In short:
NT's pygmy crocodiles aren't genetically different enough to be a separate species from freshwater crocodiles or even a subspecies, a new study suggests.
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study → suggest → crocodiles
Some scientists say this "unique ecological form" of crocodile may still need targeted conservation.
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form → say → conservation
Further research into a second pygmy crocodile site at Bullo River is needed, as well as ecosystem studies and movement patterns from both populations.
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research → need → populations
But high in Arnhem Land's remote sandstone escarpments you will also find Australia's smallest snapping handbags.
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you → find → handbags
It's here, more than 100 kilometres up the Liverpool River, that freshwater crocodiles only grow half as long as their downstream neighbours.
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crocodiles → grow → neighbours
Since this population of "pygmy crocodiles" was first reported in Western science in the 1980s, there has been debate over whether the isolated reptiles were a different species.
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reptiles → report → 1980s
"Species status can have major conservation implications," wildlife geneticist Jaime Góngora of the University of Sydney said.
"If these crocodiles were a separate species, they would likely require their own conservation assessment and management plan.
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they → have → assessment
But a new study published in Ecology and Evolution by Professor Góngora and colleagues that compared the genetics of different populations of freshwater crocodiles concluded they were one species.
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they → publish → crocodiles
Professor Góngora said the genetic differences in the small Liverpool River crocodiles was no greater than that found between freshwater crocodiles (Crocodylus johnstoni) that lived in other river systems.
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that → say → systems
"They are a unique population, but we did not find strong evidence that they are a separate species," he said.
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he → find → evidence
While not a new species, Professor Góngora said pygmy crocodiles represented an ecological form of freshwater crocodile adapted to isolated escarpments.
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crocodiles → say → escarpments
He said this could warrant more protection for the Liverpool River crocodiles and a second population of small crocodiles in the Bullo River.
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this → say → River
"Those adaptations are valuable and worth conserving," Professor Góngora said.
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Góngora → conserve → ?
The small size of pygmy crocodiles compared to other freshwater populations is thought to be an example of "phenotypic plasticity", a phenomenon whereby a species develops different characteristics based on where it lives.
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it → compare → characteristics
"A simple example is that identical plants can grow tall and lush in good soil but remain small and stunted in poor soil," Professor Góngora said.
"
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Góngora → grow → soil
They are still the same species, but they respond differently to different conditions."
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they → respond → conditions
Centre for Crocodile Research managing director Sally Isberg, who was not involved with the study, said their small size came from adapting to smaller water pools on the escarpment, where less prey was available compared to down river.
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prey → involve → river
"They've been able to mature at a smaller size because obviously the end goal is to pass on your genes,"Dr Isberg said.
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Isberg → mature → size
The original survey that described the Liverpool River's pygmy crocodiles found them in the upper reaches of the river past natural barriers, like waterfalls, where there weren't any large fish species.
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that → describe → waterfalls
The surveyors found the largest mature female pygmy crocodiles were between 1.07 metres and 1.12m long, and a male was 1.18m.
Elsewhere in the NT, female freshwater crocodiles mature at about 1.5m and grow up to 2m in length.
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crocodiles → find → length
While males mature at 1.6m to 2m and reach 2.5m.
Professor Góngora said freshwater crocodiles were known to grow larger in habitats with more permanent water and more abundant food.
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crocodiles → mature → water
For example, in the catfish-filled Lake Argyle in Western Australia, freshwater crocodiles have been recorded reaching lengths of about 3m.
Do pygmy crocodiles need more conservation effort?
Dr Isberg said there was a point of alarm a few years ago for Bullo River pygmy crocodiles when cane toads, which can kill predators when ingested, moved through the region.
The exact state of the two pygmy crocodile populations is unclear, with no current population estimates.
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state → fill → estimates
Professor Góngora said there were high levels of inbreeding in the Liverpool River population compared to other crocodiles in the genetic study.
"
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Góngora → say → study
That is not surprising given how isolated these crocodiles are," he said.
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he → give → ?
"Inbreeding can reduce genetic diversity, which may make populations less resilient to environmental change and other threats."
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populations → reduce → change
Professor Góngora said more research was needed to understand the extent of inbreeding and whether it was affecting their long-term health, survival and ability to adapt to future challenges.
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it → say → challenges
He said future research directions included more genetic sampling of the Bullo River crocodiles, studies on movement patterns and connectivity of the two populations between river systems, and ecological research into food availability, habitat quality and how the pygmy crocs responded to environmental pressures.
"Together, that would help us better understand how isolation and evolutionary processes are shaping these crocodile populations," Professor Góngora said.
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Góngora → say → populations
In the meantime, he said conservation agencies could identify pygmy crocodiles as "unique management units" and prioritise them for monitoring, threat mitigation and recovery actions.
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agencies → say → actions
A spokesperson for the flora and fauna division of the NT Department of Lands, Planning and Environment told the ABC in a written statement the concept of an "ecological" species was interesting, "but would likely apply to isolated populations of many species".
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concept → tell → species
The spokesperson said the new study confirmed pygmy crocodiles were not genetically distinct enough to be a subspecies or separate species of freshwater crocodile.
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crocodiles → say → crocodile
"This means that they could not legally be listed as a threatened species," they said.
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they → mean → species
The spokesperson said the remote pygmy populations were naturally protected from many threats.
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populations → say → threats
But they also noted the spread of cane toads was a problem for all freshwater crocodiles.
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spread → note → crocodiles
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