Populations and sustainability

The size of population of a species may remain stable, it may rise or fall quite suddenly or oscillate up and down with a regular pattern. The balance between the death rate (mortality) and the rate of reproduction determines the size of a population. The graph shows how many populations grow. 

The graph has lag, log and stationary phases.

A habitat that has reached carrying capacity cannot support a larger population, because factors limit the growth in population size. These are called limiting factors. 

Some limiting factors are density independent. These act just a strongly irrespective of the size of the population. For example particularly low temperatures may kill the same proportion of individuals in a population, irrespective of its size.

Other limiting factors are density dependent, where the factor influences population more strongly as population size increases. For example, the availability of resources like food, water, light, oxygen, nesting sites or shelter may decrease. Similarly, as population size increases, levels of parasitism and predation from other species may increase, as do the intensity of competition for resources, both with individuals of the same species and individuals of other species. The carrying capacity is the upper limit that these factors place on the population size.

Species whose carrying population size is determined by the carrying capacity is often called k-strategies. For these populations, limiting factors exert more and more significant effect as the population size gets closer to the carrying capacity, causing the population size to gradually level out. 

k-Strategists, such as birds, larger mammals, like humans, elephants and lions, and larger plants, often exhibit many of the following characteristics: 

• low reproductive rate 
• slow development 
• late reproductive age 
• long lifespan 
• large body mass.

Some species adopt a different type of population growth. In these species, the 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 to survive. Likewise, an excessive build-up of waste products may start to poison the species, and they begin to die, entering a death phase. This type of population growth is known as boom and bust. r-Strategists, such as mice, insects, spiders and weeds, tend to exhibit many of the following characteristics:

• high reproductive rate 
• quick development 
• young reproductive age 
• short life span 
• small body mass. 

 This type of growth is characteristic of species with short generation times (such as 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.

A predator is an animal that hunts other animals (prey) for food. Predation can act as a limiting factor on a prey's population size which in its turn can affect the predator's population size. 

• When the predator population gets bigger, more prey are eaten. 
• 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. 
• With fewer predators, fewer prey are eaten, and their population size increases. 
• With more prey, the predator population gets bigger, and the cycle starts again.

Competition happens when resources are not present in adequate amounts to satisfy the needs of all the individuals who depend on those resources. If a resource is in short supply in an ecosystem, there will be competition between organisms for that resource. As the intensity of competition increases, the rate of reproduction decreases (because fewer organisms have enough resources to reproduce), whilst the death rate increases (because fewer organisms have enough resources to survive). There are two types of competition: intraspecific competition and interspecific 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, while those not so well adapted fail to reproduce, or die. This slows down population growth and the population enters the stationary phase. 

Although there are slight fluctuations in population size during the stationary phase, intraspecific competition keeps the population size relatively stable. 

• If the population size drops, competition reduces and the population size increases. 
• If the population size increases, competition increases and the population size drops.

Interspecific competition happens between individuals of different species, and can affect both the population size of a species, and the distribution of species in an ecosystem. 

Gause concluded that more overlap between two species' niches results in more 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 idea became known as the competitive exclusion principle, and can be used to explain why particular species only grow in particular places. Often, though, it is not quite that simple. Other observations and experiments suggest that extinction is not necessarily inevitable. Sometimes, interspecific competition simply results in one population being much smaller than the other, with both population sizes remaining relatively constant.

It is also important to realise that in the laboratory it is easy to exclude the effects of other variables, so the habitat of the two 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 over the course of a year. 

Preservation is keeping species and habitats as they are now. The approach known as preservation focuses on keeping things 'natural' and eliminating any human effects on ecosystems that exist today. The most extreme preservationists even query whether fishing or logging should take place.

Conservation is a more active management process involving human intervention. Change in many ecosystems is almost inevitable, both through natural succession and human activity. As a result of human activity in the past, very few habitats in the UK are truly 'natural'. Therefore, adopting a preservation strategy would not preserve a natural habitat. Instead, conservation programmes (rather than preservation) focus on maintaining or improving biodiversity. This includes maintaining not just diversity between species, but also genetic diversity within species, and also maintaining a range of habitats and ecosystems.

Unfortunately, a steadily increasing human population can threaten biodiversity through:

• over-exploitation of wild populations for food (e.g cod in the North Sea), for sport (e.g. sharks), and for commerce (e.g. pearls collected from mussels); 'over-exploitation' means species are harvested at a 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.

The human population is getting larger increasingly quickly, and we have had to use more intensive methods to exploit our environment for resources. Such approaches can disrupt or destroy ecosystems, reduce biodiversity, and even completely remove the resource we originally wanted to harvest: our use of natural resources is not sustainable. 

There is potential conflict between our need for resources and conservation in, for example, wood and timber production, and in fish production. However, sustainable management and exploitation of these resources is possible, and can mean that biodiversity is maintained, whilst ensuring supplies and maintaining their economic benefits.

Coppicing provides a sustainable supply of wood. The stem of a deciduous tree (one that loses its leaves in the winter) is cut close to the ground. Once cut, new shoots grow from the cut surface and mature into narrow stems. These can be used for fencing, firewood or furniture. After cutting them off, new shoots start to grow again, and the cycle continues. 

Pollarding involves cutting the stem higher up, to prevent deer eating the emerging shoots. To provide a consistent supply of wood, woodland managers divide a wood into sections and cut one section each year. This is rotational coppicing. By the time they want to coppice the first section again, the new stems have matured and are ready to be cut. In each section, some trees are left to grow larger without coppiced. These trees are called standards, and are eventually harvested to supply larger pieces of timber.

Rotational coppicing is good for biodiversity. Left unmanaged, woodland goes through a process of succession, blocking out light to the woodland floor and reducing the number of species growing there. In rotational coppicing, different areas of woodland provide different types of habitat, letting more light in, and increasing the number and diversity of species.

In the past, large-scale production of wood for timber often involved clear felling all the trees in one area. This could destroy habitats on a large scale, reduce soil mineral levels and leave susceptible to erosion. Trees usually remove water from soil and stop soil being washed away by rain. Soil may run off into waterways, polluting them. Trees also maintain soil nutrient levels through their role in the carbon and nitrogen cycles. 

Clear felling is now rarely practised in the UK. Leaving each section of woodland to mature for 50-100 years before felling allows biodiversity to increase. However, such a time-scale is not cost-effective. Instead, modern sustainable forestry avoids this by working on the following principles:

• 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. 

Selective cutting involves removing only the largest, most valuable trees, leaving the habitat broadly unaffected.

Sustainably managing forests involves balancing conservation against the need to harvest wood, both to maintain biodiversity and to make the woodland pay for itself. If each tree supplies more wood, fewer trees need to be harvested. To achieve this, foresters:

• control pests and pathogens 
• only plant particular tree species where they know they will grow well 
• position trees an optimal distance apart. If trees are too close this causes too much competition for light, and they grow tall and thin, producing poor-quality timber. 

The fishing industry has high economic value, not just from sales of fish, but in providing livelihoods for millions of people. The Marine Stewardship Council has proposed three principles for sustainable management of fisheries:

• Fishing must take place at a level which allows it to reduce indefinitely. Over-fishing must be avoided, because it can fish populations to zero. If over-fishing happens, reducing fishing to let stocks recover can rapidly increase productivity and is good for profitability, given high stock values can support a more efficient harvest. The optimum is to maintain fish populations at the carrying capacity of their environment, while fishing excess of that capacity. 
• Fishing must be managed to maintain the structure, productivity, function and diversity of the ecosystem. This means there should not be permanent damage to the local habitat, and any effect on dependent species is minimised.
• A fishery must adapt to changes in circumstances and comply with local, national and international regulations.

Aquaculture can also provide sustainable fish stocks. Raising stocks of fish in aquaculture restricts the impact on oceanic fish stocks. Aquaculture is expanding rapidly, particularly in the developing world, is expected feed more people than and to traditional 'capture fisheries' in the near future.  

In the south of Nepal is the Terai region, made up of marshy grasslands, savannah and forests. It is densely populated, and home to endangered species including the Bengal tiger and the greater one-horned rhinoceros. There are many national parks in the region. For over 10 years, the forests in the Terai region 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. In response, the World Wide Fund for Nature (WWF) found that rural livelihoods are heavily dependent on the forests, which are also home to many of the region's endangered species. The forests can provide local people with a sustainable source of fuel, animal feed, food, building materials, agricultural and household tools, as well as medicines. Because local people have such a high stake in the forest, the WWF with the Nepalese government in the Terai-arc landscape programme focused on conservation of the forest landscape as a whole. 

To ensure conservation with development, they introduced community forestry initiatives in which local people had rights to exploit the forest as well as responsibilities to look after it. These community groups helped to create forest corridors between national parks, which are essential to the dispersal and survival of tigers, as well as taking the initiative in counteracting poachers and illegal felling. Forestry work also developed and diversified on- and off-farm activity, built entrepreneurial skills, and stimulated small credit and marketing schemes. The WWF scheme also introduced biogas plants and wood-efficient stoves to reduce demand for firewood. 

Other contributions to the Terai-arc project included constructing waterholes, monitoring endangered species, and eradicating invasive species. Community involvement combined with governmental and non-governmental (WWF) leadership appears to have been successful. Recent data from southern Nepal suggests tigers are using the corridors between national parks, and their population size steadily growing. 

The Maasai Mara in Kenya is a famous destination for wildlife watchers, with large populations of a and other large mammals. Because the Maasai Mara combines high endemic poverty with abundant wildlife populations that attract tourism, 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. 

After creation of national parks in 1945, remaining Maasai land was held in trust until 1968, when the lands were designated as 'group ranches'. Worried about their tenure on the land, many Maasai took individual title over smaller portions of land. This triggered land-use change, including intensification of agriculture. This limited wildlife to increasingly small islands and constrained the mobility of livestock. For example, wheat farms occupy 40,000 acres of the previous wet-season-range of migratory wildlife. The population of wildebeest that use the wet-season-range shrank from 150,000 in 1977 to 40,000 in 2010.

The density of other wildlife has dropped 65% over the last 30 years, while the density of sheep and goats increased. 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 (PWC) schemes. Conservancies are paid PWC revenue proportional to the area of land set aside for conservation. These conservancies are successful, because they have positive social outcomes, as well as positive conservation outcomes. 

There are some negative consequences of the conservancies. Land-owners must move their livestock out during the tourist season, which leads to increased stocking densities outside the reserve, where no one receives the PWC money. Likewise, land owners are often forced to settle elsewhere, and there are constraints on how they use their land.

Livestock have been seen as a problem for conservation. However, there is evidence that limited livestock grazing can have positive impacts on diversity. Given how important livestock are Maasai culture, it make sense to continue to integrate conservation would and livestock management more directly.

The Galapagos Islands have high numbers of native species, including Darwin's finches, specie giant tortoises and marine iguanas. Unfortunately, 50% of vertebrate and 25% of plant species are endangered. The Galapagos human population has grown in response demand to increased demand for marine products and increased tourism. 

The population size increase has placed huge demands on water, energy and sanitation services.  More waste and pollution have been produced, and the demand for oil has increased - an oil spill in 2001 had a dramatic effect on marine and coastal ecosystems. Building and conversion of land for agriculture has caused destruction and fragmentation of habitats. Forests of Scalesia trees and shrubs (a species unique to the islands) have been almost eradicated on Santa Cruz and San Christobal, make way for agricultural land. 

In the nineteenth century whaling boats and fur traders killed 200 000 tortoises in less than half a century. The Charles Darwin Research Station has a captive breeding programme to supplement tortoise numbers. The more recent boom in fishing for exotic species has depleted populations. Depletion of sea cucumber populations has a drastic effect on underwater ecology, and the international market for shark fin has led to the deaths of 150 000 shark each year around the islands, including 14 endangered species.

As well as out-competing local species, alien species can eat native species, destroy native species' habitats, or bring disease onto the islands. For example, cats hunt a number of species, including the lava lizard and young iguanas. Goats feed on Galapagos rock-purslane, a species unique to the islands, and trample and feed upon giant tortoises' food supply and disrupt their nesting sites. On northern Isabela Island, the goat has also transformed forest into grassland, leading to soil erosion. The red quinine is an aggressively invasive species on Santa Cruz Island. It occupies the highlands, and spreads rapidly - it has wind-dispersed seeds. The ecosystem in the highlands has changed from low scrub and grassland to being a closed forest canopy. Because of this, the native Cacaotillo shrub has been almost eradicated from Santa Cruz, and the Galapagos petrel has lost its nesting sites. The red quinine also successfully out-competes native Scalesia trees.

In 1999, the Charles Darwin Research Station adopted strategies - to prevent the introduction and dispersion of introduced species and to treat the problems caused by such species. They search arriving boats and tourists for foreign species. Natural predators have also been exploited to reduce the damage caused to ecosystems by pest populations - controlled release of a ladybird wiped out a scale insect, which was damaging plant communities. Culling has also been successful against feral goats on Isabela Island and pigs on Santiago Island. 

Because most residents were not born on the islands, fostering a culture of conservation and educating new arrivals about the islands is a challenge. The Galapagos marine reserve provides a is model of how local stakeholders can work to sustainably manage a resource. The reserve is managed by the National Park Service, the Charles Darwin Research Station, and representatives of local fishermen, the tourist industry and naturalist guides.  At least 36% of coastal zones have been designated 'No-Take' areas, where no extraction of resources is allowed, and communities are left undisturbed. 

Antarctica is not governed by any one country, but countries have research stations there under the terms of the Antarctic Treaty. As such, the continent is relatively untouched by human influence,  but with an increasing number of tourists and scientists, and with increased interest from the fishing industry, it remains important to actively protect the Antarctic ecosystem and biodiversity within it.

Krill are tiny shrimp-like organisms which provide food for whales, seals, penguins, albatrosses and squid. Krill are used to make nutritional supplements and for animal feed. Recent changes in technology mean that large amounts of krill can be harvested very quickly and easily. Fishing boats tend to congregate in those areas with the largest numbers of krill. However, natural predators of krill cannot adapt as easily to find krill elsewhere. For example, penguins do not migrate very far when raising young. To avoid over exploitation, there is a trigger level catch size in particular areas. When reached, fishing must be conducted equally across all areas, up to the total catch limit, to avoid a catastrophic impact on predators. However, given the size of the krill fishery, there are new recommendations to force the industry to fish evenly across all areas anyway.

To protect whales and the marine environment, a series of protected areas have been established. The Southern Ocean Whale Sanctuary was established in 1994, covering the summer feeding grounds of 80-90% of the world's whales. This followed the 1982 moratorium on whaling established by the International Whaling Commission. Within the sanctuary, it is illegal to hunt and kill whales, although monitoring of whaling activity still needs to be maintained to ensure the sanctuary is effective. Currently there is an initiative to expand a network of marine protected areas, such as in the Ross Sea (with its high biodiversity levels), which is already attracting the attention of the fishing industry. 

These majestic birds are threatened by human activities, including pollution, hunting and poaching eggs, habitat destruction and introduction of non-native predators. However, the biggest threat is long-line fishing. This is when fishermen trail a long fishing line behind their boat (up to 130km). Attached to this line are hundreds of baited hooks. When behind the boat, the birds try to eat the prey and swallow the hooks. To reduce the number of deaths, boats can use bird-scaring lines and streamers, weighted lines which sink more quickly out of reach of the birds, use lines at night to these avoid albatross and petrel feeding times, and avoid breeding and nesting time. By implementing methods, one Chilean fishery reduced its sea bird catch to zero. 

The Lake District contains an exceptionally rich diversity of species and habitats. The whole National Park is designated an Environmentally Sensitive Area. However, without grazing causing deflected succession, much of the land would revert to a climax community of oak woodland. Financial incentives are available for farmers to reduce chemical use, to safeguard hedges, and to care for hay meadows, heather moor, wetland, chalk downland and native woodland.

Spruce and pine in conifer plantations support limited biodiversity - Recent initiatives have generated more varied planting and felling patterns, giving a mosaic of smaller stands of different aged trees.

Invasive species, including rhododendron and laurel, have escaped from gardens and spread into woodland outcompeting native species. Their dense canopy reduces the light reaching the woodland floor and their roots produce toxic chemicals, which stop other plants growing (allelopathy). - They are physically removed by conservation workers.

Limestone pavement is a unique habitat characterised by solid blocks with fissures between them. Rare ferns grow well in the fissures and rare butterflies thrive in this habitat. -  The pavement is legally protected through the Limestone Pavement Orders.

Hay meadows occur in neutral grassland and support a rich diversity of flowers and grasses. They are under threat due to a preference away from haymaking and in favour of silage production, which involves use of artificial fertiliser and an earlier cut. This has caused loss of species diversity on grasslands and pastures. - Farmers are paid to maintain hay meadows.

Heathland is an open habitat with small shrubs like heather, which are important for butterflies, moths, spiders, beetles, birds and reptiles. - By burning strips of vegetation, new shoot growth is promoted, maintaining areas of different ages, which fosters a bigger variety of animals. This ensures a constant food supply for red grouse and merlin. Such areas are also managed by grazing, but overgrazing is a problem as the shoots of heather can be eaten more quickly than they regenerate. Financial incentives are provided to farmers to prevent overgrazing.

Mires are nutrient-poor, waterlogged ecosystems, within which mosses and liverworts, lichens and sedges flourish. The habitat is internationally scarce, provides a breeding ground for moorland birds, but is under threat from burning, grazing and drainage for more intensive agriculture. Peat extraction for gardeners has also threatened the habitat. - Mires are now managed more sympathetically, with some being rewetted with artificially controlled water levels. In areas with rare plants, like the bog orchid, grazing has been controlled.

Cliff, rock and scree communities support a rich diversity of plant life, providing a habitat for the stonechat and wheatear, and nesting sites for the peregrine falcon and golden eagle. These communities are easily damaged by climbers and walkers. -  To protect them, there are seasonal restrictions on walking when birds are nesting, walkers are educated to be more aware, and paths are well maintained to prevent people walking off the path. 

Snowdonia in North Wales attracts walkers and climbers. On Mount Snowdon itself, good footpaths are maintained to ensure that rare plants are not trodden on. Gutters take water from the paths, but these can get blocked by rubbish dropped by walkers. To prevent the path eroding, teams of workers clear the rubbish. National Park employees work with farmers to reduce sheep grazing on the mountain. Because they graze very low to the ground, they can leave the landscape very barren. Reducing sheep grazing gives the rare plants a better chance of survival. Feral goats are a problem for grazing, and their numbers and locations are monitored each year. Farmers are encouraged to plant hedges and conserve ancient woodland. 

As well as mountain habitat, there is also moorland and bog in Snowdonia. These habitats provide nesting sites for rare birds like the hen harrier, merlin and kestrel, and are home to a number of rare butterflies. Use of such wetland leads to humans having an impact upon it, which needs to be controlled.

Farmers dig open drainage ditches to dry the land. However, this causes poor water quality in rivers, and rain flows quickly through the habitat, increasing flood risk. - To mitigate the effect, drainage ditches can be blocked by hay bales.

Conifers are planted as cash crops. However, this dries out the moorland as the trees absorb water, and roads have to be built to carry the wood away, compacting the land. - When the trees are cut down, branches are used to block drainage ditches to slow water flow and hence keep the land moist.

The moorland was burnt to provide a varied habitat for grouse, but when sheep grazed the land, burning was stopped. Old heather burns easily, and so accidental fires can be a high risk. If the peat sets on fire, that can damage the habitat on a large scale. - Burning controlled fires before heather gets too old and dry is important to prevent such damage.