Kym Brennan

Distributional pattern of plant species endemic to the Northern Territory, Australia

The distributions of the 567 plant species considered to be endemic to the Northern Territory, Australia, were collated from a distributional database comprising about 600 000 records. Endemic species comprise a non-random taxonomic subset of all plants known from the Northern Territory. Because of substantial geographic disparity in collecting effort, we analysed geographic patterning of these endemic species by using both (1) actual records only and (2) interpolated ranges (minimum convex polygons). The geographic distribution of the number of Northern Territory endemic plant species was well predicted by a measure of topographic complexity and climate (particularly rainfall). The observed distributional patterning of endemic species was also influenced by survey effort, but this latter influence was substantially reduced by the use of minimum convex polygons. Both analyses revealed that there was a clear aggregation of endemic species in the 32 000 km2 of the sandstone plateau of western Arnhem Land. This ‘hotspot’ has been previously recognised in coarser-scale assessments of national and international centres of plant biodiversity. Our analysis concluded that 172 species are restricted to this plateau, and that the plateau comprised at least 90% of the distribution of a further 25 species. More broadly, 438 plant species are endemic to the northern part of the Northern Territory (the 316 000 km2 north of 16°S), a level of endemism that may match that of Cape York Peninsula and surpasses that of the Kimberley. The core area for Northern Territory endemic plants, the plateau of western Arnhem Land, is currently threatened, particularly by unfavourable fire regimes.

Distributional patterning of mammals on the Wessel and English Company Islands, Arnhem Land, Northern Territory, Australia

Eighteen non-marine mammal species (including seven species of bats) were recorded from a total of 49 islands in the Wessel and English Company island chains off north-eastern Arnhem Land, Northern Territory. Most individual species were restricted to, or had higher incidence on, larger islands, and species richness as a whole increased as island size increased. The most notable exception was the semi-aquatic Hydromys chrysogaster, which occurred relatively equitably across island sizes; this species, two bat species and the macropod Petrogale brachyotis were recorded from islands smaller than 10 ha. However, the variation between islands in the number of native terrestrial mammal species was not best predicted by island size, but rather by a combination of sampling effort and altitude (which explained 64% of the deviance in species richness), or altitude and distance to larger land mass (explaining 63% of deviance). Richness–area patterns for individual islands in these chains were reasonably consistent with those of other islands sampled in northern Australia. However, the fauna of the Wessel and English Company groups as a whole was less rich than that of the Pellew and Kimberley islands, and individual islands appeared to have lower species richness than comparable mainland areas. Species that were notably absent or that were recorded from relatively few islands include large macropods, Tachyglossus aculeatus, Antechinus bellus, Phascogale tapoatafa, Sminthopsis spp., Mesembriomys gouldii, Rattus colletti, Leggadina lakedownensis and Pseudomys calabyi. Some of these species may be absent through lack of suitable habitat; others have presumably disappeared since isolation, possibly due to Aboriginal hunting. Richness at the quadrat (50 × 50 m) scale was generally very low. Habitat relationships are described for the 7 species recorded from more than 5 quadrats. At a quadrat-scale, the richness of native mammals was greater on islands larger than 1000 ha than on islands smaller than 1000 ha. Quadrat-scale species richness varied significantly among the islands sampled by the most quadrats (even when the comparison was restricted to either of the two most extensive vegetation types), but this variation was not closely related to either area or altitude. The two most frequently recorded species, the rodents Melomys burtoni and Zyzomys argurus, showed distinct habitat segregation on islands where both were present, but tended to expand their habitat range on islands where only one of the species occurred. The most notable conservation feature of the mammal fauna of the Wessel and English Company Islands is the occurrence of the golden bandicoot, Isoodon auratus, a vulnerable species apparently now extinct on the Northern Territory mainland. Four feral animal species (Rattus rattus, Canis familiaris, Bubalus bubalis and Capra hircus) were recorded from a total of 6 islands.

Vegetation of the Wessel and English Company Islands, North-eastern Arnhem Land, Northern Territory, Australia

Vegetation patterning is described for two neighbouring continental island chains off Arnhem Land, Australia. A total of 684 species was recorded from 57 islands, with reasonably comprehensive lists for 36 of these islands. Almost 90% of the deviance in plant species richness was associated with island size. The richness of plants dispersed by sea and/or vertebrates was proportionally greater on small islands; that of relatively poor dispersers was greater on larger islands. Twelve vegetation communities were defined by classification of plant species composition in 226 50 × 50-m quadrats. There was no relationship between island size and species richness at the quadrat level, at least for the most extensive vegetation types. The vegetation of the islands is now composed of two main elements—an original set of communities (mostly of heath, tussock grassland and eucalypt open forest) whose species have poor inter-island dispersal, and a set of more recent colonists (mostly of strand, mangrove and coastal thicket communities). The former group is allied to the sandstone flora of western Arnhem Land, but is species-poor in comparison, as the islands lack the deep gorges which drive much of the species richness of western Arnhem Land. The latter group comprises many species with a broad geographic range across tropical coastal areas, including many species which have been shown elsewhere to be highly vagile. Many species of the former set appear to have been lost from the smaller islands. The island flora has been influenced by three sets of human managers. Aboriginal use of the islands has been long-standing and probably resulted in a fire regime which may have accentuated environmental patchiness. Centuries of wet-season use of some islands by Macassan trepang-harvesters, ending early this century, has apparently left few effects other than the persistence of occasional small populations of an introduced food tree. In contrast, European use of these islands has been minimal and fleeting, but is associated with the introduction of most weeds.

A different fauna?: captures of vertebrates in a pipeline trench, compared with conventional survey techniques; and a consideration of mortality patterns in a pipeline trench

We sampled the vertebrate fauna caught in a pipeline trench near Daly Waters, Northern Territory in the mid Dry season of 1999, and compared the species composition with that of a standardised quadrat-based survey protocol (using pitfall traps, Elliott traps and nocturnal and diurnal active searches). The proportion of records of snakes, pygopodids, frogs, two dragon species, the gecko Diplodactylus ciliaris and the rodent Pseudomys delicatulus was markedly greater in the pipeline than in the quadrat-based survey; this disparity was also evident in a pronounced size difference in the reptile species composition of the two techniques. The comparison demonstrates that the standard survey protocol has substantial biases against some species and groups of species and that additional sampling effort or specific new trapping techniques may be required for these taxa.The species composition of captures in this pipeline study differed substantially from the results from a larger section of this pipeline route con...

Demographic evaluation of translocating the threatened northern quoll to two Australian islands

Abstract Context. Translocation is widely used to help avoid extinction of species from threatening processes. A fundamental objective of translocation is to establish self-sustaining populations; estimating demographic parameters is critical to assessing success of these programs and can also be used to support future management actions. Aims. We estimated demographic parameters to evaluate the success of translocating the northern quoll Dasyurus hallucatus to two islands (Astell and Pobassoo), in response to the threat posed by the introduced cane toad Rhinella marina on the Australian mainland. Methods. We used capture–mark–recapture methods to monitor both populations at regular intervals from initial release in 2003 until 2009 and a one-off survey in 2014. Key results. Relative abundance (trap success) increased exponentially in the first 4 years, declined, and then stabilised in subsequent years. The population of female northern quolls on Astell Island peaked in 2006 with an estimate of 3640 (95% CI 3022–4257), and on Pobassoo Island the peak was 2007 with 617 (95% CI 531–703) females. In 2014 the population had decreased to 2193 (95% CI 1920–2467) on Astell and 451 (95% CI 359–543) on Pobassoo. Apparent survival and body condition decreased significantly following the population peak, possibly related to density dependence. Conclusion. Both populations of northern quolls reached their regulation phase after going through establishment and growth phases, which included exceeding carrying capacity. The pattern was similar between the populations. Implications. Increased survival and recruitment at threat-free translocation sites contributes to successful establishment of self-sustaining populations.