Factors affecting the detection of possums by spotlighting in Western Australia

Abstract

This paper describes how environmental factors, survey method procedures and differences in forest structure resulting from logging relate to the detection of koomal (common brushtail possum, Trichosurus vulpecula hypoleucus) and ngwayir (western ringtail possum, Pseudocheirus occidentalis). A total of 169 vehiclebased spotlight surveys of possums within native jarrah (Eucalyptus marginata) forest was conducted on three transects over eight years (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003). Up to 5.7 koomal and up to 3.3 ngwayir were detected per kilometre per transect side. Only one ngwayir was detected during the eight surveys conducted between 2001 and 2003. More koomal were seen in spring and autumn (i.e. September-November and March-May respectively) and more ngwayir were seen between October and April. Although surveys were not conducted on very rainy or excessively windy nights, fewer possums were nonetheless seen on nights following rainy days and on cold nights. Cloud cover also affected sightings of koomal. The time taken to complete the surveys increased in conjunction with the number of possums detected, on account of the time required to record data. The importance of standardising travelling speed also is emphasised. Possum spotlight counts differed between recently logged and unlogged areas. However, these findings were not supported by complementary koomal abundance estimates derived from trapping, suggesting that vegetation structure may affect detectability. Factors such as the lunar cycle, wind speed and survey start time after sunset did not significantly affect detection rates of either species. On the basis of these findings, specific survey conditions can be selected to improve spotlight detection efficiency.

A remarkably quick habituation and high use of a rope bridge by an endangered marsupial, the western ringtail possum

“…Greater wind speed also decreased the number of common brushtail possums (Trichosurus vulpecula) observed in open pasture (Paterson 1993). Wind speed did not influence the detection rate of P. occidentalis in a forest habitat (Wayne et al 2005); however, the rope bridge in this study was completely exposed to the wind over the road, and strong wind could deter P. occidentalis from crossing the open bridge. Given this information, we also predicted that P. occidentalis would cross the bridge more during their breeding seasons and less on well lit, cold and/or windy nights.…”

“…Whether this trend is caused by possums generally reducing their activities on bright nights or possums being discouraged to cross the exposed bridge on brighter nights cannot be known from our data. Wayne et al (2005) reported that the moon or wind had no effect on the number of possums seen by spotlighting in a forest; however, possums are likely to act differently in a completely exposed environment such as on a rope bridge compared to an environment with greater cover from predators such as the canopy in a forest. Native owl species, such as the masked owl (Tyto novaehollandiae) are thought to be present in the region (Clarke 2011), and they prey on similar sized possum species in New South Wales (Kavanagh 1996).…”

“…The crossing rate of P. occidentalis recorded in this study (8.87 crossings per night) is considerably higher than these previously reported rates, and it did not reach a clear asymptote during the monitoring period. This could be due to the high density of the species in the study area and/or their particular lack of avoidance behaviour towards unfamiliar objects such as the new rope bridge (Wayne et al 2005, Jones et al 2007, Clarke 2011. Moreover, possum species studied by Weston et al (2011) lived in rainforests that generally have greater canopy cover than our study site, and their fidelity to a dense canopy may have made these possums more reluctant to cross exposed bridges.…”

A comparison of survey methods for arboreal possums in jarrah forest, Western Australia

“…The 18 spotlight nights were generally conducted once per week. To limit environmental effects on the detection efficiency (Davey 1990;Laurance 1990;Wayne et al 2005), spotlighting was not conducted on evenings of extreme or inclement weather.…”

The influence of environmental variables on capybara (Hydrochoerus hydrochaeris: Rodentia, Hydrochoeridae) detectability in anthropogenic environments of southeastern Brazil

“…Extensive monitoring programs are conducted using population indices by assuming that the probability of detection is constant over space and time (Pollock et al 2002), which is clearly invalid in many field studies (Yoccoz et al 2001;Pollock et al 2002). Species detectability can be influenced by environmental variables, habitat features and observer bias (Bayliss 1987;Lancia et al 1996;Karanth et al 2003;Field et al 2005;Wayne et al 2005;Pinto et al 2006). These problems are generally underestimated by wildlife biologists in vertebrate surveys (Yoccoz et al 2001;Pollock et al 2002).…”