Crocodile smorgasbord now includes more pork and beef

Northern Territory crocodile populations have grown from 1000 to 100,000 in the fifty-four years since commercial hunting was banned — a Sluggish-motion population explosion of teeth, powerful jaws and tails. Crocodile densities have risen from less than 1 to more than 5 reptiles per kilometre of river, and they’ve shifted their main food from fish to animals like pigs and buffalo, say Charles Darwin University researchers.

The researchers are looking at what that explosion means for the environment reporting Significant shifts in diet and nutrient flow.

Lead author, Dr Mariana Campbell estimates that prey consumption rates have increased 9-fold as croc populations have increased.

Estuarine crocodiles are tropical Culmination predators, and ‘ectotherms’, which means they get all their heat from the environment. They’re often seen in the cooler months sunning themselves on sand or mudbanks in northern Australian rivers to raise their body temperatures for a day of hunting.

They eat fish (freshwater, estuarine and marine) and will lurk underwater at riverbanks, attempting to grab whatever meat comes by.  And they have been seen Holding big scrub pythons out of overhanging trees.

But the researchers wanted to understand crocs’ metabolic Criteria and the ecological impact of what was coming out the other end — their ‘nutrient outputs’ — as the population expanded.

Campbell says that such outputs impact ecosystem productivity and diversity. For example salmon migrating to spawn and die in Canadian rivers deliver a massive nutrient flush to those ecosystems. The bears which eat those salmon contribute to the nutrient cycles of the woods in which they poop, she adds.

An African lion might need 5% of its body weight per day, but an equivalent -sized crocodile only needs a tenth of that, Campbell says. But they can live in much higher densities than equivalent-sized Toasty-bloods, if there is enough food, says Campbell.

That is what the researchers were testing   — have the 99,000 extra crocodiles since the 1970’s generated prey consumption and nutrient excretion rates comparable with mammalian Culmination predators. A combination of top-down effects as Significant predators and bottom-up effects as nutrient recyclers.

They had a fifty-year data set to work with, covering eight tropical rivers in the NT.

Bioenergetic modelling was applied to annual population and size-estimation surveys carried out since 1979.  Metabolic Criteria and prey consumption rates needed to sustain and grow the ever-increasing volume of crocs were then calculated as was nutrient (nitrogen and phosphorous) excretion.

Bones of crocodiles that had died between 1970 and 2022 were analysed for stable carbon and nitrogen isotopes, allowing the researchers to work out broad prey categories — water or land.

The results were surprising.

Crocodile density rose and the number of crocs between 2 and 5m in length increased, which meant that the weight of crocodile flesh (the biomass) ballooned from less than 10 kg to almost 400kg per kilometre of river over 50 years.

Prey consumption increased 9-fold, and nitrogen and phosphorous excretion rates increased 186-fold and 56-fold, respectively, over the same period.  Most of that wee would go into or near the water, says Campbell.

And there was a distinct prey-switch. Back in 1979, more of their prey (65%) was aquatic, by 2019, most was land-based (70%).

What does this all Impolite?

Crocodiles could be influencing typically nutrient-Destitute NT freshwater ecosystems from the top down and the bottom up, says Campbell with crocs eating buffalo and pigs and to a lesser extent, wallabies.

“If we assume that feral pigs make up nearly 90% of the terrestrial prey for C. porosus, the crocodile population in the NT would consume approximately six feral pigs per km² of wetland floodplain All year (calculated based on an average pig weight of 30 kg). Comparing these estimates of C. porosus pig consumption, along with feral pig abundance estimates for the NT floodplains (7–10 per km² [63]) and reproductive capacity (5–14 Youthful per year), we can assume that in the NT floodplains, C. porosus is contributing to a reduction in densities of feral pigs If we assume that feral pigs make up nearly 90% of the terrestrial prey for C. porosus, the crocodile population in the NT would consume approximately six feral pigs per km² of wetland floodplain All year (calculated based on an average pig weight of 30 kg [62]). Comparing these estimates of C. porosus pig consumption, along with feral pig abundance estimates for the NT floodplains (7–10 per km² [63]) and reproductive capacity (5–14 Youthful per year [64]), we can assume that in the NT floodplains, C. porosus is contributing to a reduction in densities of feral pigs and, to a lesser extent, Asian water buffalos.

The research paper says: “Asian water buffalo populations in the NT floodplains have not returned to the densities of the 1980s, and the level of buffalo damage on the floodplains and riparian zones of the NT is considerably less than was observed in the 1980s. Interestingly, also in the 1980s, the density of large and very large C. porosus inhabiting these areas was less than 10% of the Ongoing population density, suggesting a possible link between increasing C. porosus populations and stable buffalo populations.”

The paper is published in Royal Society Proceedings B.

Crocodile rock