How to effectively monitor platypuses
18 July, 2019
Monitoring wildlife populations can be both difficult and expensive. The elusive and mobile behaviour of species, inaccessible habitat, the need for specialised training and equipment, and stringent permit requirements all create headaches for ecologists. This is especially true for our little duck-billed friend, the platypus. As a result, we have very poor data on the status of platypus populations across their geographical range. Luckily technology is constantly evolving, and we now have more options than ever before to help us monitor platypus populations. Here, I am going to cover some of the main methods for monitoring platypuses and take you through the advantages and disadvantages of these different techniques.
The effectiveness of any monitoring depends on two factors: the sensitivity of the technique and the power of the overall program design.
Sensitivity refers to the probability that the technique will detect platypuses when thye are present in an environment. No technique is 100% effective and platypus are elusive little critters. Therefore, finding no platypuses during a survey doesn’t mean they aren’t present, it may simply be that your survey method failed to detect them at that point in time. Despite years of experience, I still sometimes fail to catch any platypus during a live-trapping survey even when I know there is a population present in the area. This means that repeat surveys are often necessary to detect platypus populations or improve confidence in their absence. Techniques with low sensitivity will require more repeat sampling to achieve high confidence.
Power refers to the capacity of the monitoring program to achieve its goal with high confidence (such as detecting population changes over time). Power is usually achieved with a rigorous sampling regime that includes repetition (repeat surveys) and replication (number of sites) and is specifically designed to meet an objective. It should take into consideration the sensitivity of a monitoring technique and reflect a program’s objectives.
Live-trapping surveys remain the best method for estimating platypus abundance (measured as captures per unit of effort), however the method can have low sensitivity (20-30%; Lugg et al 2018) and highly variable capture rates.
While it has traditionally been difficult to estimate actual population size due to low numbers and few recaptures, new mark-recapture models for live-trapping have improved population estimates. Importantly, live trapping is the only method that can identify individuals. This information is used to assess demographics, juvenile recruitment, individual health, and obtain tissue samples. Tissue samples can then provide further insight on a population’s genetic health, genetic diversity, relatedness, and connectivity between populations. With long-term monitoring these metrics can be used to determine local population trajectories over time.
Unfortunately, live-trapping is also time and labour intensive, requires specialised training and equipment, as well as a number of permits, and can be restricted by weather or environmental conditions (e.g. flow, depth). Because of these limitations, live-trapping surveys can be expensive and difficult to implement over large spatial scales.
Anecdotal public sightings that are compiled into a single public repository (e.g. www.platypusSPOT.org) are a very useful and efficient method for collecting platypus presence data over large spatial scales (such as their distribution across eastern Australia). There is a risk of misidentification (usually with rakali) and unusual sightings should be followed up (as is the case for platypusSPOT). Data can also become self-verifying (i.e. easy to discount a single unusual sighting but multiple independent sightings in the same area improves confidence).
Community sightings can be useful to determine broad distribution but are naturally biased towards urban areas, towns and tourist hotspots. Also, there is no information provided on the absence of platypus. If no platypuses are recorded in an area, are they absent or is it simply that no-one is looking there?
As a result of these data limitations, public sightings have very low power to detect population trends but can be insightful and used to complement and direct other more rigorous monitoring techniques. For example, public sightings to platypusSPOT are supplementing a rigorous eDNA sampling program in the largest platypus survey ever undertaken (Great Australian Platypus Search).
Direct observation surveys
These typically involve volunteers observing a stretch of waterway for a set period of time and recording whether a platypus was seen or not. Observation surveys can be implemented in any waterbody with good visibility and are useful to determine the occurrence of platypuses over large scales and engage communities in conservation efforts. In contrast to anecdotal public sightings, observation surveys can also provide information on platypus absence (depending on number of repeat surveys conducted and confidence level). Similar to public sightings, there can be a risk of misidentification and are usually biased towards population centres and tourist hotspots, making it difficult to implement a rigorous sampling design.
However, observation surveys typically have low sensitivity to detect platypuses: 10-30% in areas of relatively high abundance (Easton et al 2008). In areas of low platypus abundance, which are generally the spots that require close monitoring, sensitivity is likely to be much lower. Therefore, repeated surveys are required to have reasonable confidence in declaring platypus present or absent and many more to provide an index of abundance (e.g. sightings per visit). Any analysis must incorporate variability across seasons, time of day, observers, and conditions (Easton et al 2002, Grant 2012), which will reduce the overall statistical power to detect trends. Another key limitation of observation surveys is the inability to identify individuals. Seeing the same individual on 10 different occasions is very different to seeing 10 different individuals once each.
In the absence of any comprehensive assessment of observation surveys, the low sensitivity, variable detection rates between sites, observer bias, and lack of rigorous design all limit the power to assess populations and detect meaningful changes over time.
Environmental DNA (eDNA)
As has been found with many aquatic species, eDNA (detection of genetic traces in the water) has very high sensitivity to detect platypuses (>95%; Lugg et al. 2018). Although eDNA generally only yields presence/absence at the local scale, site occupancy (% of sites where platypuses are present) can be used to infer “abundance” at larger scales and help detect changes over time with high statistical power. It can be a relatively cheap method of determining platypus distribution over broad spatial scales and although specialist training and equipment is required for the laboratory work, sampling can be done by anyone with limited instruction. Collecting water samples is less restricted by environmental conditions (i.e. can take samples from areas that are difficult/impossible to trap or observe) and not restricted to areas where people visit. Therefore, researchers can design a rigorous sampling regime to answer specific questions (as we are doing in the Great Australian Platypus Search) and have good confidence in results due to the high sensitivity of the technique.
Given the current global extinction crisis, it is important that we understand the status and trajectory of Australia’s wildlife populations. But before we invest in any monitoring program, it’s critical to define exactly why we are monitoring and what we are trying to find out. Is it to determine whether platypuses are present? Local abundance? Population trajectory? Is breeding occurring? Over what area? Understanding exactly what it is we’re trying to find out will influence what technique is used and the design of any monitoring program - this is what ecologists are trained to do.
With limited resources available for conservation, it is important that any monitoring efforts are effective and can achieve the desired goals so we are not wasting limited money and time, or diverting these resources from other conservation efforts.
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