PLEO Method: Measuring wildlife population densities

A massive part of our grassroots movement aims to manage and balance the needs of the local people with forests and wildlife to prevent further biodiversity loss.

Our approach is holistic which means we target the core socio-economic issues such as rural poverty to impact more symptomatic issues such as tree cover and biodiversity loss.

To see if our programs are having the intended effect, there are a lot indicators to measure. For example how many women are getting a say in family planning decisions? Are the individuals enrolled halting illegal activities? Is tree cover loss going down?

We are able to answer those questions through surveys and research projects but to see if we’re preventing biodiversity loss we need to estimate not only wildlife population numbers but population trends over a number of years.

Unfortunately, assessing wildlife density in rainforests is notoriously difficult. Why?

The incredible beauty of the Bornean Rainforests thick tree cover makes measuring wildlife population densities via transects incredibly difficult

The incredible beauty of the Bornean Rainforests thick tree cover makes measuring wildlife population densities via transects incredibly difficult

The most popular methods are transect surveys which involves monitoring activity over a fixed and marked path or point. Not only are transects extraordinarily time intensive, difficult and expensive, the accuracy of the most commonly used methods - used because there were few other options - are disputed.

The difficulty lies in the minimal visibility environments of rainforests. Furthermore, wildlife leave few presence cues that can be used.

We can use our experience with the Critically Endangered bird, the Helmeted Hornbill, as an example. Our site has been labeled a hotspot priority site for this species.

The ever elusive Helmeted Hornbill who finds life-giving protection in the rainforests opaque canopies

The ever elusive Helmeted Hornbill who finds life-giving protection in the rainforests opaque canopies

They fly around a lot and often in or above the dense tree cover of the Bornean Rainforest and make a lot of noise.

When we take audio detections along transects, the question becomes, is the same bird flying and calling? Or are multiple birds? Do we count 1 individual or 10?

Because of this many experts say that only visual detections should be used to estimate wildlife densities.

To estimate wildlife densities, we need a minimum of 60 visual detections. After 3 years of data collection, despite having collected an incredible 200+ detections, from 6 different research camps covering the massive 124,000 ha landscape…

only 11 are visual.

That data would not even cover the most important reason for measuring population density, which repeated through time allows us to look at wildlife trends. For some species, they may only reproduce every few years and only have 1 offspring.

This means that under traditional methods, wildlife monitoring is expensive, difficult, and on top of that, needs to be done almost non-stop over the better part of 10-20 years.

However, measuring wildlife density is a grossly important activity for conservation groups like us for the reasons we listed above.

That’s why our SMART Patrol and Biodiversity Teams have begun implementing the PLEO method to assess wildlife density in the nature reserve we work in.

The Pooled Local Expert Opinion (PLEO) method

(van der Hoeven, de Boer & Prins, 2004)

A citizen science tool, the PLEO method is significantly less time and money consuming compared to methods like transects but produces comparable results.

The data collected is based on the knowledge of local experts (e.g. hunters or people who spend a considerable time working in the forest).

Pak Vera, an ex-hunter who knows every nook and cranny of his village’s rainforests like the back of his hand, tells Adam about the number of Helmeted Hornbill sightings in the area.

Pak Vera, an ex-hunter who knows every nook and cranny of his village’s rainforests like the back of his hand, tells Adam about the number of Helmeted Hornbill sightings in the area.

A number of independent local experts will be asked to estimate wildlife abundance in a specified area. By calculating and combining the estimations for the surveyed area, you come to your conclusion.

In a research article “Pooling local expert opinions for estimating mammal densities in tropical rainforests”, the researchers compared PLEO with transect surveys to assess PLEO’s accuracy.

What they found was of the 24 animals surveyed, transect surveys were only able to produce enough data to determine only 2 animal species population density despite costing 7 times more and 9 months longer.

However, they were able to use past research in the area to determine that of the 85 samples studied, PLEO results fell well within the range of estimates.

The researchers determined that 95% of their estimates were within the 95% confidence interval, demonstrating the reliability of this method.

Interestingly, this method was first developed by stock market traders and economists. They found by Pooling Local Expert Opinion they could very accurately predict the price and expected changes in commodities like gold, silver, oil and with stocks of popular companies.

The methodology is simple and it can be incorporated in many tropical biodiversity and conservation projects. It can also be used for long-term monitoring of wildlife status in a given area and requires minimal formal statistics and biology training.

Most importantly, it involves local communities in conservation research and ensures strong local ownership of the data.

How it looks on the ground

We’ve already begun the processes but it is implemented through 5 main steps.

  1. Our SMART patrol will build a sampling unit map using notable man-made and natural features to determine the perimeter.

  2. Meanwhile our biodiversity team will build key species posters and other interviewing tools.

  3. After training our teams on how to conduct interviews and the research process, we will begin testing in the field.

  4. We then compile and analyse the data in the office.

  5. And repeat!

Adam MillerComment