Wednesday, 21 June 2017

Striped Possums in the Mangroves

Near Cairns, most of the mangrove swamps appear to have a good population of striped possums. The possums leave distinctive marks on dead trees, when they chew holes into the timber to get at the beetle grubs inside. Sometimes the possums leave great pits in the sides of trees. Mangrove timber is very hard and I often cannot even mark the sides of the pits with my fingernails. It must take the possums a great deal of effort to get at the beetle larvae or witchety grubs (moth larvae).

Striped possum feeding marks
Dead mangrove at end of boardwalk with possum damage
In June 2017, I was able to make a video of a striped possum (Dactylopsila trivirgata)  breaking open the bark of a mangrove tree on the Cairns Airport Mangrove Boardwalk.

During the day, the mangrove forest is revealed to be a dense, 20 m tall stilt mangrove forest. This forest has no connection to any terrestrial areas, not even stranded beach ridges, so the possum must live in the mangroves entirely. This makes me wonder how it obtains freshwater and enough variety of food to survive. The possum I observed appeared to be fairly small for a striped possum and was so hungry that it completely ignored me even though I was standing about 4 m away with a bright light.

Mangroves beside Jack Barnes Memorial Boardwalk
This tall stilt mangrove stand is about 250 m wide and 750 m long 
This particular trip into the mangroves taught me quite a lot. Firstly, there are very few flying insects in the mangroves at night (other than bloodsuckers). There also seemed be very few spider webs. Bright eyes revealed occasional wolf spiders on tree trucks. Wolf spiders jump on prey, rather than using webs. Only one sleeping bird was observed and one large Papuan Frogmouth.

Podargus papuensis
Papuana frogmouths are large and have red eyes
It is counter intuitive for a place with such exuberant vegetation to be so devoid of wildlife. However, on reflection, very few insects can eat mangrove foliage and even then, they usually only take a few bites, so it makes sense that the terrestrial food pyramid is virtually absent. Mangroves have a detrital food web that is based on plant material that has died and been reprocessed by bacteria and fungus into a less toxic form. Normally mangrove leaves which have fallen partially decompose and are then consumed by crabs and shrimps. 

Perisesarma messa feeding on fallen mangrove leaves
In a way, the striped possum is also part of a detrital food web. It feeds on beetle larvae that burrow into dead timber and spread the spores of fungus, which grows and provides food for the beetle larvae. When watching the video of the striped possum, it almost looks like the possum is either drinking or feeding on the fungus lining of the beetle tunnels in addition to feeding on the beetle larvae. Perhaps someone should research this someday.

Striped possum feeding marks
Marks created by striped possum in video

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