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.,
<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.
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,
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 et al. , 2022).
As a result, diversity is often reduced, measured as a decrease in both
species richness and relative abundance (Newbold 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 et al. ,
2003). Amazonian lowlands contain the largest biodiversity in the world
(Gentry, 1988; Wilson, 1992), including both terrestrial and aquatic
faunas (Myers et al. , 2000; Reis, Kullander and Ferraris, 2003).
Amazonia hosts about 17% and 10% of all known vascular plants and
vertebrate species, respectively (Lundberg et al. , 2000; Myerset al. , 2000). Freshwater aquatic ecosystems of the Amazon Basin
are megadiverse (Myers 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 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 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 et al. , 2019; Agudelo Hz, Barrera
and Uriel, 2023) to avoid negative conservation outcomes (Feng 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., < 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 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 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 et
al. , 2000; Bogotá-Gregory 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 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 et al. , 2018; Clericiet al. , 2019) and the high rates of endemism that characterizes
the area (Tognelli et al. , 2016). Our study sets a baseline for
evaluating future changes in Andean-Amazonian biodiversity.
Methods