Local species extinctions are often underestimated

A new study led by the University of Oldenburg and published in the journal Nature Ecology & Evolution has found that some species in seemingly healthy ecosystems with a constant or even increasing number of species may already be on the path to extinction. Due to systematic distortions in temporal trends for species numbers, even in long-term datasets, such negative trends may only become apparent after a significant delay.

“Our results are important in order to understand that the species number alone is not a reliable measure of how stable the biological balance in a given ecosystem is at the local level,” said lead author Lucie Kuczynski, an ecologist at Oldenburg. “Apparently, we have so far underestimated the negative trends for freshwater fish, for example. Species are disappearing faster than expected at the local level.”

“We find it very worrying that a constant or even increasing species number does not necessarily mean that all is well in an ecosystem and that the number of species will remain constant in the long term,” added senior author Helmut Hillebrand, an expert in biodiversity at the same university.

Focus of the study

Until recently, scientists studying biodiversity have assumed that the number of species in an ecosystem will remain constant in the long term if the environmental conditions neither improve nor deteriorate. According to Kuczynski, the main hypothesis underlying such assumptions is that there is a dynamic equilibrium between colonizations and local extinctions.

To clarify whether a constant species richness is a reliable indicator of a stable ecological balance, the researchers analyzed thousands of datasets documenting the number of species of freshwater fish and breeding birds in Europe and North America, over a period of 24 years for the fish and 37 for the birds. Then, they compared this empirical data with simulation models based on different expectations about immigrations and extinctions of various species.

What the researchers learned 

Although the scientists initially observed a general increase in species richness in both fish and birds, a comparison with the simulations revealed that this increase was smaller than what would have been expected – a phenomenon most likely caused by an imbalance between colonizations and local extinctions. 

“According to our simulations organisms such as freshwater fish which have limited potential for dispersal colonize an ecosystem faster than in neutral models, while their extinction occurs later than expected,” Kuczynski explained.

This means that, after environmental changes, species that are on the brink of extinction may remain present in an ecosystem for some time, while new species gradually move in. “There are transitional phases in ecosystems in which the number of species is higher than expected. Species extinction occurs only after these transition phases – and then usually faster than expected,” said Kuczynski.

These findings highlight the need for a reassessment of which methods are most reliable for monitoring the state of ecosystems and a redefinition of nature conservation targets. The model devised in this study could serve as a tool to distinguish between the various mechanisms influencing species richness, while providing valuable information on the extent to which observational data deviates from expected changes.

More about local species extinctions

Local species extinctions occur when a particular species ceases to exist in a specific geographic area or habitat. These extinctions can be driven by a variety of factors, including habitat loss, overexploitation, pollution, climate change, invasive species, and disease. 

Local extinctions can have significant ecological consequences, as they can disrupt the balance of ecosystems and lead to cascading effects on other species.

There have been numerous instances of local species extinctions throughout history. Some well-known examples include:

Passenger pigeon (Ectopistes migratorius)

Once the most abundant bird in North America, the passenger pigeon was hunted to extinction in the late 19th century. Its habitat destruction and mass hunting led to the disappearance of this species from its original range.

Carolina parakeet (Conuropsis carolinensis)

 Native to the southeastern United States, the Carolina parakeet was the only parrot species native to North America. It was heavily hunted for its feathers and as agricultural pests, leading to its extinction by the early 20th century.

The dodo (Raphus cucullatus)

The dodo, a flightless bird endemic to the island of Mauritius, became extinct in the late 17th century. Habitat destruction, hunting by humans, and the introduction of invasive species like rats and pigs were major contributors to its extinction.

Tasmanian tiger or thylacine (Thylacinus cynocephalus) 

The thylacine was a carnivorous marsupial native to Tasmania, Australia, and New Guinea. It was hunted and persecuted due to fears of predation on livestock, leading to its extinction in the early 20th century.

Baiji dolphin (Lipotes vexillifer)

Also known as the Yangtze River dolphin, the Baiji dolphin was endemic to the Yangtze River in China. Its decline was primarily caused by habitat degradation, pollution, overfishing, and ship traffic. It is believed to have become extinct in the early 2000s.

These examples illustrate the irreversible loss of species within specific regions. Local extinctions highlight the urgent need for conservation efforts to protect vulnerable species and their habitats to prevent further biodiversity loss.

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By Andrei Ionescu, Earth.com Staff Writer

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