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 • 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 • Species interactions • Interspecific competition • Competitive exclusion • Resource partitioning • The niche • Niche overlap • Niche width • Niche responses
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. Fitness Resource/Food (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). • 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