Integrating colony counts with NABat acoustic data to reveal the true impacts of White-Nose Syndrome on northern long-eared bats

Science Center Objects

Bat Research

Research collaboration: Brian Reichert (FORT), Wayne Thogmartin (UMESC), Winifred Frick (Bat Conservation International), Tina Cheng (Bat Conservation International)

The northern long-eared bat (Myotis septentrionalis) was listed as Threatened on the Endangered Species Act in 2014 due to rapid declines in numbers of observed hibernating bats at winter roosting sites after the arrival of white-nose syndrome. At 69% of known hibernacula in the eastern United States, northern long-eared bats disappeared entirely within four years. However, some evidence from summer surveys (including acoustic sampling) suggest that remnant populations of northern long-eared bats could be persisting in these regions.

Populations may have shifted from hibernating in observable aggregations in subterranean habitats to hibernating in small and highly dispersed aggregations in other types of features to behaviorally respond to the risk and consequences of exposure to white-nose syndrome. If this hypothesis were correct, the decline in abundance observed in winter colony counts would overestimate the consequences of the disease on total population size. Alternatively, a fraction of the population may have never used subterranean habitats for hibernating and detections of northern long-eared bats in summer surveys represent a small remnant population of those individuals that exhibit different habitat selection preferences. Making sense of these seemingly conflicting patterns of species occurrence from acoustic surveys and population trends from winter colony counts is needed to understand the true population-level impacts of WNS on northern long-eared bats and determine effective strategies for their conservation.

The North American Bat monitoring program focuses status and trend determination on these two principal sources of data, colony counts conducted in winter and acoustic occurrences gathered largely in summer. Colony counts represent the best opportunity for depicting relative total population size of a species but are inadequate for depicting the range of these hibernating species because colonies are a spatially restricted subset of the occupied landscape. Acoustic data, conversely, describe the area over which a species occurs but provide only an oblique understanding of population size. Further, an important source of data we use to diagnose the precipitous declines in populations from white-nose syndrome, winter colony counts, are sparsely available in some regions of North America. A further difficulty in using only one of these sources of data for status and trend determination is that either source depicts conditions faced in only one season. Thus, when attempting to make sense of these data, in many cases we suffer from too few data and risk observing unintuitive and potentially contradictory patterns (e.g., northern long-eared bat). The objectives of this project are to (1) formally integrate these varied sources of data into a single population model using monitoring data collected on the northern-long eared bat as a case study, (2) implement the model as a core component in NABat status and trend online reporting and assessment tools, and (3) make the statistical modeling approach publicly available along with NABat monitoring data to stakeholders through an extension of the NABat R-package. The resulting models, tools, and applications will improve our understanding of the relative influence of WNS and other threats across the entire life cycle of the species, helping to pinpoint sensitive periods in a species annual cycle. And lastly, integrating these two sources of data would allow us to leverage sparse data that may otherwise be insufficient for inference and perhaps bring inferences sooner to bear than would have been possible with either data source alone.

Integrated full-annual-cycle population models have been used frequently in understanding status and trends of birds. In these depictions, they often have the advantage of integrating similar types of data across the seasons (e.g., North American Breeding Bird Survey route counts and Christmas Bird Counts). The key innovation we will reveal with our analyses is integration of colony counts with acoustic occurrences. Research in avian ecology provided a path for this integration by modeling avian species abundance from count surveys coupled with surveys of detections-nondetections. In doing so, location- and time-specific estimates of population gain and loss (through entangled survival and immigration) are available. An important assumption of this approach is that sites are visited multiple times over the course of a season; colony counts, by and large, have historically only been visited once per season. Thus, novel development here involves relaxing the assumption of repeated sampling for the colony counts by leaning on the repeated nature of the acoustic data (species detection-nondetections). This project increases the capacity of NABat to assess the impacts of WNS and the efficacy of management actions.

Roosting northern long-eared bat (Myotis septentrionalis)

Roosting northern long-eared bat (Myotis septentrionalis)

(Credit: Dave Thomas. Public domain.)