This is the second article in a series “Shifting recollections of nature’s normal”, about ecosystem shifts in Tasmanian. The first can be found here.
My father-in-law, Richard Johnson, now in his 90s, remembers a time when abalone, then called muttonfish, could be collected in the intertidal zone off suburban Hobart’s Sandy Bay. Back then, people also fished for “couta” off the Port Huon wharf and scalloped at Wattle Grove in the Huon River. Lobsters were plentiful in the Derwent under Hobart’s city wharves.
Now, divers are lucky to find abalone of legal size or lobsters in the Derwent outside of reserve areas. Indeed, data from the Institute for Marine and Antarctic Studies (IMAS) reveals that the average daily recreational take of lobsters statewide is 0.93 – not quite a lobster.
Of course, species don’t exist in isolation. Changes to populations of key species have ripple effects through whole ecosystems, potentially causing shifts toward new states that are fundamentally different from the ecosystem that preceded the change. In the case of Tasmanian reefs, reduced numbers of rock lobsters can contribute to the formation of “urchin barrens”, which occur when urchins overgraze reefs and denude kelp forests. Rock lobsters limit such barrens because they prey on young urchins, including introduced urchin species. Careful management of the remaining lobster stocks is therefore critical to preventing loss of more kelp forest ecosystems.
Similar ecosystem shifts can take place on land. When my father-in-law was a boy, the thylacine or Tasmanian tiger went largely extinct in the wild, with the last captive thylacine dying in 1936, before he became a teenager. As a consequence of the disappearance, there was a new order of things, with Tasmanian devils taking over the role of the top, or apex, carnivore.
And now, the Tasmanian devil (Sarcophilus harrisii) is in peril due to a contagious facial-tumour disease which has caused average population declines of 80 per cent, and up to 95 per cent in long-diseased areas. When devils are absent, different behaviour patterns arise in the prey species, resulting again in ecosystem shifts.
But what if the predators were put back?
In 2012, an insurance population of Tasmanian devils was released on previously devil-free Maria Island to safeguard the species, which was threatened with extinction. Interestingly, despite initial increases in mortality of prey such as brushtail possums, University of Tasmania researchers Dr Calum Cunningham and Professors Menna Jones and Chris Johnson showed that possums employed anti-predator (predator avoidant) behaviours within a generation.
According to project lead Professor Jones, these predator avoidance behaviours commonly occurred within 1–2 years, but sometimes within months, as devils increased in number.
When devils were introduced to Maria Island, possums spent less time foraging on the ground, where they potentially impact vegetation, with ramifications for whole ecosystems.
“Possums rapidly relearned avoidant behaviours common in areas where devils were in healthy numbers,” Professor Jones said. “The finding should negate fears that prey species might go locally extinct once predators are reintroduced and their numbers start to recover.”
Professor Johnson adds the findings are important because they show that “rewilding programs that involve the reintroduction of apex predators have the capacity to restore lost top-down behaviours and controls, resulting in self-regulating and biodiverse ecosystems”.
Indeed, there are suggestions that, given the importance of top predators for managing ecosystem balance, Tasmanian devils be reintroduced to regions of the mainland, where they once existed. It is hoped the devils would keep a check on animals, such as feral cats, that are threatening local wildlife.
“Already, indications are that devils might successfully limit populations of feral cats without serious impacts to native fauna,” Professor Johnson says.
For my children, not being able to see abundant abalone or lobsters in the Derwent, snorkel through giant kelp, or hear Tasmanian devils calling at night, is their normal. They have known nothing else. They live in a beautiful state, more natural than most; nevertheless, it is a different place from that my father-in-law knew as a boy.
This idea of “sliding baselines”, or shifting ideas of what healthy, normal ecosystems should look like, was fleshed out in the pivotal research of Californian marine ecologist Paul Dayton, who wrote that, “Any measure of change in a natural ecosystem must be grounded upon a well-defined natural standard or benchmark against which potential changes are measured.”
Published in 1998, it was a cautionary tale about the need for baseline research. In northern California, as on Tasmania’s east coast, giant kelp has since all but disappeared due to a combination of warming sea water, as a result of climate change, and overgrazing by urchins.
Dayton’s point was that we cannot consider that the apparently healthy-looking ecosystems we encounter are in fact healthy functioning ecosystems if we have no idea of what came before them; if we have no picture of what is missing and what role those species played in keeping the whole together.
In his paper, famous in ecological circles, he writes. “It would be similar to studying the Serengeti after all the large grazers and carnivores were eliminated; one could still appreciate termites and other small grazers, but one’s expectations of nature pale beside what it used to be.”
The article was assisted by the Tas Writers Defining Normality digital residency, funded by the Copyright Agency.
Dr Katherine Johnson is a science writer and novelist based in Tasmania. She has published in The Conversation, Good Weekend (Sydney Morning Herald) and CSIRO’s ECOS magazine. Her fourth novel, Paris Savages, was released in the UK in July. Visit www.katherineJohnsonauthor.com and connect on social media via @KJohnsonauthor.