DOI: 10.5061/dryad.j3tx95xgm
title: Dietary shifts may underpin the recovery of a large carnivore population

description: Supporting the recovery of large carnivores is a popular yet challenging
 endeavour. Estuarine crocodiles in Australia are a large carnivore
 conservation success story, with the population having extensively
 recovered from past heavy exploitation. Here, we explored if dietary
 changes had accompanied this large population recovery by comparing the
 isotopes δ13C and δ15N in bones of crocodiles sampled 40
 to 55 years ago (small population) with bones from contemporary
 individuals (large population). We found that δ13C
 and δ15N values were significantly lower in contemporary
 crocodiles compared to the historical cohort, inferring a shift in prey
 preference away from marine and into terrestrial food webs. We propose
 that an increase in intraspecific competition within the recovering
 crocodile population, alongside an increased abundance of feral ungulates
 occupying the floodplains, may have resulted in the crocodile population
 shifting to feed predominantly upon terrestrial food sources. The number
 of feral pigs consumed to sustain and grow crocodile biomass may help
 suppress pig population growth and increase the flow of terrestrially
 derived nutrients into aquatic ecosystems. The study highlights the
 significance of prey availability in contributing to large carnivore
 population recovery. 
description: Estuarine crocodile bone samples were collected from
 north-western areas of the Northern Territory (Australia) between Darwin
 Harbour and the East Alligator River (~300km apart). Historical bone
 samples were collected from the Museum and Art Gallery of the Northern
 Territory (MAGNT, Darwin, NT, Australia). Those bones were from crocodiles
 caught and killed between 1968 and 1986 from various parts of the Northern
 Territory (n = 22). Some crocodiles were very large and were likely older
 than 50 years of age upon death. Sampled crocodile bones were from animals
 with total body lengths ranging between 120 and 513 cm. The sampled bones
 were not treated or preserved and cleaned by macerating the bones in
 water. Samples were taken from the left front leg humerus. First, an ~ 0.2
 mm outer bone layer was scraped away using a scalpel, and then ~ 0.2 g of
 bone was scraped out and placed in a sample vial for the
 analysis.  The contemporary cohort of bone samples was
 collected from crocodiles trapped and removed from around Darwin as part
 of the crocodile management program in 2016. These crocodiles ranged from
 115 to 330 cm total body length (n = 24). A small section from the left
 humerus bone was removed and prepared as the museum specimens. The bones
 were not acidified prior to analysis as carbonate removal has been shown
 to have minimal effect upon reptile
 bone <em>δ</em><sup>13</sup>C, but instead a
 correction factor was used. The mean C/N ratios for the bone samples was
 3.4 ± 0.2 S.D. (range = 3.1 to 4.3), and only 3 out of 48 samples had C/N
 ratios above a proposed threshold of 3.6 for intact collagen.
 Bone samples from both cohorts were freeze-dried at -40ºC
 (Dynavac Freeze Dryer) for 48h and then homogenised to a fine powder using
 an electric ball-mill grinder (RETSCH Mixer Mill MM400). A small
 sub-sample from each was weighed (0.8 – 1.0 mg) into tin capsules and
 analysed for stable isotopes
 (<em>δ</em><sup>13</sup>C
 and <em>δ</em><sup>15</sup>N) at the Stable
 Isotope Core Laboratory at Washington State University (WA, USA).
 The <em>δ</em><sup>13</sup>C
 and <em>δ</em><sup>15</sup>N values from those
 bone samples were corrected using diet-tissue discrimination factors (1.4
 ‰ and 3.0 ‰, respectively). The carbon isotope values were corrected for
 the Suess effect by deducting 0.4 ‰ from the “historic” cohort.
 Body size is known to
 influence <em>δ</em><sup>13</sup>C
 and <em>δ</em><sup>15</sup>N values in
 crocodilians. Our “contemporary” cohort had a reduced range of body sizes
 compared to the “historic” cohort. To provide a more comparative range of
 body sizes, we sourced additional stable isotope values from Adame et al.,
 2018. These estuarine crocodiles (n = 41, size range = 83.5 to 420 cm)
 were captured from the same region between 2012 and 2014 (details on scute
 tissue isotopic discrimination in Adame et al., 2018). We first tested if
 it was a valid assumption to combine these two groups of crocodiles, whose
 collagen had been sampled from different tissues (bone and
 scute). 
description: Data uploaded  as csv file, containing the raw isotopic values
 for δ13C and δ15N (prior to the correction factor, subtraction of
 discrimination factor and correction for Suess effect, see Methodology).
 Other variables in the file are: crocodile total body lenght (cm), site
 (location where crocodile sample was originated from), experimental
 cohort, sample/data source, year that crocodile skeleton was received by
 the Museum and Art Gallery of the Northern Territoty - MAGNT, and year of
 tissue sample collection.