6.6- populations and sustainability

What is carrying capacity?

The maximum population size that can be maintained over a period in a particular habitat

What is the lag phase?

There may only be a few individuals which are still acclimatising to their habitat. At this point, the rate of reproduction is low, and the growth in population size is slow.

What is the log phase?

Resources are plentiful, and conditions are good. Reproduction can happen quickly, with the rate of reproduction exceeding mortality. The population size increases rapidly.

What happens at carrying capacity?

The population size has levelled out at the carrying capacity of the habitat- the habitat cannot support a larger population. In this phase, the rates of reproduction and mortality are equal. The population stays stable.

What are density independent limiting factors?

They act strongly regardless of population size. For example, particularly low temperatures may kill the same proportion of individuals in a population, irrespective of its size.

What are density dependent limiting factors?

• The factor influences population more strongly as population size increases

• E.g. availability of resources like food, water, light, oxygen, nesting sites or shelter may decrease.

• As population size increases, levels of parasitism and predation from other species may increase, as do the intensity of competition for resources.

K-strategists and their characteristics

Species whose population size is determined by the carrying capacity. For these populations, limiting factors exert a more significant effect as the population size gets closer to carrying capacity. K-strategists are usually birds, larger mammals, and larger plants.

Their characteristics include:

• low reproductive rate

• Slow development

• Late reproductive age

• Long life span

• Large body mass

R-strategists and their characteristics

Population size increases so quickly it can exceed carrying capacity before limiting factors have an effect. Once carrying capacity has been exceeded, there are no longer enough resources to allow individuals to reproduce or survive. An excessive build up of waste products may start to poison the species and they begin to die, entering a death phase- this is known as boom and bust. They are usually things like mice, insects, spiders and weeds. Their characteristics include:

• High reproductive rate

• Quick development

• Young reproductive age

• Short life span

• Small body mass

Describe a predator-prey cycle

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 restarts

Why do predator prey cycles often not have a defined shape?

In the wild, predators eat more than one type of prey, and there are a number of other limiting factors.

What is intraspecific competition?

Intraspecific competition happens between individuals of the same species. As factors such as food supplies become limiting, individuals compete for food. Those individuals best adapted to obtaining food survive and reproduce, whilst those not so well adapted fail to reproduce or die.

* If the population size drops, competition reduces and the population size increases
* If the population size increases, competition increases and the population size drops

What is interspecific competition?

Interspecific competition happens between individuals of different species.

Overlap between two species’ niches results in intense competition. If two species have exactly the same niche, one is out competed by the other and dies out or becomes extinct in that habitat; two species cannot occupy the same niche.

This became known as the competitive exclusion principle. Sometimes, extinction is not inevitable and interspecific competition results in one population being smaller than the other, with both population sizes remaining relatively constant.

What is preservation?

Maintenance of habitats and ecosystems in their present condition, minimising human impact.

What is conservation?

Maintenance of biodiversity, including diversity between species, genetic diversity within species and a maintenance of a variety of habitats and ecosystems.

Threats to biodiversity

* Over-exploitation of wild populations for food, for sport, and for commerce
* 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

Outline some possible management strategies

* 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
* Restricts dispersal of individuals by fencing
* Vaccinate individuals against disease
* Preserve habitats by preventing pollution or disruption, or intervene to restrict the progress of succession, for example by coppicing, mowing or grazing

Why is simple management sometimes inappropriate?

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 fairly stable, you can clean up pollution, remove unwanted species, or recolonise with the original species. However, it is often easier and more successful to replace a disrupted community with a slightly different community than to rehabilitate the original community.

Ethical reasons for conserving

Many conservationists argue that the kinds of problem listed above make conservation essential. They believe that every species has value, and that humans have an ethical responsibility to look after them. The arguments in favour of human activities which work against conservation are objective and driven by economics. Expressing the value of conservation in economic terms is more effective in driving governments to prioritise conservation.

Economic and social reasons for conservation

• Many species have direct economic value when harvested

• Many plant and animal species provide a valuable food source + were originally domesticated from wild species

• Genetic diversity in wild strains may be needed in future to breed for disease resistance, improved yield in animals and plants, and drought tolerance in plants

• Natural environments are a valuable source of beneficial organisms. Many drugs we use today discovered in wild plant species

• Natural predators of pests act as biological control agents- preferable to causing pollution with artificial chemicals

• Many species have indirect economic value- insect species pollinate crops, without these a harvest would fail.

• Other communities are important in maintaining water quality, protecting soil + breaking down waste products.

• Reduction in biodiversity reduces climatic stability- loss of biodiversity resulting in drought / flooding

• Ecotourism / recreation in countryside have social / financial value, deriving from aesthetic value

• Ecotourism depends on the maintenance of biodiversity

Small-scale timber production

• Coppicing provides sustainable wood supply. Stem of a deciduous tree is cut close to ground, and once cut new shoots grow + mature, + these can be used for fencing, firewood or furniture.

• Pollarding- cutting the stem higher up to prevent deer eating emerging shoots.

• Rotational coppicing- consistent wood supply, wood divided into sections and one section cut each year. By the time first section can be coppiced again, new stems mature + can be cut

• Good for biodiversity- left unmanaged, woodland goes through succession, blocking light to the woodland floor + reducing species growing there

• In rotational coppicing, more light is let in, increasing diversity

Large scale timber production

* Large scale timber production often involved clear felling all the trees in one area- this could destroy habitats on a large scale, reduce soil mineral levels and cause soil erosion
* Now, any tree which is harvested is replaced by another tree, either grown naturally or planted
* The forest as a whole must maintain its ecological function, regarding biodiversity, climate and mineral and water cycles
* Local people should benefit from the forest
* Foresters control pests, only grow particular species where they know they will grow well, and position trees an optimal distance apart

Managing fish stocks

* Fishing must take place at a level which allows it to continue indefinitely. Over fishing must be avoided, and if it happens, fishing should be reduced
* Fishing must be managed to maintain the structure, productivity, function and diversity of the ecosystem
* A fishery must adapt to changes in circumstances and comply with local, national and international regulations

Aquaculture

Raising stocks of fish in aquaculture can provide sustainable fish stocks. Raising stocks of fish in aquaculture restricts the impact on oceanic fish stocks.

The Terai region

* South of Nepal, made up of marshy grasslands, savannah and forests
* It is densely populated, and home to endangered species
* The forests here have been under pressure from expansion of agriculture into forested areas, grazing from farm animals, over-exploitation of forest resources, and replacement of traditional agricultural crop varieties with modern ones
* The WWF found that rural livelihoods are heavily dependent on the forests, which are home to many of the region’s endangered species
* The forests provide local people with a sustainable source of fuel, animal feed, food, building materials and agricultural tools
* The WWF with the Nepalese government in the Terai-arc landscape programme focused on conservation in the landscape as a whole
* They introduced community forestry initiatives in which local people had rights to exploit the forest and look after it
* These groups helped to create forest corridors between national parks and counteracting poaching and illegal felling. Forestry work also diversified farming activity, built entrepreneurial skills and stimulated marketing schemes

The Maasai Mara

* The Maasai Mara combines endemic poverty with abundant wildlife populations that attract tourism, so there has been scope to develop conservation-compatible land use that rewards local people financially, whilst conserving habitats and species that are the basis for tourism
* Many Maasai took over individual title over smaller portions of land after land was held in group ranches until 1968, intensifying agriculture
* In 2005, several land owners to the north of the Maasai Mara reserve consolidated their land to form conservancies, in order to generate tourism income
* Partnerships between conservancies and tourism operators have developed payment for wildlife conservation schemes, and conservancies are paid PWC revenue proportional to the area of land set aside for conservation

Peat bogs

* Peat bogs form where a lack of oxygen prevents complete decomposition of organic matter
* Undisturbed peat bogs contain a lot of historical data, and controlled archaeological digs can reveal information about the landscape and vegetation in the past
* The ability of peat to retain moisture has led to its widespread use as compost
* Pressure from expansion of agriculture, forestry, landfill and peat extraction mean peat extraction mean that lowland peat bog now covers less than one tenth of its original area
* The UK Biodiversity Action Plan aims to enhance biodiversity through local level schemes

The Galápagos Islands

* The Galapagos human population has grown in response to increased demand for marine products and increased tourism
* The population size increase has placed huge demands on water, energy and sanitation services
* The more recently boom in fishing for exotic fishing has depleted populations, depletion of sea cucumber populations has had a drastic effect on underwater ecology, and the international market for shark fin has led to the deaths of 150000 sharks a year around the islands
* Introduced species have disrupted the islands greatly
* The Charles Darwin Research Station manages reserves and helps to manage the islands

The Antarctic

* Terms of the Antarctic Treaty have to be followed by participating countries
* Overfishing of krill damages the food chain
* Protected areas- Southern Ocean Whale Sanctuary, International Whaling Commission

The Lake District

* Designated Environmentally Sensitive area
* Financial incentives are available for farmers to reduce chemical use, safeguard hedges, and care for hay meadows, heather moor, wetland, chalk down land and native woodland
* Limestone pavement is legally protected
* Mires are being managed more sympathetically, some are re-wetted
* Seasonal restrictions on walking when birds are nesting
* Burning strips of vegetation in heathland promotes new shoot growth

Snowdonia

* Mountain, moorland and bog habitats
* Open drainage ditches cause poor water quality, so these are blocked by hay bales
* Conifers dry out the moorland and roads have to be built to carry the wood away, so branches are used to block drainage ditches to slow water flow
* Accidental fires are a big risk, so controlled fires before heather gets too old and dry prevents damage