The European Perch, brought for angling by earlier settlers, has had severe effects on a number of native Australian fish (Image Credit: Karelj)

Transport pathways shape the biogeography of alien freshwater fishes in Australia (2018) Garcia-Diaz et al., Biodiversity Research, DOI: 10.1111/ddi.12777

The Crux

Invasive species are a problem in every type of ecosystem, be it by reduction of local diversity, or negative effects on a region’s economy or human health. Freshwater rivers and lakes are no exceptions to this. Invasive fish have impacts on local habitats which include outcompeting or just flat out eating local species, changing a habitat’s entire structure (say by clearing away aquatic vegetation or increasing pH levels) and the reorganisation of the entire population of a lake or river, from the birds that nest on the shoreline to the tiny planktonic species that are the base food source of the entire ecosystem. Once an invasive species is established, it can be impossible to remove.

So naturally, understanding where and how invasive species are likely to strike is of huge benefit. This paper tries to map that out, using Australia as a case study. It’s a great example; Australia has a long history with invasive species, and this study alone looks at 33 different types of invasive fish.

Did You Know: Invasion Pathways

Historically, we’ve introduced species into new environments for a number of reasons.

Acclimitisation: This occurs when settlers bring animals with them from their homeland for nostalgic or practical purposes. Eg: The English brought foxes to Australia to hunt. They even tried moose.

Biocontrol: When an alien species is introduced to prey on and subsequently reduce populations of a pest species. Eg: The mosquito fish is the only fish species introduced to Australia for biocontrol.

Food production: Introduction of a species which is, or produces, food. Eg: Pink salmon got to Scandinavia from Russian fishing stocks. Pigs got into… well everywhere.

Ornamental: When a species is imported as a pet or for decoration. Eg: SO MANY PLANT SPECIES. Also the Goldfish.

How it Works

The authors took a database of 569 known locations of 33 different alien freshwater fish across 144 different water drainage basins across Australia. Since the introduction of invasive fish has doubled since 1970, this year was used as a focal point to see if methods of invasion have changed over time. They measured a few different things, listed below.

1. Turnover – Essentially a measurement for how many new species were found in the basin after 1970. The highest turnover occurs if every species currently found there was established after 1970, the lowest if every species was established before 1970.
2. Covariate effects – The effect of different variables on the establishment of our alien species. The variables include the Human Footprint index, which basically measures how much impact humans have in the area, and area of the basin.
3. Introduction effects – Whether the method of introduction effects the establishment of the species.

What They Found

Before 1970, most fish species that arrived were brought to Australia for acclimatization purposes, and the areas most likely to be affected had a higher rating on the Human Footprint Index, and were found in the more highly populated south-east of Australia. This makes sense, as most of these introductions would have come about as a result of human settlers introducing fish species as they colonised new areas. Since then, there has been an increase in the number of invasive fish which were imported for ornamental purposes, and the effect of the HFI has disappeared, with smaller basins more likely to be invaded. This is likely to be a result of the increase in aquariums around Australia, with these ornamental fish escaping tanks or being disposed of incorrectly. Think what Nemo was going for in that dentist’s office, if Nemo was a dangerous invasive species and Sydney Harbour was a small inland river.

A drainage basin is an area in which all water that falls as rain drains to one point. This is the Murray-Darling basin, which drains into the Murray in South Australia (Image Credit: Tim J Keegan, CC BY-SA 2.0)

Any Problems?

Sure. As the authors point out, things like the number of a certain species that are initially released and failed introductions are extremely informative, and this is info they don’t have. If you know where a species was unsuccessfully introduced, you can compare regions where introductions were successful and unsuccessful, and it becomes easier to predict where future introductions will be successful. That lessens our faith in how accurate the effects of HFI and basin area actually are. But it’s a great avenue for future research.

So What?

There are a few key takeaway messages here.

1. That how fish invade areas, and how likely they are to successfully establish, changes over time. This means we can’t automatically look at past invasions and assume they will happen in the same way. BUT, we can take some of these patterns and apply them globally. For instance, in countries where the aquarium industry has started to boom, we can now warn them to take measures to ensure regulations which prevent the spreading of those species to local waterways.
2. It also gives us a strong indicator that biologists and ecologists need to do more than understand just the physiology or ecology of an alien species. We need to be able to recognise the importance of that species to a changing society. If a certain species suddenly becomes fashionable, we need to understand the potential impacts of said species and safeguard against it.