DISCUSSION
Our results revealed that the intensive cover of human land use (i.e.,
agriculture, pasture, and urbanization) is negatively associated with
multiple biodiversity facets of three different stream assemblages.
Although our survey did not encompass all possible local environmental
factors (e.g., habitat size, drainage area, riparian condition), it
showed that impacts of human land use on taxonomic and functional
diversities of fish, arthropod, and macrophyte were consistent even
accounting for environmental, stream morphology, and climate predictors.
These results are robust and represent the first spatially extensive
analysis of the response of multiple biodiversity facets across entire
aquatic communities to intensive human land-use in the Neotropics. Our
finding is in line with previous analyses from the terrestrial realm
(Newbold et al., 2015; Gossner et al., 2016),
showing the negative impacts of
intensive human land-use also on hyperdiverse aquatic systems. Because
diversity of fish, arthropod and macrophyte assemblages responded to
different human land-use types, we believe that those different land
uses may act in concert to drive biodiversity patterns across multiple
assemblages. Consequently, the focus on isolated land-use types likely
hinders our ability to understand and manage biodiversity response to
human activities across landscapes worldwide. More studies should focus
on the combined impacts of multiple human stressors if we are to
effectively mitigate biodiversity losses and safeguard ecosystem
functioning (e.g., Benkwitt et al., 2020)
Trait diversity has been reported to decline with increasingly intensive
human land-use (Newbold et al., 2020). We demonstrated significant
declines in diversity of recruitment and life history, resource and
habitat use, and body size of different assemblages to intensive covers
of agriculture, pasture, and urbanization. Those land uses often degrade
stream environmental conditions by (i) reducing habitat and resource
availability; (ii) increasing over-exploitation, mainly of large
individuals; and (iii) compromising water quality via excessive inputs
of nutrients, pesticides, fertilizers and sewage (Allan et al., 2005;
Walker & Walters, 2019; Marques et al., 2021). Here, there was a marked
decline of sediment heterogeneity, stream depth, and water quality with
increasingly intensive land-uses. These human-induced stressors act as
filters selecting traits affecting organism resistance to disturbance,
thereby reshaping biotic community composition (Williams et al., 2020).
Lower habitat availability (due to sediment simplification) and stream
depth both reduce availability of feeding niches (Leitão et al., 2018,
Price et al., 2019), which favors generalist consumers and filters out
specialized consumers including many apex predators (Walker & Walters,
2019; Cantanhêde et al., 2021). Consequently, there was a drastic
reduction in the diversity of traits related to resource and
habitat-use. Environmental degradation also restricts the phenology of
organisms, including life histories (Morellato et al., 2016). We found
that organisms with long life spans and small geographical ranges could
not persist in streams with intensive agriculture, pasture, or
urbanization. Consequently, the diversity of life history traits
decreased. Intensive human land-use has disproportionately strong
impacts on large-sized organisms because they are more vulnerable to
loss of habitat and need a greater diversity of resources, which are
scarcer under high land-use intensity (Newbold et al., 2015). This is in
agreement with the strong declines in body size of fish and arthropods,
and macrophytes, in streams experiencing intensive agriculture, pasture
and urbanization catchment coverage. Our findings suggest a loss of
trait diversity in human-altered ecosystems that is likely to impair
ecosystem functioning (Frainer et al., 2017; Le Bagousse-Pinguet et al.,
2021).
The structural equation modelling revealed that increased cover of human
land-use affected ecosystem functioning through direct and
biodiversity-mediated, indirect pathways. The two pathways were
consistent regardless of the study area (Amazonia and Uruguay),
suggesting a broad-scale decline of standing fish biomass resulting from
human land-use intensification. The direct effect of ILUC on standing
biomass is intuitive because fish biomass often declines in
human-dominated systems as a result of fishing pressure, pollution and
eutrophication (Duffy et al., 2016). Importantly, however, intensive
land-use cover had a strong negative effect on taxonomic richness and
functional diversity, particularly of fish and macrophytes, which
ultimately resulted in net negative effects on standing fish biomass.
Considering the trophic roles of macrophytes (primary producers) and
fish (apex consumers), this indicates that intensive land-use can
disrupt the bottom-up and top-down control of ecosystem biomass
production. Bottom-up control of biomass production has been widely
reported because macrophytes are basal organisms that structure habitats
and enhance biomass production (Teixeira de Mello et al., 2015; Marsh et
al., 2021). Fish are major apex consumers in aquatic ecosystems, and
they can maintain high biomass despite relatively high human pressures
(Duffy et al., 2016). Arthropods enhanced fish taxonomic and functional
diversities, which indirectly increased standing fish biomass. This
suggests that arthropods indirectly affected stream functioning through
fish biodiversity. Arthropods are important food resources for fish, and
their diversity is often linked to greater production of fish biomass
(Correa & Winemiller, 2018). Although intensive human land use had
negative effects on biodiversity, this did not break down positive
biodiversity-ecosystem functioning relationships. This finding suggests
that biodiversity can buffer ecosystem functioning against human
pressures (Isbell et al., 2015). Thus, preserving high levels of
biodiversity, including both taxonomic and functional components, is
essential to maintain healthy ecosystem functioning in light of
increasing human pressures.
By decomposing SEM results between study areas (Amazonia and Uruguay),
we found stronger associations between diversity of different
assemblages in Amazonia than in Uruguay. In general, the positive
association between fish and arthropod diversity was stronger in
Amazonia. This can be partly explained by the higher macrophyte
diversity in Amazonia, which likely facilitates coexistence between fish
and arthropods via increasing habitat heterogeneity (García-Girón et
al., 2020; Monato et al., 2021). The taxonomic richness and functional
diversity of macrophytes were 51% and 18% higher in Amazonia (29
species; FD= 2.8) than in Uruguay (14 species; FD= 2.3). Combined with
the fact that the positive association between macrophyte on arthropod
diversity was also stronger in Amazonia, these findings imply a strong
bottom-up effect from the primary producers, favoring positive
relationships between fish and arthropod assemblages diversities.
.
Assemblage functional diversity increased with assemblage taxonomic
richness, indicating relatively low functional redundancy in the study
ecosystems. The low functional redundancy suggests that fish, arthropod,
and macrophyte are mostly composed of taxa with sets of different
traits. This agrees with the low functional redundancy that is predicted
by biogeographical hypotheses for the Neotropics (see, Leitão et al.,
2016; Rodrigues-Filho et al., 2018). This implies that Neotropical
biodiversity is particularly vulnerable to human pressures — species
loss will likely be closely accompanied by declines in functional
diversity. In addition, consistent positive associations between
diversities of different assemblages highlight that biodiversity is
closely related in these streams. Therefore, the diversity loss of any
taxonomic group would result in cascading effects on the diversity of
other taxonomic groups. We draw this conclusion based on the evidence
that the negative effect of intensive land-use on macrophyte diversity
indirectly affected fish and arthropod diversities, which likely
unraveled the interactions between these consumers (Figure 6). We argue
that preserving biodiversity requires mutual conservation of different
facets of biodiversity across multiple Neotropical assemblages.
The conversion of natural landscapes for human use is a global problem
that has transformed Earth‘s surface (Foley et al., 2005). Our dataset
revealed how intensive cover of human land-use types differentially
affect the taxonomic and functional diversities of three key stream
assemblages. We demonstrated that taxonomic richness, functional
diversity, and diversity of trait categories of fish, arthropod and
macrophyte strongly declined with intensive cover of agriculture,
pasture and urbanization. Our findings indicate that biodiversity
conservation strategies should focus on joint management of multiple
pressures at the catchment level. Biodiversity conservation will become
even more challenging in the coming decades given projected increases in
human population and climate chaos (United Nations 2018). We have shown
that intensive human land uses reduce standing fish biomass, which
occurs both directly and indirectly (mediated by taxonomic and
functional diversities), by reducing the levels of biodiversity needed
to maintain this function. This illustrates that intensive human land
use impairs ecosystem function through multiple biodiversity facets,
which suggest that biodiversity conservation alone is unlikely to
suffice for sustaining ecosystem functions if underlying human pressures
are not reduced.