Scientists have found a new explanation for Darwin’s 160-year-old mystery

The answer turned out to be not quite so simple. It is not only important to consider which species already live in a lake, forest or other ecosystem. It is also important to consider which species could live there but, for some reason, are currently absent.

Scientists refer to this invisible part of nature as ‘hidden biodiversity’. This new approach has helped to explain why different studies previously produced conflicting results.

The study has been published in the journal *Proceedings of the National Academy of Sciences*.

Details

Darwin identified the problem as far back as the 19th century. He arrived at two almost contradictory ideas.

First: an alien species is more likely to establish itself if it resembles native species. This means that the climate, food and conditions suit it.

Second: an introduced species is more likely to establish itself if it does not resemble local species too closely. In that case, it competes less with them for the same food and habitat.

Both ideas are logical. That is why ecologists have been debating for over 160 years: when does the first theory hold true, and when does the second?

New research offers a simple answer: it all depends on how ‘full’ the ecosystem is.

If, in theory, a lake can support only a few species, but almost all suitable species are already there, it is better for an introduced species to resemble the native ones. This means it is well-suited to these conditions.

But if a lake can accommodate many different species, yet some ‘niches’ in the ecosystem remain vacant, species that are less similar to the locals may be more successful. They occupy the vacant niches and face less direct competition.

What is ‘hidden biodiversity’?

We usually think of biodiversity in this way: we look at which species are actually present in a given location.

But that is not the whole picture. For example, suppose a lake is home to 10 species of fish. The conditions are actually suitable for several more species, but they are not there. These constitute ‘hidden biodiversity’.

Put simply, these are the ecosystem’s potential inhabitants. They could live there, but are currently absent.

Together, the actual species and the ‘hidden’ species show how full the ecosystem is. If most of the suitable species are already present, there are few opportunities left. If many suitable species are missing, a new species has a better chance of establishing itself.

How this was tested

The researchers used a rare dataset: a 340-year history of fish colonisation in Swedish lakes. The analysis included both successful and unsuccessful introduction attempts — that is, instances where fish were transported or released into new water bodies.

According to East China Normal University, the study covered 516 lakes, 748 cases of introduction and 22 species of non-native fish.

The result was as follows: the simple number of native species was a poorer predictor of the success of non-native fish. Two indicators were far more important: how many species could theoretically live in the lake, and what proportion of that set was already present.

It was precisely this that helped to reconcile Darwin’s two ideas. It turned out that both could be true — simply under different conditions.

Why this is important

This research is not just about fish, nor is it just about Darwin. It is important for understanding how species spread across the planet.

Today, animals, plants and microorganisms are increasingly finding themselves in new places. They are transported by people, trade, transport, climate change and the alteration of natural habitats.

Some non-native species do not survive. Others settle in without any problems. And others become invasive — they start to displace native species and alter the ecosystem.

If scientists gain a better understanding of which species are likely to establish themselves in a new location, it will be easier to assess the environmental risks. For example, which fish, plants or other organisms might become a problem as the climate warms or following accidental introduction by humans.

Background

In ecology, there is the concept of invasive species. These are species that spread beyond their usual range and begin to proliferate, sometimes causing harm to the local environment.

The classic question is: what is more important for the success of an alien species – similarity to local species or difference from them?

If a species resembles local ones, this is a sign that the environment suits it. But similarity also implies competition.

If a species is very different, it may be able to utilise different resources. But then there is a risk that the environment itself will not suit it.

A new approach based on ‘hidden biodiversity’ shows that we must look not only at visible inhabitants, but at the entire potential range of species that an ecosystem could support. This helps to explain why scientists previously obtained differing results.

Source

Study: Wen-gang Zhang et al., “Dark diversity framework reconciles Darwin’s naturalisation conundrum for freshwater fish invasions”, Proceedings of the National Academy of Sciences, 2026.