For more than 50 years, ecologists have questioned the causes behind the stability of ecosystems. Is it the result of the interaction between a wide variety of species, as the oldest theories indicate? Or can it be the cause of upheavals and extinctions of species? Ian Hatton, associate researcher in the Department of Earth and Planetary Sciences at McGill University, provides some answers in a recent study.
To the amazement of scientists, the Atlantic redfish — this bright red fish — now dominates the bottom of the St. Lawrence River, even though it was overfished and classified as an endangered species in 2010. It is also a victim of overfishing However, cod has not managed to recover and its population is constantly decreasing, despite fishing bans. Why is one more favored than the other? How can we explain that, in other ecosystems, this phenomenon does not occur? The responses of ecologists differ.
Several studies have shown that biodiversity — the variety of life forms on Earth — determines the functioning of ecosystems, those areas in which plants, animals and other organisms interact with each other and with their environment to survive and reproduce. However, the mystery remains as to the mechanisms of their long-term stability. A “stable” ecosystem results in relative constancy in the abundance and ability of species to recover from disturbances.
Modern theories suggest that greater species diversity can destabilize an ecosystem. In a published study from McGill University and the Max Planck Institute published in the journal Science, Ian Hatton and his team rather agree with the first ecologists, who believed that biodiversity does indeed promote stability. “We observed that population growth is not exponential, contrary to common belief, but that it slows down as population density increases,” explains the researcher. This means that the more species there are, the more they are able to coexist within the same space. »
A more reliable prediction model
To arrive at this conclusion, Ian Hatton and his colleagues used a different mathematical approach, called the “sublinear growth model”. This model is more commonly used in biology, to describe the growth of cells, for example. “By testing the model, we discovered that it could make solid and valid predictions,” he emphasizes. The team relied, among other things, on data relating to the abundance and growth of a variety of insects, fish and mammals, collected over the past 60 years.
“Previous ecological models did not make very good predictions,” continues Ian Hatton. It is therefore not surprising that historically it has been difficult to predict what might happen in a given place when a species disappears, as with St. Lawrence cod. » These models are based on the principle of competitive exclusion, which states that two species cannot coexist if they occupy exactly the same niche, since they are fighting for the same resources.
However, the dynamics that link species together are much more complex, according to the researcher. The results of his study lead to a better understanding of the upheavals that occur in an ecosystem as well as their impact on the living beings that inhabit it. Ultimately, this could help predict the time needed for a declining species to grow again, for example, or the possible consequences of this decline on another population.
Loss of biodiversity
If biodiversity is a factor of stability, its loss can then contribute to disrupting an ecosystem and even prevent it from recovering. The study thus warns of the devastating effects of the disappearance of numerous forests, numerous wetlands and living beings. Today, around one million animal and plant species are threatened, according to the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES). Nearly 70% of wild animal populations have disappeared in around fifty years, according to the World Wide Fund for Nature (WWF).
The main driver of biodiversity loss is the way humanity uses land and sea, eating away forests, wetlands and other natural habitats and turning them into agricultural and urban land. Overexploitation of plants and animals, climate change, pollution and invasive alien species are the four other main causes of the destruction of life, according to IPBES.
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