Interpreting past trophic ecology of a threatened alpine parrot, kea Nestor notabilis, from museum specimens

Abstract

When ecosystems are under severe pressure or environments change, trophic position and intraspecific niche width may decrease or narrow, signalling that conservation action is required. In New Zealand, alpine and subalpine ecosystems have been extensively modified through farming since 19th-century European settlement, with consequences for indigenous species such as the kea Nestor notabilis. We investigated feather stable isotope values in the kea and predicted a lower trophic position in modern kea populations, to reflect reduced lowland habitat and a mixed diet with more plant material. We predicted that size and sex would influence trophic values in this sexually dimorphic species, with larger birds more likely to have a high protein diet. We examined potential dietary changes in 68 museum collected kea from 1880s to 2000s, first recording accession details including provenance and sex and measuring culmen length. We used bulk carbon and nitrogen stable isotope analyses (BSIAs) of feathers and a further feather subset using compound-specific stable isotope analyses of amino acids (CSIA-AA) to obtain isotopic values and estimate trophic position. BSIA showed δ¹⁵ N values in kea feathers declined through time and could indicate that early century kea were highly omnivorous, with δ¹⁵ N values on average higher than in modern kea. Variance in δ¹⁵ N values was greater after 1950, driven by a few individuals. Few differences between males and females were evident, although females in the south region had lower δ¹⁵ N values. There was a tendency for large male birds to have higher trophic values, perhaps reflecting dominant male bird behaviour noted in historical records. Nonetheless, CSIA-AA performed on a subset of the data suggested that variation in BSIA is likely due to baseline changes rather than relative trophic position which may be more homogenous than these data indicate. Although there was more variability in modern kea, we suggest caution in interpretation. Stable isotope data, particularly CSIA-AA, from museum specimens can reveal potential change in ecological networks as well as sexually dimorphic feeding patterns within species. The data can reveal temporal and regional variation in species trophic position and changes in ecosystem integrity to inform conservation decision-making.

Keywords: CSIA-AA; amino acids; museum collections; niche width; omnivory; parrot; stable isotopes; trophic position.

FIGURE 1

Capture locations in the South Island of New Zealand for kea from museum collections used in this study. Recorded locations were part of the provenance data attached to museum specimens. Kea specimens used for the north group in the Canterbury region were from locations 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 and 18. Birds in the south group from the Otago region were from locations 20, 21, 22, 25, 26, 27, 28, 29, 30. Birds from the northern area and West Coast of the South Island were excluded from these groupings

FIGURE 2

Relationship of bulk δ¹⁵N values from kea feathers and time, plotted by year

FIGURE 3

(a) Relationship between kea culmen length and bulk δ¹⁵N values for adult males, adult females and immature kea. (b) Relationship between kea culmen length and C:N bulk ratios estimated from kea feathers. C:N ratios become less heterogeneous as culmen length increases in male and female kea

FIGURE 4

Isotopic niche representation of kea females and males in the North and South groups, showing the standard ellipse area. Black dots represent medians

FIGURE 5

Cross‐plot for the δ¹⁵N values of trophic and source AAs, Glx and Phe, respectively, from kea feathers. Brown depicts ‘ancient’ kea (1887–1950) and yellow depicts ‘modern’ kea (1950–2007). Trophic isoclines with a slope of 1.0 and y‐intercept intervals of 7.6‰ represent different trophic positions (TPGlx‐Phe = 1, 2 and 3) according to equation 1 from Chikaraishi et al. (2010)