The link between crocodiles and the coral reef

daintree crocodile


The story of crocodiles and coral reefs is far more interesting than the fact that the occasional animal is seen resting under a pontoon. The bigger story is centred not on the coral, but on the coast, amongst the labyrinth of mangrove forests where many of the creatures of the Reef begin their lives.

What do crocodiles really have to do with the Reef?

The environment of the coral reef is rather a terrible place to be for a baby fish. The reef is a world of wall-to-wall predators, densely packed with finely tuned carnivorous competitors on the lookout for anything they can cram into their toothy mouths.

Most newly hatched baby animals have very little chance of survival in this ferocious melee. Add to this the fact that resources are scarce, due to the very low nutrient levels on a healthy reef. In addition, the small amount of available food is greedily consumed by thousands of larger, faster, and more mature fish. They occupy every niche, leaving very little for the tiny fry to devour.

Mangroves, by contrast, are a complex, nutrient-rich habitat with a fraction of the competition, and a great deal more opportunity for little mouths to feed. About a quarter of the animals who live on the reef in their adult life, begin their lives in the sheltered, nutrient-rich mangroves.

Here they perform an incredibly important job. The reef needs nutrients that are generated by the forests along the coast. This detritus, leaf litter and mud is washed from the landscapes whenever it rains. But raw, unprocessed mulch would bury and suffocate the reef if it made it all the way out that far.

Fortunately, the complex mangrove bank sides trap this material, where it breaks down, forming the basis of the nutrient-rich ecosystem that suits the reef babies so well.

These young creatures assimilate this material, incorporating it into their growing bodies, converting terrestrial debris into living sea creatures who ultimately deliver this biomass to the reef in exactly the form the reef requires.

Nonetheless, there is a problem with this arrangement. The mangroves too, are full of predatory fish. They are also full of dead-ends and opportunities for ambush.

They are tidal channel systems in which sections and branches can become isolated, leaving any small fry trapped within to the mercy of bigger fish trapped with them. In some ways, the mangrove jeopardises survival more than the reef itself.

This is where the crocodile comes in.

The crocodile is a babysitter for these tiny young ocean-bound creatures. Fry and larvae are minute; from millimetres to centimetres in length. Such tiny creatures are not even on the crocodile’s radar, let alone its menu.

Predatory fish like barramundi and mangrove jack, on the other hand, are very much on the crocodile’s diet and are actively hunted throughout the mangrove system.

This hunting forces the predatory fish into making certain choices- they modify their behaviour to minimise the risk of predation. There are certain places that a barramundi simply can’t go (if it did, it would not be coming back out again). There are other places where they are safe to venture, and here is where they do their own hunting. There are other places too, where they can transit carefully without lingering or distraction by the feeding opportunities around them.

This dynamic has the effect of partitioning the mangrove into a series of zones. Some of these zones are perfect nurseries for baby sea life to mature. When they do, they migrate out to sea bringing with them all the converted nutrients from which they have built their bodies, whilst under the accidental protection of the biggest predator on the continent, the crocodile.

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