Do Disturbances Promote Biodiversity in the Presence of an Invasive Species?

Image Credit: Paresh Poriya, CC BY 4.0, Image Cropped (also not featuring tunicates)

Testing ecological theories in the Anthropocene: alteration of succession by an invasive marine species (2021) Christianson et al., Ecosphere, https://doi.org/10.1002/ecs2.3471

The Crux

Ecological disturbances, such as fire, floods, or storms, might seem like a catastrophe at first glance, but often they open up space for new species to take the place of dominant ones, creating a more diverse ecosystem. When a disturbance occurs matters as well – if a storm hits right before a particular species starts to reproduce, that species could take advantage of the extra space and become dominant in a short time.

In the 1970s, John Sutherland and Ronald Karlson tested this theory, looking at the invertebrate community of a coastal dock in North Carolina, USA. They found that which species dominated depended on when the community began to grow (a proxy for when disturbance opened up new space).

The area has since seen the introduction of an invasive species of tunicate, Clavelina oblonga. This week’s authors wanted to test whether the original patterns seen in the 1970s still showed up in the presence of the invader.

What They Did

The researchers lowered large tiles into the ocean at different points of the year and allowed local species to grow on them. The plates were periodically removed and photographed, with the different species present recorded. To compare community development with and without C. oblonga, the invasive tunicate was removed from one set of plates every fortnight.

The community saw two distinct disturbance events during the sampling period, one hurricane and one particularly harsh winter.

Did You Know: Climax Theory

Previously it was thought that most ecological communities built towards a natural climax state. Yet this theory was scrapped over the last few decades, and the importance of disturbance came to the forefront. Many ecosystems rely on semi-regular disturbance. Parts of Australia rely on fire not just to increase biodiversity, but to allow seeds that are triggered by smoke to germinate.

Read More: The Elusive Climax

What They Found

Plates with C. oblonga present showed consistently lower species diversity, with the invader taking up the majority of the space, and a few other dominant species present. Plates on which it was removed, however, showed a higher diversity of species, with different species dominating depending on when they were placed (much like what the original study from the 70s showed). The communities found on the plates without C. oblonga also remained more similar after the disturbance events, potentially suggesting that more diverse, invader-free communities recovered better after disturbance.

So What?

Clavelina robusta, a close relative of the invasive tunicate studied here (turns out it’s hard to find photos of specific tunicate species) (Image Credit: Rickard Zerpe, CC BY 2.0)

The authors theorised that the timing of disturbance events could alter community diversity. C. oblonga generally recruits (i.e. reproduces and multiplies) in a very specific timeframe each year, so disturbances outside of that timeframe would probably help other species regain a foothold.

Studies like these help us reinforce theory, and add valuable evidence that strengthen our understanding of the role that disturbance plays in maintaining species diversity. They also give valuabe insight into management techniques for invasive species. In this case, we know when NOT to try and perform any large-scale removal of one!

Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who definitely didn’t make any dumb climax jokes while writing this summary. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.

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