The distribution of plants and animals in a place depends on historical, ecological, and physiological factors. It varies according to the biotic and abiotic conditions present in the geographical space. It is a function of not only the fundamental niche (abiotic conditions) of the place, but also several elements such as the environmental tolerance, the presence or absence of species (pollinators, dispersers, competitors, predators, and others), and the possibilities of dispersion over time and from relevant original areas [1
]. Climate is a determining factor in the lifestyles and distribution of living beings, conditioning behavior, survival, the life cycle, reproduction, temporality, number of generations, distribution, and interspecific relationships [2
Anthropogenic climate change (CC) is a phenomenon that is altering temperature and precipitation patterns, which is affecting natural processes around the world. The need to mitigate damage and adapt to the new environment poses challenges and opportunities for various sectors [5
]. Due to global CC, between 20% and 30% of plant and animal species will face a higher risk of extinction, and a significant portion of endemic species will be extinct by 2050 [6
]. Habitats located on different environmental gradients will be subject to change, and biological populations will not be able to adapt to the speed of the climate change phenomenon. Slow-growing species will be replaced by fast-growing species, which will affect the composition of biodiversity [7
]. The increase in temperature is already causing modifications to the migration patterns of some insects. Already, some species that interact with vegetation have needed to expand their distribution as quickly as the flora does or change hosts [9
The oyamel fir (Abies religiosa
(Kunth) Schltdl. & Cham.) is a forest species that is native to the high mountains of Mexico and grows in well-drained soils of volcanic origin. It predominates in temperate climates with an average annual rainfall of greater than 1000 mm [10
]. At high elevations, it is found in pure stands, but it is sometimes mixed with Pinus montezumae
Lamb, P. hartwegii
(Lindl), and Pseudotsuga menziesii
(Beissn.) Franco. At low elevations, it is mixed with Quercus
spp., Alnus acuminata
Kunth 1817, Prunus serotina
(Cav.) McVaugh (1951), and Arbutus
]. The geographical distribution of oyamel fir forests is dispersed and localized. In most cases, the communities occur as isolated patches. Abies religiosa
forests are found in areas confined to hillsides that are usually protected from strong winds and intense radiation; they are also found in special microclimates formed by gullies or deep ravines. The largest forests are found in the Mexican Transverse Neovolcanic Axis, Sierra Madre del Sur, small regions of the Sierra Madre Occidental, and, to a lesser extent, in the Sierra Madre Oriental in Mexico [12
]. Ecologically, it is a species of great importance since it serves as the annual hibernation tree (in central Mexico) of the monarch butterfly (Danaus plexippus
Linnaeus, 1758) [7
The monarch butterfly (D. plexippus
) is one of the Lepidoptera that has attracted the attention of the scientific community due to the peculiarity of its migratory routes and hibernation sites [15
]. Following Mexico’s climatic characteristics, the monarch butterfly has historically migrated to its oyamel fir forests because the low temperature and high relative humidity are well suited to the hibernation season [7
]. Climate and weather play important but unpredictable roles in winter survival. Monarchs need cool weather to slow their metabolisms and extend their lipid reserves, but temperatures cannot fall too far. Mortality of monarchs has been found to occur at −8 and −15 °C [17
]. Besides, storms greatly decrease monarch survival by wetting the butterflies and increasing their risk of freezing [4
]. Therefore, forest quality is vital to monarchs. As fragmentation and degradation of the oyamel fir forests increases, the forests’ ability to create a suitable microclimate for the monarchs decreases.
In the last decade, oyamel fir forests within the Monarch Butterfly Biosphere Reserve (MBBR) have been under strong anthropogenic pressure due to illegal logging [13
], pest diseases [19
], and, recently, extreme weather factors such as torrential rains and droughts [17
], probably caused by climate change. The climate, to which A. religiosa
populations are adapted, is shifting. Therefore, the current forests are showing signs of decline [7
]; moreover, forest cover in the core zone of the MBBR has been reduced, and this has a negative impact on the numbers of monarch butterflies that overwinter successfully [13
]. Therefore, it is imperative to estimate areas that are potentially suitable for oyamel fir forests based on the new climate conditions.
Spatial modeling and niche ecological models combined with other analytical tools (e.g., GIS) have allowed current and future scenarios of the distribution of oyamel fir forests to be modeled [7
]. Recently, numerous modeling methods and tools have been developed [3
]. Therefore, the use of these alternatives accomplishes a double function. First, they provide knowledge about the potential distribution of the species to determine the richness and diversity of non-evaluated areas [28
]. Second, they use these predictions to choose sites of interest as biological conservation zones [29
]. The use of spatial modeling and niche ecological models has allowed us to predict potential areas for monarch butterfly hibernation outside of their regular distribution [7
Although almost all monarch butterflies that overwinter in Mexico are contained within the limits of the MBBR, some overwintering monarchs have been found beyond the protected areas [14
]. Within the past ten years, new monarch butterfly colonies have been registered outside of the regular wintering distribution, which confirms some of the predictions made about possible future distributions using spatial modeling [7
]. For instance, there are reports confirming the presence of colonies of monarch butterfly in the forests of the Sierra Nevada, Mexico State [32
], and in Real del Mar in Tijuana, Baja California [33
]. These new overwintering sites could be related to the felling of the oyamel fir forests and climate change [34
]. Therefore, it is important to characterize these reported sites to establish conservation strategies that aim to preserve the conditions that support the hibernation of monarch butterfly populations.
The objectives of the study were to recover information about the historical and new hibernation sites, reported or modeled, from different literature sources. We also aimed to perform bioclimatic and forest biometric characterization of new monarch butterfly colonies located in the Sierra Nevada of Mexico to provide information that aids conservation strategies for overwintering monarch butterflies.
The monarch butterfly is a fascinating research organism because of its peculiar migration pattern and its unique yearly life cycle. Recent concerns about the viability and conservation of the sanctuaries in Mexico have been pointed out [39
]. A statistically significant decline in monarch butterfly colonies has been documented [43
]. According to data available from the monitoring program in Mexico of the wintering of the monarch butterfly, 19 colonies have been registered as historical hibernation sites. However, it is worth noting that the number of colonies has been variable in each season, and only three of them have been consistently occupied [14
The variation in the number of colonies could be influenced by the composition and structure of the oyamel forests where the butterfly seeks its ecological niche due to their climatic and physiological characteristics, humidity, elevation, and exposure, which create unique environmental conditions suitable for its biology [18
]. Although there is no consensus on the explanation for the variation in the number of colonies in the reports, some studies have suggested that environmental conditions are changing, which could affect the presence of the butterflies, at least in the MBBR, in the future [7
Anthropogenic pressure over the biosphere reserve is causing the forest cover to diminish in a very accelerated manner. During 2001–2012, 1254 ha was deforested and 925 ha was degraded in the Monarch Butterfly Biosphere Reserve [39
]. Deforestation and forest degradation in the MBBR are factors that could cause populations of monarchs to look for new hibernation sites. Another big issue that has recently gained relevance is the land-use change within the reserve and around it because of the high demand and price of avocados driving an increase in plantations. Global Forest Watch (GFW) data show that forest clearing to establish avocado plantations has been pushed into the boundaries of the MBBR. Satellite images have revealed large areas with agricultural ponds, often with traces of burning, a distinct pattern indicative of avocado expansion [47
The existence of the new monarch butterfly colonies in Sierra Nevada could be related to the pressure suffered by their historical hibernation sites. Their establishment in different areas may be part of the seasonal migratory dynamics that existed during their journeys to their historical habitats, as indicated in the species migration map [33
]. It is worth noting that the colonies at the new locations are no more than 15 years old but have been consistently occupied according to local people from the Ejido San Miguel Atlautla.
The temperature in the colonies located in the mountains of Michoacán varies from 1 °C at night and early in the morning to 13 °C in the afternoon; during cloudy and rainy or snowy periods, the daytime temperature remains at an average of 10 °C [48
]. This temperature variation reported in Michoacán gives a view of a possible future scenario in Tlachanon and La Joya, where the minimum temperature in the period of hibernation reaches 0.8 °C, bringing a possibility of snowfall. During the hibernation period of the monarch butterfly, the temperature recorded in the Ejido San Miguel Atlautla had an average minimum of 0.8 °C and an average maximum of 17 °C. These values are very close to those indicated in other studies [31
], which pointed out that butterflies prefer temperatures that fluctuate between 3 and 18 °C.
Given the new scenarios that would arise as a result of climate change, the ideal environmental conditions for the development of oyamel fir forests may be found at higher elevations close to the highest volcanoes in Mexico [7
]. Our study may provide evidence supporting this hypothesis since the La Joya and Tlachanon sites are the first places where the monarch butterfly is migrating to in the oyamel fir forests at higher elevations.
One alternative proposed for future climatic conditions is assisted migration of fir forests [7
]. However; even if assisted migration from oyamel fir forests were to occur, it would have to be carried out in a planned manner, since the transition to a new climate pattern is an unpredictable way for the monarch butterfly to find its new ecological niche. Another option is the use of ecological niche modeling to predict or identify areas where environmental conditions are suitable for monarchs. Models predicting the presence of monarchs with considerable accuracy suggest that the ecological niche of the monarch butterfly is more widely distributed than the occurrence of known colonies [4
], as has been proved in this study. Therefore, future searching could locate new colonies and identify areas with suitable conditions for the species. Although species rarely inhabit the entire spatial distribution of their ecological niches [49
], our suggestion that additional suitable locations could exist outside the region of known wintering sites was one motivation for this study as well as future studies.
In this sense, the combination of spatial modeling and niche ecological models with other analytical tools (e.g., geographic information system (GIS)) offers an option for evaluating non-inventoried sites and modeling past and future scenarios of the distribution of species [19
]. Recently, numerous modeling methods and tools have been developed [3
], but they have mainly been used in ecology and biogeography [49
]. Therefore, the use of these alternatives accomplishes a double function. First, they provide knowledge about the potential distribution of the species to determine the richness and diversity of non-evaluated areas. Second, they can be used to predict ideal sites for biological conservation zones [29
]. These arguments represent a path and direction for future research. Finally, the study of the quality of the habitat (cover, sources of food, space, and water) in La Joya and Tlachanon is also worth highlighting as a research path.