Populations and Sustainability

A habitat cannot support a population larger than its carrying capacity because of limiting factors which place a limit on population size. These limiting factors may include:

• Resources
• Food
• Water
• Light
• Oxygen
• Nesting sites
• Shelter
• Effects of other species
• Predators
• Parasites
• Intensity of competition for resources

Extinction is more likely is a species is rare initially, if it has a slower reproductive rate and its prey numbers have reduced.

The maximum population that can be maintained over a period of time in a particular habitat. A population does not increase in size indefinately due to limiting factors determining a carrying capacity. There will be intraspecific competition for food and nesting sites and interspecific competition with other species. Larger populations attract more predators as well as parasites and diseases spread more easily.

When the predator population gets bigger, more prey are eaten. They prey population then gets smaller, leaving less food for the predators. With less food, fewer predators can survive, and their population size decreases. With fewer predators, fewer prey are eaten, and their population size increases. With more prey, the predator population gets bigger, and the cycle continues. 

Interspecific competition: Competition between individuals of different species can affect both the population size and the distribution of a species in an ecosystem as no two species can occupy the same niche. 

Red vs Grey squirrels compete for the same food. The red squirrel outcompetes the grey in conifer forests, but the grey outcompetes the red in forests with less than 75% conifers.

Intraspecific competition: Competition between individuals of the same species. 

If food supply becomes a limiting factor, the individuals best adapted to obtaining food will survive and reproduce, whereas those less well adapted will die out and fail to pass on their genes.

Conservation is the maintenance of biodiversity, but the area can still be sustainably exploited.

Preservation protects species by leaving their habitat untouched.

Selective felling: Some mature trees, diseased trees and unwanted species are harvested, leaving other trees to develop and distribute seeds to fill the gaps. This leads to a variety of heights, branch lengths etc and therefore increases biodiversity.

Stip felling: Small patches, or strips, of forest are cleared completely, leaving other patches untouched. Large areas are not felled at the same time, so loss of species and soil erosion are avoided and shade is maintained. It also limis the disturbance by machinery.

Rotational felling: Describes felling of areas or individual trees over time allowing regeneration and therefore sustainability. Felled trees are replanted. The habitat is preserved and nesting sites are not too heavily disrupted.

Coppicing/Pollarding: Trees are cut down, leaving stumps from which new shoots develop. These have a well developed root system and so grow fast. Afte a few years, the shoots are cut and yields wood for poles and fencing or burning as fuel. Can be repeated indefinately and is sustainable. Pollarding is similar but the stumps are higher than 4m or so. This prevents deer from eating the regrowth and new juicy shoots. 

Maintaining biodiversity in dynamic ecosystems requires careful management to maintain a stable community or even reclaim an ecosystem by reversing the effects of human activity. Some management strategies include:

• Raising the carrying capacity by providing extra food
• Moving individuals to enlarge populations, or encouraging natural dispersion of individuals between fragmented habitats by developing dispersal corridors of appropriate habitat
• Restricting dispersal of individuals by fencing
• Controlling predators and poachers
• Vaccinating individuals against disease
• Preserving habitats by preventing pollution or disruption, or intervening to restrict the progress of succession

• There is an aesthetic or recreational value
• It may bring in (eco)tourism - important to the economy
• It preserves biodiversity and therefore genetic diversity which may help stop extinction
• There may be interactions between species that are needed to preserve the whole habitat
• We need to preserve the gene pool as species could be useful in the future for medicine or genetic engineering (particularly applies to the rainforest)
• We should support indigenous peoples are allow them to continue their lifestyle
• It may prevent the effects of deforestation on the atmosphere, climate and soil
• Natural predators of pests can act as biological control agents
• Wild insect species pollinate crop plants

Rate breeds are maintained on farms because:

• They are a genetic resource as they have different alleles that could be used for genetic engineering, genetic modification, artificial selection or selective breeding if conditions change in the future
• Example of useful traits would be disease resistance or hardiness as well as more or better quality of wool or meat
• Rare breeds also maintain biodiversity and a large gene pool

In 1980 the population of the Galapagos Islands was 5,000, with about 4,000 tourists visiting every year. In 2005, the population was 28,000 with 10,000 tourists visiting every year.

• Dramatic increase in population size has placed huge demand on water, energy and sanitation services
• More waste and pollution is being produced
• The demand for oil has increased
• 2001 oil spill had an adverse effect on marine and coastal ecosystems
• Increased population, building and converstion of land for agriculture has caused destruction and fragmentation of habitats
• Species have been harvested faster than they could replenish themselves: giant tortoises were taken to be eaten on long voyages, fishing for exotic species has decimated the population, depletion of sea cucumbers has had a drastic effect on under-water ecology, international market for Shark-fin has led to deaths of around 150,000 sharks each year
• Humans have introduced many non-indigenous species: the red quinine tree spread rapidly and outcompetes the native species. its presence has changed the landscape from mostly low scrub and grassland to a closed canopy forest. Many native animals have lost their nesting sites. Goats eat species unique to the islands, outcompete giant tortoises for grazing, trample tortoise nesting sites, transform forest into grassland. Cats hunt indigenous species.