The Tropicalisation of Temperate Ecosystems

Anjani Ganase, marine biologist, looks at how climate change and warming ocean temperatures are impacting temperate marine ecosystems adjacent to tropical marine life.

The average land and sea surface temperature of the planet has increased by about 0.8 °C over the last one hundred years. Although this does not appear to be a significant number, a change of less than one degree can affect the timing of seasonal changes in plants and animals: breeding season, migration routes, flowering and fruiting of plants. Springtime activities in both animals and plants have been recorded to be occurring progressively earlier since the 1960s, including the arrival of butterflies and birds, delayed autumnal departures of migrating birds, as well as earlier breeding patterns in amphibians (Walther et al 2002). Apart from shifts in the timing of events, there have also been recordings of shifts or expansion in the range of plants and animals, in particular sedentary organisms, towards the poles and high altitudes as temperatures warm and providing other living conditions remain suitable.

We have also experienced the detrimental effects of tropical diseases, spread as a result of El Niño occurring on top of current warming temperatures; allowing the spread to temperate regions in the southeastern United States, China and Europe (Carminade et al 2017). What is seen on land can also be observed in the oceans.

Surveys carried out on marine ecosystems have observed changes in neighbouring temperate marine ecosystems as sea surface temperatures rise. There are places around the world that are heating up much faster, where currents originating in the tropics carry warmer waters to cooler temperate areas. The most famous in our region is the Gulf Stream, which flushes into the north Atlantic and bathes the northeast coastline of the US and Bermuda with warm water.

Other regions with similar current patterns have seen significant changes in the marine ecosystems where the current carrying warmth also brings passengers, such as coral reef fish and larvae of tropical organisms. These are able to survive for longer periods under current temperate conditions.

Tropical coastal ecosystems are primarily composed of coral reefs, which create the calcium carbonate framework used as foundational infrastructure utilised by a diverse array of other marine organisms. On the other hand, temperate ecosystems are made up of canopies formed by kelp forests and Sargassum beds, which play the infrastructural role by creating algal forests, also rich in marine life. While tropical herbivorous fish control the growth of macroalgae on corals; in temperate zones, the invasion of tropical fish to temperate kelp and Sargassum habitats will result in uncontrolled predation of the foundation species and the degradation of the ecosystem.

Underwater photographs from Tosa Bay (Southern Japan) showing: (a) well-developed Kelp bed in the early 1990s; (b) overgrazed kelp bed (‘isoyake’) in October 1997; (c) rocky barren area in January 2000; (d) coral communities present in January 2013. Image sourced from Verges et al (2014). Photograph credits: (a–c) Zenji Imoto and (d) Yohei Nakamura.
THE ISOYAKE PHENOMENON

Thousands of hectares of kelp forests have been lost over the last thirty years in southern Japan. This deforestation or the conversion of marine algal forests to barren sea bottoms is referred to as ‘Isoyake’ (literally burnt rock) by the local Japanese fisherman. Apart from warm waters being less favourable to the health of kelp forests, the algae are being fed upon by intruding tropical herbivores that graze on the palatable kelp, which prevents regrowth. Over time, the barren seascapes become occupied by large table-forming coral communities in place of the macroalgae (Verges et al. 2014). Coral may be good for tropical ecosystems but they cannot provide the same functions as kelp forest required by dependent temperate communities. The loss of these habitats also has severe repercussions in the fisheries industry that rely on the marine products that come from the kelp forest. The abalone fisheries crashed in Japan because the abalone disappeared when kelp forests were lost.

Similar trends are being observed along the Australia’s east coast where the southward currents of the EAC (Eastern Australia Currents) have increased the connectivity between the Great Barrier Reef and temperate marine ecosystems off southeast Australia, including Sydney (Figueira and Booth 2010). The EAC introduces tropical fish and larvae to temperate waters, but cold temperatures often limit the survival of these fish over winter. However, one study showed that the chances of tropical juvenile fish surviving winter will increase as winter temperatures warm, providing that other living conditions remain favourable (Figueira and Booth 2010). Hypothetically, while Nemo may currently be able to visit 42 Wallaby Way, Sydney to reminisce about his childhood adventures during the warmer months of the year; in the future, he may be able to permanently reside in some of Sydney’s prime real estate and not have to retreat to the Great Barrier Reef.

What does this mean about the future of our tropical marine ecosystems? If coral reefs are shifting pole wards, what will be replacing them in the waters that become too warm for their comfort? Unfortunately, oceans are continuing to warm and coral bleaching events are expected to increase in frequency. On the heels of the longest ever global coral bleaching event ever recorded (2015 – 2017) the future of coral reefs is bleak. The Caribbean also experienced a mass burning of coral reefs. For our coral reefs to survive, we all need to push for better climate change policies to reduce carbon emissions quickly.

Do we know what are the current targets for reduction of carbon emissions in Trinidad and Tobago and for the rest of the Caribbean? Are they enough?
Coral bleaching in the Maldives captured by The Ocean Agency, XL Catlin Seaview Survey, Richard Vevers in May 2016 during the latest major global coral bleaching event.

References:
Carminade, C., Turner, J., Metelmann, S., Hesson, J. C., Blagrove, M. S. C., Solomon, T., Morse, A. P., Baylis, M. (2017) Global risk model for vector-borne transmission of Zika virus reveals the role of El Niño 2015. Proceedings of the National Academy of Sciences of the United States of America, 114: 119 – 124.

Figuiera, W. F. and Booth, D. J. (2010), Increasing ocean temperatures allow tropical fishes to survive overwinter in temperate waters. Global Change Biology, 16: 506–516. doi:10.1111/j.1365-2486.2009.01934.x

Vergés A. et al. 2014, The tropicalization of temperate marineecosystems: climate-mediated changes in herbivory and community phase shifts. Proc. R. Soc. B 281: 20140846. http://dx.doi.org/10.1098/rspb.2014.0846

Walther, G. R., Post, E., Convey, P., Menzel, A., Parmesan, C., Beebee, T. J., Bairlein, F. (2002). Ecological responses to recent climate change. Nature, 416 (6879), 389-395.




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