Sunday, 4 September 2016

Hidden Life of Salt Pans

Marine salt pans look barren but there is life there when you look closely. The first surprise was that the salt pan was home to lots of wolf spiders.  They are the same colour as the ground and do not cast a shadow so it is hard to see them fleeing when our footsteps shake the ground.  Most of them are hiding in the cracks between the tessellating plates of parched earth.  Flies feed on the saline algal residues and I saw a spider leap several body lengths to land on a fly that was coming in to land.  However the spiders live in fear of the black wasps that hunt them both on the surface and through the cracked ground. 

A wolf spider (click to enlarge)
Whilst I was watching, a spider sensed that a wasp was looking a nearby flake of ground and abandoned its shelter to run for it across the open flats.  The wasp could smell the spider and was tracking it down.  On finding the spider’s trail the wasp would reach a frenetic pace with madly waving antennae.  The wasp was closing in on the spider and was within centimetres when it lost the trail and moved away.  Perhaps the trail rapidly becomes feint in the hot sun.  However the spider could not move when the wasp is in sight.  As the wasp moved off, I could see other spiders rising to the surface to survey the surrounds, initially mistaking the wasp for a fly before beating a rapid retreat.  I did not see the wasp find a victim and do not know where the paralysed but living victims end up, but they are probably carried by the wasp for at least 100 m to a sandy terrestrial area where they are buried in a tunnel to be slowly consumed by the growing wasp larva.

Wasp hunting wolf spiders
Wolf spider hiding in the crack (centre of photo)
Salt pans are flooded by tides at least every month and the parched ground collapses into soft mud without any cracks.  Where do the spiders go then?  Spiders can move over water quite easily and they might retreat to the patches of succulent vegetation.  The other possibility is that they hide in the ground.  On opening one of raised pimples that dot the saltpan, I found a spider inside together with a moulted skin.  The spider had been there for a while.  Perhaps it had even been trapped there, feeding on the tiny creatures that abundantly burrow through the mud.  This would be the spider equivalent of the frogs in stone phenomenon.

Small raised bumps cover much of the salt pan
On gently opening a bump to see what was inside, I found a spider and its old skin
The open salt pan seems to be habitat for spiders and insects rather than marine creatures like crabs.  Air breathing arthropods can better tolerate the harsh swings from wet to dry and from nearly freshwater after rain to hyper-saline when tides retreat and salt is concentrated by evaporation.  Many of the species present are not confined to salt pans and the spiders and wasps in particular are common in nearby vegetated sand dunes.

Flies (centre) and other insects feed on the mud and algae
The mud surface covered with tiny burrows and mounds of excavated material
Where salt pans are different is that they have a wet surface or shallow pools of water that are exposed to the full sun and grow a mat of algae.  This algal mat can support a vast population of insects. There are myriads of tiny critters that live in tubes or burrow in the mud and which feed on the algae.  Unfortunately, mosquito larvae are one of the insects that can thrive in these pools, so an understanding of the ecology of this zone is very important.  Ruts from motor vehicles driven over salt pans increase mosquito breeding potential exponentially as undisturbed salt pans usually drain and dry too quickly for mosquitoes to breed.

A wild pig foot print allows a colony of crabs to develop
Larger fauna can be found around the margins on the salt pan.  Mud crab burrows were quite common and most were surrounded by a small patch of mangroves.  It appears that the mangroves depend on the mud crab somehow.  I suspect that the crab hole helps to moderate the salinity by providing better drainage for a metre or so around the mouth of the burrow.  The mangroves persist for a while after the crab disappears but life for a mangrove in a salt pan is uncertain and without the crab hole, the trees probably perish after a period. 

mud crab burrow in north Queensland
Mud crab hole in a salt pan
Hiding under the mangroves around the edges of the salt pan are huge numbers of mud creepers and these a possibly the main prey of mud crabs.  Mud crabs also graze on vegetation and I wonder if they feed on the algal mat as well as molluscs.

Mud creepers (mainly Telescopium telescopium) sheltering under a small mangrove 
A Telescopium shell, possibly after being predated by a mud crab
The empty salt pan competes for space with great patches of succulent vegetation.  In the dry season this vegetation is like a psychedelic shag pile carpet with green, pink and purple mottles.   This part of the ecosystem supports large numbers of crabs and small air breathing molluscs.  The succulents appear to prefer the better drained areas.

A carpet of samphire vegetation on a salt pan near Port Douglas
In close-up, samphire vegetation looks like an alien forest
I suspect that the areas I have written about in this post are the areas that are shallow, brackish water ecosystems in the wet season with succulents occupying slightly raised areas which have better drainage.  Both are wetland ecosystems that are watered by tides and during the rainy season, by a film of freshwater due to water being shed from the flat landscape no faster than the water is replenished from the sky and by seepage from nearby land.

The salt pan lies between patches of samphire vegetation
The ecotone between salt pan and samphire has a parchment like algal crust

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