Data from: Designing monitoring protocols to measure population trends of threatened insects: a case study of the cryptic, flightless grasshopper Brachaspis robustus

Statistically robust monitoring of threatened populations is essential for
effective conservation management because the population trend data that
monitoring generates is often used to make decisions about when and how to
take action. Despite representing the highest proportion of threatened
animals globally, the development of best practice methods for monitoring
populations of threatened insects is relatively uncommon. Traditionally,
population trend data for the Nationally Endangered New Zealand
grasshopper Brachaspis robustus has been determined by
counting all adults and nymphs seen on a single ~1.5 km transect searched
once annually. This method lacks spatial and temporal replication, both of
which are essential to overcome detection errors in highly cryptic species
like B. robustus. It also provides no information about
changes in the grasshopper’s distribution throughout its range. Here, we
design and test new population density and site occupancy monitoring
protocols by comparing a) comprehensive plot and transect searches at one
site and b) transect searches at two sites representing two different
habitats (gravel road and natural riverbed) occupied by the species across
its remaining range. Using power analyses, we determined a) the number of
transects, b) the number of repeated visits and c) the grasshopper
demographic to count to accurately detect long term change in relative
population density. To inform a monitoring protocol design to track trends
in grasshopper distribution, we estimated the probability of detecting an
individual with respect to a) search area, b) weather and c) the
grasshopper demographic counted at each of the two sites. Density
estimates from plots and transects did not differ significantly.
Population density monitoring was found to be most informative when large
adult females present in early summer were used to index population size.
To detect a significant change in relative density with power > 0.8
at the gravel road habitat, at least seventeen spatial replicates
(transects) and four temporal replicates (visits) were required. Density
estimates at the natural braided river site performed poorly and likely
require a much higher survey effort. Detection of grasshopper presence was
highest (pg > 0.6) using a 100 m x 1 m transect at both
sites in February under optimal (no cloud) conditions. At least three
visits to a transect should be conducted per season for distribution
monitoring. Monitoring protocols that inform the management of threatened
species are crucial for better understanding and mitigation of the current
global trends of insect decline. This study provides an exemplar of how
appropriate monitoring protocols can be developed for threatened insect
species.