It’s a long way to the top and every stratum plays a role

The wet forests east of Melbourne are home to the tallest flowering plant on earth, the mountain ash. Reaching heights of over 90 metres, they dwarf many of the other plants growing alongside them. Being tall gives mountain ash first use of the sunlight that falls on the forest. This is an important advantage because energy harnessed from sunlight is what almost all plants use to fuel their growth. Closer to ground level, smaller plants have to contend with less and less light, as foliage above casts them into shadow. This has resulted in adaptations including large leaves that are rich with the light-harvesting pigment chlorophyll to maximise the amount of light captured. You may be able to see this for yourself because these leaves are usually a darker green colour. Other plants invest very little energy in woody tissue and have adapted to become climbers instead. These species piggyback on other plants to get their leaves as high as possible at the lowest cost.

Many species, such as this swamp wallaby, live in the ground layer. Despite the tangled nature of the understory vegetation, the animals that inhabit this layer have little trouble moving through it. Photo: Rowan Mott. 

Many species, such as this swamp wallaby, live in the ground layer. Despite the tangled nature of the understory vegetation, the animals that inhabit this layer have little trouble moving through it. Photo: Rowan Mott. 

There is a paradigm that habitat diversity begets biodiversity. The many ways that different plants compete for available light open up opportunities for a diversity of animals. At ground level, the vegetation is often dense and impenetrable to people. Yet, many species of birds move through the tangle of leaves and twigs with ease, and swamp wallabies crash through with little problem. Some birds, such as the superb lyrebird and Bassian thrush, pass their days almost exclusively searching through leaf litter on the forest floor. Amongst the decaying leaves, their favoured invertebrate prey items are readily found.

Slightly higher above ground, small birds such as brown thornbills and eastern yellow robins inhabit low shrubs such as kangaroo apple and snowy daisy bush. However, brown thornbills and eastern yellow robins use very different foraging strategies. The latter sit and wait on an exposed perch, darting down to the forest floor to snatch insects as they emerge from the cover of the leaf litter, whereas small parties of brown thornbills actively move through the shrub layer gleaning small insects from the foliage.

The eastern yellow robin is a denizen of the lower stratum where it can be found clinging to a vertical trunk, or perched on an exposed branch, waiting to spot an invertebrate among the leaf litter. Photo: Rowan Mott. 

The eastern yellow robin is a denizen of the lower stratum where it can be found clinging to a vertical trunk, or perched on an exposed branch, waiting to spot an invertebrate among the leaf litter. Photo: Rowan Mott. 

Above the layer of soft tree ferns and into the forest midstorey, a new suite of birds replaces those found in lower strata. Here, tall shrubs and small trees including hazel pomaderris and silver wattle provide habitat for grey fantails, golden whistlers and Lewin’s honeyeaters. This layer is also frequented by white-throated and red-browed treecreepers as they hop upwards on the trunks of even taller trees. Once again, these species each have a specialist foraging strategy. Grey fantails sally insects while in flight, whereas golden whistlers pick their insect prey from leaves and bark. Contrary to what is suggested by its name, the Lewin’s honeyeater eats mainly fruits, but it does feed on nectar from time to time and also includes insects in its diet. The two species of treecreeper probe in crevices and under loose bark on trunks and branches, hoping to find spiders and other small invertebrates.  

High up in the canopy there is more freedom to move, and this is the domain of large-bodied species such as crimson rosellas, pied currawongs, and powerful owls, but there are also smaller species such as white-naped honeyeaters and spotted pardalotes. Often, the species inhabiting this stratum are predators, nectarivores or lerp specialists. This is also the layer where arboreal mammals such as greater gliders can be found.

Except for locations where people feed crimson rosellas, this species is usually found in the canopy where they feed on eucalypt seeds and a range of other foods. Photo: Rowan Mott. 

Except for locations where people feed crimson rosellas, this species is usually found in the canopy where they feed on eucalypt seeds and a range of other foods. Photo: Rowan Mott. 

All of this diversity is made possible because of the exceptional height of mountain ash trees. This allows many distinct vegetation strata to occur, which in turn support distinct assemblages of animals. When walking along a forest path, it is easy to get caught up in your immediate surroundings. However, by taking the time to look up and down, you may be lucky enough to see a wealth of life that would have otherwise gone unnoticed. It can also be good to take a little bit of extra time where the trail is flanked by steeply sloping terrain. Here, the forest below may offer the chance to glimpse canopy-dwelling species closer to eye-level, giving the illusion that you are high amongst the mountain ash. 


Rowan Mott

Rowan is a PhD student studying seabird ecology. When he's not thinking about the ocean he likes to think about woodland birds. 

Check him out on Twitter at @roamingmoth