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\begin{document}

\title{Fish diversity of post-conflict Colombian Andes-Amazon streams as a
reference for conservation before increased land use}



\author[1]{Juan David Bogotá Gregory}%
\author[2]{David Jenkins}%
\author[1]{Astrid Acosta-Santos}%
\author[1]{Edwin Agudelo}%
\affil[1]{Instituto Amazonico de Investigaciones Cientificas SINCHI}%
\affil[2]{University of Central Florida College of Sciences}%


\vspace{-1em}



  
  \date{June 12, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Reference conditions are difficult to find in the Anthropocene but
essential for effective conservation of biodiversity. Aquatic ecosystems
in the Andes-Amazon transition zone of Colombia are now at high risk due
to expanded human activities after peace agreements in 2016 ended armed
conflict. Expanding human land use may reduce fish diversity across the
altitudinal gradient but especially in premontane streams (i.e.,
\textless{}500 m a.s.l.) because lands are more amenable to human use
than at greater altitudes. We evaluated natural fish diversity in twelve
sites over eight years bracketing the end of armed conflict. Strong
differences in community structure (measured as species richness, total
abundance, and effective species number, and multivariate analyses)
occurred as a function of altitude, as measured by. Our results provide
a baseline to identify short-term and long-term changes due to impending
human land use at a critical moment for the conservation of tropical
fish diversity.%
\end{abstract}%



\sloppy


\textbf{Fish diversity of post-conflict Colombian Andes-Amazon streams
as a reference for conservation before increased land use}

Juan David Bogota-Gregory\textsuperscript{1}, David G.
Jenkins\textsuperscript{2}, Astrid Acosta-Santos\textsuperscript{1},
Edwin Agudelo Co\selectlanguage{english}rdoba\textsuperscript{1}

\textsuperscript{1} Instituto Amaz\selectlanguage{ngerman}ónico de Investigaciones Científicas
SINCHI, Aquatic Ecosystems Group

\textsuperscript{2} University of Central Florida, Biology Department

\textbf{Abstract}

Reference conditions are difficult to find in the Anthropocene but
essential for effective conservation of biodiversity. Aquatic ecosystems
in the Andes-Amazon transition zone of Colombia are now at high risk due
to expanded human activities after peace agreements in 2016 ended armed
conflict. Expanding human land use may reduce fish diversity across the
altitudinal gradient but especially in premontane streams (i.e.,
\textless{}500 m a.s.l.) because lands are more amenable to human use
than at greater altitudes. We evaluated natural fish diversity in twelve
sites over eight years bracketing the end of armed conflict. Strong
differences in community structure (measured as species richness, total
abundance, and effective species number, and multivariate analyses)
occurred as a function of altitude, as measured by. Our results provide
a baseline to identify short-term and long-term changes due to impending
human land use at a critical moment for the conservation of tropical
fish diversity.

\textbf{Key words:} Amazon piedmont, land use, altitudinal gradient,
species richness, effective diversity, abundance, multivariate analysis,
regressions

*Corresponding author: Juan David Bogota-Gregory

email: juandbogota@gmail.com,

\textbf{Introduction}

Increased human activities have already transformed and degraded many
ecosystems worldwide due to urbanization, agriculture, and extraction of
natural resources (IPBES, 2019; Achiso, 2020; Feng \emph{et al.} ,
2022). As a result, diversity is often reduced, measured as a decrease
in both species richness and relative abundance (Newbold \emph{et al.} ,
2015) and it is difficult to understand reference conditions before
human impacts occurred. However, some areas are more affected by humans
than others, where differences may be related to human access and
landscape suitability for human uses (e.g., agriculture, urban
expansion). Among regions undergoing anthropogenic land use change, the
Amazon is known for its remarkable biodiversity and endemics
(Mittermeier \emph{et al.} , 2003). Amazonian lowlands contain the
largest biodiversity in the world (Gentry, 1988; Wilson, 1992),
including both terrestrial and aquatic faunas (Myers \emph{et al.} ,
2000; Reis, Kullander and Ferraris, 2003). Amazonia hosts about 17\% and
10\% of all known vascular plants and vertebrate species, respectively
(Lundberg \emph{et al.} , 2000; Myers\emph{et al.} , 2000). Freshwater
aquatic ecosystems of the Amazon Basin are megadiverse (Myers \emph{et
al.} , 2000) and host the most diverse ichthyofauna in the world
(Lundberg, 2001; Reis, Kullander and Ferraris, 2003). In highly diverse
systems such as the Amazon Basin, many species are relatively rare and
occupy specific niches accordingly to their morphological and
physiological traits (Hercos \emph{et al.} , 2013). In addition, most
species are not evenly distributed, whether measured in presence/absence
or in abundance (Magurran and Henderson, 2003; Magurran, 2004; Bell,
2005).

High freshwater diversity of the Amazon provides crucial ecosystem
services, and in many cases represents the sole financial income for
human communities in the region (Agudelo, Alonso and Moya, 2006; Agudelo
Córdoba \emph{et al.} , 2011). Within Colombia, Amazonian aquatic
ecosystems are now at high risk due to expanded human activities,
especially expanding agriculture after the establishment of the peace
agreements to end armed conflict in 2016 (Tellez, 2019). This scenario
sets up a critical moment for the establishment of management and
conservation policies (Clerici \emph{et al.} , 2019; Agudelo Hz, Barrera
and Uriel, 2023) to avoid negative conservation outcomes (Feng \emph{et
al.} , 2022).

Much of the Amazonian basin is lowland, but not everywhere. For example,
the Colombian Amazon includes the Andean-Amazon transition, where
natural ecosystems of basimontane altitudes (i.e., 500 - 1700 m a.s.l.)
are less affected by human activities than those in the premontane zone
(i.e., \textless{} 500 m a.s.l.) because the steep basimontane landscape
complicates human activities whereas flat and smooth premontane terrain
enables most human activities, despite infertile soils (Galvis \emph{et
al.} , 2007). Altitudinal gradients in diversity are well-recognized
worldwide, where diversity is generally expected to decrease with
altitude because more stringent environmental conditions occur at
greater altitude (Figure 1; Heegaard and Vandvik 2009, Lomolino et al.
2010, De La Barra et al. 2016). In addition, different selective
conditions across the altitudinal gradient provide different selective
conditions, leading to different diversities. For example, basimontane
fishes more often have morphological adaptions for attachment to
surfaces and behaviors and morphologies to reduce flow forces acting on
the fish body (Maldonado-Ocampo \emph{et al.} , 2005). In comparison,
lowland species are more often adapted for less flow forces and instead
develop physiological adaptations for warmer temperatures, lower
dissolved oxygen, and different pH and conductivity (Saint-Paul \emph{et
al.} , 2000; Bogotá-Gregory \emph{et al.} , 2020).

Thus, the Andes-Amazon transition zone provides a double opportunity to
evaluate natural diversity patterns across a strong altitudinal gradient
and to evaluate reference conditions for differential and impending
anthropogenic effects. The natural altitudinal diversity gradient should
interact with spatially-biased anthropogenic effects of land use to
further modify diversity patterns (Figure 1), where diversity reduction
may not be uniform across altitudes (Penjor, Jamtsho and Sherub, 2022).
Anthropogenic effects may be local and point-source (e.g., mining;
Rehmana et al. 2024), but more generally extensive, nonpoint-source
anthropogenic effects occur, related to land use (e.g., agriculture,
urban systems). Land use change is often greatest and earliest in
lowlands due to simple economics of access and labor (Shively, 2001).
Well-known effects include habitat loss and fragmentation on lands
(Plieninger, 2006; Adhikari and Hansen, 2018) and non-point source
pollution (e.g., sediment loading, nutrient runoff) in streams and
rivers (Martinelli \emph{et al.} , 1989; Ikeda, Osawa and Akamatsu,
2009). This bias of human activities among terrains seems general, given
it is repeated in other landscapes (e.g., Mclain et al. 2013, Bürgi et
al. 2017).

Expanding human land use in the Colombian Andes-Amazon transition zone
may reduce diversity but is unlikely to cause biological homogenization
between altitudinal levels because the altitudinal gradient is selective
(e.g., fishes adapted to low-slope streams may not move into high-slope
conditions). Instead, premontane streams should become more similar to
each other in species richness and abundance, though with different
suites of species in lowlands than in uplands (Figure 1). For practical
reasons, premontane land use may also be expected to precede that in
basimontane regions, with matching timing for diversity effects.

Here we evaluate fish diversity patterns in basimontane and premontane
streams of the Caquetá River basin in the Colombian Andes-Amazon
transition zone, using data from samples collected between 2013 and 2022
in 12 localities. We expected greater fish diversity in premontane
streams than in basimontane streams, corresponding to an altitudinal
gradient. We also expected no great change pre- and post-2016 in
patterns because land use had not yet accelerated. Data represent
barely-impacted conditions soon after the conflict ended and before
development of formerly avoided lands (Calle-Rendón, Moreno and Hilário,
2018), especially given the COVID-19 pandemic. Results are especially
relevant considering the importance of Andean-Amazonian connectivity
(Melack and Fosberg, 2001; Anderson \emph{et al.} , 2018;
Clerici\emph{et al.} , 2019) and the high rates of endemism that
characterizes the area (Tognelli \emph{et al.} , 2016). Our study sets a
baseline for evaluating future changes in Andean-Amazonian biodiversity.

\textbf{Methods}

\emph{Study Area}

The Caquetá River basin is a western Amazonian affluent of Andean
origin, formed at 3,850 m above sea level (a.s.l.) by the confluence of
three different minor tributaries in the Peñas Blancas Páramo, located
in the East Mountain Chain in the Southeast Region in Colombia (IGAC,
1996). It runs over 1,200 km in a southeast direction before it merges
to the main channel of the Amazon River, crossing the Caquetá, Putumayo,
and Amazonas departments in Colombia, until it's named the Rio Japura\selectlanguage{english} at
the Colombian-Brazilian border (IGAC, 1996, 1999).

The upper section of the Caquet\selectlanguage{ngerman}á River drains most of the western uplift
of the Guyana Shield, a formation from the Miocene characterized by a
crystalline basement. Shields are very evolved soils with low nutrient
and organic contents. Above 500 m a.s.l. (Figure 2a), the aquatic
systems of the Caquetá\selectlanguage{english} basin are typically Andean ecosystems with
dominant rocky substrates, abrupt slopes, and high flow (Figure 2b).
Below the 500 m a.s.l., denoted as Amazonian Piedmont, aquatic
ecosystems are characterized by basement alluvial fans (Figure 2b) of
volcanic origin with elements from the Andes Mountain chain (Hoorn,
1994; Galvis \emph{et al.} , 2007).

\emph{Data collection in situ}

Sample sites were selected based on location (basimontane and premontane
levels) and access. All sample sites are located in the Caquet\selectlanguage{ngerman}á
Department in the municipalities of Bele\selectlanguage{english}n de los Andaqui\selectlanguage{english}es, Florencia,
and Morelia, between 200 and 1500 m a.s.l. (Figure 2, Table 1). Twelve
sites (four sites at the basimontane level and eight at the premontane
level) were sampled during the falling water season in each of eight
years (2013, 2015-2019, 2021, and 2022). However, three sites were not
sampled in 2019 due to access restriction because of weather conditions.
In total, data represent 91 sample events.

Fish were sampled using SAMUS725M electrofishing equipment, along a 100
m stretch in each of the sample sites. This sampling methodology follows
the standardized sampling technique of Maldonado-Ocampo et al. (2005)
for subsequent data comparisons. Fish were euthanized with clove oil and
fixed in a 10\% formaldehyde solution. Prior to species taxonomic
identifications, specimen vouchers were transferred for preservation in
a 75\% ethanol solution. All the fish specimens were deposited in the
Ichthyological Collection of the Colombian Amazon (CIACOL ) at the
Amazonian Scientific Research Institute (SINCHI) in Leticia, Amazonas,
Colombia (https://sinchi.org.co/ciacol).

\emph{Data analysis}

We evaluated the efficiency of our overall sampling effort for
basimontane and premontane levels using rarefaction curves and the Chao1
species estimator for data pooled across sample years (Chao, 1984).
Extrapolating a species accumulation curve to its asymptote and
estimating species richness via rarefaction and the Chao 1,
respectively, both provide an estimate of the performance of the
proposed sampling method (Magurran, 2004; Chao \emph{et al.} , 2009)

We evaluated diversity components using both univariate and multivariate
analyses. We used generalized linear mixed effects models to evaluate
the effect of altitude on species richness, total abundance, and
effective diversity (i.e., e\textsuperscript{H'}, where H' is the
Shannon diversity index; Jost 2006). Sampling year was a random effect
because we used a repeated measures sampling design. Sample sites were
also included as a random effect to address unmeasured differences in
sites. As a result of this analytical model, results are general for
altitudinal effects on fish diversity in the Caquet\selectlanguage{ngerman}á River through
years, including pre and post conflict time-periods, and among sites.
Alternative models were compared using the corrected Akaike's
Information Criterion (AICc) with the bbmle package (Bolker, Team and
Giné-Vázquez, 2022), where we emphasized AICc weights to identify the
most plausible model after discounting for model complexity (Burnham and
Anderson, 2002). Different alternative residual distributions (e.g.,
Gaussian, Gamma, negative binomial) in generalized linear models were
iteratively evaluated and compared to log-transforms of response
variables to best meet model assumptions (using check\_model in the
performance package; Lüdecke 2023). Given a chosen distribution,
alternative models were then compared by AICc, where compared models
included a null, random effects only, potential fixed effects (i.e., a
dummy variable representing pre- and post-cease-fire conditions and
altitude effects), and combinations of fixed and random effects.
Finally, we used nonmetric multidimensional scaling (NMDS) with
PERMANOVA to confirm differences in community structure between
basimontane and premontane sites.

Data management and statistical analyses were performed in R (R Core
Team, 2021) using functions from the glmmTMB (Magnusson \emph{et al.} ,
2020), vegan (Oksanen \emph{et al.} , 2020), devtools (Hadley, Hester
and Chang, 2022), iNEXT (Hsieh, Ma and Chao, 2016), ggplot2 (Kassambara,
2018), MASS (Ripley \emph{et al.} , 2019), multcompView (Graves and
Piepho, 2022), and performance (Lüdecke, 2023) packages.

\textbf{Results}

Samples included 4216 fish, belonging to 100 species, 58 genera, 24
families, and six orders (Supplementary file F1). Rarefaction curves
reached asymptotes for both basimontane and premontane levels (Figure
3a). Premontane sites had \textgreater{}3 times more fish and
\textgreater{}5 times more species than those recorded in basimontane
sites (Figure 3b). In addition, basimontane sites had a different
taxonomic composition than premontane sites. Basimontane sites had
similar numbers of Characiformes and Siluriformes and some
(\textless{}10\%) of the Blenniiformes (mainly in the Cichlidae family;
Figure 3c). In contrast, the premontane level was clearly dominated
(\textgreater{}75\%) by the Characiformes fishes which comprised a
higher diversity at the order level (Figure 3c). Fish community
structure disparities between altitudinal levels are confirmed by the
PERMANOVA (\emph{F = 3.04, R = 0.23, P\textless{}0.05} ), and the NMDS
in which two clear groups, corresponding to the basimontane and
premontane sites, are depicted in the multidimensional space
representing the analysis (Figure 3d).

Species richness was most efficiently modeled as a power law (i.e.,
log-log) function of altitude and the random effects of year and site
(supplementary Table S1; Table 2). Although altitude alone has a good
predictive power (R\textsuperscript{2} = 0.42) predictions improve with
the random effects (R\textsuperscript{2} = 0.62), as confirmed by the
model performance procedure (supplementary Figure S1). The model output
suggests a significant mean decrease by 0.98 log(species richness) per
unit of log-altitude (Table 2; see Figure 4a for regression line).

As for species richness, a power law model for total abundance using
altitude as a fixed effect and the random effects was most predictive
compared to other models (Supplementary Table S2; Table 3). Altitude
alone represented 42\% of variation in species richness, but model
predictions improve with random effects (R\textsuperscript{2} = 0.62;
supplementary Figure S2). The model output suggests a 0.72 significant
decrease in log(fish abundance) per unit of log-altitude (Table 3; see
Figure 4b for regression line).

Effective diversity was also most plausibly modeled as a power law
(Supplementary Table S3; Table 4). Altitude alone has substantial
predictive power (R-squared = 0.433), but adding year as a random effect
slightly improved fit (R-squared = 0.456; Supplementary Figure S3). The
model output suggests a significant 0.834 mean significant decrease in
log(effective diversity) per unit of log-altitude (Table 4; see Figure
4c for regression line).

Overall, for all of the diversity components evaluated herein, using
altitudinal level as fixed and site and year as mixed effects was rather
predictive, and no pre- and post-conflict difference was apparent in
results (i.e., results here represent conditions before further human
land use effects). Fish diversity predictably decreased with altitude as
a power law function.

\textbf{Discussion}

Our study is the first of its kind to evaluate fish community structure
in the Colombian Andes-Amazon transition zone. Results here provide a
baseline for conservation of regional streams across premontane and
basimontane zones because data represent the period when armed conflict
ended but before potential development of formerly avoided lands
(Calle-Rendón, Moreno and Hilário, 2018).

Our results confirmed that altitudinal gradients in fish community
assembly are important (van der Hammen and dos Santos 1995) and that
fish diversity is greater at lower altitudes, no matter how we estimated
diversity (e.g., Jaramillo-Villa et al. 2010, Lomolino et al. 2010,
Lujan et al. 2013). Considering that effective mitigation of
anthropogenic pressures on streams and fish assemblages should account
for site-specific conditions and at different scales (Poiani \emph{et
al.} , 2000; Newbold \emph{et al.} , 2015), altitudinal gradients in the
study area may be predictive for conservation and management. We note
that the basimontane streams are important to conserve despite their
relatively low diversity because fishes there are notably different from
those downstream.

Changes in community composition through time are typically complex and
depend on spatial and land use contexts (Allen \emph{et al.} , 2019;
Hill\emph{et al.} , 2021). We expect that fish diversity might be most
heavily affected at the premontane level over time due to human
activities that will stronger and earlier at lower altitudes.
Conservation practices (e.g., riparian buffer zones, runoff settling
ponds) to maintain stream conditions can be implemented early in
regional land changes to best conserve diversity.

Given species extinction rates due to habitat degeneration in ecosystems
similar to those sampled here (Manjarrés-hernández \emph{et al.} ,
2021), this study is valuable as a baseline of barely-impacted
conditions soon after the conflict ended in Colombia and before
encroaching development of formerly avoided lands (Calle-Rendón, Moreno
and Hilário, 2018). We expect future sampling will compare results to
those reported here to document changes in fish assemblages due to land
use changes. We also expect land use to be most changed at lower
altitudes, where soils and slopes are more amenable to agriculture,
roads, and housing. If so, then it is possible that fish assemblages
will be degraded by increased land use to become simpler and later
affected by upstream effects (Figure 1). If ongoing deforestation and
encroaching anthropogenic land use in the hyper-diverse Amazon basin is
to be managed to minimize species losses, then some lands and streams
must be preserved (Mckinney and Lockwood, 1999; Granado-Lorencio, Cerviá
and Lima, 2007; Rull, 2007). Results here suggest that land surrounding
premontane streams should take priority to conserve the most species in
areas that are most vulnerable. Upstream catchments in basimontane areas
can also be preserved to maintain water quality flowing into premontane
streams and conserve unique fishes in those upstream reaches(Booth and
Jackson 1997, Klauda et al. 1998, Araújo and Tejerina-Garro 2009, de
Melo et al. 2009). Future work in the study area should evaluate
relative effects of changing land use in upper and lower catchments on
fish diversity in the lower reaches.

So far, most emphasis on ecological studies on understanding diversity
has been on spatial patterns of biological diversity rather than
analyzing temporal patterns. Understanding temporal changes is essential
to predict possible scenarios of the most diverse world's biota in
natural ecosystems of the Amazon. Fish assemblages are affected by
processes occurring at multiple scales, including those covered in our
analyses (Livingstone, Rowland and Bailey, 1982; Jackson, Peres-Neto and
Olden, 2001; Tedesco \emph{et al.} , 2005; Oberdoff \emph{et al.} ,
2011). Future surveys could integrate data at scales obtained here with
even greater spatial and temporal extents. We regard results here as a
potential baseline for future work in the same drainages, where future
work may reveal effects of coming anthropogenic land use in the Amazon
basin.

\textbf{Acknowledgements}

This study was funded by the projects ``Investigacio\selectlanguage{english}n Conservacio\selectlanguage{english}n y
Aprovechamiento Sostenible de la Diversidad Biolo\selectlanguage{english}gica Socioecono\selectlanguage{english}mica y
Cultural de la Amazonia Colombiana-Amazonas, Caqueta, \selectlanguage{english}Putumayo,
Guaviare, Vaupe\selectlanguage{english}s y Guaini\selectlanguage{english}a'' and ``Expedicio\selectlanguage{english}n Colombia-Bio a la
Biodiversidad en la Transicio\selectlanguage{english}n Andino-Amazo\selectlanguage{english}nica del Departamento del
Caqueta\selectlanguage{english}'. Grupo de Investigacio\selectlanguage{english}n en Ecosistemas Acua\selectlanguage{english}ticos Amazo\selectlanguage{english}nicos of
the SINCHI Institute. Marcela Nu\selectlanguage{english}n\selectlanguage{english}ez and Iva\selectlanguage{english}n Gonza\selectlanguage{english}lez for their support
during the fieldwork. Members of the headquarters of the SINCHI
Institute at Leticia, Amazonas and Florencia, Caqueta.

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\textbf{Tables}

Table 1. Sample sites localities, geographical coordinates and altitude
(m a.s.l.)\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
\emph{Sites} & \emph{Locality} & \emph{Latitude} & \emph{Longitude} &
\emph{Altitude}\tabularnewline
\midrule
\endhead
S1 & La Portada Stream & 1.8139 & -75.6591 & 1253\tabularnewline
S2 & Sucre River & 1.7953 & -75.6465 & 1013\tabularnewline
S3 & Para\selectlanguage{ngerman}íso River & 1.7472 & -75.6283 & 676\tabularnewline
S4 & Las Doradas Stream & 1.7289 & -75.6464 & 530\tabularnewline
S5 & La Sardina Stream & 1.6797 & -75.6225 & 386\tabularnewline
S6 & La Carbona Stream & 1.7064 & -75.6253 & 439\tabularnewline
S7 & La Yuca Stream & 1.6078 & -75.6383 & 277\tabularnewline
S8 & La Mochilero Stream & 1.5508 & -75.6756 & 285\tabularnewline
S9 & Aguas Calientes Stream & 1.4758 & -75.7694 & 286\tabularnewline
S10 & La Chocho Stream & 1.4472 & -75.8117 & 316\tabularnewline
S11 & La Arenosa Stream & 1.4644 & -75.8633 & 360\tabularnewline
S12 & Bodoquerito River & 1.4971 & -75.8741 & 382\tabularnewline
\bottomrule
\end{longtable}

Table 2. Details for the most efficient model to predict species
richness, total abundance, and effective diversity (\textsuperscript{1}D
= exp(Shannon entropy) of fishes in the Caquet\selectlanguage{ngerman}á River, Colombia, based
on 91 samples among 12 sites collected in 8 years during the 2013-2022
interval.\selectlanguage{english}
\begin{longtable}[]{@{}lllllllll@{}}
\toprule
& \emph{Model distribution} & \emph{Fixed Effects R\textsuperscript{2}}
& \emph{Fixed + Random R\textsuperscript{2}} & \emph{Random Effects} &
\emph{Std. Deviation} & \emph{Fixed Effects} & \emph{Coefficient} &
\emph{95\% C.I.}\tabularnewline
\midrule
\endhead
Species richness & Gaussian & 0.42 & 0.62 & Year & 0.1950 & Intercept &
7.724 & 2.229\tabularnewline
& & & & Sites & 0.2612 & log(Altitude) & -0.979 & 0.363\tabularnewline
Total abundance & Gamma (log link) & 0.13 & 0.48 & Year & 0.359 &
Intercept & 7.987 & 3.657\tabularnewline
& & & & Sites & 0.437 & log(Altitude) & -0.716 & 0.594\tabularnewline
Effective diversity & Gamma (log link) & 0.43 & 0.57 & Year & 0.146 &
Intercept & 6.526 & 1.622\tabularnewline
& & & & Sites & 0.170 & log(Altitude) & -0.834 & 1.164\tabularnewline
\bottomrule
\end{longtable}

\textbf{Figure legends}

Figure 1: Hypothetical effects of potential land use on fish local
(alpha) diversity across elevation gradients. The solid curve represents
a decay of diversity with altitude. The lower dashed curve represents
potential diversity after natural lands are converted to agriculture and
other human uses, where greatest diversity loss (arrows) occurs at lower
altitude.

Figure 2: a) Geographical location of the sample sites. b) Examples of
the aquatic ecosystems within the altitudinal levels, which are the ones
recognized by van der Hammen and dos Santos (1995).

Figure 3. Comparisons of premontane and basimontane fish communities. a)
Rarefaction curves approach asymptotes, indicating representative
sampling. Premontane sites were inhabited by more species, as supported
by evidence in b) for total abundance, richness, and effective
diversity. c) Taxonomic composition differed between premontane and
basimontane sites, where Siluriformes were more prominent in basimontane
sites than in premontane sites. Characiformes were dominant in both site
types, but in different proportions. d) NMDS of fish communities from
the sampled sites. Dots represent pooled temporal data (2013-2022) for
sites. Polygons represent fish communities based on altitudinal levels
recognized by van der Hammen and dos Santos (1995).

Figure 4. Regression lines for species richness (a), total abundance
(b), and effective diversity (c) \textasciitilde{} altitude, where
regression lines represent GLMM predicted values and error bars. Note
log axes; regressions represent power law functions.

\textbf{Data accessibility:} Supporting data are available at
https://doi.org/10.5281/zenodo.10424407

\textbf{Competing Interests Statement:} All authors declare that there
are no competing interests.

\textbf{Author contributions:}

\selectlanguage{ngerman}\emph{Juan David Bogotá Gregory:} Conceptualization (Lead); Data
curation (Lead); Formal analysis (Lead); Investigation (Lead);
Methodology (Lead); Project Administration (Lead); Resources (Lead);
Writing - original draft (Lead). \emph{David G.
Jenkins:}Conceptualization (Lead); Formal analysis (Lead); Methodology
(Lead); Writing -- review \& editing (Lead). \emph{Astrid
Acosta-Santos:} Data curation (Equal). \emph{Edwin Agudelo:} Resources
(Supporting).\selectlanguage{english}
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