The East Australia Current (EAC) transports warm tropical water southwards along the east coast of Australia (a). During the warmer months, the EAC extends as far as south-eastern Australia and brings with it a suite of tropical fishes including Indo-Pacific sergeant (Abudefduf vaigiensis) (b) and Threadfin butterflyfish (Chaetodon auriga) (c). These fishes are termed “vagrants” because they are moving outside of their typical geographic range and generally only survive in temperate ecosystems whilst the water is warm. SST map sourced from: www.bom.gov.au/oceanography/forecasts/idyoc300.shtml?region=Aus&forecast=SST, Abudefduf vaigiensis image sourced from: Mark Rosenstein / iNaturalist.org, Chaetodon auriga image sourced from: orangkucing / http://orangkucing.tumblr.com
The redistribution of marine species from the tropics to the poles is a particularly interesting impact of climate change, with broad implications for marine ecosystems and ocean users alike. The dramatic effects that redistributing species can have on native marine ecosystems (Ling 2008, Vergés, Doropoulos et al. 2016), have driven a huge investment of research into understanding which species will or won’t successfully shift as temperatures continue to rise. This research has given us considerable insight into the complexities of species redistributions and highlights multiple factors, which need to be considered when attempting to predict redistributions (Feary, Pratchett et al. 2014).
Overlap between seasonal water temperatures and the physiological tolerance of a species is the primary determinant of whether cold-blooded species may or may not establish in a new area. However, there are other determinants, such as dispersal potential, habitat suitability and interactions with novel species, which are also highly influential. The occurrence of tropical ‘vagrant’ fishes in south-eastern Australia (Figure 1), provide unique opportunities to investigate the multiple factors influencing species redistributions. Although vagrant fishes are only seasonally abundant in south-eastern Australia, their persistence over summer provides a window of opportunity to investigate interactions between novel tropical fishes and temperate ecosystems. Interactions that will likely be important determinants of whether species redistributions succeed with ongoing climate change. Here we highlight a few interesting findings taken from research which has used tropical vagrants to investigate the factors influencing the success of novel tropical species in south-eastern Australia.
Dispersal potential is a key determinant of redistribution success. For pelagic species that can swim long distances, such as yellowtail kingfish, dispersal probably isn’t a significant limitation. But for smaller site attached species, dispersal can be a significant barrier in successful redistribution. An interesting example is seen in butterflyfishes (Chaetodon aurigaand C. flavirostris), which generally spend their adult lives on coral reefs. However, juvenile butterflyfish occur as far south as Merimbula in south-eastern NSW, a distance of ~1,500km from the nearest breeding adults! The incredible journey is made by butterfly fishes during a pelagic larval stage. When they first hatch and are only a few milometers long, butterflyfishes spend ~45 days as plankton. During this time, they are at the mercy of the east Australia current which transports the juvenile’s southwards on their 1,500 km journey along the east coast of Australia (Booth and Parkinson 2011).
Even when temperatures are suitable and dispersal mechanisms do allow for the transport of species into new areas, there are additional factors which have the potential to either facilitate or inhibit new species. Firstly, there’s the question of habitat suitability. For tropical species which have evolved to specialise in coral habitats, establishment on higher latitude temperate rocky reefs is unlikely. It is therefore unsurprising that the tropical fishes we commonly observed in these temperate reefs are those with more generalist habitat requirements, likely to be able to find adequate food and shelter in these novel environments.
On top of an understanding of how novel species interact with native habitats, it’s important to think about how these species interact with native fishes. We could assume that behavioural interactions between native-temperate and novel-tropical species could be detrimental. For instance, predation or aggressive competition for food and shelter. Alternatively, interactions between temperate and tropical fishes may be beneficial to the successful establishment of vagrants. Research published earlier this year (Smith, Fox et al. 2018) showed that the association of tropical damselfish with temperate species in shoals benefits the novel tropical species. Surveys conducted around Sydney, found that the tropical Indo-Pacific sergeant (Abudefduf vaigiensis) displayed increased growth rates when associating with temperate fish in shoals. The reason for this increase in growth is currently under investigation.
Hopefully these interesting cases will inspire you to think about how novel vagrant species in your local marine environment are interacting with the native ecosystem and if those interactions are likely to either facilitate or inhibit permanent redistributions.
1. In order to predict species redistributions, researchers need to investigate the thermal tolerance and dispersal potential of that species, and how that species interacts with the environment into which it is moving.
2. Interactions can be complex and species specific, with native habitats and residents potentially facilitating or inhibiting the establishment of novel species in an area.
For updates on our vagrant research you can follow us on twitter: @Kai_Paijmans @paloma_matis
Booth, D. and K. Parkinson (2011). "Pelagic larval duration is similar across 23 of latitude for two species of butterflyfish (Chaetodontidae) in eastern Australia." Coral Reefs30(4): 1071-1075.Feary, D. A., M. S.
Pratchett, M. J Emslie, A. M. Fowler, W. F. Figueira, O. J. Luiz, Y. Nakamura and D. J. Booth (2014). "Latitudinal shifts in coral reef fishes: why some species do and others do not shift." Fish and Fisheries15(4): 593-615.
Ling, S. D. (2008). "Range expansion of a habitat-modifying species leads to loss of taxonomic diversity: a new and impoverished reef state." Oecologia 156(4): 883-894.
Smith, S. M., R. J. Fox, D. J. Booth and J. M. Donelson (2018). "‘Stick with your own kind, or hang with the locals?’Implications of shoaling strategy for tropical reef fish on a range-expansion frontline." Global Change Biology.
Vergés, A., C. Doropoulos, H. A. Malcolm, M. Skye, M. Garcia-Pizá, E. M. Marzinelli, A. H. Campbell, E. Ballesteros, A. S. Hoey, A. Vila-Concejo, Y.-M. Bozec and P. D. Steinberg (2016). "Long-term empirical evidence of ocean warming leading to tropicalization of fish communities, increased herbivory, and loss of kelp." Proceedings of the National Academy of Sciences.