Are there old-growth dependent species?

December 1998

• Forest dependent species - species that rely on some particular forest structure (eg. multistorey tree canopies), forest vegetation associations (eg. the association of the Koala and certain species of eucalyptus) or ecological process (eg. the relationship between pyrophilic tree species and frequent wildfire).
• Keystone species - species known to be very important to an ecosystem, for example pollinators, food species for large carnivores, and key decomposers.
• Representative species - species with habitat dependencies typical of a group of similar species and which are likely torespond to changes in availability of those habitats or resources. Examples are wetland dependent species and tree-hollow dependent species.

The discussion in response to this question considered defintions of ‘old-growth’ and the evidence concerning dependent species. A summary of the discussion follows.

Old-growth

The term ‘old-growth’ originated in North America where it refers to the last successional stage of forests, typified by old-field succession in North-Eastern USA. Thus the term ‘old-growth’ has notions of climax and succession, in the traditional Clementsian sense. However, ‘old-growth’ and Clementsian succession do not apply to Australian forests. Rather old-growth is characterised by structural properties rather than by species succession.
      In Australia 'old growth' has been defined in the National Forest Policy Statement (1992) as “ecologically mature forest where the effects of disturbances are now negligible”. A more quantitative definition has old-growth forest as that which has at least 10% of the total crown cover as senescing trees in the upper canopy, less than 10% of total crown cover as regrowth, and has not been significantly disturbed (eg Woodgate et al A study of the old-growth forests of East Gippsland, Department of Conservation and Natural Resources, Victora 1994).
      It was argued that such definitions do not acknowledge that there is a continuum of forest structures, in which "old-growth" sits only at one end of a gradient and that other forest structures may contain many of the structural elements and attributes of ‘old-growth’. From a scientific and a management point of view it may make much more sense to classify forests using a"structural diversity scale". Forest structures could score on this scale according to the presence of structural elements such as hollow bearing trees, downed logs, species mix, understorey density and age. It was also argued that interpretation of local forest type is required as there is not a definitive condition across all types or regions.

Dependent species

With regard to 'dependencies', it was felt that we should be careful to relate these to the components of the forest, not to a composite of components such as 'old growth', however defined. An example of an encompassing term used in the North American context is "species associated with late-successional forest" (Marcot, B.G. in: Creating a Forestry for the 21st Century. Kohm, KA and Franklin JF (eds),pp 87-105). It was suggested that in the Australian context, the term "Late successional" could be replaced by "structurally diverse" or more specific descriptions of habitat attributes. Using the term "associated" implies that they can also occur in other forest structures that meet their habitat requirements.
      For example, there are many southern Australian vertebrate species that depend on elements of old forest (e.g. tree hollows; abundant epiphytes such as mistletoe; wide spaces between old trees) but these elements can be maintained or recreated in other environments. There are some species for which old forest is a very important habitat. The large forest owls (especially Sooty Owl) and Yellow-bellied Glider are two species that tend to be commoner in extensive areas of old forest than in regrowth. Any definition needs to consider scale. "Hollow-dependent" is a much more easily defined term, and hence probably more useful for these species but other species present more complex requirements. For example, the presence of epiphytes which are found particularly on the tree fern Dicksonia antarctica in Victorian and Tasmanian mountain ash forests appears to be a function of humidity. Since the understorey in old-growth generally provides a more humid atmosphere than that in re-growth forests, epiphytes are by and large restricted to old-growth. The difficulty is - not always. There are younger forests with epiphytes on tree ferns (though not in such abundance as in old-growth) and there are old-growth forests where there are no epiphytes. Again, perhaps it is a matter of scale rather than rigid definition. One could say that epiphytes are in much greater abundance in old-growth than in younger forests. As with the hollow-dependent species, epiphytes require a substrate and the best substrates are in old-growth forest.
      It was felt that the tendency to use the term "dependent" rather loosely exacerbates the problem, in that it makes it impossible to either prove or discount the notion that certain taxa are reliant on "old growth" for their continued existence and that the term should not be used without a suitable qualifier ( the terms obligate or facultative dependence were suggested) or, alternatively its meaning should be restricted to refer only to total reliance. A workable definition would allow for some of the hypotheses regarding the relationship of species to not only the attributes that comprise "old growth" but the impact of their spatial patterning on species distribution and abundance to be tested.

We would welcome more contributions to this debate through the Scientists for Sustainability Forum page.