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