The PESM is located north of Toluca city (19°20'00"N–19°17'47"N, 99°39'00"W–99°43'25"W), approximately 63 kilometers southwest of Mexico City (Fig. 1). The PESM ranges in elevation from 2,630 to 3,040 m above sea level (Sierra-Domínguez et al. 2018). The dominant climate is temperate subhumid, with summer rains. The dry season spans from November to May, and the rainy season spans from June to the end of October. The average annual rainfall from the nearest weather station (located less than 1 km away in the community of Calixtlahuaca) is 840 mm. The wettest month is July, with 167 mm of rain, whereas the driest month is December, with 9.6 mm of rain. Due to the high elevation, daily temperature fluctuations are very pronounced throughout the year. The annual average temperature oscillates between 4.9 and 22.3 °C (Servicio Meteorológico Nacional 2021).

PESM was declared a natural protected area in 1976 with an original extent of 394.96 ha (Gobierno del Estado de México 1976). In 1981, an additional area of 860.13 ha was incorporated into the park (Gobierno del Estado de México 1981), reaching 1,255 ha. When the natural protected area was originally declared, approximately 12.65 ha of human settlements (isolated buildings separated by cultivated terraces and backyards) were already present within the polygon. Since that time, the urban built-up area has expanded in the PESM as cultivated land and backyards were replaced by new buildings (Sierra-Domínguez et al. 2018).

The current landscape of the PESM consists of a small plain and multiple hills covering the municipalities of Toluca (98% of the PESM) and Zinacantepec (2%) (Olvera-Viscaino 2018). The hills originated during the Miocene and are formed by highly permeable volcanic rocks that favor water infiltration (Niño-Gutiérrez et al. 2007), thus playing an important hydrological role. In contrast, the soil composition of the plain has a high clay content, which favors the flooding of the land during the rainy season and the formation of lagoons. Most of the PESM is covered by grassland and shrubland ecosystems. The forest cover is composed of introduced species that form forest stands, mainly Cupressus lusitanica, Eucalyptus sp., and Pinus radiata, which are considered fire-prone species, particularly the latter two (McWethy et al. 2018). There are also native tree species in the area, such as Buddleja cordata, B. microphylla, wild black cherry (Prunus serotina), Mexican hawthorn (Crataegus mexicana), shrub oaks (such as Quercus frutex), and arboreal oaks (Q. mexicana and rugosa), but they mostly grow isolated rather than as forest stands, with the exception of shrub oaks (Sierra-Domínguez et al. 2018). Scientific information about the biodiversity of Sierra Morelos is scarce; to date, the records published in iNaturalist probably constitute the most reliable information on diversity in PESM and consist of 441 species of plants, 62 fungi, 542 arthropods, and 213 vertebrates. Fourteen reptile species have been reported in the area.

Of the 14 species reported for the PESM, five belong to the genus Thamnophis (Garter snakes, Colubridae family). The most common Thamnophis species in the PESM are T. scaliger (adult snout-vent length, SVL of 29–38 cm), T. eques (adult SVL of 39.5–92 cm), and T. scalaris (adult SVL of 34.5–50 cm) (Reguera et al. 2011; Manjarrez et al. 2007, 2014). They are slow-moving, and ground-dwelling species that can usually be found in grasslands and can use natural cavities or burrows on the ground. The lined Toluca earth snake (Conopsis lineata) is also a ground-dwelling snake from the Colubridae family that can be found in grasslands and forests (Sunny et al. 2023), usually under rocks. The Mexican dusky rattlesnake (Crotalus triseriatus) is also present in the area (Sunny et al. 2019; Rubio-Blanco et al. 2024) and can usually be found in bunchgrass, such as Muhlenbergia sp. Four reptile species present in the PESM belong to the Sceloporus genus (Phrynosomatidae family); the most common is the torquate lizard (S. torquatus), a lapidicolous species with an adult SVL of 8.4 to 9.7 (Ortiz et al. 2001). The graphic spiny lizard (S. grammicus) is smaller and much less abundant in the area and can usually be found in fallen logs. The Mexican plateau horned lizard (Phrynosoma orbiculare, also from the Phrynosomatidae family) is a lizard that typically reaches an adult SVL of 6.5 to 8.5 cm (Hernández-Navarrete 2018). This species is classified as threatened in Mexico under NOM-059-SEMARNAT-2010 and inhabits open areas and grasslands, using bunchgrass for perching and shelter (Gómez-Benítez et al. 2021). The transvolcanic alligator lizard (Barisia imbricata), which belongs to the Anguidae family, has adults with SVLs ranging from 9.38 to 9.7 cm (Woolrich-Piña et al. 2021). It is a ground-dwelling species commonly found among bunchgrasses (Sunny et al. 2017).

The methodology consisted of mapping the burned areas in 2021 and 2022 within the PESM and searching on the ground for live and dead reptiles throughout the entire extent of the burned areas after the fires occurred.

Assessing burned areas:

We sampled burned areas shortly after the fires occurred (from the same day up to three weeks; see Table 1). To assess the surveyed area, we recorded the perimeter of the sampled areas using GPS. Unburned areas within the burned area polygon were also registered and excluded. The area of the surveyed polygons was calculated using a Geographic Information System in UTM Zone 14 N, WGS84. To map all the other burned areas that we could not survey, Sentinel-2 satellite images were acquired at the end of the dry season in 2021 and 2022. We displayed the images in RGB colors in a Geographic Information System with the following band combinations: shortwave infrared (band 12: R), near-infrared (band 8: G), and green (band 3: B). We visually outlined the burned areas and calculated their extent. While we knew when the fire occurred at some sites, for others, the timing was uncertain. Thus, we used the Sentinel-2 satellite images to approximate the dates of the fires in cases of uncertainty. This approach allowed us to determine the time between the fire and the sampling.

Prescribed burned areas (conducted as a fire management program of the PESM) were differentiated from unplanned burned areas (e.g., man-made illegal or accidental burns) because the former were very similar in shape, extent, and location both years. In addition, we corroborated prescribed burns with local people and with the authorities of the PESM. The main objective of these prescribed burning practices was to preserve areas planted with non-native trees. While prescribed burning practices have been conducted in the PESM since at least 2013, in 2021 and 2022, the amount of burned area under this type of management increased considerably (from an average of 2.65 ha per year between 2013 and 2020 to 35.9 ha in 2021).

We carried out the surveys from the end of January to the beginning of April 2021 and in January 2022 between 09:30 h and 15:00 h (except on one occasion when the fire started as we were leaving, and we stayed to extinguish it and survey the burned area at approximately 16:00 h). The sampling was conducted on sunny days, which are most common during the dry season. Since most fires occurred in grasslands, the majority of the surveys were conducted in this ecosystem. Only sites 12 and 14 were extensively covered by trees (cedars and eucalyptus, respectively).

Usually, during the first two weeks following a fire (but in some cases, up to three weeks), we sampled the entire burned area in search of reptiles. The burned area was divided among two to four people with experience in surveying herpetofauna. We walked across the entire area in a zigzag pattern, maintaining a distance of approximately two meters between passes to ensure that no areas were overlooked. In large burned areas, we needed to conduct surveys over several days. In 2021, the surveys were conducted over 18 days (approximately 100 hours; equivalent to 300 person-hours), and in 2022, they were conducted over four days (approximately 22 hours; equivalent to 66 person-hours).

When we found a reptile in the burned area, we identified the species, noted whether it was dead or alive, took a photograph, recorded the coordinates using GPS, and documented the type of habitat where it was found. When possible, we displaced live and/or dead individuals outside the burned area to avoid counting them twice. Carcasses were not collected because they represent a food resource for predators. Live animals were released near the edges of burned areas so that they could avoid short- and medium-term fire effects but could eventually return to the area. When needed, we poured clean water over the survivors to cool them and remove the ashes. The density of carcasses was obtained by dividing the number of carcasses by the area surveyed (the area recorded with GPS). Animals that died from a cause other than fire were excluded from the analyses and tables presented here. For example, one Sceloporus torquatus and one Thamnophis sp. were excluded because they seemed to have been crushed.

We conducted a Mann-Whitney U test to compare the density of dead individuals in prescribed burned areas versus unplanned burned areas. Finally, we assessed the relationship between the density of dead reptiles and burn shape (area/perimeter).

A substantial proportion (more than 30%) of all reptiles found in burned areas between 2021 and 2022 were dead. Although the surveys were conducted on sunny days, minor changes in climatic conditions during the samplings should be considered a potential source of bias in the relationship between live and dead animals. Additionally, our findings should be interpreted with caution, as some carcasses may have been eaten by predators/scavengers, and some survivors may have abandoned the site, especially fast-moving species such as S. torquatus. However, this seems more difficult for slow-moving reptiles such as P. orbiculare or Thamnophis sp., which were often found in bad physical conditions, e.g., slower than normal, dehydrated, and stained with ash.

The density of dead reptiles found in our study area is lower than that of other studies, such as in the Pantanal wetland in Brazil, where Tomas et al. (2021) estimated 2.48 dead small reptiles/ha—a value described as astonishing by the authors—and Lapalala wilderness, South Africa (7.77 dead reptiles/ha; Jordaan et al. 2019). Nevertheless, the biodiversity in the PESM is much lower than the biodiversity at these study sites. For example, the Pantanal is the largest wetland in the world, with 113 reptile species reported (including more than 30 species of snakes; Alho 2008), whereas the PESM is a small natural protected area (1,255 hectares) with a low level of protection, situated within a densely populated city and heavily impacted by anthropogenic activities (exotic forest plantations, water pollution, illegal garbage dumps). In the context of lower and more vulnerable biodiversity, the mortality densities found at our study site may alert us to the serious risk that anthropogenic fires can pose to native herpetofauna in fragile ecosystems. At least, our findings contrast with those of several studies that report direct fire-induced mortality in reptiles as rare, negligible, or nonexistent (Costa et al. 2013; Harper et al. 2016; Greenberg et al. 2018; Certini et al. 2021; Jolly et al. 2022). Our findings are rather in line with studies that have already warned about the impacts of fire on reptile populations (Popgeorgiev 2008; Smith et al. 2012; Abom and Schwarzkopf 2016; Jordaan et al. 2019, 2020; Santos et al. 2022; Ballouard et al. 2023) and on vertebrates in general (Berlinck et al. 2021; Tomas et al. 2021).

We consider that the mortality densities obtained can be highly underestimated: carcasses are difficult to detect within a burned area, some may be covered by ashes, and some may have been completely calcined (Fig. 3). Some initial survivors, especially those that were injured, may have died after the survey due to wounds or indirect fire effects, and some animals may have died underground or in other refugia; inhalation of hot or toxic gasses produced by fire has been suggested as a source of mortality for snakes that have fled underground (Durbian 2006), making the mortality of these animals nearly impossible to determine. Additionally, surveys were conducted by area (searching across the entire area in a zigzag pattern) rather than by transect, which leaves the possibility that some sites in the largest burned areas were not surveyed. Moreover, it was not always possible to sample the areas during the first week after the fire, extending into the second and, in some cases, the third week; thus, some carcasses may have been eaten by predators/scavengers. For example, Ballouard et al. (2023) reported the disappearance of 20% of carcasses during the second week after a fire. Thus, the overall impact on reptile communities should be considered higher than our counts. Even if mortality by fire has been underestimated in our study, our results still show the major deleterious effects of fire on the reptile community of the PESM.

Phrynosoma orbiculare was the most common carcass found. The high number of carcasses can be attributed to the abundance of this species in certain areas of the natural protected area and its slow movement (Presch 1969), which makes it vulnerable to fire. Fair and Henke (1997) also expressed concerns about fire-induced mortality in Texas horned lizards (Phrynosoma cornutum), and other slow-moving lizard species, such as the Australian thorny devil (Moloch horridus), have also been reported dead after fire (Smith et al. 2012). The density of P. orbiculare carcasses found in 2021 (0.17 ind./ha, mean = 0.30 ind./ha ± 0.51) is similar to that reported for other slow-moving species, such as Testudo hermanni (0.19 ind./ha) (Ballouard et al. 2023). In contrast, S. torquatus, another lizard of the Phrynosomatidae family that is similar in size and very abundant in the area, experienced markedly lower mortality. This is likely due to its greater speed and ability as a lapidicolous species, enabling it to climb logs, shrubs, agaves, and rock walls.

Thamnophis sp. carcasses were also among the most commonly found. Although they can retreat to underground burrows, ground-dwelling and slow-moving species are more vulnerable to fire than lapidicolous species. Other studies have reported severe fire-induced mortality in snakes (Durbian 2006; Lyet et al. 2009; Cross 2015). For example, small snakes represented 55% of the dead vertebrates recorded in wildfires that hit the Brazilian portion of the Pantanal wetland in 2020 (Tomas et al. 2021). Thamnophis sp. were the only species among which some dead individuals (three) were found without signs of burns. These individuals also did not exhibit any other signs of visible damage and were found on a completely burned surface (Fig. 3). A study carried out in the PESM reported that T. scalaris can retreat in underground hollows or burrows deeper than one meter (Mundo-Hernández et al. 2017), and studies with better-known Thamnophis species from other regions, such as T. sirtalis, reported that they use two-meter-deep hollows (Shine and Mason 2004). The dead Thamnophis individuals found in our study with no visible burns may have been below ground during the fire and came out after the flames consumed the vegetation and passed by; however, they probably died as a consequence of smoke inhalation or noxious gas inhalation. This is consistent with the findings of other studies on the effects of fire on fossorial reptiles, where dead reptiles were found on the soil surface, some in positions suggesting these animals attempted to reach the surface (Jordaan et al. 2020). Such observations have led to the hypothesis that burrowing reptiles are not necessarily protected from fire effects (Engstrom 2010). Deaths by smoke inhalation and inhalation of noxious gases produced during burning in association with anoxia have been reported for snakes (Durbian 2006; Lyet et al. 2009; Jordaan and Steyl 2023) and other reptiles (Jordaan et al. 2019; Jordaan and Steyl 2023). Finally, we found a relatively high number of B. imbricata carcasses (10), considering that this species is not very abundant in the PESM, which suggests that this ground-dwelling species may also be highly vulnerable to fire.

Site 13 had the highest number of dead reptiles (20 individuals). The fire was caused by stubble burning, which went out of control. However, the mortality density at this site was lower than in other sites. This may be because the zigzag survey pattern is insufficiently thorough for such a large area. The highest mortality density was found at site 8, which was a firebreak located alongside a canal that carries water during the wet season. We found three Thamnophis sp. carcasses in this small firebreak. This may be due to the greater density of Thamnophis near the canal than in other areas without water bodies. We found no relationship between carcass density and fire shape. This could be attributed to the limitations of the simple measure (area/perimeter), which does not account for landscape elements such as the presence and distribution of rocky areas, agave plants, and other potential refuges. These elements could serve as more effective indicators for explaining variations in mortality density across sites. For example, Hale et al. (2022) and Robinson et al. (2013) found that the presence of refugia in burned areas is a key factor that explains wildlife resistance to fire. Other variables not measured in this study, such as wind direction and speed during the fire, as well as the severity of the fire, are likely to have also impacted mortality rates at each study site. Slower and lower-intensity fires would have provided greater opportunities for animal survival.

Prescribed burning is the intentional use of fire under specific environmental conditions (e.g., during the dormant season) to achieve various land management goals. It has been reported as an appropriate management tool to benefit herpetofauna by restoring a historical mosaic of successional stages (Russell et al. 1999) and by generating more open habitats with greater insolation (Matthews et al. 2010). However, our study questions these views. We report no statistically significant differences in the densities of dead individuals between unplanned and prescribed burned areas. This may be because, in our study, both prescribed and unplanned fires occurred during the same season (the fire-prone season), and it is likely that the fire severities were similar. As evidence, an individual of P. orbiculare was found completely calcinated in a prescribed burned area.

Some studies suggest that prescribed fire effects on herpetofauna can be reduced by conducting burning practices during winter, when many terrestrial herpetofaunal species are inactive and are likely below ground or otherwise insulated from high temperatures and the potential desiccating effects of fire (Harper et al. 2016). However, in our study, all the carcasses found in the prescribed burned areas were from fires conducted during the cold and dry seasons (January and February). We must consider that although the climate of our study site is classified as temperate, it exhibits important differences compared to temperate climates at higher latitudes. At night, temperatures can drop below zero degrees Celsius, but during the hottest hours of the day, they can reach more than twenty degrees. Fires occurring when reptiles are less active can cause less mortality only if inactive reptiles are protected from fire. At our study site, reptiles are less active during the cold and dry seasons but are not completely inactive, and they are not necessarily hidden at a sufficient depth as in colder climates. For example, when P. orbiculare is less active, it shelters under bunchgrass or leaf litter (Gómez-Benítez et al. 2021), increasing its vulnerability to fire compared to when it is active (e.g., in areas with less vegetation coverage, looking for ants).

On the basis of our findings, we seriously question prescribed burning practices in the PESM and in similar ecosystems of the region, e.g., grasslands that share a similar reptile community composition. Prescribed burning has been carried out in the same grassland areas of the PESM during consecutive years, causing deaths in the same reptile populations, with the main objective of preserving areas with planted trees that are not native and, moreover, fire-dependent (which is related to the promotion of tree planting in the country as a “green” policy that does not adhere to ecological criteria, stemming from the misconception that grasslands or shrublands are less important than forests). Whelan (2002) affirms that populations of many species are less threatened by a single high-intensity fire event than by a sequence of lower-intensity fires in close succession. In the study by Abom and Schwarzkopf (2016), reptile numbers in native grass were found to decline after fire and failed to return to previous numbers with revegetation even in extremely fire-prone, often-burned environments, suggesting that longer periods without burning may be beneficial to reptile assemblages. Furthermore, annual fires disrupt the natural succession of vegetation and hinder regrowth, even for fire-adapted species such as native Quercus populations (Trejo 2008).

In the PESM, unplanned fires (e.g., man-made illegal and accidental burns) continue to occur every year despite prescribed burning practices. Unplanned fires are related to a variety of factors, including the use of fire in agriculture and grazing, the practice of trash burning in the vicinity of precarious settlements, the presence of remaining campfires, and the disposal of cigarettes. The practice of systematic ‘weed clearance’ in natural and semi-natural areas around settlements has led to burning every vacant lot, roadside, and land between crops every year, and is likely the main contributor to unplanned fires in the region, as these fires often spread out of control. Both planned and unplanned fires may have a cumulative impact on the most affected reptile species, such as P. orbiculare (which is an endangered species in Mexico), Thamnophis sp. (which also suffer from direct killing by humans), and B. imbricata (which is less abundant in the natural protected area). The recovery of these populations will also be hindered by the fact that the PESM is a small natural protected area surrounded by urbanized land and agricultural fields and disconnected from other natural environments. This situation can potentially affect reptile diversity and the community composition of PESM in the long term.

Stopping prescribed burns as part of the PESM management program is important. However, if we aim to protect these reptile species, it is also necessary to halt the unplanned fires that have been observed to burn a greater proportion of the natural protected area annually.

More systematic surveys that quantify direct fire-induced reptile mortality and reptile densities before and after the fires in the PESM and similar ecosystems in the region may allow for better direct comparisons of mortality rates and the direct impacts of fires on reptile populations. With the objective of lowering the occurrence and extent of anthropogenic fires and preserving the native biodiversity in the PESM, we suggest several actions be considered. 1) Maintaining a well-distributed bike path network in the area, as bike paths can act as firebreaks. 2) Rotating the existing small-scale livestock farming, as it can reduce fuel accumulation while reducing grazing pressure in specific areas of the PESM. 3) Progressively replacing exotic and flammable plantations of Pinus radiata and eucalyptus with native vegetation, as the resins of these commonly planted species are highly flammable, increasing the likelihood and rate of wildfire propagation (McWethy et al. 2018). 4) Implementing a collaborative network and awareness campaign to enhance vigilance across multiple communities during the dry season. 5) Restoring biophysical structures in the landscape, such as agaves, which serve as refuges for some reptiles during fires. 6) Conducting active postfire searches for reptiles, pouring some water over them, and relocating survivors to safer areas outside the burned zone.