breeding season. The curve did not reach a horizontal level, presumably because the vole population was not high enough for the predator to reach saturation level.


Type II functional responses.

(c) The response of wolves to changing moose density.

(d) Mean daily rates of aspen cutting by beavers in relation to density in three experimental treatments.


Type III model - generally applies when more than one prey species is available and assumes a complex response (sigmoid) involving learning behavior, switching between prey items, and the establishment of a search image by a predator.


Type III Response- the number of prey taken is low at first and then increases in a sigmoid fashion, approaching an asymptote. It has been associated with predators that can learn to concentrate on a prey when it becomes more abundant.Switching and search image are important aspects of the response. It invariably involves two or more prey species.

(e) Encarsia formosa parasitizing Trialeurodes vaporariorum.

(f) Shrew feeding on sawfly larvae.

(g) Deer mice preying on sawfly larvae.


Switching – turning of a predator to an alternative, more abundant prey species.

Search image - a perceptual change in the ability of a predator to detect a familiar cryptic prey.

(i.e. cats and “string”)


A numerical response in which the density of predators will change in response to prey density

  • (predators increase as a result of increased food supply)
  • and can be short-term in response to localized prey concentrations (aggregative response)
  • or long-term in response to a change in predator emigration/immigration patterns
  • or from an increase or decrease in rates of predator mortality or natality.

Aggregative response

At the lower plateau of prey density, predators do not distinguish among low (unprofitable) prey areas – don’t spend much time in them;

at intermediate densities, predators discriminate markedly - spending increasingly more time in high density areas;

at the upper plateau, predators do not discriminate among high-density (profitable) areas – lots of prey everywhere, so no need to concentrate on a specific area


An example of a numerical response

Aggregative response in the redshank. The curve plots the density of the predator against the average density of arthropod prey.


Basic forms of numerical response – some definitions

No response means that the number of predators remains the same in the face of increasing prey density.

A direct response implies that predators increase in response to an increasing prey density.

Inverse response means that the number of predators per unit area decreases as prey density increases.


Types of long-term numerical response (recap):

    • Immigration or emigration of predators in response to changes in prey density.
    • Increase or decrease in the rate of predator natality and mortality in response to changing prey densities.

Optimal foraging (optimal diet and foraging efficiency) relate to rules of “decision making” that one would expect predators to follow with regard to prey choice and foraging efforts (i.e. costs vs benefits)

  • But predators are constrained by and respond to other aspects of their environment to the extent that strict adherence to these rules is unlikely in many cases.

Optimal diet decision rules:

    • 1. prefer the most profitable prey (items that yield the greatest net energy gain);
    • 2. feed more selectively when profitable prey or food items are abundant;
    • 3. include less profitable items in the diet when the most profitable foods are scarce; and
    • 4. ignore unprofitable items, however common, when profitable prey are abundant.

Optimal diet

  • Pied wagtails show a definite preference for medium-sized prey.
  • Prey size chosen by pied wagtails is the optimal size for maximum energy per handling time.

Most profitable prey




Foraging efficiency decision rules:

    • 1. Concentrate foraging activity in the most productive patches;
    • 2. Stay with those patches until their profitability falls to a level equal to the average for the foraging area as a whole;
    • 3. Leave the patch once it has been reduced to a level of average productivity (for a more than average patch); and
    • 4. Ignore patches of low productivity.

Risk-sensitive foraging

    • Risk-sensitive foraging- because the quality of patches varies over space and time, the animal has to decide whether to go back to a patch that gives it a constant rate of return

or visit a new patch where the return is unknown.

(i.e. the devil you know or the devil you don’t)


Expected energy budget rule- the animal will likely be risk-prone if the daily energy budget is negative

but be risk-adverse if it is positive.

i.e. when you’re very hungry, you will be more likely to take risky strategies to find food than when you have sufficient energy – stealing bread when you’re starving

  • Predation risk- in deciding where it will feed, the forager must balance its energy gains vs the risk of being eaten.