Biodiversity & Extinction

Biodiversity is the coexistence of many different species in an ecosystem. The many interrelationships between species keep the ecosystem stable — for example, by maintaining the numbers of predators and prey in food webs — and are essential for the perpetuation of communities.

Within one species, diversity is genetic. A gene pool is all the alleles in a population, and a large gene pool means high genetic diversity — more chance that some individuals carry the traits needed to survive a new selection pressure. Genetic diversity is the main indicator of a species' potential for extinction: low numbers mean low diversity, and low diversity means a changed environment can eliminate the whole population at once.

A population bottleneck strips genetic diversity, leaving the survivors more vulnerable to change.

Even if a low-diversity population survives a crisis, recovery is slow: it takes many generations, and new mutations during DNA replication, to introduce new alleles back into the gene pool.

EXTINCTION AND ENDANGERED SPECIES

Extinction is the permanent loss of a species — the population equals zero. A species is endangered when, at the current rate of decline, it is likely to become extinct in the near future. Before extinction, species pass through risk categories: vulnerable, critically endangered, and extinct in the wild (surviving only in captivity). Conservation programs aim to breed and reintroduce extinct-in-the-wild species, but this can be difficult because the captive gene pool is so diminished.

Attempts to preserve species in isolation (usually in zoos) have rarely succeeded, because organisms require complex interactions with their environment in order to survive and perpetuate. The best way to preserve a species is to preserve its natural habitat — with its food supplies, breeding grounds and interconnected species. Maintaining an unaltered habitat also reduces selection pressures on the species, reducing the risk of extinction.

Extinction is not only caused by humans. Natural events — meteorite impacts, ice ages and volcanic eruptions — have driven species extinct throughout history. What is different now is the rate of extinction, and its cause.

HUMANS AS A SELECTION PRESSURE

Human activities act as selection pressures on other species. Natural selection favours individuals with the traits needed to survive the new conditions, shifting allele frequencies. Over many generations the species either adapts — or, if no individuals carry the required traits, the population dies out, which can mean extinction of the species.

Adapt or die out: a new selection pressure only shifts allele frequencies if the needed traits already exist in the gene pool.

The human influences that can push species toward extinction go well beyond habitat clearing:

• Habitat destruction - deforestation and urban development.
• Pollution - runoff, rising carbon dioxide emissions, plastic use and waste.
• Hunting - poaching of species.
• Farming - land clearance, fertilisers and other chemicals.
• Removal of water - taking water from natural water systems, and reducing water quality.
• Depletion of natural resources - removing habitats and resources.
• Decreasing air quality - transport and the burning of fossil fuels.
• Increasing waste - food packaging, more landfill, less recycling.

Behind all of these sits exponential human population growth, which is not sustainable and drives enormous demand for food, water and energy. Unlike past mass extinctions, the current crisis is considered the culmination of human impacts over the last several thousand years: modifying the landscape, using species unsustainably, extracting natural resources, and moving species between environments.

CLIMATE CHANGE

Climate change is already visible: rising sea levels in low-lying Pacific islands are forcing communities to higher ground, and global temperatures have already risen 1 °C, with a predicted rise of 3–5 °C in the next century. The consequences include:

1. Hotter, longer summers with more frequent natural disasters.
2. Evaporation and drought in places that cannot afford to lose water, reducing food supply.
3. Erratic, unpredictable rainfall, and extreme flooding from cyclones and hurricanes.
4. Melting polar ice caps, further raising sea levels.

CASE STUDIES: LOW GENETIC DIVERSITY

The Tasmanian devil's case is the most striking: the tumour spreads as living cancer cells transferred between animals. This is possible only because the devils' immune systems are too genetically similar to recognise the cells as foreign.

WHY MAINTAIN BIODIVERSITY?

Maintaining biodiversity is an ethical issue with long-term biological and environmental consequences, and there are concrete reasons behind the obligation: