Byron Coast Koala Habitat Study
The Byron Coast Koala Habitat Study is the first step in the preparation of the Byron Coast Comprehensive Koala Plan of Management (CKPoM). The study provides the scientific basis for the CKPoM providing detail of koala population location and size, historic and current population trends, extent of habitat, preferred feed trees and threats impacting on koalas.
The specific aims of the study were to:
- examine current and past koala distribution within the whole Byron Shire local government area (LGA)
- determine the preferred koala food trees for the study area
- map potential and core koala habitat
- examine key threats to koalas including road mortality black-spots
- identify important linkage areas, and
- provide an assessment of population size and local population viability, and recommendations to inform preparation of a Comprehensive Koala Plan of Management for the study area.
This study was carried out on the coastal portion of the Byron Shire LGA, an area of approximately 13,790 hectares including the entire coastal strip extending from Billinudgel Nature Reserve in the north, south of Broken Head, and west to Mullumbimby. This study area included the major urban growth areas and therefore the areas where highest potential impact on koalas exist.
Analyses of 1,471 records of koala sightings over the period 1900 to 2011, revealed that koalas have a long history of occupation in Byron Shire and are more widespread today than they have been in the past. A 17% increase in Extent of Occurrence was recorded for both the Shire and the study area, while an apparent doubling in Area of Occupancy has occurred over the last three koala generations. At the Shire wide level, population expansion has generally been from the west to the south and east, whilst a contraction of range has occurred in the north.
Field based survey
Field based surveys were carried out using a systematic habitat sampling strategy that gathereddata on evidence of koala presence, food tree preferences, and koala density. Sixty three (63) field sites were sampled; 18 of which recorded evidence of habitat use by koalas. Koala activity was highest in the area between Brunswick Heads and West Byron, with four disjunct sub-populations occupying an area of approximately 1,470ha within which habitat is highly fragmented and of variable quality. Two major koala population centres were identified; Myocum – Tyagarah, and West Byron while activity was also recorded at Mullumbimby and Broken Head. Modelling of fielddata provided a broad population estimate of approximately 240 koalas existing within currently occupied habitat.
Food tree preference and habitat mapping
Data from 1,565 trees collected during the course of the study were augmented by that from other studies in order to identify preferred koala food trees. Collectively, a total of 2,543 trees comprised the tree use data set. Supported by previous work, four species – Forest Red Gum Eucalyptus tereticornis, Swamp Mahogany E. robusta, Tallowwood E. microcorys and Grey Gum E. propinqua – were identified as the most preferred species within the study area. Swamp Mahogany, Tallowwood and Forest Red Gum were confirmed as primary food tree species across the majority of the study area, while Tallowwood and Grey Gum were expected to act as secondary food trees when growing on low-nutrient soils.
Tree preferences, vegetation mapping and soil landscape mapping informed classification of koala habitat which identified approximately 2,060ha of potential koala habitat in the habitat classes Primary, Secondary (A) and Secondary (B) within the study area.
The study identified 5 key threats to koalas:
Habitat loss and fragmentation – historic and ongoing clearing of vegetation and development, including the Pacific Hwy, has lead to a decrease in available habitat and increase in isolation of koala populations.
Fire- Wildfire and poor fire management can negatively impact on koalas leading to direct mortality as well as reduction and temporary loss of feed trees. Fire is considered a major factor in the decline in koala population north of the BrunswickRiver.
Road mortalities – Of the 374 koala mortalities recorded by Friends of the Koala between 1989-2011, 19% are attributed to motor vehicle strike. Analysis this data and from other sources indicates that the numbers of koalas killed on roads has been increasing annually over the last decade. The study identified a number of ‘black spots’ where road mortalities were frequently reported.
Dog attack – domestic dog attack continues to contribute to koala mortality and increased with urban expansion. A single bit can lead to the death of a koala as a result of shock or infection.
Disease – disease such as Chlamydia and Koala retro-virus are present in koala populations and lead to high levels of mortality. The increase level of disease is considered symptomatic of disturbance and cumulative impacts from the range of threats koala experience, particularly isolation of populations leading to inbreeding.
A history of land clearing has resulted in a landscape that is highly fragmented with the coastal strip effectively disconnected from habitat in the west of the shire. The persistence of koala populations could be considered surprising given the extent of habitat fragmentation on the coastal plain, and the barriers presented predominantly cleared farmland and the Pacific Highway.
While the population expansion over the entire shire is positive and encouraging, examination of the situation for Byron’s coastal koala populations has identified a number of important issues to be addresses to ensure their long term survival. The low occupancy rate and level of isolation of sub-populations suggest that coastal populations may be unsustainable in the absence of improved connectivity and an increase in habitat cover. The enhancement of linkages to allow gene flow between coastal and hinterland populations will be a fundamental tool for increasing their probability of persistence. Road mortality is also identified as a major barrier to the recovery potential and the isolation of coastal koala populations.
The study identifies a number of recommendations to ensure the conservation of Byron Coastal koala populations. The full study can be found at the top of this page under the item Links.