With temperatures rising, here comes ‘global weirding’

A great concern with changes of these kinds is their potential irreversibility. Known as “regime shifts”, these changes often result in “alternative stable states” that resist going backwards. That is, to get back to an earlier state, not only must conditions be returned to what they were when the ecological changes began, but they must go an undetermined distance further back in time to overcome the resilience of the new stable state. Yet where warming is the driver forcing regime changes, the effect of higher temperatures has a forward momentum unlikely to be reversed anytime soon.

A fine example of regime shifts is in the Baltic Sea, for which the Helsinki Commission conducted analyses of a rich, multi-decadal data set of 75 NASA physical, chemical, and biological ecosystem indicators that show unambiguous changes among alternative stable states. In the 1970’s, the Baltic was cooler and more saline than it is today. In recent years it has moved to a warmer, low salinity ecosystem. What’s striking is not the range of conditions seen in this brackish sea - where fresh and salt waters meet and, thus, variability would be expected - as much as the sheer stability across years once a new state has been reached.

Where the Baltic goes from here is of more than academic interest. Two major commercial fishes in the sea are bottom-dwelling codfish and a mid-water herring, called sprat. Studies show that warmer sea temperatures tend to favour sprat at the expense of cod, but it remains to be seen whether the large-scale climate variations to come - which also influence currents and mixing with North Sea waters - will push the Baltic towards cod- or sprat-dominated systems. But we can be sure that the winner’s identity will be both an outcome and a driver of surrounding ecosystem changes.

Add to this climate-driven “weirding” one more element - the ongoing invasions via human vectors of non-native species, many of which are capable of deleterious interactions with native species, with a few causing profound ecological changes. The US has been woefully ineffective in preventing these colonisations. But our failures do instruct; we should view these additions from afar as lessons, as the equivalent to the sudden appearance of new species in an ecosystem because of warming.

A poster species for the seriousness of these invasions is the freshwater zebra mussel in North America. In its native Eurasian range it exists in balance with other aquatic creatures. But in New York's Hudson River these mussels went from zero to an estimated 550 billion individuals in only two years. Zebra mussels now reduce the river's phytoplankton by more than 80 per cent, an ecological insult that echoes through its food chain, including reducing available food for newly hatched shad and striped bass.

Indeed, climate change may be linked directly to the facilitation of invasive species - a worrisome synergy. For example, it has been shown for sea squirts in Long Island Sound that warm winters provide an earlier start in spring for three non-native species of sea squirts as they settle and claim bottom territory. Warm summers also allow them to grow faster than native species. A maximum of only a three-degree Celsius change caused a 10-fold difference across 12 years in the ratios of native and non-native sea squirts. Sea squirt species changes may seem innocuous (although we don’t really know this), but if responses to warming like these can be extrapolated to other marine invertebrates, greater temperature alterations may strongly upset the “playing field” in the ecological contests between native and aliens species.

Although I focused on what I know best - the marine world - these principles apply equally to the terrestrial realm, with land-based examples being numerous and increasing as more studies are launched. The more we look, the more we find. Norwegian researcher Jöel Durant and his colleagues wrote, “All components of a food chain cannot be expected to shift their phenology at the same rate, and thus are unlikely to remain synchronous”. The implications of this simple statement are forbidding - the biotic scrambling to come will indeed be weird, or worse.