Hydrological change is a conspicuous signal of land use intensification in human-dominated landscapes (Allan, 2004). The importance of water as a resource for human, livestock and crops, as a source of energy, and as pathway for transportation makes water management a quintessential element of human activities. In addition to the deliberate management of water bodies, the many interventions in the terrestrial environment that follow human occupation indirectly influence hydrology by altering the partitioning of rainfall among the processes of evapotranspiration, infiltration and runoff (Wohl et al., 2012).

Because dramatic changes in the watershed follow the conversion of native forests, important insights into the causes and consequences of anthropogenic hydrological change can be gained by analyzing agricultural frontiers. In agricultural frontiers, ongoing deforestation permits side-by-side comparisons of native and converted habitats, and strong biodiversity responses are expected from the occurrence of many sensitive, forest specialists (Schiesari et al., 2013).

The Amazon Basin is the largest area of the world currently undergoing frontier settlement and, within the Brazilian portion of the Amazon Basin, the state of Mato Grosso responds to one third of all deforested land (32.8%; INPE, 2019). Building on published hydrological studies (Scheffler et al., 2011; Macedo et al., 2013; Hayhoe et al., 2011, Neill et al., 2013), we characterize changes in freshwater habitat and biodiversity that are consistent with the hypothesis that deforestation is promoting the ‘lentification’ of Southern Amazonian landscapes in the Upper Xingu Basin. The term ‘lentification’ was coined to the lentic characteristics naturally taken on by small, intermittent floodplain channels during low waters (Drago et al., 2003) and later used to the transformation of the habitat character of rivers from lotic (i.e. flowing water) to lentic (i.e. standing water) through processes such as flow regulation, unsustainable water extraction and climate change (Sabater, 2008; Sabater et al., 2016). The term does not appear to have been embraced by the scientific community though (only three papers with ‘lentification’ were retrieved in a Web of Science search; September 11, 2019) and lacks an operational definition. We here define ‘lentification’ as an increase in the availability (i.e. density and/or area cover) of lentic freshwater habitats in the landscape, directly or indirectly caused by human activities. This definition is inclusive to any driver, and highlights that lentification is a property of the landscape and not only of the drainage network as it includes freshwater habitat change in interfluves (i.e., in areas of higher land between two streams or rivers in the same drainage system) as well. Because few processes lead to the formation of novel lotic habitats, an increase in the availability of lentic habitat almost invariably implies on an increase in the proportion of lentic habitats over lotic habitats in the landscape.

We hypothesize that at least four drivers of lentification could be operating in converted Upper Xingu landscapes: stream and river damming, increased water yields, soil compaction, and the deliberate construction of cattle and drainage ponds. Among these, river damming is the most readily recognized driver of lentification as the Amazon Basin is the new ‘hydroelectric frontier’ in Brazil with 154 hydroelectric dams in operation, 21 under construction and 277 planned (Castello et al., 2013). Less attention is given to the myriad small dams found today in converted land. Yet, in the Upper Xingu Basin alone there are ∼10,000 small dams (i.e., dams with a median impounded area ∼0,01km2), most of which built to provide water to cattle and generate electricity for local consumption (Macedo et al., 2013). Lentification is a clear product of river damming as reservoirs have attributes typical of lentic systems including slower currents, higher water residence times, greater depth and surface area, open canopy, higher surface temperatures, thermal stratification, and net accumulation of sediment and nutrients (Henry, 1999).

Deforestation could also promote lentification through increased water yields. Local deforestation has little effect on precipitation but strongly reduces per-unit-area evapotranspiration due to the lower height, shallower rooting depth, and lower leaf area index of grasses and crops relative to trees. Thus, proportionally more rainfall becomes river discharge (Wohl et al., 2012). In the Upper Xingu Basin, mean daily water yields increased four times in soybean watersheds relative to forest watersheds via an increased contribution of groundwater-driven baseflow to stream discharge (Hayhoe et al., 2011). Lentification could result if deforestation-driven increased water yields promote the expansion of semi-lentic riparian floodplains.

In interfluves away from streams and rivers, deforestation could also promote lentification via soil compaction. In the Upper Xingu Basin soil infiltrability was markedly reduced in the conversion of forests to pastures both in terms of infiltrability (from 1258mm/h to 100mm/h) and saturated hydraulic conductivity, probably due to cattle trampling. Shallow disking in the conversion of pastures to soybean fields increased infiltrability (469mm/h) but further reduced subsoil saturated hydraulic conductivity (Scheffler et al., 2011). Lentification could result if deforestation-driven soil compaction increases the frequency in which precipitation exceeds infiltration, thereby forming ponds and puddles. An additional driver of lentification in interfluves is the deliberate construction of ‘cacimbas’, artificial ponds providing water to cattle and draining stormwater runoff from dirt roads.

Taken together, it is apparent that there are several drivers in place capable of creating novel lentic water bodies in converted Amazonian landscapes, and that such novel lentic water bodies span the full spectrum of the hydroperiod gradient – from ephemeral to semi-permanent to permanent water bodies – in such a way that could consistently favor quite distinct elements of lentic freshwater fauna (Wellborn et al., 1996).

In this article we test, for the first time, the hypotheses (i) that land conversion in the Upper Xingu Basin is associated with an increase in the availability of lentic freshwater habitats both along drainage networks (by creating reservoirs and expanding stream floodplains) and in interfluves (by creating ponds and puddles), and (ii) that this landscape-level lentification increases the abundance of freshwater fauna typically associated with lentic habitats. We tested these hypotheses by habitat characterization and biological surveys of fish and amphibians in forests, pastures and soybean fields in the headwaters of the Xingu River in Mato Grosso, Brazil. Fish and amphibians are excellent indicators of hydrological change (Hering et al., 2006) with high complementarity in habitat use (Wellborn et al., 1996): fish are usually restricted to permanent water bodies whereas most amphibians reproduce in temporary and semi-permanent lentic water bodies. Importantly, our study was conducted at the same site where the hydrological mechanisms we envision as promoting lentification were demonstrated. Hayhoe et al. (2011) demonstrated that deforestation increases water yields but not whether this phenomenon increases the area of shallow riparian floodplains. Scheffler et al. (2011) demonstrated that deforestation decreases soil infiltrability but not whether this phenomenon promotes the formation of ponds and puddles. Finally, Macedo et al. (2013) quantified the number of impoundments in the Upper Xingu Basin as a whole; we here confirm their findings at our specific study location before linking habitat with biodiversity change.

This study provides evidence, for the first time, that deforestation promotes an increase in the availability of lentic freshwater habitats in Southern Amazonian landscapes. Streams in converted watersheds had numerous impoundments, and riparian floodplains twice as wide than streams in preserved watersheds. At the same time, converted interfluves had numerous ponds and puddles, in a striking contrast to the complete absence of water bodies in sampled forested interfluves. Importantly, lentification of streams and interfluves was accompanied by strong responses by fish and amphibian assemblages.

Reservoir conditions favored half of the six dominant fish species, and disfavored the other half. The characid Astyanax multidens and the cichlid Aequidens michaeli were 8 and 2.3-fold more abundant in reservoirs than in lotic reaches; several other less common cichlids were also predominantly or exclusively found in reservoirs, as were the trahira Hoplias malabaricus and the curimatid Steindachnerina sp. At the other extreme, not a single individual of characids Hyphessobrycon mutabilis and Moenkhausia phaeonota, otherwise common in stream reaches, was found in reservoirs. These patterns are consistent with large-scale studies showing that cichlids, characids and other characiforms, but also erithrynids, serrasalmids, loricariids and curimatids that inhabit more lentic habitats such as backwaters, shallow river margins and lakes, are sometimes favored by river damming (Agostinho et al., 2007).

Expanded stream floodplains, in turn, had a strong positive effect on the abundance of Melanorivulus megaroni, a fish species associated with shallow waters (>90% of individuals were sampled in water <10cm deep) and streamside pools. Other fishes could be influenced by floodplain expansion as well given that many small-sized species, or larvae and juveniles of larger species, rely on these habitats as nurseries and shelters against predation and water dragging (Schiemer et al., 2003).

Conversion of forests to pastures and soybean fields had a surprising positive effect on streamside amphibian abundance and richness. Both response variables were associated with number of dams around calling sampling stations. Thus, landscape lentification via reservoir formation is a plausible contributor to this phenomenon, although other hydrological and non-hydrological changes associated with deforestation, including the expansion of stream floodplains (not quantified along surveyed streamside transects) and canopy opening (Schiesari, 2006), could also be involved.

Even more striking was the influence of land conversion in interfluves on amphibian richness and abundance. No single amphibian species was heard in forested plateaus, as opposed to 12 in pastures and six in soybean fields. Because calling involves not only benefits (mate attraction and selection) but also costs (energetic costs, increased risk of predation and parasitism), males usually call in, at or close to their breeding sites. Therefore, the lack of water bodies is a sufficient explanation for the absence of calling amphibians in forested plateaus, as 35 of 38 amphibian species in the region depend on water for egg laying and/or premetamorphic development (Bitar et al., 2012; incidentally, Pristimantis aff. fenestratus and Leptodactylus cf. andreae, species that do not depend on water bodies for reproduction, were heard calling from forested plateaus in occasions other than our calling sampling surveys). We suspect the lack of a statistical effect of density of ponds and puddles on amphibian richness and abundance results primarily from the need to exclude forested plateaus from the analysis, and secondarily from a mismatch in the scale of habitat assessment (ponds that could be seen from transect, i.e., tens of meters) and that of faunal assessment (frogs that could be heard from transect, i.e., hundreds of meters).

Contrary to streams, where amphibian richness and abundance were similar in pastures and soybean fields, in interfluves pastures had twice as many amphibian species and severalfold greater abundance than soybean fields. At least two factors could be involved. First, several of the water bodies in pastures were cattle ponds, which, contrary to roadside puddles, are constructed to effectively hold water – i.e., they are deeper and compacted with tractors. As a consequence, part of the water bodies in pastures held water for one month longer than in soybean fields (personal observation), a difference in hydroperiod of considerable biological significance considering that a primary cause of mortality in temporary ponds is desiccation (Wellborn et al., 1996). Second, environmental conditions in plateaus - effectively contrasting primary forests, pastures and soybean fields - are much harsher than along streams, protected by law by a 25-m wide vegetated buffer zone. Specifically, the intensive management of soybean plantations inflicts strong, periodical disturbance to the fauna that inhabits, visits or traverses interfluves. Conversion of pastures to soybean fields is made by clearing with fire, tilling and liming, whereas plantation management annually involves sowing, applying fertilizers (∼300kg NPK/ha) and pesticides (13.7kg/ha including 39 active ingredients) and harvesting (Schiesari and Grillitsch, 2011; Schiesari et al., 2013). Considering that puddles in soybean fields are subject to direct pesticide overspraying and runoff, it is even surprising that there are amphibian species colonizing ponds in plantations – although, we do not know whether these are viable habitats for metamorph production or simply amphibian population sinks (as is the case for aquatic insects; see Negri 2015).

In summary, the creation of new reproductive habitat is a plausible hypothesis for the increased amphibian abundance in converted land. The increase in species richness is, by contrast, at first sight surprising. Differences in observed gamma diversity are likely artifacts of abundance; Chao estimators predict 29 species in forests, less than the 38 recorded (Bitar et al., 2012) but more than the 22 predicted in converted land. Differences in alpha diversity, in turn, do not appear to be artifacts and could be due to the particular biogeographical scenario of our study site and/or the spread of generalistic open area species with deforestation (see also Bitar et al., 2015). Our study site is at the edge of Amazonian closed-canopy forests; the savanna line is as close as ∼20km south and it is likely that in the Pleistocene this area has alternated open-canopy/closed-canopy phytophysiognomies. The observation that several species found in closed-canopy forests are more typical of open areas (Leptodactylus labirinthicus, L. mystaceus, Hypsiboas albopunctatus, Rhinella schneideri) is a testimony for this mixed amphibian assemblage. On top of that, converted streamside transects have the addition of at least seven species exclusive of open areas (Leptodactylus fuscus, Physalaemus cuvieri, P. centralis, P. nattereri, Pseudopaludicola mystacalis, Scinax fuscovarius and S. fuscomarginatus). Converted watersheds could therefore be housing an enriched amphibian fauna through the mixture of closed-canopy species of Amazonian origin (e.g., Osteocephalus taurinus, Phyllomedusa vaillanti, Pristimantis fenestratus), open-area cerrado species capable of colonizing closed-canopy forests, and generalistic open-area cerrado species that accompany the line of deforestation and are favored by landscape lentification.

Although our research is local, it is tempting to speculate that the lentification of freshwater systems and their biota could be occurring in many locations of deforested Amazonia. This is because all proposed drivers of lentification are quite general, according to a literature review of the available evidence linking land conversion and the density of dams, soil compaction and stream discharges in the Amazon Basin (and its sub-basins Xingu, Tapajós, Madeira, Solimões, Negro and Amazonas) and surrounding basins (Tocantins, Marajó and the NW Coastal South Atlantic to the Southeast; Orinoco and Coastal North Atlantic to the North) (Box 1 and Table 1 and references therein).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.