Comparison of human modified and native forest habitats in the Hunua Ranges, Auckland : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Auckland, New Zealand
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Date
2010
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Massey University
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Abstract
Understanding the trophic structure of a habitat is vital to understanding the species
composition and interactions of species and individuals within that habitat. It dictates
which organisms may survive, their abundance, and biotic interactions. Pine (Pinus
radiata) (hereafter pine) plantations in New Zealand are the most common type of
silviculture, and, although primarily a commercial forestry enterprise, they are
recognised as an ecosystem able to provide habitat for some native species. It is
therefore pertinent to evaluate the ecological value of this habitat while keeping in mind
its lack of permanence. New Zealand’s native forests are a natural comparison for
mature pine plantation, and I have tracked the diet and behaviour of selected species
across both habitats and their contiguous boundary. This study utilised multiple
techniques and collected two years of behavioural and prey availability data to compare
the habitats of interest on a variety of trophic levels (TLs) and temporal scales.
Research was conducted in the Hunua Ranges, New Zealand, between March 2006 and
June 2009 and considered three habitats (pine plantation, native forest, and the
contiguous boundary of these habitats). Vegetation samples from leaf litter (hereafter
vegetation), Lepidopteran larvae (hereafter caterpillars), predacious adult Coleoptera
(hereafter beetles), rats (Rattus rattus) (hereafter rats), house mice (Mus musculus)
(hereafter mice), and North Island tomtits (Petroica macrocephala toitoi) (hereafter
tomtits) were analysed in terms of δ 13C, and δ 15N values. Comparisons between
habitats, taxa, seasons, and sexes were conducted. Stable isotope analyses showed
samples from native habitat had the lowest δ 15N levels within taxa, with boundary
samples usually showing an intermediate value, and pine plantation samples commonly
having the highest δ 15N levels. This suggests that the native forest provides a lesser
amount of available nitrogen to the fauna inhabiting it, whereas the pine plantation
(potentially due to fertilisation) contains a higher level of available nitrogen. Significant
separation of taxa was seen between habitats for δ 13C values of rat and tomtit samples,
and for δ 15N values of vegetation, rat, and tomtit samples. Within habitats, taxa were
distinctly separated for both δ 13C and δ 15N, and their foraging ranges spanned three to
four TLs. The caterpillar and mouse samples collected did not show significant
seasonal fluctuations in δ 13C or δ 15N values, and ship rats showed seasonal differences only for δ 13C values. Seasonal difference in ship rat isotope signatures may indicate
season related foraging locations with variation occurring between summer and autumn
compared to winter and spring. Stomach content analyses for rats and mice did not
show separation by habitat within species, but did show significant differences between
rat and mouse diet in the boundary habitat. The volume of invertebrates, vertebrate
remains, and vegetation in rat stomachs showed significant differences between seasons
with a greater proportion of vegetation found during winter; however no evidence of
this was seen for mice. Neither technique showed evidence of intersexual dietary
differences for rodents, and isotopic values were also similar between tomtit sexes
within each major habitat type. The use of stable isotope and stomach content analyses
to assess rodent diet was a valuable combination as it clarified this aspect better than
either method alone.
Tomtit sexes differed in foraging behaviour, with males observed foraging more
frequently on the ground than females and females using vegetation (in particular
substrates between 0 - 3 m) more than males. Foraging by both sexes varied between
breeding and non-breeding season in 2006, with more ground use occurring in the non-
breeding season and more vegetation use (males: 3 - 6 m; females: 0 - 3 m) in the
breeding season. Tomtit foraging behaviour in three habitats (pine plantation, native
forest, and the contiguous boundary of these habitats) was compared. Overall, tomtit
foraging in native forest occurred more frequently in vegetation 3 - 6 m compared to
the use of this strata in either pine or boundary habitat. Males showed inter-annual
differences in foraging, using the ground significantly more in 2006 than 2007. The
research described tomtit foraging and habitat use, illustrating the complexity of
foraging behaviour and the difficulty of understanding sex, habitat, and season
associated foraging variation.
The availability of the ground-prey items for tomtits differed most widely between
habitats. Annual and seasonal differences were also found within pine and native forest
habitat. Prey availability varied between seasons within pine (spring versus summer),
native (winter versus spring), and boundary (winter versus summer) habitats. No
differences between prey availability were found for male and female tomtits.
However, male foraging samples showed annual separation in the pine and native
habitats, and between some seasons within the pine (winter versus summer) and native
(winter versus spring) forests. No significant seasonal differences were found for
female comparisons. Through comparison of habitat and temporal prey availability for
tomtits I have begun to determine the role that pine plantation invertebrates play in the
diet of insectivorous native birds. Many questions have been raised by this study, and
there is much scope for future research into the trophic structure of pine versus native
forest.
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Keywords
Petroica macrocephala toitoi, Forest ecology, Forest habitat