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RESEARCH ARTICLE
Standard deviation: Standardized bat
monitoring techniques work better in some
ecosystems
Danny HaelewatersID
1,2,3,4
*, Morgan Hughes
3,5,6
, Jose
´Anto
´nio Lemos Barão-No
´brega
3,7
,
Kathy Slater
3,7
, Thomas Edward Martin
3,8
1Department of Ecology and Evolutionary Biology, University of Colorado Boulder, Boulder, Colorado,
United States of America, 2Faculty of Science, University of South Bohemia, Česke
´Budějovice, Czech
Republic, 3Operation Wallacea Ltd, Old Bolingbroke, Spilsby, Lincolnshire, United Kingdom, 4Biology
Centre of the Czech Academy of Sciences, Institute of Entomology, Česke
´Budějovice, Czech Republic,
5Department of Ecology and Environmental Science, UmeåUniversity, Umeå, Sweden, 6Faculty of
Science and Engineering, University of Wolverhampton, Wolverhampton, United Kingdom, 7rePLANET Ltd,
Old Bolingbroke, Spilsby, Lincolnshire, United Kingdom, 8School of Natural Sciences, College of
Environmental Sciences and Engineering, Bangor University, Bangor, United Kingdom
*danny.haelewaters@gmail.com
Abstract
Standardized monitoring strategies are often used to study spatial and temporal ecological
patterns and trends. Such approaches are applied for many study taxa, including bats
(Mammalia, Chiroptera). However, local characteristics of individual field sites, including
species assemblages, terrain, climatic factors, and presence or lack of landscape features,
may affect the efficacy of these standardized surveys. In this paper, we completed mist-net-
ting surveys for bats in two widely separated field sites, Calakmul Biosphere Reserve
(CBR), a Mexican lowland tropical forest, and Krka National Park (KNP), a Mediterranean
dry scrub forest in Croatia. Standardized surveys were conducted along predefined tran-
sects for six hours. We also completed targeted surveys in KNP that focused on the key bat
activity period (the first two to three hours after sunset), with nets being deployed at sites of
known or assumed value to bats (independent of predefined transects). We analyzed how
survey success differed in standardized surveys between CBR and KNP and between stan-
dardized and targeted surveys in KNP. Survey success was measured through three
parameters: capture rate = the number of individual bats captured per net hour, inventory
rate = the number of unique bat species recorded per net hour, and inventory efficacy = the
percentage of known species assemblage recorded per net hour across all surveys. Results
for all three parameters indicate that standardized surveys in CBR were vastly more effec-
tive than those in KNP (e.g., mist-netting in CBR detected 69.8% of the species assem-
blage, compared to just 8.3% in KNP), and it was only by employing targeted mist-netting in
KNP that meaningful capture rates could be achieved. This study contributes further evi-
dence to discussions around how and when standardized survey methods should be
employed, and the alternative approaches that can be taken in ecosystems where generally
effective methods underperform.
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OPEN ACCESS
Citation: Haelewaters D, Hughes M, Barão-
No
´brega JAL, Slater K, Martin TE (2024) Standard
deviation: Standardized bat monitoring techniques
work better in some ecosystems. PLoS ONE
19(12): e0311553. https://doi.org/10.1371/journal.
pone.0311553
Editor: Lyi Mingyang, Institute of Geographic
Sciences and Natural Resources Research Chinese
Academy of Sciences, CHINA
Received: July 8, 2024
Accepted: September 22, 2024
Published: December 12, 2024
Peer Review History: PLOS recognizes the
benefits of transparency in the peer review
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all of the content of peer review and author
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editorial history of this article is available here:
https://doi.org/10.1371/journal.pone.0311553
Copyright: ©2024 Haelewaters et al. This is an
open access article distributed under the terms of
the Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the manuscript and its Supporting
Information files (in particular S1 Table).
Introduction
Long-term monitoring is vital for studying ecological patterns and trends on a global scale [1–
3]. Standardized monitoring strategies are often recommended to generate the data necessary
to evaluate changes in richness, abundance, and distribution of species, to increase the explan-
atory power of ecological drivers and threats, and to develop conservation strategies [4,5].
Standardized sampling techniques are used in many different fields, including canopy biology
[6,7], entomology [8], mycology [9], ornithology [10], mammalogy [11,12], and parasitology
[13]. However, an underlying assumption of all these standardized survey methods is that they
provide comparable results in all ecosystems.
Assumptions regarding uniform performance of standardized survey methods may be
problematic, given that variability in ecological and geographical factors offer challenges and
present obstacles to standardized survey efficacy [14,15]. Different species assemblages may
possess different traits that influence their susceptibility to detection [16]. Similarly, terrain
may affect the type of equipment that can be used during surveys, climatic factors and season-
ality may influence survey effectiveness, and the presence or lack of features attractive to the
survey taxa can heavily influence the likelihood of species encounter. For example, Martin and
colleagues [10] showed for birds that local characteristics of individual sites significantly influ-
ence the efficacy of standardized surveys. To overcome these limitations, bespoke approaches
may be more effective. Here we explore the effectiveness of standardized surveys for describing
biological communities across multiple sites in another group of organisms: bats (Mammalia,
Chiroptera).
With regard to bats, whilst surveillance using passive methods (e.g., acoustic monitoring,
infra-red/thermal technology, roost emergence counts) is effective in many situations for
meeting the needs of some studies [17,18], the capture of bats is required to accurately identify
cryptic species, to determine demographic and breeding parameters of a sampled population,
to obtain samples (DNA, chemical samples, fecal samples, ectoparasites), and to facilitate radio
telemetry studies [19–22]. Standard and bespoke equipment exists to facilitate the capture of
bats, with mist nets in particular (as opposed to harp traps) being widely used in remote loca-
tions owing to their versatility, portability, and affordability [21].
Standardized mist-netting techniques for bats typically utilize mist nets deployed with set
spacing for set periods. The placement of nets often focuses on landscape characteristics of
interest to bats, including water features (riparian and lentic), roosting sites, and linear fea-
tures, such as woodland edges [23,24]. Survey designs aiming to compare sites will standardize
the amount and placement of equipment as well as the timing and duration of deployment.
Best practices and guidelines for standardized mist-netting for bats are presented by Kunz and
Kurta [23], Barlow [24], Walsh and Catto [19], Battersby [21], and Collins [20]. Survey effort
for mist-netting has been explored in further detail by Weller and Lee [25] and Hughes and
colleagues [26].
In this study, we explored the effectiveness of standardized mist-netting in ecologically and
structurally dissimilar ecosystems. We compared the number of individual bats and unique
species captured as proxies for trapping efficacy, as well as the proportion of the known bat
species assemblage captured based on available data from each site. We also compared the
effectiveness of standardized mist-netting with more bespoke, targeted approaches to mist-
netting. This will allow for the first quantitative assessment of the performance of standardized
mist-netting as a monitoring tool between widely separated and ecologically dissimilar study
sites. We formulated two hypotheses: (1) standardized mist-netting is more effective for cap-
turing bats in a landscape with greater heterogeneity and abundant features suitable for com-
muting (e.g., rivers, forest edges) than in a homogenous landscape; and (2) in homogenous
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Funding: DH acknowledges support from the
Systematics Research Fund of the Linnean Society
of London and the Systematics Association. The
funders had no role in study design, data collection
and analysis, decision to publish, or preparation of
the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
landscapes, targeted surveys result in greater efficacy in capturing bats compared to standard-
ized surveys.
Materials and methods
Ethics and permits
The Ethical Committee for Animal Experimentation at the Faculty of Science of Ghent Uni-
versity approved all capture and sampling procedures under reference EC2021-062 (license
number LA1400452). Fieldwork in Croatia was supported by the Croatian Ministry of Envi-
ronment and Energy (research permit KLASA: UP/I-612-07/20-48/138, URBROJ: 517-05-1-1-
20-4, 16.09.2020). Fieldwork in Mexico was supported by the Secretarı
´a de Medio Ambiente y
Recursos Naturales and Comisio
´n Nacional de A
´reas Naturales Protegidas (research permit
SGPA/DGVS/03535/20).
Description of study sites
Study sites comprised lowland deciduous tropical forest in Mexico and Mediterranean dry
scrub-dominated vegetation in Croatia (Fig 1).
Calakmul Biosphere Reserve (CBR) in Mexico (18.60583 N 89.94444 W) is a large (723,000
ha) expanse of lowland deciduous tropical forest in the southern portion of the Yucata
´n Penin-
sula (Campeche). CBR is part of the Selva Maya that spans over 10.6 million ha in Mexico,
Guatemala, and Belize, making it the largest continuous tract of tropical forest in Mesoamerica
[27]. The southern Yucata
´n Peninsula is characterized by a warm, sub-humid climate with a
mean annual temperature of 24.6˚C. A precipitation ecocline goes from the northwest (ca. 900
mm) to the southeast (ca. 1400 mm) of the reserve [27], over the 120 km from the north of the
reserve to the Guatemalan border [28], significantly influencing forest structure and tree spe-
cies composition [29,30]. To date, the species list for bats within the Calakmul study site com-
prises 53 extant species of which 13 are Phyllostominae, 13 are Stenodermatinae, eight are
Vespertilionidae, five are Molossidae, with three species each of Carollinae and Mormoopidae,
and two species each of Desmodontinae, Emballonuridae, Glossophaginae, and Natalidae [31].
This list includes recently confirmed records of Gardnerycteris crenulatum,Glossophaga com-
missarisi,Micronycteris minuta, and Trinycteris nicefori (Ivan Samayoa Gomez, Fabia
´n Mora,
Gabriel Oviedo, Kathy Slater, unpubl. data) (S1 Table).
Krka National Park (KNP) in Croatia (43.943109 N 15.991765 E) is in an ecotone between
the evergreen Mediterranean and the sub-Mediterranean deciduous vegetation zones. The
study area predominantly comprises rough Mediterranean scrub characterized by Carpinus
betulus,Juniperus oxycedrus, and Quercus cerris. The region experiences a hot-summer climate
with temperatures between 5˚C in January and 23˚C in July. Precipitation averages 1,078 mm
per year and falls largely in autumn and winter months [32]. The bat species list for KNP com-
prises 24 extant species of which 18 are Vespertilionidae, four are Rhinolophidae, and one spe-
cies each of Miniopteridae and Molossidae [33,34]. This list includes recently confirmed
records of Myotis aurascens and Myotis daubentonii [35] (S1 Table).
Standardized surveys
Standardized mist-netting involved the deployment of a set length (site-specific) of 36-mm
mesh mist nets during each survey along predefined transects to represent different habitat
types based on botanical assemblage. GPS locations of each net were recorded. At both sites,
mist-netting was conducted for six hours, starting before or shortly after sunset. Nets were
checked every 15 to 30 minutes. Surveys were not carried out during heavy or prolonged rain,
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Fig 1. Map of study sites with inset photos showing main vegetation characteristics. A. Krka National Park in Croatia. B.
Calakmul Biosphere Reserve in Mexico.
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nor with heavy wind [36,37]. No acoustic lures were used for any surveys. Surveys were car-
ried out in the summers (June to August) of 2014 and 2021, at CBR and KNP, respectively.
Targeted surveys
Targeted surveys (KNP only) broadly followed the same approach as standardized surveys.
However, nets were deployed at sites of known or assumed value to bats (e.g., confirmed
roosts, mines, caves, water bodies, and riparian corridors) [24], independent of the predefined
transects used for the standardized surveys. To facilitate capture of bats at these targeted sites,
bespoke methods were used, including the use of a suspended net over a water body [38] and
the utilization of a raised net at a mine entrance. Targeted surveys took place for only two to
three hours after sunset, as their focus was on emerging bats and post-emergence commuting
and drinking/foraging activities. Targeted surveys also did not utilize acoustic lures.
Processing of bats
Bats were extracted from nets and held in capture bags until processed as per standard proto-
cols [24,39]. Processing of bats included the identification of species; assessment of sex, age
class, and breeding condition of each individual; and the recording of morphometric measure-
ments (body mass and forearm length). In the case of cryptic species, additional measurements
(e.g., tragus width, length of tibia) were taken as appropriate. Each bat was marked by fur-clip-
ping or wing punch to ensure that re-captured bats were immediately recognized and released
without being subject to repeat processing.
Quantification of survey effort
As the deployment of nets varied in length among sites, and the length of surveys also varied
according to survey type, survey effort was quantified in net hours (NH), defined as 12 m of
net deployed for 1 hour [25,40]. This facilitated the comparison of survey efficacy among sites
and between methods.
Statisical analyses
We examined two measures of survey success: capture rate (hereafter “CR”), being the number
of individual bats captured per NH, and inventory rate (hereafter “IR”), being the number of
unique bat species recorded per NH. To remove any inherent bias caused by the fact that CBR
has a larger bat species assemblage than KNP (53 and 24 species, respectively), we used a third
measure of success as inventory efficacy (hereafter “IE”), being the percentage of known spe-
cies assemblage recorded per NH across all surveys.
We ran a pairwise comparison using Mann-Whitney U tests to explore significant differ-
ences in capture rates between the countries. For the KNP data, we also ran Mann-Whitney U
tests to determine if the CR, IR, and IE values for standardized surveys were statistically differ-
ent to those of targeted surveys. All analyses were done in the R language and environment for
statistical computing [41]. Figures were produced using ggplot() implemented in the R pack-
age ggplot2 [42].
Results
The total number of bat individuals captured during standardized surveys in both countries
was 1,837, comprising 39 species. CBR represented the greater abundance and species rich-
ness, with 1,786 individual bats captured representing 37 species. KNP yielded a meager four
individual bats representing two species. When adjusted to account for survey effort in NH,
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the CR values for standardized surveys in CBR and KNP were 1.93 and 0.02, respectively. IR
values for CBR and KNP standardized surveys were 0.04 and 0.01, respectively. IE values for
standardized surveys at CBR and KNP were 0.08% and 0.04%, respectively. The targeted sur-
vey data from KNP comprised 47 individual bats of nine species, representing a CR of 1.52, an
IR of 0.29, and an IE of 1.21%. These results are also comparatively shown presented in
Table 1. Mann-Whitney U tests (Fig 2) showed significant differences between standardized
surveys in KNP and CBR in CR (W = 6, p<0.001), IR (W = 6, p<0.001), and IE (W = 6,
p<0.001) values. Within our comparative standardized versus targeted netting datasets for
KNP, our Mann-Whitney U tests showed that there was a significant difference between CR
values (W = 6, p<0.002), IR values (W = 6, p= 0.002), and IE values (W = 6, p= 0.002),
respectively (Fig 3).
Discussion
Results demonstrate that standardized surveys in CBR were much more successful compared
to standardized surveys in KNP. This was the case for all parameters examined: capture rate,
inventory rate, and inventory efficacy. In contrast, the targeted surveys in KNP yielded the
highest species inventory rate and species inventory efficacy overall—even higher than those
of standardized surveys in CBR. The key to the interpretation of these data lies in the landscape
of the study sites included here. A recent paper that compared ornithological survey methods
between CNP and lowland forests of Buton Forest Reserves (Buton Island, Indonesia) also
found that their efficacy is affected by habitat structure, in addition to composition of local
bird communities [10].
We identified three limitations that are associated with our study. First, the number of
study sites is limited (two sites: CBR and KNP). This is in part due to the fact that most stan-
dardized bat surveys use methodologies that cannot be compared one-to-one. Second, the
method selected for our standardized and targeted surveys (i.e., mist-netting) is more likely to
capture species and feeding guilds that are more likely to fly within 5 m above ground-level.
This excludes high-flying foraging bat species (e.g., in the genera Eptesicus,Nyctalus,Tadar-
ida). While this can be compensated for using acoustic lures [43], these were not employed in
any of our surveys. Finally, our site inventories of known species assemblage may not be 100%
complete. If an area is relatively understudied, fewer of the species occurring in it will have
been recorded, particularly acoustically cryptic species [44,45]. As a result, with understudied
sites, the calculations of survey inventory rate may be overestimated. This factor is, however,
unlikely to be a significant factor in our results as multiple years of mist-netting were under-
taken at the studied sites, and both sites are ecologically fairly well-studied. In addition, the
comparison of standardized versus targeted surveys in KNP is not affected by this factor.
Regardless of these limiting factors, the results of our study are very clear; standardized
mist-netting in CBR vastly outperforms the same standardized methods in KNP across all
Table 1. Survey success of standardized and targeted surveys in Calakmul Biosphere Reserve (CBR), Mexico and Krka National Park (KNP), Croatia. Columns indi-
cate: number of net hours (NH, 1 NH = 12 m of mist net deployed for 1 hour), number of bats captured (Bats), number of species captured (Species), total known species
assemblage for each site (Assemblage) (S1 Table), species captured as percentage of known species assemblage for each site (%), capture rate (CR = number of individual
bats captured per NH), inventory rate (IR = number of unique bat species recorded per NH), and inventory efficacy (IE = percentage of known species assemblage
recorded per NH across all surveys).
Site Type NH Bats Species Assemblage % CR IR IE
CBR Standardized 926.37 1,786 37 53 69.8% 1.93 0.04 0.08%
KNP Standardized 202.50 4 2 24 8.3% 0.02 0.01 0.04%
KNP Targeted 30.88 47 9 24 37.5% 1.52 0.29 1.21%
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Fig 2. Box and whisker plots showing comparative results of Mann-Whitney U tests for A) capture rate, B) inventory rate, and C)
inventory efficacy of standardized surveys. Abbreviations: CBR, Calakmul Biosphere Reserve, Mexico; KNP, Krka National Park, Croatia;
PNH, per net hour.
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Fig 3. Bar plots showing comparative results of Mann-Whitney U tests showed in Krka National Park, Croatia for A) capture rate, B)
inventory rate, and C) inventory efficacy of standardized versus targeted surveys. Abbreviation: PNH, per net hour.
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measured parameters. In likelihood, these highly disparate results are due to the nature of the
landscape in KNP, which differs from CBR in terms of its homogeneity, openness, aridity, and
lack of linear features. This is illustrated by the photo taken by a DJI Mavic Mini drone with
built-in FC7203 camera (Fig 4). These site-specific characteristics may have contributed to the
poor capture and inventory rates in the KNP standardized surveys as the landscape presents
few opportunities to place nets along transects in places where bats are likely to be present.
The pressures of arid landscapes can affect bat distributions [46] and may force increased bat
abundance and species richness at the few water bodies present [47], making even small ponds
particularly lucrative, as shown by our data. Similar effects have been observed for avifauna in
arid areas, with species richness being negatively associated with distance to water [48].
Whilst the inventory efficacy of targeted surveys at KNP are persuasive, there are behavioral
idiosyncrasies at play affecting our data. In the arid karst landscape, all bats must find water
daily. Where lentic and riparian habitats are limited (as in KNP), this naturally forces a greater
species diversity at fewer existing water sources than might be found at abundant water
sources in other landscapes, such as that of CBR. This would likely create a steeper species
accumulation curve than in other habitats, and as such the inventory efficacy of targeted sur-
veys at KNP is likely to be skewed to that effect. This idea is somewhat reinforced by the fact
that targeted surveys in KNP, whilst much more effective than standardized surveys at KNP,
resulted in CR, IR, and IE values that were still lower than those of CBR.
Our results may also be influenced by the overall abundance (or lack) of bats in each of
these regions or by the proximity to standardized trapping sites of features of value to bats. In
KNP, our standardized trapping sites are located on a plateau above and around the Krka
River. All standardized trapping locations in KNP were located within 2 km distance from this
Fig 4. Homogeneity of Mediterranean scrub landscape at Krka National Park, Croatia.
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important feature, and between 2.5 and 3.7 km from the Miljacka II cave, where 6,000 bats of
eight species roost [49]. This implies that a lack of bat abundance is not a potential cause of
low capture rates; rather, it is their distribution across the landscape. Flying is energetically
expensive [50], with energetic pressures on bats increasing in summer during lactation and
spermatogenesis [51]. Bats tend to fly directly and at great speeds from roosting sites to forag-
ing grounds on their nightly commute to reach optimal energy expenditure [52]. For their
commute, they select linear features along which they navigate, such as tree lines, rivers, and
forest edges. Bat abundance has indeed been shown to be greater on such linear fly paths [53–
55]. The lack of linear features in the KNP landscape greatly reduces the chances that netting
will capture bats (particularly as transects are often selected for the monitoring of multiple
organismal groups and do not consider the likelihood to encounter bats [56]).
When a new field site is being surveyed for bats for the first time, it is advised to initially use
targeted surveys to gain a good first idea of the species assemblage that is present. For example,
the first bat inventory in Chucantı
´Nature Reserve in the remote Darie
´n province, Panama
[57] was the result of a targeted survey with mist nets that were positioned over existing trails
that were assessed for their potential use by bats as fly paths [55]. Despite a low survey effort
(total NH was 34), this survey resulted in 227 bat individuals representing 17 species; the CR
value was 6.68 and the IR value was 0.50. While the bat species assemblages of both Chucantı
´
Nature Reserve and the Darie
´n province are unknown, a total of 118 bats are known in Pan-
ama [58], resulting in an IE of 0.42%. We note that the local bat species assemblage will be
lower given that Chucantı
´is a cloud forest system, thus with a specific assemblage of bats that
occur between 600 and 1480 m altitude [59,60]. With this in mind, despite the IE value at
Chucantı
´being lower than that for targeted surveys at KNP, it is still higher than those for
standardized surveys at CBR and KNP. Also the CR and IR values for the targeted survey at
Chucantı
´are considerably higher than those for standardized surveys at CBR and KNP.
In summary, for the reasons discussed above, our results closely matched the expectations
laid out in our hypotheses; (1) that standardized mist-netting performed better in the (more
heterogenous) habitats of CBR than in the (more homogenous) landscapes of KNP, and (2)
that targeted surveys yielded better results than standardized surveys in the homogenous land-
scapes of KNP. While it is likely that employing targeted surveys at CBR will significantly
increase the capture rate, inventory rate, and inventory efficacy of surveys as it did for KNP in
this study and Chucantı
´Nature Reserve [57], this would need to be weighed against the bene-
fits standardized, comparable datasets being lost [4,61,62]. The suitability of standardized sur-
vey methods for deployment in different ecosystem types can be hard to judge. It is down to
the professional judgment of the individual researcher whether the landscape will lend itself to
contributing towards such datasets, or if the results of standardized surveys are likely to be
inhibited by landscape, homogeneity, climate, and scarcity of features. This study demon-
strates that there is not a “one-size-fits-all" approach to monitoring biodiversity, highlighting
the challenges in the development of surveying methods on a global scale. We emphasize that
researchers have a responsibility to weigh the costs and benefits of selecting one monitoring
approach over another to best effect positive change for species conservation.
Supporting information
S1 Table. Overview of all reported species of bats in our study sites. All reported bat spe-
cies–including unpublished ones–from Calakmul Biosphere Reserve, Mexico and Krka
National Park, Croatia.
(XLSX)
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Acknowledgments
This research was conducted as part of Operation Wallacea’s long-term biodiversity monitor-
ing project in collaboration with Biota Ltd in Croatia and Pronatura Peninsula de Yucatan in
Mexico. Operation Wallacea provided the logistical support necessary to complete this study.
We thank the following individuals for logistical support and assistance with fieldwork: Mark
P. Aquilina, Anneka Goeter, Bernice Hyett, Dus
ˇan Jelić, Fabia
´n Mora, Gabriel Oviedo,
Tommy Saunders, Carmen Sorina, Anna Suvorova, and Oliver Thomas. Sasha Karabasova
and Jalal Khan are thanked for permission to use their photographs of Krka National Park and
Calakmul Biosphere Reserve, respectively.
Author Contributions
Conceptualization: Danny Haelewaters, Thomas Edward Martin.
Formal analysis: Morgan Hughes.
Investigation: Danny Haelewaters, Morgan Hughes, Jose
´Anto
´nio Lemos Barão-No
´brega,
Kathy Slater.
Visualization: Morgan Hughes.
Writing – original draft: Danny Haelewaters, Morgan Hughes.
Writing – review & editing: Danny Haelewaters, Morgan Hughes, Thomas Edward Martin.
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