6.6- Populations Flashcards
What determines population size
The balance between mortality rate and rate of reproduction
Name different ways in which population size can change
- stay stable
- rise/fall suddenly
- oscillate up/down in regular pattern
Graph of how many populations grow, explanation
A:
- lag phase
- may only be a few individuals who are still acclimatising to their habitat
- rate of reproduction low
- growth in population size slow
B:
- log phase
- resources are plentiful
- conditions are good
- reproduction can happen quickly, rate of reproduction exceeds mortality
- population size increases rapidly
C:
- stationary phase
- population size levelled out to carrying capacity of habitat- habitat can’t support larger population
- rates of reproduction and mortality equal
- population size therefore stays stable (or fluctuates up/down slight due to small variations in environmental conditions
What prevents a habitat that has reached carrying capacity from supporting a larger population
limiting factors
Name 2 types of limiting factors
Density dependent, density independent
Describe density-independent limiting factors
- act just as strongly irrespective of population size
- e.g. temperature drop effects same proportion regardless of size
Describe density-dependent limiting factors
- factor influences population more strongly as population size increases
- e.g. availability of resources- food, water, light, oxygen, nesting sites, shelter
- similarly, as population size increases, leaves of parasitism, predation, and competition from other species may increase (and with own species for competition)
- carrying capacity is the upper limit that these factors place on population size
Name 2 types of stratagists
R and K stratagists- represent 2 ends of a continuum of strategies adopted by living things
Describe K strategists characteristics
- low reproductive rate
- slow development
- late reproductive age
- long lifespan
- large body mass
- examples- birds, larger plants, larger mammals like humans, elephants and lions
Describe R strategists characteristics
- high reproductive rate
- quick development
- young reproductive age
- short life span
- small body mass
- examples- mice, insects, spiders, weeds
Describe K strategists population growth
- population size is determined by the carrying capacity
- limiting factors extert a more and more significant effect as the population size gets closer to the carrying capacity, causing population size to gradually level out
Describe R strategists population growth
- population size increases so quickly that it can exceed the carrying capacity of the habitat before the limiting factors start to have an effect
- once the carrying capacity has been exceeded, there are no longer enough resources to allow individuals to reproduce, or even survive
- excessive build-up of waste products may start to poison the species, and they begin to die- entering a death phase
- population growth called boom and burst
- most important influence on population growth is the physical rate (r) at which individuals can reproduce
- this type of growth is characteristic of species with short generation times (e.g. bacteria) and of pioneer species
- quick population growth means pioneer r-strategist species colonise a disturbed habitat before k-strategists, dispersing to other habitats once limiting factors start to have an effect
R and K strategists population growth graph
What is a predator
An animal that hunts other animals (prey) for food- predation can act as a limiting factor on preys population size which in turn can affect the predators population size
Describe the stages of predator-prey interactions
1) when the predator population gets bigger, more prey are eaten
2) The prey population then gets smaller, leaving less food for the predators
3) With less food, fewer predators can survive and their population size reduces
4) With fewer predators, fewer prey are eaten and their population size increases
5) with more prey, the predator population gets bigger, and the cycle starts again
Predator prey graph, describe conditions of this
- comes from experiment conducted in laboratory, where the predators only ate one type of prey, and predation was the main limiting factor on the preys population
- however, in the wild, predators often eat more than one type of prey, and there are a number of other limiting factors
- because of this, studies of predators and prey in the wild yield graphs of a similar, but not so well defined, shape
Predator-prey graph over number of years
Describe competition
- happens when resources (e.g. food/water) are not present in adequate amounts to satisfy the needs of all individuals who depend on those resources
- if a resource is in short supply in an ecosystem, there will be competition between organisms for the resource
- as the intensity of competition increases, the rate of reproduction decreases (because fewer organisms have enough resources to reproduce)
- death rate increases (as fewer organisms have enough resources to survive)
Name two types of competition
- interspecific
- intraspecific
Describe intraspecific competition
- between individuals of same species
- as factors e.g. food supplies become limiting, individuals compete for food
- those individuals better adapted to obtaining food survive and reproduce, while those not so well adapted fail to reproduce, or die
- this slows down population growth, and the population enters the stationary phase
- intraspecific competition keeps the population size relatively stable but there are slight fluctuations
- keeps stable as if population size drops, competition reduces and population size increases, and if population size increases, competition increases and population size drops
Describe interspecific competition
- happens between individuals of different species, and can affect both the population size of a species and the distribution of a species within an ecosystem
Describe an example of research into interspecific competition
- Gause (1934)
- grew 2 species of paramecium- P. Causatum and P. Aurelia, both separately and together
- when together, there was competition for food, with P.A. obtaining ffod more effectively than P.C.
- over 2- days, the population of P.C. reduced and died out, while the population of P.A. increased- eventually being only species remaining
Describe Gause’s conclusions
Competitive exclusion principle:
- more overlap between 2 species niches results in more intense competition
- if 2 species have exactly the same niche, one is out-competed by the other and dies out or becomes extinct in that habitat- 2 species can’t occupy same niche
- principle explains why particular species only grow in particular places
Why isn’t the competitive exclusion principle always straight forward in the wild
- other reserach suggests extinction isn’t inevitable
- sometimes, interspecific competition simply results in one population being much smaller than the other, with both population sizes remaining fairly constant
- also, in lab it is easy to exclude the effects of other variables, so the habitat of the 2 species remains stable
- in the wild, however, a wide range of variables may act as limiting factors for the growth of different populations and may change on a daily basis or river the course of a year
- e.g. experiments on the competition between flour beetles Tribolium confusum and T. castaneum initially confirmed the competitive exclusion principle- the T.Cas population size increased while the T.Con population died out, but even a small chang in the temperature could change the outcome, so T.Con would survive instead
Gause experiment/ competitive exclusion principle graphs
Flour beetles population interactions graph
Describe preservation
Maintenance of habitats and ecosystems in their present condition, minimising human impact
Describe conservation
- maintenance of biodiversity, including diversity between species, genetic diversity with species, and maintenance of a variety of habitats and ecosystems
- involves active human management and intervention
- change in many ecosystems is inevitable- due to human activity in past and natural succession- very few UK habitats truly ‘natural’- adopting preservation strategy wouldn’t preserve a natural habitat
- therefore, conservation programs focus on maintaining or improving biodiversity
Describe threats to biodiversity
- over exploitation of wild populations for food (e.g. cod in North sea), for sport (e.g. sharks), and for commerce (e.g. pearls from mussels)- species harvested at faster rate than they can replenish themselves
- habitat disruption and fragmentation as a result of more intensive agricultural practices, increased pollution, or widespread building
- species introduced to an ecosystem by humans that out-compete other native species, leading to their extinction
What does successful conservation require
- consideration of the social and economic costs to the local community, and effective education and liaison with the community
- maintaining biodiversity in dynamic ecosystems requires careful management to maintain a stable community, or even reclamation of an ecosystem to reverse the effects of human activity
List different methods of conservation
- establishing protected areas like National parks, green belt land or sites of special scientific interest (SSSIs)
- giving legal protection to endangered species, or conserving them ex situ in zoos or botanic gardens
List management strategies for maintaining biodiversity
- raise carrying capacity by providing extra food
- move individuals to enlarge populations, or encourage natural dispersion of individuals between fragmented habitats by developing dispersal corridors of appropriate habitat
- restrict dispersal of individuals by fencing
- control predators and poachers
- vaccinate individuals against disease
- preserve habitats by presenting pollution or disruption, or intervene to restrict the progress of succession e.g. by coppicing, mowing, or grazing
What does which management strategies are adopted depend on
The specific characteristics of the ecosystem and the species involved
Why is simple management sometimes inappropriate, what is done instead
- disruption of a community may have gone too far
- understanding which species was part of the original community is not always clear, and succession is likely to take a long time before it allows such a community to survive again
- short-cutting that process requires detailed knowledge of all species involved
- where environmental conditions have remained entirely stable, it is possible to clean up pollution, remove unwanted species, or reconolise with the original species e.g. from captive breeding programmes
- however, it is often easier, and more successful, to ‘replace’ a disrupted community with a slightly different community rather than to rehabilitate the original community