Ecology and conservation of the northern hopping-mouse and sympatric vertebrates of Groote Eylandt

Abstract

Effective monitoring is a critical component of wildlife management. The conservation of the enigmatic northern hopping-mouse (Notomys aquilo) has been impeded by poor understanding of the species’ ecology; an issue compounded by the lack of appropriate survey methods. Within a genus of largely arid-dwelling rodents, N. aquilo is the only extant species in Australia’s northern, monsoonal tropics where it is likely restricted to Groote Eylandt and a small area on the adjacent mainland. Surveys for N. aquilo have traditionally involved recording the location and abundance of indirect signs, as the species is trap-shy towards box-style traps, such as Elliott traps. One of the most important signs is the ‘spoil heap’ left by the construction of hopping-mouse burrows. Recent surveys for N. aquilo have relied almost entirely upon the identification of spoil heaps; however, the effectiveness of these methods has been questioned. This study aimed to fill key gaps in this knowledge by: (1) evaluating the effectiveness of spoil heap surveys for determining the presence and abundance of N. aquilo; (2) developing more-reliable survey methods for N. aquilo and sympatric vertebrates; and (3) describing the general ecology of N. aquilo with views to its conservation on Groote Eylandt. A novel use of video camera traps allowed the recording of burrow construction by N. aquilo. This confirmed that conspicuous spoil heaps are associated with N. aquilo burrows and these remain visible many months after construction is complete. However, further monitoring confirmed that the sympatric delicate mouse (Pseudomys delicatulus) constructs similar burrows to N. aquilo on Groote Eylandt and spoil heaps made by the two species cannot reliably be differentiated in field surveys. These results emphasised the pressing need for unambiguous survey methods for N. aquilo and suggested that the abundance of the species has potentially been overestimated in the past. Camera trapping and pitfall trapping were identified as possible alternative methods for monitoring N. aquilo. An experiment was conducted to determine if the rate of detection of N. aquilo and other mammals could be increased by the use of a particular bait at camera trap stations. Detection rates for N. aquilo were low and no significant effect of bait type was found for this species. However, sesame oil resulted in the equal highest number of N. aquilo detections, had better longevity, and attracted fewer non-target bandicoots than other baits trialled and, therefore, may be the most useful bait for standardised camera trapping surveys targeting N. aquilo. Trapping with three different sized pitfall traps was conducted in two different habitats over two years. The results from this study demonstrate that general fauna surveys may benefit from the use of a deep, wide pitfall trap that is effective in capturing herpetofauna as well as rare mammals such as N. aquilo. The general ecology of N. aquilo was described using pitfall trapping and radio-tracking. It was found that N. aquilo, like its relative Notomys alexis, displays female-biased sexual size dimorphism; an uncommon trait in murid rodents. Radio-tracked individuals utilised discrete home ranges of 0.39–23.95 ha and used open microhabitat proportionally more than was available. Camera trapping was more effective than live trapping at estimating abundance and could be employed with lower effort and long term costs. A combination of sampling techniques confirmed the presence of N. aquilo in three out of 32 locations on Groote Eylandt. This illustrated a stark decline in the abundance and distribution of this species on the island that was once thought to be its stronghold. Further investigation was conducted into the ecology of N. aquilo and sympatric mammals by analysing data from camera trap images collected over three years. Diel activity patterns of five species of small mammal were estimated. Analyses provided insight into how different species, all physiologically entrained to nocturnality, show varying levels of plasticity in response to environmental factors such as season and habitat. This study made significant advances in the understanding of the ecology and conservation of the poorly understood N. aquilo as well as improved survey methods for a number of north Australian vertebrates. The results can be used to develop more-constructive conservation and monitoring strategies for these species. Importantly, the previously inferred decline of N. aquilo has been confirmed by the results of this study and recommendations are provided for a change in conservation status from ‘vulnerable’ to ‘endangered’ under the Environment Protection and Biodiversity Conservation Act 1999.