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How does climate change cause range shifts among marine species, and what are the consequences?

Prof David Schoeman discusses how climate-driven range shifts arise and what the consequences are.
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In this section, you’ll learn about one of the most common responses of marine life to climate change, range shifts. Hi, I’m Professor David Schoeman, a specialist in ecological modelling and climate-change ecology from the University of the Sunshine Coast, Australia. As ecologists have known for centuries, most plants and animals live in specific places. So we know that corals occur in tropical oceans, for example, whereas polar bears live where there’s snow and ice. Where these species’ ranges are not bounded by geographical obstacles like mountains, or coastlines. Their boundaries generally coincide with conditions that would be unfavourable to the organism in question.
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For this reason, we’ve come to understand that the geographic distribution of species is somewhat static, except of course, where we as humans deliberately move species from one place to another. But things are changing. The Intergovernmental Panel on Climate Change, also known as the IPCC, concluded this year, that it is unequivocal that human influence has warmed the atmosphere, ocean and land. By pumping fossil carbon into our atmosphere, we’ve already warmed the Earth’s atmosphere and oceans. Over the past 50 years or so. We’ve warmed the ocean by about 0.4ºC, although some areas have warmed by more than one and a half degrees in this period.
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The overall effect of ocean warming is that temperature bands generally move away from the tropics and towards the poles. If species stay in their traditional ranges while this happens, they’ll find themselves in unfavourable temperatures, which will cause them to lose condition and potentially even to die. To avoid this outcome, species tend to shift their ranges to track their favoured temperature zone also called a temperature niche. Although I’ve illustrated this, as species actively moving from one place to another, this doesn’t happen overnight, but rather over decades.
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And except in the case of long-lived very mobile species, it isn’t a case of the same individual moving from one place to another, but rather environmental space becoming available where it was previously to cool, and the reduction of population size at the warm edge of a species’ range. To illustrate, let’s consider the geographic range of a coral. Corals not only need the right temperature, but also sufficient light, so they tend to be restricted to shallow waters of the continental shelf. In this diagrammatic map of the northeast coast of Australia, the continental shelf is illustrated by the dotted line with shallow, well-lit waters to the landward side, but depth increasing sharply to the seaward side.
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Given that a coral species has a specific temperature niche, that will translate into a geographic range that contains coastal waters having suitable temperatures. Let’s suppose that this range is illustrated here by the yellow polygon. As global warming shifts temperature zones poleward, the thermal niche of this hypothetical coral species would move southward. The southward-moving warm range edge is often called the leading edge of the range. The green polygon now represents the potential new thermal habitat for our coral species. But some of it is unavailable because it extends beyond the continental shelf into deeper water. Let’s remove that part of the polygon. In the north of the coral’s range, increasing temperatures will mean increasing stress on corals that remain there.
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Eventually, temperatures will become so high that corals there die, and we have a trailing range edge contraction. This local loss of a species is called a species extirpation, as opposed to the more commonly known species extinction, which refers to the global loss of a species. Adding the newly available area to the existing range and subtracting the area experiencing extirpation. We’re left with the new species range for the amount of warming experienced. A general feature of range shifts illustrated here is that the area available for range expansion is often larger than the area lost to access of heat. While this might superficially seem like good news it isn’t.
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Extirpations of species in the tropics have already resulted in a decline in tropical marine species richness, with a concomitant increase in the number of species at mid latitudes. This is illustrated by our recent analysis of the total number of free swimming marine species by latitudinal bands in two time periods – the first before major warming of the ocean, and the second period being a recent 21 year period. There is a clear drop in marine species richness in the tropics, representing tropical extirpations due to range shifts driven by ocean warming. Range shifts are also evident in the mid-latitude peaks in richness, shifting poleward in both the northern hemisphere and the southern hemisphere.
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The gains and losses associated with range shifts also manifest in fisheries. Many tropical islands risk losing access to fish stocks as their waters warm, potentially destroying important fisheries upon which local communities depend. By contrast, some temperate areas a gaining new fishery species are experiencing growth in existing fish stocks as waters warm. Ironically, nations that stand to lose the most have contributed least to the fossil-fuel emissions that are causing climate change, whereas nations that stand to benefit are often amongst those that caused the problem in the first place. This highlights the importance of global action on climate change, with all nations participating actively honestly, and with the overarching aim of finding just solutions.

Species on the move

In this short video, Prof David Schoeman explains what range shifts are, explores how and why they arise, and discusses some of the basic consequences for ecosystems and humanity.

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Life Below Water: Conservation, Current Issues, Possible Solutions

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