Saltwater crocodiles' feral pig diet may be changing NT waterways: study
By environment reporter Peter de Kruijff
ABC Science
Topic:Ecology
16m ago16 minutes agoWed 12 Mar 2025 at 12:04am
Crocodile with a feed at Cahills Crossing
Saltwater crocodiles in the Northern Territory are eating more pigs, buffalo and cows compared to the fish diet of the relatively few individuals that lived there in the 1970s. (ABC News: Jayden O'Neill)
In short:
Excrement from the Northern Territory's saltwater crocodile population might be supporting plant and algae growth in Top End waterways, according to a new study.
A feral-mammal-based diet, coupled with a dramatic recovery of crocodile numbers, could also be reducing ecosystem damage in wetlands.
What's next?
The amount of croc poop-delivered nutrients is thought to have "significant" impacts on the NT's nutrient-poor rivers, but further research will examine if vegetation is benefiting from increased nitrogen and phosphorous levels.
With booming saltwater crocodile (Crocodylus porosus) numbers in the Northern Territory comes a whole lot of faeces.
So much, in fact, that a new study, published in the Proceedings of the Royal Society B: Biological Sciences, claims it could be helping support the base of the food chain in the Top End's flood plains.
Predation pressure and a "landscape of fear" created by crocodiles on feral pigs could also be limiting the latter's damage to sensitive ecosystems, the study suggested.
But the research did not make any suggestions about crocodile management and culling strategies.
A crocodile in brown water looking straight on at the camera close-up like a portrait.
Male saltwater crocs can grow around 6m long and weigh a tonne. (Supplied: Cameron Baker)
More crocs, more poop
Salties were nearly hunted to extinction in the NT in the 1960s. At one point, there were only an estimated 3,000 left.
The species was protected in 1971 and their numbers rebounded exponentially.
There's now thought to be more than 100,000 saltwater crocs in the Top End — and quite a lot more croc poop.
Crocodile excrement, like that of most animals, contains nitrogen and phosphorous, nutrients that help algae and aquatic plants grow.
A dried out white piece of crocodile poo on leaf litter as a big as a man's hand that is next to it.
Urine and faeces tends to come out as one combined mass from a crocodile's cloaca. (iNaturalist: lorcanmc, Crocodile faeces, CC BY-NC 4.0)
But crocodiles weren't considered to have major impacts on freshwater ecosystems as they didn't poop enough. Large individuals can go a year without eating.
So there are few studies into crocs' role as ecosystem engineers or movers of nutrients, which is important information when considering how to approach species conservation.
To get an idea of how NT salties affect their surroundings, ecologist Hamish Campbell and his team at Charles Darwin University took data from half a century of croc surveys.
The data indicated each kilometre of river currently contains an average of 400 kilograms of collective croc weight or biomass.
This allowed the researchers to calculate how much food the crocs needed annually to achieve these weights.
Research published by Professor Campbell's team three years ago showed the diet of saltwater crocodiles shifted from being predominantly aquatic-based (fish) to terrestrial-based (pork) with the rise of feral pigs in the NT.
The new study estimated the territory's crocodiles could now be munching through as many as six feral pigs per square kilometre of wetland annually.
This meant there were far more nutrients being expelled by present-day NT crocodile populations than previously thought. And because NT waterways don't normally contain many nutrients, that poop had the potential to make profound changes to the environment.
"If you look at the sheer volumes, it's got to have significant effects on the ecosystem from crocodiles," Professor Campbell said.
"That's probably creating algal blooms, and the base-building blocks of the food chain are being highly supported by crocodile excretion rates."
Professor Campbell said there probably wasn't much of an algal-bloom effect in the northern wet season when the sometimes-disconnected rivers are in full flow.
But in the dry season, he said, crocodiles would congregate in more isolated basins, which meant excrement could be highly concentrated.
A large saltwater crocodile on a bank, eyes closed, it's mouth open pointed upwards, a barramundi tail and body sticking out.
Fish such as barramundi form part of a saltwater crocodile's diet. (iNaturalist: Will Hunt, Saltwater crocodile, CC BY-NC 4.0)
Research in the US has shown that alligators can create small-scale nutrient-enriched "hotspots" in similarly nutrient-poor systems.
Too much nitrogen and phosphorous can overload a small water body leading to huge algal blooms, a lack of oxygen and fish die-offs.
Professor Campbell said that wouldn't necessarily happen in the NT, but no-one has yet quantified what croc excrement meant for ecosystem health.
"The excretion rates are different to the 1970s, but we don't have evidence if that's better or worse [for the environment]," he said.
CSIRO global change ecologist Bruce Webber, who was not involved in the study, said the research was another good example of how interactions between species can influence ecosystem stability and change.
"Interactions are the glue that hold ecosystems together but equally, novel interactions are catalysts for ecosystem change," he said.
"Not all ecosystem interactions have direct impacts, and the study highlights how non-native invasive species, such as pigs and buffalo, can change these ecosystems in both direct and indirect ways."
Pressure on pigs
Judging the full extent of saltwater croc impacts on the NT environment is tricky.
A lack of baseline data means ecologists don't precisely know how many crocs there were before mass hunting, nor what exactly a "healthy" NT river system should look like.
Over the past 50 years, there have been changes to the climate, as well as the amount of weeds and feral mammals in the region.
A mother feral pig with three piglets in a muddy patch they have dug up.
There's an estimated seven to 10 pigs per square kilometre of NT flood plains. (Supplied: Neil Edwards )
Professor Campbell said he thought there were probably more crocodiles now than before they ate pigs, but without any saltwater crocodiles, the NT would look a lot different.
"We really believe those rivers, the billabongs in the north, would be overrun with pigs and buffalo," he said.
"If you're a young buffalo or pig in the 1970s you could wallow around. Not now, you'd get smashed by a 5-metre crocodile.
"I imagine more animals are wary of river edges and ... [riverside] vegetation, and because of that we have much healthier vegetation."
Dr Webber said the suggestion a landscape of fear created by crocodiles could benefit wetlands was plausible.
"However, the full ecological impacts of the work, including how increased nutrient load impacts on vegetation, would need to be verified with field studies to understand the scale of impact on these freshwater ecosystems," he said.
An NT Department of Lands, Planning and Environment spokesperson said it was possible crocodiles were both helping suppress feral pigs and enriching freshwater flood plains with nutrients.
"Although the degree of these influences is still uncertain, this is important evidence of their ecological roles as apex predators," they said.
But the spokesperson said the predation pressure was not enough for meaningful control of pigs and buffalo.
"These feral animals need to be directly managed through targeted reduction programs," they said.
Target pigs to target crocs
As croc numbers grow, the debate over bringing back mass culls because of concerns for human safety continues to circulate.
There were 76 crocodile attacks in the NT, 23 of them fatal, between 1975 and 2022.
Research last year from Professor Campbell's team suggested that if crocodile populations maintained their current growth rate, there would likely to be little or no statistic change in the frequency of attacks per year to 2033 when compared to 2022.
An aerial view of five grey coloured crocodiles in muddy brown mangrove with green tree tops.
There are about five crocodiles for every kilometre of river in the Northern Territory. (Supplied: Cameron Baker)
Similarly, reducing populations by 15 per cent would have no detectable effect on attack frequency, while reducing them by 90 per cent would only drop the attack rate from 2.16 per year to 1.16.
Since 1997, the NT government has allowed for up to 1,200 juvenile and adult crocodiles to be removed from the environment each year.
But the take has tended to max out around the 300 mark and mostly involved removing crocodiles from Darwin Harbour.
Since April last year, 218 crocodiles have been removed.
Professor Campbell said if people wanted to reduce crocodile numbers, they could look at removing their feral food.
"In 20 years' time the crocodile population is going to be bigger than it is now," he said.
"If you take away pigs and buffalo you'd manage crocs."
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Posted16m ago16 minutes agoWed 12 Mar 2025 at 12:04am
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