EPOW - Ecology Picture of the Week

Brown Tree Frog: A Lesson in Nuance

Brown Tree Frog (Litoria ewingii), Family Hylidae
New South Wales, Australia

Explanation:  This unassuming little tree frog was clinging to an insect trap hanging in a tree, likely trying in vain to secure some of the netted prey.  

We are in the heart of the Otway Range in southern New South Wales, Australia.  I am following biologist Sandra Penman who, along with her husband Trent, are hosting me for a coastal exploration and stay.  Sandra has put out a series of insect traps in trees throughout the forests here, in a study of bats.  But here we have caught -- well, just encountered -- this tree frog.

But what a lesson it has to teach.  Tree frogs of the family Hylidae have become remarkably diverse in Australia.  Just this one genus alone, Litoria, has evolved into no less than 34 species that collectively occupy every Australian state as well as also occurring in Papua New Guinea.  This is adaptive radiation at its finest, where some original form spreads out and specializes in various environments, evolving into a suite of different species.  Australia is a wonderful natural laboratory for studying adaptations and evolution.  

But with so many species, identification can be difficult.  Our friend here can be differentiated from 6 similar-appearing species only by a set of identification markings, most of which can be seen in this week's main photo, above.  These differentiating markings include:  the dark eye stripe that covers the tympanum ("eardrum"); the white stripe below that dark stripe; the broad darker brown strip down the back that lacks green specks but has small darker flecks; and the greenish color of the hind thigh with some orange-yellow.  Can you find all these features in the photo?  Additional features separating the brown tree frog from its cousins include the color the belly, small toe discs, and its different call.  

Whew.  

We rescued this amphibian from its perch on the insect trap net and returned it to the forest floor.  And thought, how interesting it would be if we could return here in another few thousand, or hundred, years, and see if more species had developed ... 

Information:  
     Robinson, M. 1999. A field guide to frogs. Reed New Holland Publishers, Sydney, Australia. 112 pp.

                   Acknowledgments:  
                          My thanks Sandra and Trent Penman for so kindly hosting and teaching me.