"Avoiding mortality: timing prescribed burns in ornate box turtle habitat"
This open access article was published October 10, 2023, in Journal of Wildlife Management. Access the article through the permanent web address (DOI). (https://doi.org/10.1002/jwmg.22510)
Abstract
Fire is a vital management tool for maintaining prairie ecosystems. Prescribed burns control invasive species, regulate succession, stimulate plant growth, and are a cheap and effective method for removing excess biomass; however, fire can also inadvertently cause wildlife mortality, placing land managers in a challenging situation. Turtles are especially at risk of mortality from fire because of their low mobility and population sensitivity to reductions in adult survival. We studied ornate box turtles (Terrapene ornata) at 3 sites in Illinois, USA, from 2019–2022 to determine the best predictors of above-ground activity so land managers can conduct prescribed burns when turtles are underground. We used turtle shell temperature, air temperature, soil temperature, and precipitation data to develop a predictive model of above-ground activity. The best model for predicting above-ground activity included an interaction between day of year and current air temperature. Earlier in spring and later in fall, above-ground activity is more likely at higher air temperatures compared to later in spring and earlier in fall when the same likelihood of above-ground activity is predicted at lower air temperatures. In spring, we recommend burning in Illinois ornate box turtle habitat before 1 April when air temperature is <10°C and in fall after 1 November when air temperature is <15°C. Above these temperature thresholds, there is a >5% likelihood that turtles in northern populations are above ground.
Until the nineteenth century, fire was a major process shaping ecosystems throughout the Great Plains and midwestern United States (Axelrod 1985, Frost 1998). Today native grasslands in the region have mostly been converted to agriculture, with as little as 0.1% of historical tallgrass prairie remaining (Samson and Knopf 1994). Land managers commonly use fire to maintain and restore prairie ecosystems, control invasive species, remove biomass, regulate succession, and stimulate plant growth (Hulbert 1988, Brockway et al. 2002). While prescribed burns have many benefits, especially related to vegetation, they also inadvertently cause vertebrate mortality (Erwin and Stasiak 1979, Buchanan et al. 2021). Considering many species of wildlife inhabiting grasslands are now threatened and confined to isolated habitat fragments, land managers face a dilemma whereby they must use fire to maintain ecosystem health but risk causing mortality of threatened wildlife species from burning.
Varying fire intensity, speed, and timing are all strategies to avoid harming wildlife during prescribed burns. Less-intense fires moving into the wind (backfires) are slower and may offer time for wildlife to flee; however, backfires more completely burn the area, leaving little shelter or refuge for species with low mobility. Because fire speed and intensity do not affect all species equally, it is important for managers to consider local assemblages in an area and species' responses to fire (Smith et al. 2001, Sutton et al. 2013, Greenberg et al. 2019). For birds and mammals, often adults can escape fire, whereas their young cannot, and so burning before or after seasonal reproduction can reduce mortality risk. Similarly, for reptiles and amphibians sheltering underground during winter, burns can be targeted to coincide with their inactive period. However, early spring and late fall burns may not stimulate plant growth as well as summer burns (Copeland et al. 2002), and so decisions about the risk to threatened species from summer fires need to be balanced with any benefits to restoring or maintaining their habitat. For most reptiles and amphibians, we have an incomplete understanding of their basic life history and behavior, making it challenging to predict if a threatened species is above ground and at risk of mortality from fire on days when conditions are otherwise favorable for burning.
Turtles and tortoises are among the most threatened vertebrates, with 67.5% of assessed species at risk of extinction (International Union for Conservation of Nature [IUCN] 2022). They are also especially susceptible to mortality from fire because of their limited mobility and their slow life-history pace. Turtles typically have a long generation time, low fecundity, and high adult survival, meaning population persistence depends heavily on the survival of adults (Heppell 1998). When adult survival declines, so do populations. While some have argued reptile mortality resulting from prescribed burns is generally outweighed by the benefits of improving habitat quality (Russell et al. 1999), when turtle populations are small and isolated, any additive mortality to adults can be the difference between a stable and a declining population (Daigle and Jutras 2005, Howell and Seigel 2019). As such, it is imperative to only burn in the habitat of threatened turtle species during times of the year when animals are likely underground, in water, or otherwise at low risk of mortality.
North American box turtles (Terrapene spp.) are experiencing declining populations range-wide and occur in habitats where fire is often used as a prescriptive management tool. Several studies have examined the effects of prescribed burns on eastern box turtles (T. carolina), documenting injuries from burns, reduced annual survival in burned versus unburned areas, and recording a mass mortality event of 47 dead individuals after a summer burn (Howey and Roosenburg 2013, Roe et al. 2019, Buchanan et al. 2021, Cross et al. 2021). Eastern box turtles can survive prescribed burns if sheltering during winter dormancy and if they can take refuge in moist hardwood forests near streams (Roe et al. 2019, Harris et al. 2020, Roe and Bayles 2021). Similarly, no mortality was recorded in a Florida box turtle (T. c. bauri) population as long as burns were conducted during the dry season when turtles were dormant (Platt et al. 2010). Thus, determining the factors influencing when box turtles are active and susceptible to mortality from fire is important for guiding burn decisions.
Few studies have examined overwintering behavior in the ornate box turtle (T. ornata), a grassland species with a declining population trend (IUCN 2022). The species ranges from the Sonoran Desert northeast across the Great Plains and Midwest, with northern populations overwintering underground 6–7 months per year (Doroff and Keith 1990). In Missouri, USA, captive ornate box turtles maintained in outdoor pens emerged in spring following a 5-day warming period when the soil temperature reached 7°C (Grobman 1990). Fieldwork in Iowa, USA, did not support soil temperature triggering emergence (Bernstein and Black 2005). Instead, the authors proposed above-ground activity in spring is initiated by a combination of environmental factors, including air temperature and precipitation. In Illinois, USA, Milanovich et al. (2017) identified microhabitat characteristics of overwintering sites and the date range of winter ingress and egress, with turtles completely underground by 15 October and emerging in spring starting 10 April. Further research is warranted on ornate box turtle overwintering behavior considering the limited number of studies and their varied results.
Our objective was to determine the best predictor(s) for when ornate box turtles are above ground so land managers can avoid burning when turtles are at risk from fire. Based on previous studies, we hypothesized air temperature, soil temperature, and precipitation could be associated with above-ground activity. Additionally, we anticipated that the day of year and study site could influence the effects of environmental variables because temperature and precipitation vary seasonally and locally.
Management Implications
When conducting prescribed burns in the habitat of ornate box turtles, we recommend considering the day of year and the current air temperature to decide the risk of fire to turtles. In Illinois, by 1 March at 15°C, there is a 10% likelihood of a turtle being above ground. By 1 April at the same temperature, the predicted likelihood rises to 20%. In fall, on 1 October at 16°C, there is a 10% likelihood of a turtle moving above ground, which decreases to 6% at the same temperature on 1 November. Our results support burning in our study region earlier in spring and later in fall at cooler temperatures to avoid mortality. Still, we recognize land managers must balance a range of costs, benefits, and constraints when deciding the timing of prescribed burns. Historically, large, connected metapopulations of ornate box turtles should have been able to absorb incidental mortality from fire. Many turtle populations today are small, isolated, and likely declining. Thus, we recommend managers prioritize ornate box turtles when burning in their habitat to avoid mortality, especially of adults.
Citation
Edmonds, D. A., Bach, E. M., Colton, A. L., Jaquet, I. S., Kessler, E. J., and Dreslik, M. J.. 2023. Avoiding mortality: timing prescribed burns in ornate box turtle habitat. Journal of Wildlife Management e22510.
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