When searching an area for scat, invasive plants, or other data points, many researchers and conservation detection dog teams employ a transect search. In order to determine the appropriate spacing of your transects, you need to understand the local conditions influencing air flow and the probability of detection for your target in that space.
There is no correct answer for transect spacing that can be universally applied.
Depending on local conditions, the consequence of missing a sample, and the size/behavior of your target odor, appropriate transect spacing may range from just 3-5 meters to over 100 meters.
For example, in consistently windy conditions with relatively stinky targets (think dogs finding bat carcasses on wind farms), dogs can often find targets from 70m away. But when the target odor is small and there is lots of 3D interference in odor movement (imagine finding bumblebee nests in thick undercover), detection distances may be mere centimeters.
Dogs are not machines; they get tired. Many studies are limited by time or budget. All of these factors mean it’s best to optimize transect spacing to get the most out of the valuable “tool” of a conservation detection dog team.
What is a Transect Search?
When conservation detection dog teams conduct a search, they often walk along a specific GPS line. The line walked by the team is called a “transect.”
Detection dogs can detect odor emanating from a source quite far away from the transect. While there is some variability in what human searchers can see from a line transect as well (based on vegetation, topography, and what they’re looking for), there is much more variety in how odor moves and therefore detection distances. This makes strategic transect spacing extremely important.
In some cases, a transect search may not be the optimal strategy for conducing your research or acquiring samples. An experienced conservation dog handler can help you strategize search setup to optimize results.
Factors for Deciding on Transect Widths
If you and the conservation detection dog handler decide that line transects are your optimal strategy, the next question is likely what transect width or spacing is best for your study.
There are many factors to consider when deciding on transect spacing, which we will explore below.
- Study Goals. Do you want to find every single individual sample? In that case, you want narrower transects to ensure that samples are not missed. This is common when working with invasive plants, where a miss of a single plant could result in tens of thousands of seeds being released into the ecosystem. If your goal is to rely on the dogs to find “hotspots” for further investigation or monitoring, wider transects may allow the dog team to cover more ground and find more of these hotspots. Similarly, if your goal is simply to collect scat samples for other analysis and it’s ok if you miss some, wider transects may be appropriate. When the consequence of missing a sample is high, err on the side of narrower transect width.
- Time and Financial Budget. Unfortunately, money and time often limit what we can accomplish during our research. If you have limited time or money to survey a given area, a “hasty search” with wider transects may be a simple reality of your situation. This may only give presence/absence of a species or yield a few samples, but this information could be used to target further funding and research next season. That said, in some cases it may make more sense to reduce the study area and use narrower transects rather than increase transect width to survey a larger area. This comes down to study goals and the other factors below.
- Target Odor Availability. Samples that are buried, covered by duff, hidden inside of tree knots, tucked under bark, or otherwise protected from air flow will have less available scent for a conservation detection dog. It’s not that a buried scat doesn’t stink; it’s that not as much of the odor can leech out of the soil as quickly. If you expect many of your samples to be tucked away, buried, or otherwise hidden from wind, expect to need narrower transects.
- Target Odor Volatility. When you walk into a room containing a dead mouse or fresh flowers, you know it right away. But if you walk into a room containing a bowl of uncooked greens or even dried flowers, you likely wouldn’t notice the odor. Part of this is based on the scent receptors in your nose and how they’re interpreted by your brain, but part of this is also because of the volatility of the molecules emanating from a sample. More volatile molecules mean that there is more odor available in the air for a dog to detect. This is partially why scat detection dogs and carcass detection dogs often have a greater detection distance (and therefore can tolerate wider transects) than dogs finding plants, cryptic animals, and nests. This (plus humidity) relates to why fresher scat samples are often detectable from further away than desiccated samples.
- Target Odor Size. It’s intuitively clear that a dead whale smells more than a dead bat and that adult male grizzly bear scat is easier to sniff out than that of a young cub. With increased surface size of a target, more odor is given off for the dogs to detect. Generally, larger targets stink more and therefore allow for wider transects.
- Local Weather Conditions. We’ll dig more into weather conditions below, because this gets complicated. At an extremely basic level, higher humidity and consistent winds make for easier scenting and therefore facilitate wider transects. Blustery conditions, shifting wind, low humidity, and inversions can all complicate a search and may require narrower transects. Since the weather shifts constantly, it is often wise to devise study methods that allow the handler to update search strategy and transect width based on the conditions on the ground if your goal is to locate as many samples as possible.
- Local Topography. Odor flowing with the wind may get channeled down a riverbed, caught in an eddy behind a hill, or pool in a depression in the ground. Air flow (and odor with it) can change throughout the day based on slope direction and aspect. Varied topography makes for challenging search conditions and may require narrower transect spacing.
- If your goal is to find as many samples as possible, it’s best to give handlers permission to deviate from their transects to allow their dogs to access air flow that may not fall on-transect. For example, when searching wind farms in Nebraska in 2021, I often deviated off-transect to allow my dog to search steep gullies that may have trapped odor.
- Local Vegetation. Thick underbrush can limit air flow. The edges of forest can suck in or expel air depending on other local conditions and time of day. Trees can create eddies of scent in their wake, suck odor up in a “chimney” effect, or capture odor in their canopy. Forests can also protect the understory from overhead weather conditions. Extremely short grass or pavement on a flat surface can allow odor to move very far without collecting in many spots, making it difficult for the dog to “read” the scent plume. This stuff is complicated! Generally speaking, denser vegetation needs narrower transect spacing. Handlers should allow their dogs to investigate vegetation that may have collected odor in its bark or in its eddy, and use their knowledge of air flow to aid the dog in locating the final source.
- Individual Dog Team Characteristics. Not all dog-handler teams are created equal. Some dogs excel at detailed, intensive searches. Other dogs naturally work fast and far. Some handlers are extremely in-tune and read their dog’s minor changes of behavior with ease; other handlers expect the dog to spell everything out for them. Some dogs tire easily or lose motivation; others are absolute workhorses. Knowing the strengths and weaknesses of your conservation detection dog team will help you design methodology that meets your needs.
How to Determine Appropriate Transect Spacing for Conservation Detection Dogs
Given all of the factors above, you may already have an idea of what transect spacing may be appropriate for your study.
The best way to determine transect spacing is to actually get into the field and determine the typical detection distance and/or probability of detection for your situation. It is extremely important to do this in conditions that mimic their operational search: time of day and season both influence air flow too much to be ignored.
Broadly speaking, you can estimate typical detection distance by placing samples at a variety of distances from a transect without the handler’s knowledge of where they are. As the handler walks the transect and the dog finds (or misses) the samples, you will have an idea of what distance allows the dog to find samples.
For example, at a given field site during this test the dog may find all samples placed 10, 15, and 20m from the transect, find some samples at 25m, find few at 30m, and find none at greater than 35m. You may opt for 20m (or even 15m to be safe) spacing if you want the dog to find every single sample in the field, or you may opt for wider transect spacing and then calculate estimated population based on what you expect the dog to miss.
Research on Transect Spacing and Detection Distance for Conservation Detection Dogs
Although limited, there have been scientific studies conducted that attempt to identify optimal transect spacing for various targets and local areas. We’ll outline a few studies and their findings below.
- A study looking at dogs finding live human subjects classified the detection distance based on the air stability class. We can’t dig too deeply into air stability class here, but essentially classifications range from very unstable (Class A) to very stable (Class F). The study found that at 100m, dogs had a 95% detection rate for very stable air and 13% for very unstable air. At 25m from the subject, dogs had an 82% detection rate for very unstable wind ranging up to 99% for very stable air (Graham 1994). This paper underlines the importance of air stability, not just topography, for determining transect spacing. We would expect detection distances to vary for different targets.
- Scat detection dogs working in northern California oak woodlands found >75% of scats that were within 11 yards of the transect (Reed 2010). The dogs’ performance dropped to 30-40% when scats were 27 yards off-transect.
- Detection distance for knapweed dogs were as high as 68 yards, although the paper does not specify percentage detected at various distances – 68 yards is just a maximum (Goodwin 2010).
- Desert tortoise detection dogs could locate tortoises from up to 70 yards away but mean detection distance was 15 yards. The paper also found that detection rates were 70% when humidity was 16-18% and that higher wind allowed for greater detection distance (Cablk et al. 2008 and Nussear et al 2008).
- Dogs trained to find tree snakes in Guam found an average detection rate of 35% and that 30% of alerts were within 1 yard of the snake (Savidge et al. 2011). This suggests that in hot, humid, calm conditions with an elevated target, extremely small detection distances can be expected.
- Whale scat detection dogs found detection distances at distances of 164 to 616 yards (Rolland et al. 2006). With a large sample, good scenting conditions, and very little turbulence or thermals over open ocean, the scent cone traveled very far.
These papers and their findings are summarized in the table below. Because Graham 1994 had such different study design than the other papers, it is not included in the table. Please comment if you know of other research that should be included!
|Paper Authors||Target||Max Detection Distance||Average Detection Distance||Detection Rate||Search Conditions|
|Reed 2010||scat||not given||not given||70% at 10m||northern California oak woodlands|
|Goodwin 2010||knapweed (plant)||62m||not given||77%||short vegetation, flat area|
|Cablk et al. 2008; Nussear et al. 2008||desert tortoises (live)||64m||13.7m||70% when humidity was 16-18%||open shrub desert|
|Savidge et al. 2011||tree snakes (live)||11.9m||not given, 30% were within 1 yard||35% (26-44% average between teams)||hot. humid, calm air; elevated source|
|Rolland et al. 2008||whale scat||1930m||not given||not given||open ocean|
Weather conditions are the most volatile factor and the hardest to plan for. If possible, you may want to plan a study that allows the handler to use their judgement to assist the dog and increase probability of detection rather than sticking to strict transect widths.