Birds, fruit and nectar: spatio-temporal patterns of regional bird abundance and food availability in subtropical eastern Australia
thesisposted on 23.02.2017 by Hawkins, Brian Anthony
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
Spatio-temporal patterns of animal abundance, and the factors explaining them, have seldom been studied at a regional scale. I sought to describe and explain patterns of bird abundance (for all birds, and for frugivores and nectarivores) by counting birds and measuring fruit and flower availability monthly for 24 months at 83 sites across a 300 000 ha region in subtropical eastern Australia. In particular, I wanted to examine the effects on abundance of climate, primary productivity, vegetation and food availability. Patterns of fruit availability were similar in both years, but the spatio-temporal pattern of flowering differed between years because of irregular blossoming by eucalypts. Most variation in fruit and flowers was spatial; spatio-temporal variation was also important, but there was relatively little temporal variation. Vegetation type and primary productivity were the greatest influences on fruit availability; flowering was chiefly influenced by primary productivity and rainfall. As with food availability, most variation in bird abundance was spatial: there were more birds in certain vegetation types and where mean food (fruit and flower) availability was higher. Spatio-temporal variation resulted from food tracking, whereby frugivores and nectarivores moved among localities and vegetation types in response to seasonal changes in the availability of fruit and flowers. However, spatio-temporal variation in consumer abundance was not as great as might have been expected, given the degree of spatio-temporal variation in food availability; this suggests that fruit and nectar were generally in over-supply during the study. Temporal variation in bird abundance was less marked than spatial or spatio-temporal variation, and was chiefly associated with variation in primary productivity, probably because recruitment of juveniles and influxes of migratory insectivores occurred during times of high productivity; there was little net migration into or out of the study region by frugivores or nectarivores. Of the explanatory factors I examined, food (fruit and flower) availability had the greatest influence on bird abundance. Because food availability differed among vegetation types, the association between vegetation and bird abundance was also strong. Although variables relating to primary productivity were important influences on food availability, they had only weak effects on bird abundance. The weakness of the productivity-abundance relationship was partly due to a combination of low spatial (but high temporal) variation in productivity and low temporal (but high spatial) variation in bird abundance. I would expect the relationship to be stronger in areas with greater spatial variation in productivity, or where long-distance migrants were a greater component of the avifauna. My study represents an important advance in our understanding of the factors that influence animal abundance over large areas. Abundance is strongly influenced by food availability, which is in turn affected by climate, primary productivity and vegetation characteristics. Seasonal changes in food availability drive intra-regional bird movements among vegetation types and localities; such movements need to be considered in conservation planning, which is often premised on species having static distributions.