DOI: 10.5061/dryad.fbg79cnrb
title: Data from: Designing monitoring protocols to measure population trends of threatened insects: a case study of the cryptic, flightless grasshopper Brachaspis robustus

description: 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 &gt; 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 &gt; 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.
description: Plot and transect searches for <em>B.
 robustus</em> were conducted at Patersons Terrace during the austral
 summer (November to March) for three consecutive seasons (2015–2018).
 Three 100 m x 1 m (100 m<sup>2</sup>) transects spaced ~1 km
 apart were marked out along the centre of the gravel road. Along each 100
 m transect, four evenly spaced 5 m x 5 m (25 m<sup>2</sup>)
 plots were defined on alternating edges of the road using metal corner
 pegs. Henceforth, a group of 4 plots (equal to 100
 m<sup>2</sup>) is referred to as a “plot unit”. The plots and
 transects remained in the same location over the duration of the
 study. Transect searches were conducted at Snowy River
 during the austral summers of 2016–17 and 2017–18 only. Three 100 m x 1 m
 transects spaced &gt; 200 m apart were set up longitudinally along the
 riverbed in locations where grasshoppers were known to occur. Transects
 were marked with plastic pegs placed at 20 m intervals. In the third
 season, two additional 100 m x 1 m transects were set up at the site. The
 three original transects were set up as close as possible to their initial
 location. However, changes in channel morphology resulted in minor
 deviations of less than 8 m. Monitoring took place from
 November to March each summer on days of suitable weather (ground
 temperature &gt; 13.6°C and not during gale force winds or
 precipitation). Each transect and plot unit was searched on at least 6
 days per month when feasible (Patersons Terrace, mean visits per month =
 6, range = 2 to 8; Snowy River, mean = 5.5, range = 3 to 7) but the number
 of monthly searches achieved and duration between visits (1 to 11 days)
 varied according to the occurrence of favourable weather. All plots and
 transects were searched by the same observer throughout the study. Both
 sites were searched on the same day except where weather was not
 permitting (e.g., rain at one of the sites). Prior to commencing a search,
 barometric pressure, air temperature at 1 m above the ground, and ground
 surface temperature in the shade were measured using a Kestrel 3500 Pocket
 Weather Metre (GeoSystems New Zealand Ltd). Cloud cover (categories: none;
 high cloud, when cloud was high in the sky but did not cause shadows;
 patchy cloud, when clouds were lower in the sky and caused shadows;
 overcast) and start time were recorded for each search. During the search,
 the observer walked slowly sweeping their front foot over the ground in
 front of them and moved in a direction such that their shadow fell on the
 area already searched. This method made visual detection possible by
 eliciting a jump response from the grasshopper. When detected,
 grasshoppers were captured, their body length (from the top of the head to
 the tip of the abdomen) and femur length measured, and sex and transect
 location recorded. Each grasshopper was then released behind the observer
 to ensure it was not re-counted, and the remainder of the area was
 searched. The minimum time required to complete a search of a 100
 m<sup>2</sup> transect was 5 mins, and for each plot was ~1.5
 mins (~6 mins per 100 m<sup>2</sup> plot unit including time
 taken to walk between plots) but this increased with the number of
 grasshoppers found. During this study, adulthood was determined by
 measuring hind femur length. Females with a femur length of ≥ 15 mm, and
 males with a femur length of ≥ 9 mm were considered adults. Individuals
 less than 8 mm in body length were excluded from the study because the
 risk of causing a fatal injury during capture and handling was too
 high.
description: For usage notes, please refer to
 README_Schori_et_al_2020_dataset.txt.