280 Monitoring Threatened Species and Ecological Communities
appropriate models to assess the robustness of population trends and key drivers
should be constructed to assist in making decisions about management
intervention. More resources and input from stakeholders are needed to lift
monitoring of threatened, irruptive desert species above current levels.
Introduction
Determination of trends in species’ population sizes or distributions can be
achieved only if a monitoring program is in place. Effective monitoring should
allow managers to detect a deleterious trend early so they can then intervene to
rectify the situation. Threatened species pose particular challenges for effective
monitoring. Unless they occur in discrete locations, such as within fenced
enclosures or on offshore islands, population trends in scarce or sparsely distributed
species can be difficult to detect with any reliability. These difficulties can
sometimes be overcome using extra effort and resources, or by use of monitoring
techniques that have been tailored to the target species such as remote cameras or
sound recorders. Ensuring that survey timing is appropriate (e.g. for species of frog
or bird that are present or active only in certain seasons) is also important.
Even with dedicated effort and targeted sampling, population trends in some
threatened species can be uniquely difficult to discern: these are species that irrupt
in large numbers from low population levels at irregular intervals, only to collapse
again soon after. Numerical declines from ephemeral population peaks in these
species can exceed 90% within a single generation (Dickman et al. 2014). Irruptive
populations occur in resource-pulse environments worldwide (Yang et al. 2010),
but in Australia are most evident in arid regions. Here, irruptions usually follow
large rainfall events and collapses are complete only a few months later (Letnic and
Dickman 2010). Ephemeral desert f loras provide excellent examples of such ‘boom
and bust’ life histories (Silcock et al. 2011). Many plant species are detectable for
long periods only in the soil seed bank, but monitoring of this life history stage is
likely to be resource-intensive and is seldom carried out. Quiescent life history
stages characterise some animals too, such as burrowing desert frogs and shield
shrimps. Small mammals and nomadic birds provide other examples of species
that undergo brief, but dramatic, irruptions.
This chapter presents insights from long-term monitoring of two species of
dasyurid marsupials in the Simpson Desert, central Australia, at both local and
regional scales. These species vary in the extent to which they irrupt: neither
achieves the high numbers that can be seen in sympatric rodents, but both f luctuate
between apparent absence for variable periods to densities of ~5 ha (Greenville et al.
2016). Although neither of the study species is listed currently as threatened, the
population trends that are discussed are likely to be representative of other members
of the study genera that are at risk. The monitoring methods and results are first