Volume 31, Issue 3, Fall 2013
Finding Priority Sites for Conservation
Computer models point the way to Mexican long-nosed bats
By Emma Gomez-Ruiz
The rare, nectar-eating Mexican long-nosed bat (Leptonycteris nivalis) is listed as endangered by the United States and Mexico and by the International Union for the Conservation of Nature (IUCN), which cites estimated population declines of up to 50 percent in the past 10 years. Conserving this important species is especially challenging because reliable information about its numbers, habitats and distribution is in short supply.
In Mexico, where long-nosed bats are thinly scattered along the length of the country, pregnant females migrate northward each spring along “nectar corridors,” following the blooming of agaves, their primary source of nectar.
Some of these bats migrate across the international border into the southwestern United States, where, the IUCN reports, the only documented colonial roost of Mexican long-nosed bats is in Emory Cave on the Big Bend National Park in West Texas. The summer bat population there fluctuates wildly from year to year, ranging from zero to about 10,000 long-nosed bats. The reason for that is unclear. It may simply reflect the timing of the surveys or, the IUCN notes, the Texas population may be a “spillover” colony that visits when the bat population is high or food supplies are low.
With support from a Bat Conservation International Student Research Scholarship, I am working to fill in some key gaps in our knowledge of Mexican long-nosed bats – and to demonstrate the conservation potential of multilayered computer modeling for identifying critical habitats. A variety of databases now provide a wealth of data on animal and plant species, as well as topographical and climate records. Increasingly sophisticated modeling software improves our ability to combine and interpret such data.
Understanding the distribution of potential roosting and foraging sites along the species’ migratory route is fundamental for developing and implementing effective conservation programs to recover these beleaguered bats. And protecting nectar bats ensures they will continue providing vital pollination services in this semiarid region.
Agaves increase nectar production during the night, an adaptation that clearly favors nocturnal pollinators such as bats. Nectar-feeding bats carry pollen from plant to plant, ensuring the cross-pollination that enhances agaves’ genetic diversity and increases resilience to environmental stress. Agaves, which die after blooming, are critical in maintaining soil stability in deserts, scrublands and subtropical forests. They are also economically important producers of food, fibers, tequila and mescal.
Food availability (agaves) and cave conditions for roosts are the two main factors that must be considered when identifying critical sites. This research is providing insights into the distribution and status of agave populations in my study area (West Texas in the U.S. and the states of Coahuila and Nuevo Leon in northeastern Mexico). Agaves are also an important food source for the Mexican long-tongued bat (Choeronycteris mexicana), the other nectarivorous bat in the region, so its conservation should be enhanced through these results, as well. I will also use the models to explore the likely impact of climate change on the species.
My goal is to identify the most critical foraging and roosting sites along L. nivalis’ migratory routes and assess potential threats to those key habitats. The IUCN reports that major threats include disturbance of roosts and loss of food sources due to expanding agriculture and overharvesting of wild agave for tequila and mescal. Wildfires also sometimes destroy agaves, and beneficial bats such as these are often intentionally destroyed under the erroneous belief that they are vampire bats.
During this first year of the study, I modeled the potential distribution of the agaves along the nectar corridor used by Mexican long-nosed bats in their migration. I selected six of the most common agave species in the region and obtained data on plant locations from two repositories – the Global Biodiversity Information Facility and Mexico’s Red Mundial de Información sobre Biodiversidad. Then I expanded those results by searching the scientific literature and conducting on-the-ground research.
To that database I added topographical layers of elevations, slope and soil moisture from the U.S. Geological Survey. Then I mixed in climate data from WorldClim’s database of monthly temperature and rainfall values in order to include annual trends, seasonal impacts and environmental factors.
I used DesktopGarp, a software package developed by the University of Kansas Biodiversity Research Center to analyze and predict the distribution of wild species. Using this tool, we developed 100 models of potential distribution for each of the six agave species, then selected the 10 best models for each species and “summarized” them into one map per species. We ultimately combined the grids into a single map, which displayed agave species diversity by area.
We overlaid published “presence data” of Mexican long-nosed bats on the agave map and began to visualize a potential migratory corridor for the bats (see the map on this page).
With the distribution model in hand, we began fieldwork to assess the status of the agave populations and the foraging habitat in general. We designated a series of study “quadrats,” each about 50 x 82 feet (15 by 25 meters), where we counted and measured the agave plants to asses their density, cover and life-cycle stage (the presence of a flowering stalk, for example). We also collected and stored leaves, flowers and fruits for additional species identification.
Among other things, we counted dead agave plants that had not produced a flowering stalk to gauge mortality in the agave population. In areas located in the northern portion of the state of Coahuila, we found 14 percent of agaves in that condition, which can result from severe drought and record freezes of previous years. Continuous monitoring of the agave populations is needed to better understand long-term trends.
During our first field season, we used mist nets to capture and identify bats at four of our target locations to confirm the presence of Mexican long-nosed bats. We captured about 130 bats of 8 species, including 39 L. nivalis, during this initial fieldwork.
The resulting “nectar corridor” map suggests that areas with higher agave richness overlap with the known maternity/roosting sites for the long-nosed bats in its northern range. This makes sense from two perspectives. On one hand, it has been recognized that bats are effective pollinators of agaves and have played an important role in the plants’ speciation. So higher numbers of agave species are likely where the bat is present. On the other hand, a higher number of agave species allows longer periods of nectar availability, which could attract the bats.
We will continue our fieldwork to develop a more complete assessment of agave conditions and bat species and numbers. Priority sampling areas will be within a 31-mile (50-kilometer) radius of caves known to be used by Mexican long-nosed bats. That is reported as the potential distance these bats can fly each night to find foraging areas. Our habitat maps suggest where additional sampling is most needed.
By taking advantage of available databases and powerful ecological modeling tools, then expanding on the results with on-the-ground research, we are beginning to answer key questions that should enhance the conservation of these endangered, border-crossing bats. And we are proving the value of these tools in identifying the most critical sites for the investment of always-scarce conservation resources for Mexican long-nosed bats – and for bats around the world.
EMMA GOMEZ-RUIZ is a graduate student at the Department of Wildlife and Fisheries Sciences at Texas A&M University. The Global Biodiversity Information Facility, based in Denmark, recently honored her with its 2013 Young Researcher Award for this continuing study.
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