Monthly Archives: February 2015

Tigers and Territory: The Issues We See with Home Range Size and Conservation in these Big Cats

Many big cats have large home ranges or territories. Cats like mountain lions or pumas, tigers and bobcats don’t live in big groups like lion prides. Instead, they live by themselves most of the time, except when a mother is raising cubs or sometimes when a male and female hang out together around mating time. Many people think tigers are beautiful and majestic, but keeping them in the wild seems to be kind of difficult—they are currently listed as endangered because tiger populations have decreased by 50% in the last 30 years or so4. Tigers are native to Asia, and Asia has a lot of people (4.4 billion in 2014). When human populations get really big, it’s had for other animals to coexist with us.

For tigers, population density depends upon the density of their prey, which may also be scarcer where human populations are expanding. Tigers commonly eat other mammals, like deer and other hoofstock which are known for being skittish and can be hard to catch. One model suggests that male Amur tigers must consume at least 25 prey per year, non-reproductive females consume 20 prey/year and mothers raising 4 cubs require 54 prey/year8. That is quite a lot when your prey is hard to find and catch, so it might make sense that tigers have a big home range. They have to search pretty far to find enough food when the food is also trying not to be found. Prey density is declining in native tiger habitat6. That makes getting enough to eat even harder. Sometimes people’s livestock are easier to get, which means that livestock depredation is one of the biggest reasons we see conflict between humans and tigers.

A tiger’s home range size can vary a lot depending on the place you’re looking at. One study in Thailand found that male tigers’ ranges were around 267–294 km and female’s ranges were around 70–84 km7. But a study in Bangladesh found that a couple females had an average home range of 14.2 km2, which is much smaller.

According to World Wildlife Fund (WWF), some of the current conservation projects for tigers in the wild are focused around preserving their habitat and reducing tiger-human conflict10. Because tigers need so much habitat for those big home ranges, one of the first steps in their conservation is preserving enough habitat to support a good sized population. In order to do that, organizations like WWF are working to try to influence agriculture, paper and timber industries, among others, to use sustainable practices that minimize loss of forest land. To reduce conflict with local communities, organizations like WWF rely on innovative solutions and education. In regions where the conflict most often occurs, some strategies might include scare tactics, negative conditioning and capturing tigers to assess their condition because tigers in good condition are not usually the ones who are desperate enough to attack humans and livestock9.

So what do these big sizes and big differences in home ranges mean for conservation? A lot! We know that in order to conserve tigers in the wild, we need to conserve big enough patches of their habitat for them to move through, ideally without running into and having conflicts with people. Because the density of their prey influences their feeding behavior and potentially the size of their range, we might have to consider how to maintain those species at good levels (and figure out what those good levels are). In order to reduce conflict with humans and hence reduce killings by humans, we need to learn more about how tiger home ranges change around people, and how tigers move through populated areas. Helping people build safe enclosures for their livestock might be one thing to try. Do you have any good ideas? Leave them in a comment below!!

Here are some of the sources I used. If you’re interested in tiger behavior or tiger conservation, you should let us know what articles and papers you like, too!

– Leorah McGinnis

Sources:

  1. Athreya, Vidya. et al. “Movement and Activity Pattern of a Collared Tigress in a Human-dominated Landscape in Central India.” TROPICAL CONSERVATION SCIENCE1 (2014): 75-86. Web.
  2. Barlow, Adam. et al. “Femalle Tiger Panthera Tigris Home Range Size in the Bangladesh Sundarbans: The Value of This Mangrove Ecosystem for the Species’ Conservation.” ORYX1 (2011): 125-28. Web.
  3. Bhattarai, Babu, and Klaus Fischer. “Human-tiger Panthera Tigris Conflict and Its Perception in Bardia National Park, Nepal.” ORYX4 (2014): 522-28. Web.
  4. Chundawat, R.S., Habib, B., Karanth, U., Kawanishi, K., Ahmad Khan, J., Lynam, T., Miquelle, D., Nyhus, P., Sunarto, S., Tilson, R. & Sonam Wang 2011.Panthera tigris. The IUCN Red List of Threatened Species. Version 2014.3. <iucnredlist.org>. Downloaded on11 February 2015.
  5. Miller, C.s., M. Hebblewhite, Y.k. Petrunenko, I.v. Seryodkin, J.m. Goodrich, and D.g. Miquelle. “Amur Tiger (Panthera Tigris Altaica) Energetic Requirements: Implications for Conserving Wild Tigers.”Biological Conservation170 (2014): 120-29. Web.
  6. Simcharoen, Achara. et al. “Ecological Factors That Influence Sambar (Rusa Unicolor) Distribution and Abundance in Western Thailand: Implications for Tiger Conservation.” RAFFLES BULLETIN OF ZOOLOGY62 (2014): 100-06. Web.
  7. Simcharoen, Achara. et al. “Female Tiger Panthera Tigris Home Range Size and Prey Abundance: Important Metrics for Management.” ORYX3 (2014): 370-77. Web.
  8. Soh, Yi Hui, Luis Roman Carrasco, Dale G. Miquelle, Jinsong Jiang, Jun Yang, Emma J. Stokes, Jirong Tang, Aili Kang, Peiqi Liu, and Madhu Rao. “Spatial Correlates of Livestock Depredation by Amur Tigers in Hunchun, China: Relevance of Prey Density and Implications for Protected Area Management.” Biological Conservation169 (2014): 117-27. Web.
  9. “Solutions.” Save Tigers Now. World Wildlife Fund. < http://www.savetigersnow.org/solutions>. Accessed 21 February 2015.
  10. “The Siberian Tiger Project: Managing Tiger-Human Conflicts.” WCS Russia. Wildlife Conservation Society, 2008. Web. 21 Feb. 2015.

 

The Problem of Brown Tree Snakes: Why Trojan horses only work in Troy

Several years ago, while working as an Animal Care Technician for the USDA’s National Wildlife Research Center (NWRC), I became intimately familiar with the case of the Brown Tree Snake (Boiga irregularis). For those of you who are unfamiliar with the story, it goes something like this:

At the end of World War II, war-time ships were busy with activities around Guam, an island about a thousand miles south of Japan. A few Brown Tree Snakes hitched a ride to Guam, an island their species had never been to. The snakes were probably drawn to the mice already stowing-away on the ships. However, once on Guam the species started to flourish. Their numbers, with more abundant prey resources than in their native ranges, started to show exponential growth.

This snake has wiped out several bird and lizard species, including some that were endemic to Guam. This has caused an increase in the abundance of spiders on the island. Determined to regain our beautiful Helen taken by the Trojans (and also to alleviate the infrastructure problems caused by the snakes) Uncle Sam was rallied to war, and set out across the ocean to destroy its enemy.

The first attempts at controlling the snakes focused on using trained dogs, and eliminating individual snakes by hand. These measures were put into effect in 1994, 40 years after the introduction of the snake to the island. These methods were largely unsuccessful in lowering snake populations. By 1999, no headway had been made in crashing the gates of Troy, and a new strategy was needed.

Researchers from the NWRC quickly collected specimens and started testing the most effective control methods. It was found that over-the-counter grade aced into a dead mouse was enough to kill brown tree snakes. Over the next 14 years, different delivery methods were tested and implemented.

As of 2015[1], the newest tree snake eradication efforts involve injecting acetaminophen into dead mice, attaching them to a parachute, and dispersing them, by helicopter, throughout the jungle of Guam[2]. Here’s how it is supposed to work:

The little parachutes get caught in the canopy, dangling their deceptive prize for the snakes to find. Snakes that eat the mice are exposed to the toxin inside, and quickly die. It is more or less the population control equivalent of the Trojan horse, or in this case the “Trojan mouse”. The dead snakes reduce the size of the reproducing population, and (hopefully) the total population can be brought under control. It seems simple and straight-forward. However, remembering some simple concepts from evolution and behavior, the story gets more complicated.

For instance, Charles Darwin noted great variation among individuals within a species, and used this to develop his theory of natural selection. The process of natural selection is one by which the varieties of a species that are most successful in an environment reproduce more, causing an increase in the abundance of that variety, compared to other varieties. Through the many environmental pressures, species change through time to fit the constraints of their surroundings. Modern biologists have recognized that many behaviors act as variable traits that are influenced by heredity. In other words, natural selection acts on behaviors as well.

Given this, consider the snakes on Guam. Prey selection is a variable behavior in brown tree snakes, with snakes “eating a wide variety of vertebrate prey, including reptiles, birds, and small mammals…”3. In relation to the quality and abundance of these food sources, when one food source becomes scarce or dangerous, snakes that don’t rely on this food source will have higher reproductive success than a snake that does.

By dropping our “Trojan mice”, we are simply adding a human selection pressure to snakes. We are not eradicating the brown tree snake, because we cannot possibly get every snake to take the bait.  By killing the snakes that will sometimes eat a dead mouse, we have simply given a leg up to the snakes that avoid mice as a prey item. These include the snakes that eat only birds and lizards. Effectively, we have increased the relative abundance of snakes that prefer “rare meats”, and are punishing snakes that are likely to provide beneficial services, such as rodent control.

Other activities employed against the brown tree snake include “detector dogs, traps, nighttime fence searches, oral toxicants, barriers, prey reduction, and public outreach activities.[3]” The total costs of these programs is about $3,000,000 annually (although last year the budget was increased by $500,000 “for the manufacturing of an automated bait delivery system for the Toxicant Bait Drop Project[4]”). Seems like a lot of money to control some snakes.

Given that there are roughly 2,745,400 brown tree snakes on Guam (50 snakes/hectare x 54908 hectares on Guam), I propose a simpler and more effective solution. Put a bounty of $1 on snakes.

Considering the high density of snakes of the island, to make a salary of $31,000[5], a hunter would have to collect about 124 snakes per working day (31,000 snakes/250 working days in a year). To trap this many snakes, a hunter would need to cover an area of only 6.2 acres per day, or roughly 6 football fields. Given an 8 hour day, traps, trained dogs, and the human technological advantage, it seems reasonable that the activity of snake hunting could become economically profitable. Profitable hunting has been a proven eradication method of any species; look at the history Passenger Pigeon[6] if you need proof that it works.

Therefore, it seems unreasonable that we spend $3,000,000 a year applying a selection pressure, when we could spend $2,745,400 to eliminate all the snakes and provide around 85 islanders with jobs until the snakes are eliminated. If they really are a threat to biodiversity, and could potentially cost us billions of dollars, it would make sense to pick a more proven and effective method of eradication. Let’s leave the elegant deception to the ancient Greeks and focus on methods modern capitalists can support.

–Gabe Buckley
B.S. Zoology with a minor in Business Administration
Profession Science Masters in Zoo and Aquarium Management, class of 2015

[1] See page 73. http://www.doi.gov/oia/budget/upload/2015-OIA-Greenbook-Master.pdf

[2] http://www.npr.org/blogs/thetwo-way/2013/02/22/172695707/dead-mice-are-going-to-be-dropped-on-guam-from-helicopters-really

[3] See page 75. http://www.doi.gov/oia/budget/upload/2015-OIA-Greenbook-Master.pdf

[4] See page 73. http://www.doi.gov/oia/budget/upload/2015-OIA-Greenbook-Master.pdf

[5] the average salary of Fishing and Forestry Occupations in Guam. See “Annual mean wage” in “Farming, Fishing, and Forestry Occupations”. http://www.bls.gov/oes/current/oes_gu.htm#45-0000

[6] http://www.audubon.org/magazine/may-june-2014/why-passenger-pigeon-went-extinct