1. Introduction
Globally, wildfire regimes are changing, with increasingly longer seasons, mainly induced by climate and human activities [
1,
2]. Recent estimates obtained from remote sensing reveal that the area affected annually by wildfire ranges from 3.5 to 5 million km
2 [
3,
4]. In South America, a significant increase in burning and wildfire activity has been detected during the period 2001–2018, and a severe wildfire crisis strongly linked to deforestation and forest degradation was recorded in 2019 [
4].
Fire is a fundamental component of most of the Earth’s terrestrial ecosystems [
5], and the relationship between wildfire and biodiversity is complex [
6]. Biodiversity loss is associated with high frequency, large-scale intensity, or absence of fires [
7]. The new era of wildfire poses a global challenge in understanding how to maintain biodiversity [
8]. Fire is an important ecological factor for numerous species inhabiting some dry ecosystems, such as the Cerrado [
9,
10,
11], but for other ecosystems such as tropical rainforests, wildfire can lead to the destruction or loss of native species and habitats [
12]. In both cases, the response of these ecosystems when the seasonality or frequency of burning exceeds their resilience is poorly understood [
13].
Large-scale forest wildfires have become the main cause of forest degradation in Bolivia in the last two decades. These wildfires have occurred annually [
14], mainly in the lowlands, and have become more frequent and severe in recent years [
15]. Although wildfires originate mainly in areas that were recently deforested [
16], these spread to protected areas (national and subnational). The subjacent causes for the increase and severity of these are inadequate fire management in the
chaqueos (slash-and-burn agriculture to prepare land for planting), mechanized agriculture for commercial purposes, and inadequate pasture management for cattle ranching [
17,
18,
19,
20]. Between 2000 and 2015, the absolute area impacted by wildfires in the country was 16 million ha, of which 28% occurred in forested areas [
17].
Due to the combination of environmentally destructive agricultural–economic policies [
21] and the increasing severity of the droughts [
22], wildfires have increased in recent years. The burned area in 2019 was three times larger than in 2018 and exceeded the average of the 2001–2018 period by 51% [
4]. These events reopened the debate on the problem of wildfires in Bolivia, the impact they generate on biodiversity, and the potential strategies that could be developed for the effective restoration of impacted ecosystems.
In Bolivia, progress has been made in the development of spatial analyses to determine the impact of wildfires on biodiversity [
23,
24,
25,
26]. However, it is necessary to prioritize the evaluation of areas that have been identified as the most important biodiversity areas for species of conservation concern [
27]. The Key Biodiversity Areas (KBAs) are a global network of more than 16,000 sites that contribute significantly to the global persistence of biodiversity (
http://www.keybiodiversityareas.org/kba-data, accessed on 15 August 2021). These sites were identified using standardized criteria and quantitative thresholds [
28]. In Bolivia, there are 58 KBAs, most of which were initially identified as Important Bird Areas (IBAs), and they are distributed across all ecoregions of the country [
29,
30]. The pressure exerted on these sites by human activities threatens the survival of many species dependent on these areas.
In this study, we analyze the impact that wildfires have had on the geographic patterns of general bird species richness, endemic bird species richness, and threatened bird species richness between 2001 and 2020. The geographic scope of the present study is national, but we place particular emphasis on the impact of the wildfires on all Bolivian KBAs and the bird species that inhabit these areas of high biodiversity importance. We hope that the results presented here can serve as a basis for the development of strategies for immediate conservation and monitoring actions for those KBAs most threatened by wildfires.
4. Discussion
The patterns of bird species richness in Bolivia are consistent with the results found by Herzog et al. [
42], in the sense that the areas with the highest concentration of species are located in the Amazonia and mountain humid forest (Yungas) regions. The highest concentration of threatened species is found in the Yungas of La Paz and in the northeastern region of the department of Santa Cruz, where the transition from Amazonian rainforest to Chiquitano dry forest takes place. These regions should be prioritized for the establishment of effective protected areas. The Bajo Paraguá region (Chiquitano dry forest) that lies to the west of the KBA Parque Nacional Noel Kempff Mercado is a region that is experiencing very high levels of human pressure. Fortunately, a municipal protected area was declared in this area in 2021 to address increasing deforestation pressure and the construction of a national highway [
43].
The geographic pattern of endemic species richness in Bolivia indicates that the existing KBAs in the western Andean region of the country, especially in the department of La Paz, do not coincide with the geographic distributions of these species. This suggests that it is necessary to evaluate the location and extent of some KBA in this region of Bolivia.
Just under a half of the KBAs (44%) are found within national protected areas, and six (12%) overlap [
30]. In principle, this implies that at least part of the KBA’s protection should be guaranteed by the state. However, the protected area’s capacities to safeguard biodiversity within their territories needs to be improved significantly for at least three protected areas, where over 2.5 million ha were burnt at least once between 2001 and 2020 (KBAs San Matías, Otuquis and Noel Kempff Mercado). It is also important to highlight that most wildfires take place in the Bolivian lowlands, where the geographical overlapping between KBAs and protected areas is smaller compared with the Andean region of Bolivia. Finally, ten KBAs (5.1 million ha) do not overlap with any national protected area and are among the most vulnerable ones to wildfires. The protection and proper management of all KBAs can offer a major opportunity to combat biodiversity loss [
44].
All KBAs are a priority for conservation due to their unique biodiversity characteristics [
30]. However, there are not enough economic and human resources to effectively protect all KBAs, so it is necessary to prioritize them based on their biological value, vulnerability, and other social–economic criteria. Yepez et al. [
45] carried out prioritization of the IBAs (Important Bird Areas) in the Amazonia region and determined that 9 of the 13 IBAs in Bolivia are at a critical priority level. For most of these, Soria-Auza and Hennessey [
29] identified fire-associated activities among the pressures over these IBAs in Bolivia (including annual burns for pasture replacement, out-of-control grassland burns, grassland burns, burns for the renovation of cultivated areas, and arson). The only known study on the historical incidence of wildfires in IBAs was mentioned by Maillard et al. [
30]. These authors reported the highest concentration of fires to be located in San Matías (KBA 2). The results obtained in our study can serve, together with other environmental and socioeconomic variables, as a basis for updating a national prioritization list of KBAs.
Wildfires are more frequent and widespread in the Bolivian lowlands, and, consequently, their impacts are also far more extensive in the KBAs located in this part of the country. On the other hand, wildfires are less frequent in the Bolivian Andes, and therefore their impact on Andean KBAs are also less extensive. However, a much more detailed analysis needs to be conducted for range-restricted bird species, or birds that have very local and patchy distributions, whose key habitats (e.g., Polylepis forests and high Andean scrublands) are frequently affected by uncontrolled wildfires (e.g., Poospiza garleppi).
Controlled burns have been the most frequent agriculture practice to clear crops areas or renew pastures for many years [
16,
46]. These agriculture activities take place mostly between July and October [
15,
17,
20]. However, strategies used by farmers to control burns are not effective, and very frequently, burns initiate large-scale wildfires that affect millions of hectares every year [
15,
17,
19,
22]. These are causing a series of ecological, economic and social problems [
44]. The scenario is worrisome, especially for the Chiquitano dry forests, where wildfires have been more frequent in recent years. Devisscher et al. [
47] simulated possible future risk scenarios for the Chiquitano forest and estimated that until 2025, the probability of increased wildfire risk will be approximately up to 1.8 times more than the estimates for 2010.
For most dry ecosystems, fire is an important ecological force that plays a role in maintaining biodiversity [
48]. Generally, fire has beneficial effects for some species but detrimental effects for others [
8,
9,
12,
49,
50,
51]. Most vertebrates are able to escape from wildfires, but mortality rates due to starvation or predation while wandering around until new territories are established increase [
51,
52]. Loss of habitat, territories, and food [
49] provoked by fire might even become a driving force for the local extinction of some species [
53]. Species whose main habitat are forests are especially vulnerable, as these are less abundant and more sensitive to disturbance and habitat-specialists [
54,
55]. In Bolivia, very few studies have examined the impacts of fires on vertebrates. Some investigations carried out in the Chiquitano forest demonstrated that some groups (e.g., small mammals) increase in abundance or even species richness, while abundance and richness for other groups (e.g., birds) decrease [
54,
56]. The dry and humid forests in the north of the Santa Cruz department changed in structure and composition of tree species as a response to wildfires. These changes are related to plant species-specific abilities to survive and regenerate after wildfires [
47,
57,
58,
59,
60,
61]. All in all, it is evident that visible changes in habitat take place after wildfires affect forests, and it is still poorly understood whether forest structure and composition might return to its previous state (and how long might this process take). This raises the question of how IUCN vulnerable-to-extinction bird species such as
Tinamus tao,
Crax fasciolata,
Patagioenas subvinacea and
Hypocnemis ochrogyna might cope with new habitat conditions/characteristics.
Common sense might lead us to consider that threatened species inhabiting in ecoregions where fire plays a key role in ecosystem dynamics might not be at risk. However, the situation for these species is not straight. It is known that
Geositta poeciloptera (also Vulnerable of extinction) also inhabits grasslands regularly burnt [
62,
63], but the effect that changes in wildlife regimes (i.e., increasing in the frequency of fires) might have over this species remains to be studied. In the case of
Ara glaucogularis (Critically Endangered) that inhabits in the Llanos de Moxos, a tropical seasonally flooded savanna that contains small palm forest islands (key for this species), wildfires are key in the ecosystem dynamics of this savanna. However, the frequency of provoked fires during the XXI century is presumably higher than during the XX century and more extensive as well. Therefore, the higher frequency of fires might be affecting the habitat quality for this species or its capacity to recover. Something similar has been documented about the effect of bad cattle-management practices over palm-forest islands regeneration in the Llanos de Moxos [
64]. However, it remains to be evaluated the specific role of provoked fires within the whole package of bad cattle-management practices. A similar situation might be expected for
Anodorhynchus hyacinthinus (vulnerable to extinction) that in Bolivia inhabits in the Pantanal of San Matías (also a tropical seasonally flooded savanna), Cerrado, Mauritia palm stands and gallery forests.
Monitoring is an essential component of conservation and is necessary to identify threats to biodiversity [
65]. Monitoring land surface change has been one of the main uses of remote sensing over the past decades [
66], and the multiscale capability of remote sensors makes them particularly suitable for quantifying patterns of variation of wildfire impacts in space and time [
67]. However, it is a priority to continue field monitoring of bird populations, as they represent one of the best indicators to assess the state of the environment since it is a group sensitive to environmental changes and therefore useful to measure biodiversity trends [
44]. Future research should consider studies of bird populations in the field, in combination with remote sensing analyses, in order to understand how sensitive species respond to wildfire regimes at different temporal and spatial scales. In these analyses, it is important to assess fuel consumption and fire spread patterns, intensity (energy release), severity (ecosystem impact), frequency, and seasonality [
5]. These studies can help researchers and policymakers to guide actions in the restoration planning of fire-impacted ecosystems in KBAs.