Thursday, 31 July 2014

Grassy Salt Pans of Cairns

Almost exactly 100 miles/160 km down the coast from the succulent covered salt pan near Cooktown is one of the few remaining salt pans near Cairns. Despite being so close that a lazy crocodile could drift from one to another in less than a week, the botany of these two salt pans is very different. At Cooktown, it was all succulents, near Cairns it is usually nearly all grass. In Google Earth the photo pattern of each vegetation type is remarkably similar. Surprises like this remind me that neither I nor anyone else I know of can fully understand why things grow where the do. Until we can understand why what grows where, environmental management is just educated guess work.  I think students and rangers could do mini science projects to find out why some places that should be similar are entirely different.  Professional scientists don't answer these questions as they need to maintain their fame and are not interested in easy research.

Sporobolus virginicus
Saltwater couch grassland/salt marsh at Yorkeys Knob
Most of the original salt pans and samphire flats around Cairns have been lots to development so I have to be careful about deciding that Cooktowns salt pans are different to those found near Cairns. Many salt pans within Cairns City were reclaimed. Cairns Airport is on what once was a salt pan and the major agricultural folly of East Trinity also included saltpans. Fortunatelly, we have a couple of good examples left and I think the remaining examples are similar to the ones lost. It is also unlikely that any halophytic (salt tolerant) species would be lost from the entire region. Salt pan plants are great colonists and show up even in salty ruts around the edges of sugarcane fields. For these reasons, I suspect that the succulent flats like those near Cooktown never occurred on salt pans near Cairns.

The natural park lands are quite pleasant provided you wear long clothes and don't dally around after sunset
Cooktown is in the Cape York Peninsula Bioregion and Cairns is in the Wet Tropics Bioregion. Bioregions are like countries for flora and fauna so this suggests that the salt pan vegetation could be different. Queensland has 13 bioregions. The bioregion boundaries were originally drawn by smart people with marker pens on paper maps. They were not something that came from a sophisticated analysis. Yet it is strange that even intertidal communities seem to have a sudden transition at the bioregion boundary. It is strange because most mangrove plants and animal species (fish, crabs, molluscs) occur from here to Africa and on almost every coastline in between. Given that most species are so widespread, why to comparable salt pans that are so close by natures standards have such different vegetation? I can only guess. Climate and geology and tidal inundation cycles are all quite similar. The founder effect may be at play. The founder effect is natures version of losers/keepers. The founder effect is when the first plant species to find a new island or in this case salt pan multiplies and occupies all the space making it very hard for new species to establish.  In very flat places, I have noticed that even a few centimetres in altitude makes a big difference to how long an area is flooded in inland areas or how many tides per month are experienced in coastal areas and I would start my investigations here. Succulents (Halosarcia sp.) do grow on slightly lower ground around the margins of the grassy swards in Cairns but are low and straggly.

Saltwater couch (Sporobolus virginicus) is the grass that covers the salt pan. We also have a second species of saltwater couch (Paspalum vaginatum) which often grows with the first species and looks very similar. The leaves of the first species are spirally arranged on the stems and the on the second only grow on opposite sides (called distichous).

Sporobolus virginicus has little tufts of leaves from underground runners.
Crabs and mud lobsters thrive in the saltwater couch and the ground is so full of hummock and hollows that I stumble through the grass rather than walking through it.  I wonder how the productivity of this marine grassland compares with mangrove ecosystems.  

Mud lobster mound peppered with crab holes
Shed exoskeletons were hanging in the grass
Ant plants also like the margins of salt pans and I suspect this is related to mists but I don't really know.  

Spiny ant plant
Myrmecodia beccarii - the spiny ant plant

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