Alaska Fish & Wildlife News
March 2017

How Biologists Estimate Populations of Animals

Past Kari Rasmussen

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Biologists use a variety of methods to guess populations. Counting animals like this camouflaged snowshoe hare can be hard. Kari Rasmussen photo.

1 of the groovy perks of living in Alaska is sharing our space with a variety of wildlife. Whether we are curious about our side by side hunting opportunities, or eager to watch wildlife, we often wonder about the animals living in our ain backyards. Have you ever considered how many bears are hibernating near you on common cold winter evenings? When you run across a moose outside your window do you imagine there might be more yous exercise not see? Wildlife managers and researchers work diligently to discover out how many individuals of a detail species alive in a certain surface area. Their jobs are necessary to ensure the conservation of wild animals, particularly for animals that are hunted.

Tim Peltier, a biologist for the Alaska Department of Fish and Game (ADF&M) in Palmer, explains one of the reasons why biologists have such an important task.

"We count moose, caribou, sheep, and to a bottom extent bears and wolves," he said. "The Board (of Game) has determined the population and harvest objective in each area and we are tasked with determining how close we are to those objectives. We demand to have an idea of the population size."

How do biologists determine the number of animals living in an area? A diverseness of methods are used to determine populations. A biologist must consider the ecosystem, terrain, the species being counted; and must also factor in weather, costs, and time.

Information technology would be ideal to see and count each individual animal in a population. This is chosen a census. Unfortunately, there is no simple way to do this. Animals move from place to place, they hide, they hibernate, and they are often camouflaged and difficult to see in their surroundings. The closest affair to conducting a census in Alaska is counting caribou such as the Western Arctic Herd on the N Slope. During the summer, caribou huddle upwards on snow fields for relief from mosquitoes and other insect pests. Viewed from in a higher place, the brown caribou on the white, snowy background are relatively easy to run across. Biologists fly over these groups of caribou, systematically taking pictures as they become. Back in the office, every picture is analyzed and biologists zoom in and count each caribou individually. Information technology is an important and fourth dimension consuming project – hundreds of thousands of caribou can be counted in more than 1,000 pictures.

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The ADF&One thousand beaver flies over the western Arctic Caribou Herd. Photo by Geoff Carroll.

Another efficient method researchers take developed to estimate populations is called "capture-mark-recapture." Instead of trying to count every animal, biologists randomly capture a sample group of the population, marking it, release it, and so do a series of recaptures that volition permit them to estimate the entire population in a particular report area. Ane easy fashion to empathize this method is to see how Alaska teachers are learning to utilize ADF&Chiliad'south curriculum Wildlife for the Futurity in their classroom lessons.

Terminal fall in Nome, sixteen teachers were each given a container filled with beans. Their get-go job was to write down the number of beans they thought were in their container. The teachers used a diverseness of strategies to come upwardly with their visual estimates. Many took random guesses without much forethought, some relied solely on their sight past counting every bit many beans as were visible to them, and others counted the rows and columns of the beans and multiplied to make up one's mind their judge.

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Teachers Josie Bourdon and Justin Heinrich in Nome, Alaska employ ADF&G'south Wild animals for the Future lessons to learn about the population gauge method: capture-mark-recapture.

Then they applied the "mark-recapture" technique. Each teacher reached into the bowl of beans to "capture" a scattering of "animals." They counted the captured beans and recorded this number on a data sheet. Before putting the beans back in the bowl, they "marked" the captured beans past replacing them with the aforementioned number of beans of a dissimilar color – these then became the "marked animals." Afterward thoroughly mixing the beans they did a serial of "recaptures" past once again grabbing a handful of beans, counting the total number of "animals" captured, recording that number, and then recording the number of marked "animals" in each recapture. They did this 10 times. Once all 10 recaptures were finished they calculated the average number of beans for the recaptures and then the average number of "marked," or different colored beans, in the recaptures. They put these ii averages along with the number in the first sample count in to a mathematical formula and calculated the scientific approximate of the population of beans in the container. Finally, they dumped the beans out and counted each individual edible bean for an accurate count, or census, of the bean "population." Nearly every instructor came up with a less than five percentage mistake rate between their scientific estimate and the actual count, a much narrower margin of error than their ain visual estimate.

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A Baranof Island mountain goat, equipped with a GPS collar. Phil Mooney photo.

Land wildlife biologist Stephen Bethune, based in Sitka, explains how this works in real life. On Baranof Island mount goats are captured and fitted with brilliant reddish or orangish collars and ear tags.

"We know we never come across all the goats in a given area, but having marked goats on the landscape allows us to determine the sightability of goats during that survey," he said. "For example, if in that location are 10 marked goats in an surface area and we see seven, we would utilize a 70% sightability correction cistron to a survey. And so if we counted 50 goats, applying the correction factor we tin estimate there were 71 goats in the area."

Biologists then utilise the collars to carry what are called aeriform surveys, where they monitor survival and child production to help build population models. "The more sightability surveys we bear, the more refined our sightability factors tin be calculated," Bethune said. "We are able to monitor the general health of the population to assistance set appropriate harvest quotas in hunt zones."

Other animals, such every bit dark-brown bears on Kodiak Island, are surveyed from above. Biologist Nathan Svoboda uses intensive aeriform surveys (ISA) to derive estimates for chocolate-brown bears on Kodiak Isle. His goal is to manage for a sustainable harvest and population and, "to ensure a healthy, robust, viable brownish deport population exists in perpetuity, every bit chocolate-brown bears are the cornerstone of ecological integrity on the archipelago," he said.

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A Kodiak acquit photographed by a remote, motility-triggered trail camera. Photo past Robin Overall.

These ISA's must exist done after bears exit winter dens in the spring, merely before vegetation dark-green-upwards so surveyors tin identify both private bears and family groups without vegetation obstructing their view. Time is of the essence. In contempo years, earlier green-up than normal has presented some challenges for biologists. They must start surveying earlier in the year while making sure they acquire accurate estimates of all bears, equally some bears may not have exited dens notwithstanding, specifically females with young. In spite of challenges, Svoboda continues managing for a sustainable harvest and sustainable population and is investigating options to develop a robust population estimate for the Kodiak Island chocolate-brown bears.

Obtaining a population judge is a large job for management and enquiry biologists. Whether conducting a census, an aeriform survey, a capture-mark-recapture survey or one of the many other methods used to obtain an accurate estimate, each i provides important information to better understanding wildlife. So, next time you run into a moose outside your door and yous wonder if more are out there, rest assured that Alaska's wild animals biologists are curious almost that besides. And, they will continue to work to help keep Alaska's wildlife populations healthy and sustainable, at present, and for time to come generations of wildlife enthusiasts.

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