The species living on earth represent between 85% and 95% of the biodiversity on the planet. This is particularly impressive when we consider that the continents cover only 30% of the surface of our planet. And that most terrestrial species are the descendants of a small number of pioneer groups that invaded the land about 400 million years ago.
Surprisingly, scientists disagree about when terrestrial biodiversity will reach modern levels. What we see today is typical of the last tens, even hundreds of millions of years? Or has diversity grown exponentially, with more species alive today than ever before?
In a new article in Nature Ecology & Evolution, my co-authors and I examined the evolution of the diversity of terrestrial vertebrate species living in "local" ecosystems (also called "ecological communities") during 375 million years past. We analyzed nearly 30,000 fossil sites that produced fossils of tetrapods, terrestrial vertebrate animals, such as mammals, birds, reptiles (including dinosaurs) and amphibians. Counting species within fossils allowed us to estimate the diversity of ancient ecological communities.
Our results show that the wealth of terrestrial biodiversity observed around the world is not a recent phenomenon. The diversity of tetrapod ecosystems has been similar for at least 60 million years, shortly after dinosaur extinction. This suggests that the prevailing view that biodiversity within ecosystems increases more or less continuously over time is incorrect. Instead, it is likely that the way species interact – for example, by competing for resources such as space and food – tends to limit the number of species that can be integrated into local ecosystems.
This does not mean that the local diversity of tetrapods has not increased over the previous 375 million years. Our results also show that this diversity is at least three times greater today than it was about 300 million years ago, when tetrapods first developed both key innovations for life on earth (like the amniotic egg, which allowed reproduction away from water sources). However, we have discovered that increases in diversity are rare and occur relatively abruptly in geological terms. They are also usually followed by tens of millions of years without any increase.
Counter-intuitively, the greatest increase in local diversity occurred after the massive extinction that wiped out dinosaurs 66 million years ago. Only a few million years after this event, local diversity had increased by a factor of two or three from pre-extinction levels, largely due to the spectacular success of modern mammals, which have evolved to fill the ecological space left by the dinosaurs. But after this strong increase, the local diversity of tetrapods has not increased over the next 60 years.
The competing animal diversity models provide clear predictions of how diversity at the local scale is expected to change over geological time, with long-term stability or continuous increases. By demonstrating that local diversity has limits that persist for millions of years, our results pose a challenge to models that show that diversification continues more or less unchecked. But diversity at the continental or global level can follow a distinct pattern, so our results do not necessarily apply to these scales.
For example, after a massive extinction, most species of a continent could be eliminated. However, a relatively small number of surviving species could be very successful and spread widely. In this scenario, diversity across the continent would collapse, but local diversity might seem unchanged, as the same small group of species would be found everywhere.
In fact, a similar process seems to be happening now in response to the destruction of human-caused habitat. Invasive species are spreading widely, sometimes pushing for local diversity even as regional diversity is declining. But previous work by my terrestrial diversity research group of Mesozoic and early Cenozoic continental vertebrates (about 250 to 47 m) suggests that, over long periods, species counts on continents show a trend similar to that observed at the local level. . This means that the spectacular diversity on Earth today – at least in vertebrates – is probably not a recent innovation.