Frog Population Surveys
Hochstetter’s frogs (Leiopelma hochstetteri) are one of three surviving species of endemic New Zealand frogs. While they are the most widespread and least threatened of the remaining New Zealand species, their range has become considerably reduced due to habitat destruction and the introduction of mammalian pests. Rats, mustelids, goats, pigs and cats are suspected to prey on frogs, and can also degrade their fragile bush stream habitats.
Hochstetter’s frog are classified as “At Risk-Declining” and this has been consistent across the last three revisions of their New Zealand threat status ranking. The Aotea / Great Barrier Island Hochstetter’s frogs represent the only island population for this species, and are genetically distinct from mainland populations. It has therefore been recommended that the Great Barrier form of these frogs be treated as an ‘Evolutionary Significant Unit’ for conservation management. On Aotea/Great Barrier Island Hochstetter’s frogs occur as a metapopulation – a group of spatially separated subpopulations that interact through dispersal (immigration and emigration).
Hochstetter’s frogs are nocturnal and seek refuge during the day in damp spaces underneath rocks, logs and leaf litter. In dry weather they are typically found within and around stream beds and banks, although they can also be found away from streams under bush canopies. One problem with monitoring Hochstetter’s frogs is that it is very difficult to estimate their population parameters. This is because; (1) it is difficult to distinguish one individual from another using natural patterning or human addition of marks, (2) they are cryptic meaning that within-season recapture rates can be low, and (3) they also can move between different streams over longer timeframes meaning that between-season recapture rates in long-term studies can be low. Most monitoring therefore relies on an index, rather than an estimate of abundance. Data for the index is collected using single or double-observer counts of individuals. However, there has been some recent work on developing protocols for collecting data that can be used to more rigorously estimate their occupancy and abundance.
Searching a streambed
2021 Survey Method
Four people carried out the 2021 frog survey in Te Paparahi between the 13 and 23 of April 2021. The survey team on the first week of surveying comprised Henry Cookson (Windy Hill), Blair Balsom (Wildlands), and Sarah Herbert (Wildlands). Claire Johnson (Massey University) took Sarah’s place in the survey during the second week. The survey team were joined by Justin Howie (Ngāti Rehua Ngātiwai ki Aotea) and Sarah Dwyer (DOC Aotea) on the 23 of April as observers, and to assist with carrying the field gear out. The survey covered the 15, 100 m, streambed transects that were surveyed in 2012 and 2015 (Herbert et al. 2014, Herbert and Gilbert 2015). The location of each transect is shown in Figure 1. To prevent the spread of disease, the surveyors’ boots and field gear were disinfected with >1% Sterigene solution prior to entering Te Paparahi, and at the end of each day.
The 2021 survey differed slightly from the 2012 and 2015 survey in that each transect was surveyed for frogs by three, rather than two, observers. To do this, a 100 metre transect tape was run along the length of the transect to allow the observers to record the position of each frog found. Each 100-metre streambed transect was split into thirds (0–34 metres, 34–67 metres and 67–100 metres). Each observer searched a third of the transect simultaneously, then swapped over to re-survey the remaining two thirds. All three surveys were completed within five hours of each other to reduce the likelihood of frogs moving around between searches. Observers did not share information until they had all surveyed the entire transect; this was important to ensure the observers were not influenced by each others’ observations. Search methods for frogs followed Bell (1996) and are detailed in Herbert et al. (2014). Briefly, these consisted of systematically searching underneath rocks and plant debris within the streambed, and stream banks up to 1 m above the water level. Observers were careful to replace rocks and debris into their original position.
When a frog was encountered, the observers recorded the frogs:
- • Position along the transect to the nearest centimetre1
- • Snout-vent length (SVL)
- • Frogs’ sex and approximate age (juvenile, sub-adult, adult, adult female).
Table 1: Total number of frog observations and the number of juvenile frog observations by transect, April 2021.
Number of frog observations
Number of observations of juvenile frogs (% of total)
A total of 428 observations of Hochstetter’s frogs were made during the 2021 survey. Frogs were detected on 13 of the 15 transects (87%, Table 1). The majority of the frog observations were made in the transects in catchments A and C and in transects B1 and B2 (Figure 1). The number of sightings is not equal to the number of actual frogs present, because some individuals may have been counted more than once, and some individuals may have been missed. Of the total frog sightings, 17% were juvenile frogs (that is, frogs with a SVL of less than 19 millimetres; Table 1). The presence of juvenile frogs indicated that recruitment through births or sub-adult immigration was occurring on eight of the 15 transects (53%). Recruitment was highest on transects where the overall number of frog sightings was highest (Table 1).
When compared with the previous surveys, this survey suggests that the observed occupancy and recruitment of the Te Paparahi Hochstetter’s frog metapopulation appears to be stable. The frogs have been found on 13 of the fifteen monitoring transects (87%) in all three surveys (Herbert et al. 2014, Herbert and Gilbert 2015). Evidence of recruitment (presence of juvenile or sub-adult frogs) has been observed each year, ranging from 53% (2015, 2021) to 60% (2012) of the transects (Herbert et al. 2014, Herbert and Gilbert 2015). This recruitment is occurring despite there currently being a lack of intensive rodent and pig control in the Te Paparahi area. However, despite this observed stability, pigs, rodents and other introduced mammals may still be limiting the abundance of Hochstetter’s frogs, and preventing them from establishing in other available habitat. Whether this is true could only be determined by experimental removal of introduced mammals from some of the catchments.
Further comparison of the 2021 data with the previous surveys, and a more in-depth analysis of the nine-year trend will be conducted by Claire Johnson for her MSc thesis research at Massey University, with supervision from Doug Armstrong and Sarah Herbert. Claire will apply different statistical approaches for estimating abundance and occupancy to the 2021 data to determine whether the approach used this year is viable for estimating population parameters from unmarked populations of this species.
The Hochstetter’s frog population at Te Paparahi, Aotea/Great Barrier Island should continue to be monitored at 3-5 yearly intervals to inform the Department of Conservation, Ngāti Rehua, and the Great Barrier Island community on the status of the population in the future. This will help them to make informed decisions on whether the Te Paparahi population needs targeted conservation management. Additionally, the nine-year data set from the Te Paparahi monitoring programme contributes valuable long-term information for assessing the national threat status of Hochstetter’s frogs.
Long-term monitoring will also be beneficial for understanding to the role of key threats such as predation from introduced mammals and the potential effects of climate change.