The invisible majority: Invertebrate animals
Invertebrate animals (Invertebrata), as the name suggests, are characterized by the absence of a backbone, unlike vertebrates (Chordata). This group encompasses a wide variety of organisms, including insects. Also included among invertebrates are, for example, spiders, snails, clams, jellyfish, octopuses, scorpions, crabs, starfish, worms, corals, or tardigrades. The exact number of invertebrate species on Earth is difficult to determine as many species have not yet been discovered or scientifically described. It is estimated that there are millions of invertebrate species, with more than 95 percent of all animal species being invertebrates.
The diversity of invertebrate species enables them to inhabit a wide range of habitats spanning across various ecosystems. Some live in hydrothermal vents, on the ocean floor, or in coral reefs, while others inhabit freshwater habitats such as rivers, lakes, or wetlands. Additionally, invertebrate animals are found in terrestrial habitats such as forests, meadows, deserts, and the Arctic tundra. Some species have adapted to extreme conditions such as the aridity of deserts or the cold of polar regions.
This diversity and the often small size of invertebrate animals make it difficult to accurately determine their conservation status. The International Union for Conservation of Nature (IUCN) has listed 384 invertebrate species as extinct, including 60 species of insects. Most of these extinct species (297) are mollusks such as snails, clams, squids, and octopuses. It is presumed that many more invertebrate species have become extinct as the IUCN is extremely cautious in its assessments and lists some species as “critically endangered” even though they have not been observed for over 100 years.
Habitat loss – one of the greatest threats
The reasons for the disappearance of invertebrate animals are as diverse as the animals themselves, ranging from habitat loss to threats from parasites. A major factor is often human intervention in their habitats, whether through deforestation, mining, or urbanization. The overuse of resources such as overgrazing, overfishing, or the collection of animals for trade also contributes to species disappearance. Climate change and environmental pollution further exacerbate the situation by affecting the animals’ living conditions and making them more susceptible to diseases.
The mussel crayfish Namibcypris costata from Namibia likely went extinct in the mid-1990s due to the cleaning of the springs where it lived. (© Apokryltaros, CC BY-SA 4.0, via Wikimedia Commons)
Some invertebrate species live only in a single small location, and if that habitat is destroyed, it leads to the extinction of the species. This was the case with the Pasadena freshwater shrimp: the water body where it lived was completely destroyed by canalization and urbanization. The site where the shrimp species was discovered in 1897 is now occupied by the Rose Bowl Stadium.
Another example is the Lake Pedder earthworm, which was found exclusively in Lake Pedder, a lake in Tasmania, and went extinct in 1972 shortly after dams were built on the lake. The former sandy beaches around the water body, where the earthworm lived, are now underwater.
The giant Brazilian earthworm, which grew to over two meters long, likely had a very limited range as well, so habitat loss led to its extinction in the early 20th century. The Cascade funnel-web spider, endemic to Tasmania, also likely disappeared around 1926 due to urban developments that destroyed its habitat.
Extinct invertebrates due to invaders
Endemic species that occur only on islands are particularly vulnerable. The destruction of their habitats and the introduction of animals and plants often lead to their demise. The list of extinct species that were once native to islands like the Seychelles or Hawaii is enormously long.
Before humans settled the Hawaiian Islands, there were about 750 species of tree snails there. Today, most of them are considered endangered or already extinct. Of the 41 nocturnal snail species of the genus Achatinella, which live exclusively on the island of Oahu, at least 16 are extinct. Two examples are Budd’s Oahu tree snail, which has not been sighted since 1900, and Yellow-tipped Oahu tree snail, the last specimen of which died in 2019.
The main reasons for their disappearance, besides snail collection and loss of their island habitat, are introduced invasive animals and plants, such as the rosy wolfsnail (Euglandina rosea) and black rats (Rattus rattus). The rosy wolfsnail was introduced to Oʻahu in the 1950s to combat the previously introduced giant African land snail but ended up preying on tree snails as well.
Also, on the Seychelles, where more than 1,000 invertebrate species are endemic, numerous animals like rats, pigs, cattle, or chickens, as well as plants like the cinnamon tree, were introduced by humans, partly destroying the native flora and fauna. Since the late 19th century, at least five harvestman species have become extinct on the Seychelles, including Brauer’s spiky harvestman.
The sooty crayfish went extinct after the signal crayfish, along with crayfish plague pathogens, a deadly fungal disease in crayfish, reached California. Today, the signal crayfish is found in most European countries as a non-native species. It poses a significant threat to the three native crayfish species of Central Europe.
When hosts of parasites go extinct
The extinction of a species of animal or plant can trigger a chain reaction, leading to the disappearance of at least one more species. This is particularly true for parasites classified among invertebrates that are closely associated with a host. If the host species becomes extinct, the parasite is also lost. For example, with the extinction of the Passenger pigeon, native to North America, the Passenger pigeon mite also went extinct in 1914. Similarly, the Caribbean monk seal nasal mite, which exclusively inhabited the respiratory passages of the Caribbean monk seal, extinct around 1952, faced a similar fate. Other examples include the feather mite Chiasmalges carolinensis, which lived parasitically on the feathers of the Carolina parakeet, or the chewing louse Rallicola extinctus, which specialized on the huia as a host.
For some lesser-known invertebrate species, only speculations can be made about why they became extinct. This includes, for example, the eyeless cave scorpion Akrav israchanani, which presumably only occurred in two cave systems in Israel. The extinction of the land snail species Plegma duponti and Tropidophora carinata, native to Mauritius, in the 19th century, remains unclear as well.