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Edge effects and birds across karri forest (Eucalyptus diversicolor) clear-fell edges: a study of theory and conservation management

Atkinson, Penelope Ingrid (2003) Edge effects and birds across karri forest (Eucalyptus diversicolor) clear-fell edges: a study of theory and conservation management. PhD thesis, Murdoch University.

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Theoretical and empirical studies on logging impacts internationally indicate that edges between mature forest and regrowth may alter the abundance and distribution of fauna, with implications for biodiversity conservation. The south-west Western Australian karri (Eucalyptus diversicolor) forest bird community is characterised by high degrees of endemism, cooperative breeding and reliance on nest hollows. It may therefore show different responses to clearfelled edges than bird communities in Northern Hemisphere forests, where most edge studies have been carried out.

The term Edge Effect was originally defined specifically as an increase in abundance and diversity at the edge but has since been used indiscriminately to describe any change in distribution across an edge, confusing any assessment of the types and prevalence of these changes. For clarity, in this thesis the term Edge Response was used to describe the response of a community, guild or species to edges, and the term Edge Effect restricted to its original definition. I surveyed the international literature to determine the types and prevalence of community level changes in bird distribution across edges. Edge Effects at the community level are not common internationally, while there are many other documented types of edge response. Foraging or functional guilds can differ in their edge responses, and at the species level, edge responses have been demonstrated to differ between habitats for the same species, and may also change seasonally.

Therefore, I monitored the distribution of birds at the Community, Guild and Species levels across replicated clear-fell edges (120m and 60m into regrowth, at the edge and 60m into mature forest) between mature forest and 1) Establishment regrowth, 2) Juvenile regrowth and 3) mature forest (control, no edge) in three karri forest areas over four seasons (one year), and one forest area over eight seasons (two IV years). A five minute count of birds within each 30m radius circular plot was repeated approximately five times each season (108 plots in the first year, 36 plots over two years). Mean values of abundance (for the whole bird community and for individual species) and diversity measures for each census point were calculated and analysed with repeated measures ANOVA. Bird behavioural and vegetation data and Mist-netting captures across edges were also investigated.

My aims were to 1) describe and quantify any edge-related changes in the distribution and abundance of the birds 2) suggest possible explanations for the patterns observed and 3) propose management actions to minimise negative impacts.

There was no Community Edge Effect. The bird community as a whole did not respond to edges themselves, but was most abundant where mature forest was present (including at edges), least abundant and diverse in Establishment regrowth and intermediate in abundance and diversity in Juvenile regrowth. Guilds were poor predictors of species edge responses. Only the Nectarivores showed a unified response to edges - all five species avoided regrowth and four species avoided Juvenile edges. The Granivore/Frugivore guild and the whole Insectivore guild were less abundant in regrowth than in mature forest but did not have higher or lower abundance at the edge. Insectivorous species' edge responses were diverse and not reliably attributed to foraging-strata guilds (Understorey; Shrub; Canopy; All-Levels; Aerial; Bark). The Tree Martin, an Aerial foraging insectivore, showed an Edge Effect at Establishment edges in the first year of the study only. Individual vegetation variables were poor predictors of bird species distributions but overall, floristics changed with forest area not with edge distance, while habitat structural variables varied with edge, forest age and distance from the edge indicating that overall, structural variables were most strongly related to bird edge responses. The proportion of trees of 10-24cm diameterwas highest at Juvenile edges, which were avoided by four Nectarivore species and the bark-foraging Rufous Treecreeper.

Ordination grouped bird species by six edge response categories - Avoid edges; Prefer edges; Prefer Mature-forest; Prefer Mature-forest/Juvenile-regrowth; Prefer Establishment-regrowth; Neutral. However for individual species, these edge responses changed both temporally and spatially, emphasising the need for edge studies to be well replicated in both time and space. Some bird species also showed behavioural changes across edges. Species potentially disadvantaged by the creation of clear-fell edges are those avoiding Juvenile edges (Purple-crowned Lorikeet, New Holland Honeyeater, White-naped Honeyeater, Little Wattlebird and Rufous Treecreeper), and those most abundant in Mature forest habitat, but which show great spatial variation in abundance and therefore have unclear responses to edges (Whitebrowed Babbler, Restless Flycatcher). Several management options to ameliorate the effects of edge creation within production forest areas for these species will be discussed.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Environmental Science
Supervisor(s): Calver, Michael
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