How Many Species Are There?

Have you ever wondered just how many different species of living things there are on our planet? This is a hard question to answer! The actual number of living organisms on Earth is unknown to us, but so far, scientists have described up to 1.7 million species of living organisms. Of those, over 1.3 million are animals, 100,000 are fungi, over 300,000 are plants, plus countless species of bacteria, algae, and viruses [1]. This incredible variety of life forms is what we know as biodiversity, that is, all kinds of life existing in a certain place.

It is difficult to know the actual number of living organisms on Earth, or what percentage of these organisms are already known to us. In some cases, there is quite a lot of debate. For example, the estimated number of animals varies from 2 to 11 million species. And while the average estimate of known plant species is somewhere around 300,000, the predicted number of plants, including yet unknown species, is around 450,000 species [1]. In any case, scientists agree that, despite the great number of species that have already been discovered, it is likely that there is an even larger number of species yet to be discovered.

Now you might wonder, “Okay, that is nice to know, but what difference does it make?” It makes all the difference! When we are trying to protect the biodiversity on our planet, knowing how many species are out there is as important as knowing how and where they are distributed around the globe, so that we can decide which areas of the planet are priorities for conservation, especially if we cannot protect every area.

Biodiversity Is Unevenly Distributed Around the Globe

With the enormous number of species that exists on Earth, it is remarkable that the distribution of these species is so highly concentrated in specific areas. Species richness, the total number of species found in an area, is not evenly distributed around the globe: two-thirds of all known species occur in tropical areas, especially in tropical forests [2], even though the causes of such uneven distribution are still a matter of debate.

In order to prioritize the areas that should be protected, scientists look for areas that are home to a large number of species, especially those species that are under threat of extinction or that are currently being destroyed at a fast pace. These areas that are particularly important for biodiversity conservation are called biodiversity hotspots. Two things are crucial when determining that a place is a biodiversity hotspot: (i) the number of different species there; and (ii) whether species in that area are endangered or currently being destroyed. Figure 1 shows the location of 36 biodiversity hotspots, according to the Critical Ecosystem Partnership Fund.

Scientists have observed that, even though biodiversity hotspots make up only approximately 1.4% of land on our planet, they are home to 60% of Earth’s plant, bird, mammal, and reptile species [3]. Just the endangered species in the tropics account for 43% of vertebrates (animals with backbones and its close relatives) and 80% of all amphibians [4]. Species that found only in a certain geographical area are known as endemic species, and biodiversity hotspots are full of them! For example, the Banana Tree Frog can only be found in Ethiopia, and you will only find lemurs in Madagascar [5]. Tropical forests are typically biodiversity hotspots and are usually filled with endemic species. The Upper Amazonia/Guyana Shield, the Congo Basin, and the New Guinea/Melanesian Islands have the highest number of endemic terrestrial (land-living) species on Earth [6]. Figure 2 shows some examples of biodiversity hotspots and some of their endemic animals and plants.

In addition to land, the waters surrounding these tropical regions are just as important, and equally in danger [4]. Tropical coral reefs are currently being threatened by climate change and habitat destruction. These areas are some of the most biodiverse ecosystems on our planet! Scientific studies of 3,235 marine species in these areas, including fishes, corals, snails, and lobsters, show that high percentages of these species are at serious risk of becoming extinct [7]. Conservation of species living in fresh or seawater is especially difficult, because many bodies of water are interconnected. For example, all the oceans are connected through sea currents that allow the movement of species, minerals, and pollution across the entire globe [8].

Extinctions and a Look Into the Future

Interestingly, the organisms that we know today represent only a very small portion of all the living creatures that have inhabited the planet since life began. All the species living today represent only 5% of all the species that have roamed the Earth during its history! This is a reminder that extinction is a constant force shaping Earth’s biodiversity.

However, many scientists would agree that, today, we are facing a rate of species extinction that is faster than has ever been seen before. Plant and animal species in biodiversity hotspots are currently suffering devastating losses. In fact, by definition, a biodiversity hotspot must have lost at least 70% of its habitat [9]. Biodiversity hotspots now cover only 1.4% of the land on Earth, when they originally covered 12% of the land [10]. Factors, such as pollution, exploitation of land, invasive species, deforestation, and climate change are the leading causes of habitat loss and destruction [11]. The fact that these factors are widespread creates challenges for the species that manage to survive; and with an ever-changing climate and unpredictable circumstances, species that cannot resist the changing environment or move to a more suitable habitat will likely become extinct [4].

When we think about the future of biodiversity on Earth, we need to consider the role we play in climate change. Some scientists predict that up to 54% of species are at risk of extinction due to climate change. The consequences of climate change are extremely widespread, threatening even places untouched by humans [12]. In order to protect our planet, we can start by making small changes in our daily lives. Taking action by recycling, picking up trash, being conservative with our water consumption, and limiting pollution by walking, biking, or taking public transportation are ways that we can help the environment. We can also come up with our own ideas, as we educate ourselves on biodiversity by reading about different places and living things.

Summary

As we have learned, a region with a large number of species is considered to be biodiverse. There are 36 biodiversity hotspots on our planet, and these areas are dazzling, unique, and full of life. Plants, animals, and other living organisms that populate these places are rare and many of them are only found in these specific geographic areas. These biodiversity hotspots are currently at risk of being destroyed [13]. Habitat destruction directly caused by humans, as well as destruction due to climate change, are the leading causes of the increasing extinction of Earth’s species.

Glossary

Conservation: ↑ Taking care and preserving species and the environment.

Species Richness: ↑ The number of species living in a specific location.

Tropical Area or Tropical Zone: ↑ Area situated between the Tropics of Cancer and Capricorn. See Figure 1 for the location of the location of the Tropics on the globe.

Extinction: ↑ The death of an entire species (like dinosaurs).

Biodiversity Hotspots: ↑ Geographical areas with very large numbers of species that are threatened by destruction.

Endemic Species: ↑ Species that occurs only in a specific geographic region.

Climate Change: ↑ The change of weather and temperature patterns around the world.

Deforestation: ↑ The destruction of forests or the removal of great number of trees.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

AA would like to thank INCT (IN-TREE, 465767/2014-1) for support.