The disappearance of one species often has a cascading impact on entire ecosystems. Researchers have calculated using a computer model that more than half of the food web connections of terrestrial mammals have been lost in the past 130,000 years due to the extinction of species or reduced ranges.

The emergence and spread of humans led to a sharp decrease in the complexity of food webs in the respective region, as the scientists working with Evan Fricke wrote in the journal Science at the time of the study at Rice University in Houston. While the decline in mammals is a well-documented feature of the biodiversity crisis, it has not been clear to what extent these losses have already impacted global networks.

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The food web is defined as all the connections between predators and their prey in a geographic area. There are simpler systems in which, for example, an animal species has only one or a few predators or a predator specializes in a specific prey. A network is complex if, for example, a predator has a whole range of species on its menu.

If such a predator dies out, several connections are lost at the same time, the network becomes simpler – and thus more unstable, as Eoin O’Gorman explains in a comment on the study. Both in the past and in the present, species with many connections have disappeared the most, as Fricke’s researchers write.

A well-known example of a cascade of consequences when a species becomes extinct is the overhunting of sea otters. The predators live near the coast of Alaska and the North American Pacific coast, where they are at the top of the food chain. Hunted to the point of extinction, there were hardly any sea otters that ate sea urchins. These spread rapidly and consumed far greater amounts of seaweed than before. This in turn is the habitat of a number of species and the nursery of many fish. In addition, seaweed binds large amounts of climate-damaging carbon dioxide and protects coasts from storms.

There have also been similar consequences in other cases in recent history – which often also reflected back on their causer, the people. Because many lions and cheetahs were shot in Africa, baboons multiplied and spread and got more contact with humans – with more parasite transmissions as a result. Hunting sharks in a coastal ecosystem led to the proliferation of rays, which in turn led to the collapse of mussel populations. And when India’s vulture populations shrank by 90 percent or more because of the toxic painkiller diclofenac used in cattle, feral dogs proliferated because of the now often abandoned carcasses – which resulted in more rabies transmissions.

The results of Fricke’s team not only underline the effects of species loss on the long-term survival and functioning of ecosystems: They also show the possibility of restoration, as the scientists emphasize. About half of the calculated declines in the food web are due to the extinction of species – but the others are due to the reduction of the range of existing species. Reintroducing these species throughout their original habitat has great potential to restore more complex food webs.

The findings of ecosystem researchers are mostly based on long-term observations, stomach content analyzes and experiments to clarify the complex structures within the many food webs worldwide. The scientists working with Fricke used a different approach: they compiled a global database with characteristics of extinct and extant mammals, geographic ranges and predator-prey interactions. Based on this, they used a machine learning approach to model changes in the global food webs of terrestrial mammals since the Late Pleistocene.

Although only about 6 percent of terrestrial mammal species have died out since then, more than half of the global food web connections have been lost. “When an animal disappears from an ecosystem, its loss affects the entire web of connections that connect all species in that ecosystem,” says Fricke. His team’s work offers new tools “to measure what has been lost and what will be lost as endangered species become extinct.” It is also possible to see the ecological complexity that can arise again, for example as a result of settlement projects.