Discussion
Estimating population density is fundamental to ecology and conservation
but arboreal nocturnal animals pose unique challenges for achieving
this. Cryptic behaviour and efficacy of capture/detection can be a
hindrance for accurate density estimation in these animals. We address
this challenge for the sugar glider in the context of its unusual role
as a threat to a critically endangered species (Owens et al. ,
2022, Stojanovic et al. , 2014). Our literature review revealed
that four decades worth of effort have not changed capture or trapping
rates, with little innovation in baiting approaches. In our field study,
we showed that the traditional bait used by practitioners are less
effective than a new bait with fish oil.
Using fish-oil baits, we tripled the usual detection likelihood of past
studies and showed that it is possible to estimate density in
environments where typically detections are too sparse to support
rigorous analysis. Our modification of the lure incorporates
contemporary knowledge of the preferred diet of sugar gliders. Within
our field study, fish oil improved glider detection probability 33-fold
(0.33) compared to honey (0.01). Moreover, we detected nearly three
times as many individuals and recorded 23.6 times more detections with
fish oil than honey, which improved confidence in density estimates.
Gliders may be protein-limited by their typical diet (Smith & Green,
1987), necessitating the consumption of invertebrates (Smith, 1982) or
vertebrates (Stojanovic et al. , 2014). Fish oil is known to
appeal to carnivores worldwide (Austin et al. , 2017, Heinleinet al. , 2020), and the oil-based emulsion likely persisted longer
on trees than soluble honey. Our study is a promising new approach for
implementing field-surveys for this species.
Our literature meta-analysis of previous studies explored the range of
approaches used in the field and their effectiveness. Methods developed
long ago were subsequently applied to most studies with relatively
little innovation in trapping techniques. Although this improved
comparability among studies, generic baits yielded low capture and
trapping rates. Our estimated glider density (0.12
ha-1) is within the range reported in other studies in
continuous habitat (0.09-0.54 ha-1) but amongst the
lowest reported (Gracanin et al. , 2022, Jackson, 2000b, Quin,
1995). Comparison of densities is complicated, however, by the different
analytical approaches between studies (e.g. smaller buffers and sampling
area calculations) (Jackson, 2000b, Quin, 1995) compared to SECR (Effordet al. , 2009). Gracanin et al. (2022) provides the only
SECR-derived estimate (range: 0.09 ha-1 in continuous
forest to 0.7 ha-1 in fragments) and had too few
detections in one continuous forest patch to calculate density. Fewer
sugar gliders have been estimated in continuous forests than in
fragmented forests across multiple studies (Jackson, 2000a, Lindenmayeret al. , 2021).
The accuracy and precision of density and home range estimates using
SECR increases with the number of recaptures. Our data suggest that
fish-bait improved recaptures, but not universally, which hindered home
range estimation for rarely-observed individuals (i.e. those with home
range centres outside the grids). Increased confidence in our estimates
may have been attained with a longer study (which may violate closure)
or a larger trapping area. For individuals that had sufficient
detections, estimated core home range was 1.3ha-1which is narrowly within the range of 1.2-2.4ha-1 in
fragmented forests reported by Gracanin & Mikac (2022b). Average
movements indicate that sugar gliders in our study travelled
approximately 170m per night. The maximum linear movement of 552m we
observed is both greater (Suckling, 1984) and less (Gracanin & Mikac,
2022b) than the maximum displacement elsewhere in fragmented forests.
Our captures of new individuals in the fish survey increased
substantially, and our capture rates were comparable to the literature.
It is feasible that there is a maximum possible capture/detection rate,
which may be an artefact of underlying density and behavioural variation
of gliders. Other studies have also found 65-80% of sugar gliders known
to be alive in a given area are regularly recaptured, but the remainder
are rarely detected (Quin, 1995, Suckling, 1984). Indeed, in our study,
two gliders that were marked in the week before camera deployment were
not sighted again. Our meta-analysis found that trapping outcomes
increase with decreased forest connectivity. Across the literature,
models of trapping success, defined as repeat captures over time,
improved with the inclusion of trap nights and year, in addition to
habitat. Our meta-analyses found that captures of unique gliders
likewise increased in fragmented habitat, suggesting a density effect,
and that unique captures also increased with minimum trap height. This
was supported by our findings that detectability improved with greater
trap height, even though this was not an important model of density
estimation (Table 1).
Our field study makes three key assumptions. The first is that the order
of surveys did not impact on the observed differences in detectability.
Applying experimental treatments in separate surveys, rather than within
surveys has been used effectively by similar studies contrasting control
and treatment baits (du Preez, Loveridge & Macdonald, 2014).
Nonetheless, there is likely to be some effect of habituation (Gracanin
& Mikac, 2022a), and we accounted for this by including temporal and
behavioural effects in our models and restraining our sampling duration.
Second, we assume the “selfie-traps” did not impact on detection
probability, which we cannot rule out. Third, we assume that unmarked
individuals were always correctly identified but cannot exclude the
possibility of misidentification. We controlled for this by excluding
any recordings that were difficult to identify (19.3% of videos) which
negatively biased our estimates of density. However, marking gliders
prior to the SECR trial increased the efficiency and confidence of
resighting gliders on videos.
We have several recommendations for practitioners. First, we recommend
fish-oil in sugar glider lures and baits. Furthermore, we encourage the
trial of protein-based or novel olfactory lures for other Petaurus spp ., which are also traditionally lured by honey
baits (Goldingay & Sharpe, 2004, McBurney, 2019, Stobo-Wilson et
al. , 2021). Second, we recommend further study for potential novel
attractants, as have been done for other mammals (Heinlein et
al. , 2020, Jackson, Hartley & Linklater, 2016, Morgan, 1990). Third,
we advise limiting the duration of studies to minimize conditioning
behavioural responses (especially if baits are accessible for
consumption) and violating assumptions of population closure. Fourth,
for studies that do not explicitly identify individuals, we suggest
spacing cameras beyond the mean maximum distance moved. Lastly, traps in
the mid-strata and above are more likely to capture a higher number of
individuals than traps close to the ground, but ultimately, bait type is
the most important factor for detectability.
Accurate estimation of density is fundamental in biology and our study
shows the benefits of carefully scrutinizing the methods used to achieve
this goal by increasing the underlying detection rates. We show that
substantial improvements in the accuracy of estimation can be achieved
when the methods are tailored to suit the specific biology of the target
species. This sometimes may not be obvious from long-established
practices in the literature and we encourage other practitioners to
re-evaluate the techniques used for detecting species based on
contemporary knowledge of their biology. Unattractive baits may mean the
difference between success/failure of a study because low detections
render density and home range estimates inaccurate or incalculable. This
can be a major problem given the importance of good detections to
understanding animal-habitat requirements, evaluate threats, prioritize
actions and allocate limited conservation resources. Failure to adopt
survey approaches suited to the ecology of the target species may
inadvertently sabotage projects seeking to gain new ecological insights.