Population distribution and abundance and life histories Flashcards
Populations
Population – group of individuals of the same species that live within a particular area and interact with one another
Populations are dynamic entities that vary in size over time and space
Abundance can be reported as population size (number of individuals) or density (number of individuals per unit area)
Abundance can change over time and space
Some species vary more than others
Populations may exist in patches that are spatially isolated but linked by dispersal
This can result from physical features of the environment and from human activities that subdivide populations
e.g heathlands in England have been fragmented by human development
Dispersal
Dispersal – process in which individuals move away from a population in which they were born to another location where they will settle and reproduce
Species vary in their ability to disperse
For most plants, dispersal occurs by seed movement (small distances)
Other species can disperse 100’s of km
Individuals
For some species, it is difficult to determine what an individual is
Some species produce clones, which makes counting individuals in such a population difficult
Not all individuals in a population are alike
Life cycle stages
Unitary organisms – highly determinate fertilisation, birth, growth, maturity, reproduction, death (most animas)
Modular organisms – unpredictable form and growth, develop according to environment (most pants, some animals)
Counting individuals
Population boundaries
Arbitrary boundary of study sites
Sticklebacks in a lake – per m3 of water
Aphids on a leaf/tree – per cm2 of leaf
Population densities
Distribution and abundance of organisms
Distributions and abundances of organisms are limited by habitat suitability, historical factors and dispersal
Abiotic factors (conditions)
Species have varying tolerance ranges
Abiotic and biotic features of the environment can act together to determine distribution and abundance
Some species may be restricted by temp for survival and reproduction, but competition from other species precludes them from some areas that have suitable temp
Some species distributions depend on disturbance events that kill or damage some individuals, creating opportunities for other individuals to grow and reproduce
e.g some species persist only where there are periodic fires
Evolutionary history, dispersal abilities and geologic events all affect the modern distribution of species
e.g polar bears evolved from brown bears in the arctic – not Antarctica due to inability to disperse tropical regions
Dispersal also affects density of populations, and vice versa
Allowing dispersal may remove competition
Geographic range of organisms
Geographic range – the entire geographic region over which a species is found
Many species have patchy distribution of populations across their geographic range
Few species are found on all continents
Includes areas occupied by all life stages of a species
Some species migrate long distances between summer and winter habitat
Many species have patchy distributions because not all habitat within the range is suitable
Can operate at different spatial scales
At large scales, climate may dictate locations of populations
At small scales, soils, topography, other species etc can determine patchiness
Dispersion of organisms
The spatial arrangement of individuals within a population
Regular – individuals evenly spaces
Random – individuals scattered randomly
Clumped – most common pattern
Depends on location of essential resources, dispersal and behavioural interactions
Life histories
A record of events relating to growth, development, reproduction and survival of an organism
Defining characteristics – age and size at sexual maturity, amount and timing of reproduction, survival and mortality rates
Individuals within a species show variation in life history traits due to genetic variation or environmental conditions
Natural selection favours individuals whose life history traits improve their survival and chance of reproduction
Life history diversity
Phenotypic plasticity – one genotype may produce different phenotypes under different environmental factors
Phenotypic plasticity may produce a continuous range of growth rates; or discrete types – morphs
Polyphenism – a single genotype produces several distinct morphs
Spadefoot toad tadpoles have both omnivore morphs and larger carnivore morphs – carnivore tadpoles grow fast and metamorphose early (ephemeral ponds that dry up quickly), the omnivores grow more slowly (ponds that persist longer), metamorphose in better conditions and have better chances of survival
Complex modes of reproduction have evolved
Asexual reproduction (cell division) – prokaryotes and many protists
Sexual reproduction (multicellular) – promotes genetic variation but population growth slower, isogamny (equal gametes), anisogamy (unequal gametes)
Most multicellular organisms produce anisogametes
Complex life cycles involve at least two distinct stages (different body forms and habitats)
Transition between stages may be abrupt – e.g metamorphosis
Most vertebrates have simple life cycles without abrupt transitions but complex life cycles are common in insects, marine invertebrates, amphibians and some fish
Life history continua
Reproductive patterns can be classified along several continua
Semelparous – a single, distinct reproductive period then death, e.g annual plants and Pacific salmon
Iteroparous – several reproductive events, e.g perennial plants and Atlantic salmon
Semelparous species include – annual plants, agave (century plant) vegetative growth up to 25 years (also produces clones asexually), giant pacific octopus female lays single clutch of eggs and broods them for 6 months and dies after they hatch
Iteroparous species include – trees such as oak, pink and giant sequoia, most large mammals, several reproductive events
r-selection and K-selection describes two ends of a continuum of reproductive patterns
The r-K continuum is a spectrum of population growth rates, from fast to slow
dN/dt = rN(1 – N/K)
r is the intrinsic rate of increase of a population
r-selection is selection for high population growth rates; in unstable habitats, less crowded niches
K is the carrying capacity for a population
K-selection is selection for slower growth rates in populations that are at or near K; crowded conditions, heavy investment in few offspring
r – short life spans, rapid development, early maturation, low parental investment, high rates of reproduction
K – long-lived, developed slowly, delayed maturation, invest heavily in each offspring, and low rates of reproduction
Trade-offs
Trade offs shape and constrain life history evolution
Organisms allocate limited energy or resources to one structure or function at the expense of another
The larger an organisms investment in each individual offspring, the fewer offspring it can produce
Investment includes energy, resources, time, and loss of chances to engage in alternative activities such as foraging
Lack clutch size – maximum number of offspring a parent can successfully raise to maturity
Names for studies by David Lack (1947) – clutch size is limited by the maximum number of offspring the parents can raise at one time
Lack noticed that clutch size increased at higher latitudes
Longer periods of daylight allowed parents more time for foraging, and they could feed greater numbers of offspring in a day
Experimental manipulation of clutch size in lesser black-backed gulls showed that in larger clutches offspring have less chances of survival (Nager et al 2000)
In species without parental care, reproductive invested is measured as resources invested in propagules (eggs or seeds)
Size of propagule is a trade-off with the number produced
In plants, seed size is negatively correlated with the number of seeds produced
Under what conditions should an organism allocate energy to growth rather than reproduction – long life span, high adult survival rates and increasing fecundity with body size, if rates of adult survival are low, future reproduction may never occur, so early reproduction rather than growth would be favoured
Senescence – decline in fitness of an organism with age and physiological deterioration
Can set an upper age limit for reproduction
Semelparous species undergo very rapid senescence and death following reproduction
Senescence may occur earlier in populations with high mortality rates due to disease or predation
In some large social animals (e.g African elephants) post reproductive individuals contribute to the success of the group through parental and grandparental care
