The Wollumbin Bioregion refers to the area roughly corresponding to the Tweed Valley and the surrounding rim of mountains. This includes Lismore and Ballina to the South, Byron Bay in the East, Mount Tambourine in the north and the Border Ranges to the north west. The shaping of the region and its ecologies owes a great deal to geomorphic processeses. A large volcano once towered over the valley but, in a major geomorphological event, subsided to create the Tweed Valley. The rim of the eroded volcano is visible to the west. At the centre is Wollumbin, or Mt Warning, the massive volcanic plug.

The volcanic process that shaped the landscape also influenced the evolution of natural ecosystems in the area. Subtropical rainforests evolved on the deep basaltic and alluvial soils, giving the area its rich biodiversity. Some people suggest that Indigenous people of the Bundjalung language group had an understanding of the bioregion that roughtly corresponds to the Wollumbin bioregion, as understood through through the eyes of modern science.

Ecologies of the Wollumbin Bioregion

Ecology is the study of organisms in relation to their environment. It focusses on relationships between organisms and natural elements such as climate, geology and hydrology. Ecologists take a holistic perspective on how systems function. The theory of succession demonstrates how ecosystems develop and respond to disturbance over time and become self-maintaining. The Wollumbin Bioregion has many rainforest ecologies as well as estuarine ecologies. European settlement impacts ecologies through land clearing, introduction of pests and development.

Rainforest ecologies

Prior to European settlement in the mid 19^th century, a massive expanse of subtropical rainforest covered the bulk of the bioregion. It has come to be known as the Big Scrub. This covered some 75,000 ha of the Wollumbin bioregion stretching from the foothills north of the Nightcap Range, to Byron Bay, Ballina and Lismore

There were, and still are, a number of variations in rainforest types that evolved in particular niches. Unique types of rainforest evolved in areas with particular climactic and geological features including high altitude, coasts (littoral rainforest), cool climates, dry areas (vine forest), as well as river and creek ecosystems (riparian) and areas with poor drainage (palm forest). Each of these has a different structure and dominant species.

Rainforests are characterised by high levels of rainfall but also by their multilayered structure, plus the presence of vines and epiphytes (plants that live in low light conditions). This includes an emergent layer where very large trees that grow above the general canopy; a canopy layer, home majority of the largest trees; the understory layer which lies between the canopy and the forest floor; and the forest floor populated by fungi and decaying vegetation.

Rainforest species are adapted to thrive in a particular layer of the rainforest. For example, epiphytes (such as ferns and orchids) are adapted to low light, and live in the understory. They rely on branches of trees for support and capture nutrients and moisture from above. Vines can survive in lower light condititions but use the trees as a structure to climb up for more light exposure.

The theory of succession is central to understanding the built-in disturbance response of rainforests, and how rainforests achieve self-maintenance. After a disturbance, such as when a tree falls and creates an opening in the canopy, early successional species (pioneer, early secondary) respond to the abundance of light, germinate and grow quickly. They have a relatively short life span and are thought to create conditions for later stage species to establish and grow. Such species include Sweet Pittosporum, Pittosporum Undulatum and Kangaroo Apple, Solanum Aviculare. Pioneers are gradually replaced by later successional (later secondary) species which are slower growing, generally longer lived and can survive in lower light conditions. Silky Oak, Grevillia Robusta is a later successional species. The forest eventually reaches what is referred to a mature phase consisting of species that are longer lived and have a higher tolerance for shade during the seedling and sapling stages (eg black bean). This phase is typically the canopy layer. However, this is only a rough guide as the whole forest will be patchwork each at different stages of succession.

Reproduction within an ecology

Seedbanks persist in the soil over a long time. Pioneer species (for example Acacia melanoxylon) have longer lived seeds. Medium successional species have medium to short term viability. Mature phase species have much shorter viability. To compensate for their short term viability, mature phase species usually have some suppressed successors waiting in the understory.

Seed dispersal occurs through a variety of ways. Fruit eating birds and bats, eat and excrete seeds over very long distances (eg. figs, laurels, quandongs lilly pillies, white cedar). Wind can disperse seeds over very large distances (eg. teak, red cedar, cudgerie). Water can move both short of long distances. Larger marsupials, possums and rodents can move bigger seeds small distances (black bean, macadamia etc). Ants and other invertibrates can move small seeds like wattles.

Insects are the dominant pollinators in a subtropical rainforest, playing an essential role. Many plants make use of a variety of pollinators present in a mature forest. Essentially it's an insurance policy to cope with disturbance. Often the understory is neglected reafforestation but this plays a major role in pollination by bringing in a huge variety of pollinators.

Animals predate on seeds, playing a major role in maintaining the balance and diversity of ecosystems. Forest floor nutrient recyclers, like macropods, assist with the recycling of nutrients. Many niches in the rainforest are filled by creatures. Bird species have co-adapted to particular plants. For example native lime have spikes adapted to bird life which don't seem to give the plant much protection but provide safety for small bird species. The ancient history of rainforest allows these niches to be filled.

Humans & weeds: towards ecosynthesis

Weed species can spread because they find a niche in the ecosystem and have no predators to keep populations in balance. Weed species often perform important ecological functions, such as stabilising soil, building nutrients and providing habitat (eg lantana). I have observed river banks grow multilayered weed ecologies comprising understory ferns and lillies, cassia, lantana, a low canopy of acacia melanoxylon and umbrella tree and emergent camphor laurels. It seems that grazing encourages dominance of aggressive pioneer species like camphor and the removal of most other species.

Ecological concepts and patterns help us in restoring damaged ecologies, managing agricultural systems and management of weedscapes. Ecological lessons can even apply to humans and cultural institutions. While the Bundalung language group perhaps represent a mature phase of climax evolution, pioneers and their descendents represent early and later successions. The movement towards sustainainability perhaps represents a transition towards another mature stage. It is tempting to want to recreate the Bug Scrub. But a more realistic goal is eco-synthesis: finding a balance between introduced 'weed species', like us, and fragile pre-existing ecologies.