Bats Exploit Dynamic Forests
Timothy Divoll
There’s no better time than now to consider how we may help surviving bats keep calm and carry on. Several
Figure 1. Map depicting the spread of white-nose syndrome (WNS) since its discovery near Albany, NY in 2006. County confirmations are based on winter hibernacula surveys and declines in summer populations are typically observed 2-3 years after initial detection of WNS in nearby caves. Millions of bats have died from this epidemic, most of which were little brown, northern long-eared, and Indiana bats. Forest bats at the HEE most likely overwinter in southern Indiana and/or Kentucky. Compare this 2015 WNS map with the map in Dr. Tim Carter’s 2010 Woodland Steward article: www.inwoodlands.org/bats-are-dying-by-the-thousand/ bat species in danger occur regularly in the forested matrix of the Hardwood Ecosystem Experiment, or HEE (heeforeststudy.org), in Morgan, Monroe, and Brown counties, Indiana. The northern long-eared bat, Myotis septentrionalis, is a small forest-dependent bat at the focus of several current HEE projects, alongside the Indiana bat, Myotis sodalis, a federally Endangered Species. Not only is the northern long-eared (northern Myotis) bat a focal point of the HEE, it is in the federal spotlight as well. This species has declined so much from white-nose syndrome that it is was very recently listed as a threatened species under the Endangered Species Act.
White-nose syndrome survivors
Several million bats have died from the deadly fungus that finds its way into bats’ tissues while they’re fast asleep for winter. The bulk of mortalities have been little brown (Myotis lucifugus) northern long-eared, Indiana, and tri-colored (Perimyotis subflavus) bats. Bats typically disappear from summer landscapes 2-3 years after detection of the disease in nearby caves. (See Figure 1.) Hibernating bats with white-nose syndrome wake up more often than usual and this excessive activity burns precious fat stored for hibernation. Some bats may exit caves in winter to search for food, burning even more stored fat. In northern locations, bats likely don’t find much to eat but recent studies suggest bats hibernating in southeastern caves may find limited food in winter. In either case, many bats will not be able to restore fat reserves. However, some do make it through the winter and emerge in spring to head on to their summer habitats. It’s likely that survivors emerge with very low energy reserves, and thus, quality habitat and forage are necessary for bats to gain strength for migration, birthing, and nursing pups to independence.
HEE Captures and Roost Selection
Both northern long-eared and Indiana bats have migrated back to the HEE landscape and settled in by the time research crews from Ball State University and Indiana State University show up in mid-May each year. Both bats have been captured consistently since 2006, though we capture tenfold more northern long-eared bats in our mist-nets each year. (See Figure 2.)
Declines in captures rates over the last few years have led us to focus our mist-netting activities at small forested ponds where capture rates are higher than on forest roads and our efforts are maximized. The fact that
Figure 2. Eight forest-dwelling bat species have been captured during ongoing surveys at the Hardwood Ecosystem Experiment in south-central Indiana. Bar widths are relative to the number of total bats captured per year, comparing among years. Bar heights relate to the number of each species captured within the same year, comparing among species. The Indiana Myotis (Indiana bat) is a Federally Endangered species and all others are State Special Concern with the exception of the big brown bat. Northern Myotis = northern long-eared bat. northern long-eared and Indiana bats favor small ponds as foraging sites suggests that permanent water sources may be an important resource for bats stressed by white-nose syndrome. Water is limited in the HEE landscape, where most creeks are dry for most of the summer. It is also important to point out that bats seem to visit small ponds under the forest canopy rather than ponds and lakes without tree cover.
Another limiting resource for forest-dependent northern long-eared and Indiana bats is suitable roosting habitat, but these bats have many roost options in the mixed deciduous forest of the HEE. In 3 seasons of tracking, we have learned that both northern long-eared and Indiana bats roost primarily in oak or maple snags with sloughing bark or cavities, but sometimes use live trees with crevices and/or cavities, such as sassafras and hickory. Northern long-eared bats were more versatile, selecting trees smaller in diameter than Indiana bats, which typically selectively roost under exfoliating bark with high solar exposure in larger dead trees. On the HEE, Indiana bats used 13 different tree species, including oaks, maples, hickories, elm, and tulip poplar. Northern long-eared bats roosted in trees of 11 different genera, but used mostly oaks, maples and sassafras–which
Figure 3. This small forest pond, approximately ½ acre in size, has been a productive capture site for mist-netting at the HEE. There are several of these small ponds scattered about the HEE and water is otherwise limited, with the exception of a few small streams. Bats simply love to visit these waterholes in the forest matrix, which require minimal management. As long the pond edge remains cleared and aquatic vegetation does not cover the water’s surface, bats maintain the ability to drink and forage under the safety of canopy cover. provide many bat-sized roosting cavities. Both bats roost in interior forest or near the edge of small openings, such as patch cuts. High solar exposure is important for bats in maternity roosts, which rely on the heat of the sun for passive thermoregulation. Both bat species will take advantage of available resources in different forest types. For example, in southern forests, both bat species will roost in pines and northern long-eared bats will roost in sugar maple and yellow birch in northern forests. If forest bats have access to critical resources such as water and a variety of snags, they will exploit various forest types while foraging. Small Myotis bats are highly mobile (they can fly, after all) and have the opportunity to utilize different structural components with differing heights. Timber management strategies such as single-tree selection, patch cutting, and shelterwood harvesting all create horizontal as well as vertical edges and gaps for bats to forage in.
HEE foraging –acoustics and telemetry
Based on acoustic surveys used to record bat activity as an index of species-specific echolocation recordings, we know that Indiana bat activity is highest in the HEE matrix near patch cuts (1, 3, and 5 acre openings) and first stage shelterwood treatments compared to larger clearcuts and control units. Northern long-eared bat activity is relatively equal among forest treatments types, suggesting that this species is also able to use forests with a variety of age-classes and openings. In 2014, we used radio telemetry to track both bat species at night; both selectively foraged in harvested areas, mainly around the edges of patch cuts and within single-tree selection harvests. Tracked bats did not use first stage shelterwood cuts in 2014, but we will investigate this further in 2015, as well as targeting areas harvested and/or burned in spring 2015 to study use of newly disturbed areas.
Bats and management in harmony
Our case study with bats at the HEE demonstrates that heterogeneity is critical to maintaining roosting and foraging habitats needed by bats during the summer maternity season. There is an optimal distance dynamic between roosting areas and foraging areas. We found northern long-eared bats foraging no more than 2 km from roost trees and Indiana bats no more than 3.3 km, suggesting that bats will travel to forage, but should thrive in foraging areas that already contain many roost tree options. Although many bat species will forage or commute along larger edges, northern long-eared and Indiana bats prefer patch and shelterwood edges that are more representative of natural disturbances such as fire, tree fall gaps, and storm destruction. Situating small openings near large tracts of mature forests with suitable roost trees (e.g., dead oaks or cavity-bearing sassafras trees) should provide quality of habitat for these Myotis bats. However, too much forest fragmentation may force Myotis bats to find new summer areas.
Although bats use harvested areas, it is important to consider timing of forest management activities in relation to the maternity season of bats to minimize disturbance while bats are present. Indiana bats (Myotis sodalis) are a federally protected species that drive seasonal harvest restrictions. Rules related to Northern long-eared bats (Myotis septentrionalis) are currently under consideration. For more information on endangered bat species and management recommendations, visit the U.S. Fish & Wildlife, Bloomingtion, IN Field Office website or review their timber management guidance document at: http://efotg.sc.egov.usda.gov/references/public/IN/BFO_Forest_Management_Guidelines2-14-08.pdf
Whether endangered or common, all bats benefit from healthy forests and studies such as the HEE show that low-impact forest management can provide quality habitat for a diverse assemblage of bats.
Timothy Divoll is a current PhD student at the Center for Bat Research, Outreach and Conservation located at Indiana State University.