Unit 1 - Life processes in the Biosphere - Species Diversity and ecological stability

What is species diversity and ecological stability like in abiotically extreme environments?

There are low biodiversity levels in abiotically extreme environments. Abiotic conditions dominate and determine the overall ecological features of ecosystems. Populations of species are more unstable as one change like the loss of one species can have knock-on effects that change the availability of food in the wider ecosystem.

What is species diversity and ecological stability like in less abiotically extreme environments?

Biotic factors are more dominant. There are higher levels of diversity and population sizes of species result in increased levels of competition between species, with higher rates of disease and predation being the main determinants of the ecosystem characteristics. Biological species in less extreme environments affect the physical habitat more e.g. beavers build dams to affect river flow speed and channel sinuosity.

Which abiotic factors might limit population sizes?

• Low soil pH/acidity - might become outside of more species range of tolerance. Fewer plants can grow and gain the nutrients that they need.

• Water salinity

• Water turbidity

• Low water availability

• Increasing temperatures - more extreme weather events

• Low soil nutrients

• Low oxygenation in water

• Low light levels

Which biotic factors might limit population sizes?

• Lack of food and shelter

• An increase in predators

• Introduced species - may outcompete native species

• Pathogens

• Fewer pollinators

• Parasites

• Competition

How do birth rates and death rates influence changes in population sizes?

If death rates are higher than birth rates, population size will decrease. Whereas if birth rates are higher than death rates, population size will increase.

What are death rates determined by?

Environmental factors such as disease, drought, predation and shortages of food

How do birth rates influence a species chance of survival?

Maximum birth rate is determined by the natural ability of a species to reproduce. Species with higher birth rates have a higher chance of survival e.g. male seahorses produce 2000 babies at a time.

What are r-selected species?

R-selected species can reproduce rapidly and respond quickly to low survival (high death) rates. They reach sexual maturity quickly, they produce many young and they can disperse widely.

What are some examples of r-selected species?

Mice, locusts, greenfly

What are k-selected species?

They reproduce slowly and respond slowly to low survival (high death) rates. They reach sexual maturity at an older age, produce few young and cannot disperse widely.

Give some examples of k-selected species

Whales, elephants, rhinos. These are all endangered species.

How can population change be forecasted? What calculation is used?

Estimating future populations rely on: Current population size, birth and death rates, number of individuals emigrating and immigrating into the habitat.
- Population = (starting population + births + migrants) - (deaths + emigrants)

What are density-dependent factors and how do they influence population sizes?

They are factors where the chance of an individual surviving depend on the population density of the species. Survival chances are usually higher when the population density is low and lower when population density is high. This includes food supply - intra-species competition for food is greatest when population density is high; diseases spread more easily between individuals when they are close together; predator density - low populations of prey

What are density-independent factors?

They are not influenced by a species population size e.g. climate conditions, pollutants in the environment

What is carrying capacity?

The greatest population that an area can support indefinitely without damaging or over-exploiting the environment. The mortality rate in a population changes if the population is above or below the carrying capacity, so that the population size changes back to the carrying capacity.

What is a sigmoidal population growth curve? What are its four stages?

A sigmoidal population growth curve is a model of population growth which many species follow. Its four stages are the: Lag phase, exponential growth phase, transitional phase, plateau phase.

What occurs during the lag phase?

There is a slight increase in population as the population is becoming established. At this point, the population is low and stable. The population is finding food, building nests, establishing territories.

What occurs during the exponential phase?

There is a rapid increase in population - at this point, the population has become established and is able to grow. Birth rates are much higher than death rates. There are not many limitations to the populations growth at this point.

What happens during the transitional phase?

Very slow population growth - perhaps the population is facing more difficulties due to its high population size such as higher competition, food shortages, disease. At this point, the population has reached its carrying capacity.

What happens during the plateau phase?

The population begins to stabilise with slight increases and decreases in populations which fluctuate slightly around the carrying capacity of the population. At this point, density dependent factors are controlling the size of the population.

What factors might cause a habitat to have a higher carrying capacity?

A large amount of space or territory in its habitat, a larger amount of suitable breeding and nesting sites

Predator-prey relationships: How have hare and lynx population sizes changed in relation to one another between 1845 and 1935

As hare populations increase, it provides food for lynx and so lynx populations increases. As the lynx reach carrying capacity, the hare populations decrease and then as a result, lynx populations have less to eat and so their populations decrease. This allows the hare population to rise again and this process continues in a cycle as long as there are no other external influences. There has to be a change in the prey population first and then the predator population will change accordingly.

What is artificial population control and why might it be required (general)?

Intervention to artificially control the population e.g. through culling might be required to enable the species or habitats to survive where natural control mechanisms can no longer regulate the population

In what cases might artificial population control be necessary?

If:
- The breeding rate of an endangered species is low, so a captive breeding and release programme is needed to maintain or increase the population
- A non-indigenous species is introduced which reduces populations of indigenous species as it is a predator, competitor or pathogen
- An indigenous predator has been removed so its prey species becomes over-populated and needs to be culled to avoid the ecological damage it might cause to other species

What are some specific examples of artificial population control for each of these three reasons?

1 - Populations of Pandas have experienced an increase from 50 Giant Pandas in 2005 to 190 in 2017 as a result of captive breeding and human intervention
2 - Rabbits were introduced to Australia from European ships and there are now approximately 200 million wild rabbits in Australia despite efforts to cull their populations.
3 - Wolves in Scotland had been exterminated and their prey species, Red Deer, must be culled to prevent their population rising too high which would lead to overgrazing, decreasing food availability for other species.

What issues did rabbits cause in Australia?

They out-competed many native species, negatively impacted agricultural crops and local environments through overgrazing which causes high levels of soil erosion meaning that plants cannot grow and farming is made more difficult - farming produced lower yields.

Which methods were used to try to control rabbit populations in Australia? How have these measures worked?

Initially, farmers and the government built fences to try and keep the rabbits from destroying their crops and the government commissioned the construction of a fence which stretched across Western Australia. This fencing was not very effective and merely fenced in rabbits already in Western Australia.
Destroying rabbit warrens, where rabbits breed and raise young, has been effective in controlling rabbit populations found on accessible lands.
Biocontrol - released rabbits infected with myxoma - a rabbit-specific virus - into Southern Australia and though it did lead to the deaths of many rabbits, they eventually developed immunity, rendering it ineffective. Releasing RHDV in 1996, another rabbit specific pathogen, was able to reduce rabbit numbers by up to 90% but rabbits have began to develop resistance to it. Introducing viruses seems to be the most cost-effective way to control numbers of rabbits.