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Sandhill cranes gather in a wildlife preserve.Anthony Souffle \/ Minnesota Star Tribune \/ ZUMA<\/p>\n
\t\t\t\tGet your news from a source that\u2019s not owned and controlled by oligarchs. Sign up for the free Mother Jones Daily.<\/a><\/p>\n This story was originally published by\u00a0Yale E360<\/a>\u00a0and is reproduced here as part of the\u00a0Climate Desk<\/a>\u00a0collaboration.<\/p>\n Nature is slowing down, and its ability to regenerate is failing in the face of climate change, according to the authors of a new analysis of the speed of species turnover in ecosystems across the world.\u00a0<\/p>\n The finding comes as a big surprise to many ecologists. They have long predicted that nature will respond to climate change and humanity\u2019s other assaults by ramping up the rate of turnover, with existing species moving out and new ones moving in. Some studies have appeared to confirm this is happening.<\/p>\n But the latest and largest\u00a0analysis<\/a>, published last month by researchers at Queen Mary University of London, has found the opposite. And the slowdown is big. Measured as comings and goings over a time frame of up to five years, species turnover is down by a third since the mid-1970s, when the current trend of rapidly rising global temperatures began. \u00a0<\/p>\n This is bad news, says the lead author Emmanuel Nwankwo. \u201cNature functions like a self-repairing engine, constantly swapping out old parts for new ones. But we found this engine is now grinding to a halt.\u201d\u00a0<\/p>\n Species turnover, most ecologists now agree, is a sign of\u00a0\u201cthe ongoing back-and-forth of a healthy ecosystem.\u201d <\/p>\n Nwankwo and his Queen Mary University colleague Axel Rossberg analyzed research assembled in\u00a0BioTIME<\/a>, a unique global database of hundreds of separate studies of the composition of ecosystems. Encompassing everything from North American birds to terrestrial plants, freshwater ecosystems, and fish on the seabed, it contains records from more than half a million locations, gathered over the past 150 years.<\/p>\n \u201cWe were very surprised at the discovery,\u201d says Rossberg. \u201cWe did not expect at all to see the slowdown.\u201d\u00a0Such findings are \u201cthe opposite of existing expectations,\u201d agrees ecologist Christopher Terry of the University of Oxford, who in a separate\u00a0study<\/a>\u00a0with Rossberg found short-term turnover decline in the data on North American birds in habitats modified by humans.\u00a0<\/p>\n Most ecologists contacted by\u00a0Yale Environment 360\u00a0endorsed the new findings. \u201cThe results look quite convincing to me,\u201d says Ryan Chisholm, a theoretical ecologist at the\u00a0National University of Singapore.\u00a0<\/p>\n These scientists suggested that, by concentrating on changes on a short timescale of five years rather than the longer periods of other research, the new study had identified an important phenomenon of slowdown in natural \u201cintrinsic\u201d species turnover that could influence how ecosystems are able to respond to external forces such as climate change.\u00a0<\/p>\n Yet the head of BioTIME\u2019s leadership council, biologist Maria Dornelas, says her analysis of data from two large, long-term studies in her database\u2014the North Sea International Bottom Trawl Survey and the 60-year old North American Breeding Bird Survey run by the US Geological Survey with Canadian and Mexican counterparts\u2014found an increase in species turnover in recent decades. Comparing her work with that of Nwankwo, she says: \u201cI am finding it difficult to reconcile the two findings.\u201d<\/p>\n Other researchers attributed any discrepancies in estimates of the rate of species turnover to the time frames of the different studies.<\/p>\n \u201cYou might get pushback from people who have observed a net increase in turnover rates using a long time window,\u201d says Jacob O\u2019Sullivan, an ecology modeler at Forest Research, an agency of the British government. \u201cBut slowdown does appear to be the correct interpretation [of] their results.\u201d<\/p>\n The new research findings have rekindled a long-running argument among ecologists about how communities of species in natural environments function. At root is the question of whether changes in the composition of natural ecosystems are prevalent, and whether they should be seen as good or bad for the ecosystem.<\/p>\n Most ecologists once believed that healthy ecosystems are naturally stable, with a low turnover of species. Left to themselves, they reach a perfected equilibrium that the influential 19th-century American botanist\u00a0Frederic Clements termed a\u00a0\u201cclimax ecosystem.\u201d\u00a0 After any disruption, they return to this stable state, a process called succession.<\/p>\n This stability was seen as vital, because the functioning of the ecosystem\u2014whether tropical rainforests, temperate grasslands, or polar tundra\u2014depended on\u00a0tight living relationships between species that had evolved together and were mutually dependent.\u00a0Predators and prey, and plants and the insects that fed on them and pollinated them, were inseparable. Species turnover disrupted those associations and could lead to ecological breakdown.\u00a0<\/p>\n Echoing this idea, many ecologists still quote the \u201crivet-popper hypothesis,\u201d first articulated in the 1980s by biologist Paul Ehrlich of Stanford University. He likened an ecosystem to an aircraft, in which each part, down to the smallest rivet, is vital for the plane to fly safely. Removing a single apparently insignificant species from an ecosystem might be like popping a rivet in the aircraft. It could cause the aircraft to crash\u2014or the ecosystem to collapse. In this framing, species turnover looks like a bad thing.<\/p>\n In an ecological sporting analogy, there are fewer players \u201con the bench\u201d to make tactical substitutions.<\/p>\n But detailed long-running studies of unperturbed ecosystems have shown that even the most pristine ecosystems are not so unchanging. In a famous long-term\u00a0tracking<\/a>\u00a0of nature on an island in Lake Superior, Daniel Botkin at the University of California, Santa Barbara, documented ecosystems constantly changing their composition. \u201cSpecies in ecosystems are not fixed entities, even without human-induced change,\u201d says\u00a0Anne Magurran of St Andrews University, the founder of BioTIME. \u201cAll ecosystems experience natural turnover.\u201d<\/p>\n Ecologists call this natural churn within unstressed ecosystems \u201cintrinsic turnover.\u201d\u00a0And most now agree that this turnover is a sign not of fragility and imminent breakdown, but, as Terry puts it,\u00a0\u201cthe ongoing back-and-forth of a healthy ecosystem.\u201d It can be driven by internal dynamics such as natural fire regimes that periodically wipe out forests, cycles in predator-prey relationships, or periodic outbreaks of disease. Or it can be essentially random.<\/p>\n Many species, from lemmings to jellyfish, have boom-and-bust cycles with no obvious external cause. Whole ecosystems can also be made up of mosaics of habitat patches that spontaneously shift, such as the regular\u00a0switches<\/a>\u00a0between woodland and grassland seen in some savannah regions of Africa.\u00a0<\/p>\n Rossberg likens these internal dynamics to a giant, unending game of rock paper scissors. And far from being a sideshow, he says, they appear to be the dominant cause of short-term species turnover.\u00a0Moreover, in a world of growing external pressures such as a changing climate, increased turnover may indicate that the ecosystem is responding and adapting, with\u00a0some species going locally extinct or migrating out, while other colonizers move in.\u00a0Which makes it a positively good thing.\u00a0<\/p>\n If those who adhere to the rivet-popper hypothesis are right, then the new evidence of slowdown might be good news. For it would suggest that most ecosystems are growing more stable, and remain largely uninfluenced by human activities, including climate change. But if, on the other hand, significant species turnover is the healthy norm for ecosystems, a sign of resilience rather than fragility, then any decline in turnover is bad news, especially when turnover is increasingly needed as a survival strategy in a world of widespread human interference in nature.<\/p>\n So, what is causing this unexpected slowdown? Terry says it likely shows that \u201chumans are disrupting the background [intrinsic] turnover of these systems.\u201d Rossberg says the central problem is probably that natural landscapes are increasingly fragmented. So, as some species disappear, there are fewer opportunities for replacement colonists to migrate from nearby. This \u201cslows the pace at which species replace one another,\u201d he says, reducing the ability of the isolated ecosystems to conduct running repairs and threatening their long-term survival.\u00a0<\/p>\n O\u2019Sullivan agrees. His own research has found that species turnover \u201cincreases with both the size of the regional species pool and the connectivity of the landscape. The new study makes the case that a regional decline in [species] richness may explain the local decline in turnover.\u201d<\/p>\n \u201cOnce we accept natural turnover as a force, we must accept change as natural and not to be fought against.\u201d<\/p>\n In a sporting analogy popular with some ecologists, there are fewer players \u201con the bench\u201d to make tactical substitutions if things are not going well on the field. A less fragmented landscape will have a larger species pool \u201con the bench,\u201d ready to replace those disappearing, and allowing increased species turnover if the situation demands.<\/p>\n