Why Do Mass Extinctions Occur? This Study Could Provide the Answers
Why do mass extinctions occur? This is the question that researchers have attempted to shed light on in a new study published by the journal Science.
An international team led by academics from the University of Leicester examined sudden ecological transitions throughout Earth's history in an attempt to address the long-standing mystery of why they occur. The new findings could help to predict upcoming ecological catastrophes, according to the researchers.
"Species actually go extinct all the time, this is thought to be a normal property of biological evolution, however, the matter is that extinction rates have accelerated significantly over the last few hundred years," co-author of the study, Sergei V. Petrovskii, from Leicester's Department of Mathematics, told Newsweek.
"In fact, the extinction rates have become so high that there is an opinion that we are witnessing the sixth mass extinction," he said. "Correspondingly, understanding the details of the scenarios that a mass extinction may follow then becomes a problem of a practical importance, not only a matter scientific curiosity."
Ecological transitions are periods where ecosystems experience sudden changes in their properties or function, which can result in the extinction of species and a loss of biodiversity.
So far, researchers have had a hard time explaining why such significant changes occur, in large part because these transitions often take place under apparently steady, constant conditions. As such, they cannot be linked to specific environmental shifts over the last century, such as anthropogenic climate change.
"There are two factors that make mass extinctions—there were five of them when more than 90% of the Earth's biota disappeared—challenging to explain," Petrovskii said. "First, is the generic complexity of ecological dynamics. An ecosystem is a very complex system and how the dynamics of the whole emerges from the interaction between its constituting parts—for example, between different species or between species and their environment—is not always clear."
"Second, is the mismatch between the 'short' timescale on which the ecological changes happen—for example, the extinctions that happened in modern times usually happened on a scale of a hundred years or faster—and the much longer timescale on which the paleontological data (fossil records) are available: in palaeontology, one hundred thousand years is regarded as a very short time step."
In the latest research, the team combined knowledge from ecological theory with mathematical models and empirical data, finding that sudden transitions in ecosystems can be explained by long-term transient dynamics—the patterns of change as a system moves from one equilibrium state to another.
"Ecological science usually treats ecosystems as stationary systems, i.e., systems where the properties—such as species abundance or population size, for instance—are either approximately stationary, show little change with time, or follow a clear well defined scenario, such as seasonal changes," Petrovskii said.
"A large part of ecological theory is built based on this assumption. Recently, however, there has been growing recognition that it is not always true—in fact, rarely true—and in many cases the ecosystems are actually far away from their stationary state," he said. "The corresponding behaviour far away from equilibrium is called transient dynamics."
These dynamics are described by mathematical terms known as "attractors," "ghost attractors" and "crawl-bys".
"'Attractor' is the mathematical term to refer to a stable steady state, where 'stable' means that the system will return back to its state if it is slightly perturbed, 'pushed' out of the equilibrium, according to Petrovskii. "'Ghost attractor' is a bit more tricky: it is the term to refer to a situation where the system behaves like it is at a steady state but in fact there is no steady state—for instance, it may have disappeared as a result of a catastrophe," he said.
"Crawl-by" refers to a situtation where the system is close to a steady state but it is, in fact, unstable, i.e., in the long run, the system will move away from this state but at a very slow rate. Thus, the system may give the impression of being at equilibrium, despite the fact it is not.
"Correspondingly, what we would observe in real ecosystems controlled by a ghost attractor or a crawl-by [is that] the system would start changing its properties—for example, some species would go extinct—apparently without any reason," Petrovskii said.
The team found evidence of ghost attractors and crawl-bys in their mathematical models of ecosystems, a discovery which could have important implications, they say.
"An ecological catastrophe emerging from a 'ghost attractor' or a 'crawl-by' may be a debt that we have to pay for the actions or mistakes—for example unsustainable use of natural resources— made many generations ago," Petrovskii said in a statement.
"Our research shows that a healthy ecosystem will not necessarily remain healthy, even in the absence of any significant environmental change," he said. "Therefore, better monitoring of the state of an ecosystem is required to mitigate potential disasters."
The researchers say that their work can help predict an approaching catastrophe by showing what signs to look for in data collected from ecosystems.
"It is whole new understanding of ecological dynamics that is likely to, eventually, significantly increase the predictive power of ecological theory," Petrovskii said. "It will also have important implication for ecological management where the existence of long transients may lead to a mismatch between the timescale of observations and decision making and the inherent timescale of the ecological dynamics."
