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]
Monogamy is the formation of a pair bond between one male and one female.
Honest signals – the females choose males according to displays that give an honest indication of the male’s fitness
Leks – males hold small territories with no resources and display for females
Reproductive effort - the nature and amount of resource and time allocations to reproduction over a period of time.
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;
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)
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
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
Percentage of male offspring born to individual red deer hinds differing in social rank. High-ranking females tend to have sons.
k-strategists are competitive species with stable populations of long-lived individuals where selection favors genotypes that:
The Lotka-Volterra model of interspecific competition is an extension of the logistic growth model with coefficients to account for inter- and intraspecific competition.
There are four outcomes of competition between two species as predicted by the Lotka-Volterra model:
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.
Each species inhibits its own population growth more than it inhibits the population growth of the other species.
So not really likely in nature
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.
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
(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
The niche: An organism’s place and function in the environment.
Fundamental niche - the full range of environmental conditions, biological and physical, under which an organism can exist (absent competition).
Two niches could theoretically be adjacent, and there would be no competition between the species
If the niche of one species (A) lies completely within a larger niche of another species (B)
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.
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.
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).
Population oscillations of host and parasitoid predicted by the Nicholson-Bailey equations.
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.
The central component of each of these models is that predator and prey density patterns are potentially cyclic in nature.
Predators respond to increasing prey density in two ways: functional or numerical
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.
(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
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
Aggregative response in the redshank. The curve plots the density of the predator against the average density of arthropod prey.
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.
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)
Most profitable prey
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