The Number of Species on Earth

Estimating Numbers of Species

• Current global species estimates vary, but it is generally accepted that there are between 5 and 10 million species on Earth

• As of December 2022, just over 2.16 million species have been described and classified

• However, the actual number of species could be higher or lower depending on a variety of factors, including classification issues, under-sampling of certain groups, and differences in taxonomic opinions

Challenges of Estimating Species Numbers

• Estimates of the total number of species on the planet vary considerably due to a number of factors, including:

  • Classification issues

  • Lack of funding for scientific research

  • Limited access to remote and inaccessible habitats

• The classification of species is not always clear-cut, and different taxonomists may have different opinions on what constitutes a species

  • As a result, there may be significant variation in the number of species recognised within a particular group

  • Additionally, the discovery of new species can lead to changes in classification, which can further complicate the process of estimating the total number of species

• Another factor that contributes to the variation in global species estimates is the limited funding for scientific research

  • Many areas of the world remain relatively unexplored, and new species are still being discovered in remote and inaccessible habitats

  • Without adequate funding for research and exploration, many of these species may go undiscovered and unrecorded, making it difficult to estimate the total number of species on the planet

• In addition, some groups of organisms are more difficult to study than others, which can result in a significant underestimation of the total number of species within those groups

  • For example, the diversity of microorganisms is poorly understood, and many species have yet to be discovered and described

  • Similarly, deep-sea organisms are significantly under-sampled

Pie chart showing the estimated proportions of different groups of organisms on Earth today, by numbers of species

Current Species Loss

• Current rates of species loss are far greater now than in the recent past, due to increased human influence

• The human activities that cause species' extinction include:

  • Habitat destruction

  • Introducing invasive species

  • Pollution

  • Overharvesting

  • Hunting

• The loss has been compounded by rapid human population growth on Earth since the 18th Century because:

  • Humans are highly intelligent and can colonise many types of habitat

  • Humans have developed the ability to travel across continents and transport other species with them

  • Industrialisation has led to the more rapid consumption of natural resources by humans

  • Humans need food, space for infrastructure (eg. houses, roads), grazing land for livestock, clean water, and natural resources (eg. fossil fuels, rocks, wood)

• This has created selection pressures on many other species, which are being outcompeted

  • For many species, the inevitable outcome is extinction

Habitat Destruction

• Natural events such as hurricanes, landslides and tsunamis can cause major damage to habitats; however, it is often only temporary and habitats can recover

• Over the past 1000 years humans have made major advances in agriculture, construction and industry but it has had major permanent consequences for habitats

• Causes of aquatic habitat loss include: destructive fishing techniques, dredging of wetlands, damage from ships, tourism and pollution

• Causes of terrestrial habitat loss include: inland dams, deforestation, desertification, agriculture and pollution

• When a species’ habitat is destroyed or degraded then they no longer have the support systems and resources they need to survive

Introducing invasive species

Photo by Shannon Kunkle on Unsplash 

Purple loosestrife is considered an invasive species in North America and New Zealand where it can out compete native species and cause disruption to the ecosystem in canals and rivers

• When humans travel between countries and continents, they often exchange (either intentionally or unintentionally) animal and plant species between their home country and the foreign country

  • These introduced species are non-native

  • Non-native species can be highly problematic as they often have no natural competitors, predators or pathogens that help limit population growth

  • Without these natural population checks, non-native species can massively increase in number

  • The large numbers of non-native species can negatively affect the native species through factors such as competition and disease

• For example, grey squirrels have led to the decimation of the native red squirrel population in the UK

  • Grey squirrels were introduced to the UK in 1876

  • They quickly grew in numbers

  • The larger grey squirrels compete with red squirrels for food

  • They also carry and transmit a disease known as squirrelpox which is fatal to red squirrels

Pollution

Photo by Nithin Sameer on Unsplash 

Oil spills cause devastating impacts on ecosystems

• Pollution is the addition of contaminants into a natural environment that negatively impact that environment

• Fertilisers, fossil fuels and non-biodegradable plastics are all examples of pollution that can seriously harm ecosystems

  • For example, many species within aquatic ecosystems have been negatively affected by large crude oil spills

  • The oil doesn’t break down for many years and it can kill thousands of birds and fish

• Sewage and runoff from factories can contaminate the soil and upset the nutrient balance in terrestrial and aquatic ecosystems

Overharvesting

• Humans harvest many plant and animal species

• When humans remove individuals from a natural population at a rate greater than the population’s natural growth rate, overexploitation is occurring

• Continued overexploitation of a species can drive it to become extinct

  • For example, the popular commercial fish tuna has seen a dramatic decline in numbers in Europe due to overfishing

  • To combat this, the Council of Fisheries in the European Commission has introduced total allowable catches (TACs) or catch limits (measured by weight or number of fish)

• Many tropical rainforests are under threat from overexploitation

  • They have major ecological and economic value

  • The trees are being cut down and harvested at a rate much faster than reforestation takes place

  • There are many initiatives and organisations around the world with the aim of protecting tropical rainforests

Hunting

• In the past humans would have hunted, killed and eaten wild animal species in order to survive

• Nowadays most of the meat that humans consume comes from domesticated livestock like cattle, sheep and chicken

  • The development of livestock and the food processing industry has negated the need for many humans to hunt wild animals

• Some humans in underdeveloped countries still have to hunt animals for survival

• The hunting of wild animals is still common and has become a sport for some individuals

  • The rarer and more vulnerable species are often more desirable for a sport hunter

• If too many individuals within a species are killed then the population will become so small that it is no longer able to survive

Human activities that lead to a loss of biodiversity