Introduction
The high rates of change in natural environments direct and indirectly
caused by human activities may exert negative effects on terrestrial and
aquatic ecosystems (Secretariat of the Convention on Biological
Diversity 2020). In lotic environments, human modifications at local
(e.g., removal of riparian vegetation), regional (e.g., river dam) and
global scales (e.g., climate change) can change water flow, and
regularity of flood and flash flood events (Allan 2004, Dudgeon et al.
2006, Poff et al. 2007, IPCC 2014). Changes made to the surroundings of
lotic systems, such as the removal of riparian vegetation and the
linearization of water courses, reduces the area available for water
infiltration, increasing the occurrence of flood and flash flood events
(Resh et al. 1988). Furthermore, global climate changes tend to alter
the precipitation regime on a global scale (IPCC 2014, Fischer and
Knutti 2015, Donat et al. 2016). In tropical regions, precipitation
tends to become more concentrated in shorter periods of time, and,
consequently, the amounts of heavy precipitation events is expected to
increase in the coming years (IPCC 2014, Fischer and Knutti 2015). Then,
the flow regime of lotic systems will change due to an expected increase
in disturbance events caused by altered precipitation. Despite this
general expectation, few studies have experimentally addressed the
impact of such changes in the biodiversity of tropical streams.
Manipulative experiments are key for understanding the mechanisms behind
biodiversity response to environmental changes because, they can
separate the effects of different drivers related with the disturbance.
For a long time aquatic environments have been undergoing hydrological
disturbances, and studies addressing this topic are essential to
understand ecosystem dynamics and biodiversity patterns (Townsend et al.
1997b, Lake 2000, Melo et al. 2003, Sutherland and Freckleton 2013,
Shabarova et al. 2021). Historically, knowledge on the effects of
hydrological disturbances on biodiversity (e.g., water withdrawal or
flash floods) usually focus on taxonomic measures such as species
richness and beta diversity indices (Death 1996, Lake 2000, Melo and
Froehlich 2004). However, environmental disturbances can affect species
differently depending on their functional traits, which still needs to
be further studied for aquatic systems (Menezes et al. 2010). Community
functionality is an important component of biodiversity, once function
considers not only the identity of organisms, but the physiological,
behavioral and/or ecological responses of species to the environment
(Verberk et al. 2013).
Traits related to permanence and colonization are important for aquatic
communities when facing hydrological disturbances. Permanence and
colonization traits can be inserted in the so-called
resistance/resilience axis, which is relative to the organism occurrence
(Townsend et al. 1997a, Verberk et al. 2013, Sarremejane et al. 2020,
Wilkes et al. 2020). Resistance traits, such as morphological
adaptations for fixation on substrates and body shape, allow organisms
to maintain persistent population sizes when resisting disturbances
(Mazzucco et al. 2015). Resilience traits are related to life cycle
length such that short life cycles enable faster colonization in newly
impacted environments. The relationship between resistance and
resilience traits is a trade-off, once organisms that have a high
resistance to disturbance tend to present reduced resilience
characteristics, whereas organisms with low resistance to disturbance
tend to have increased resilience (Kappes et al. 2014, Ferzoco 2019).
Functional traits related to the use of the environment (Wilkes et al.
2020), such as for food acquisition and to avoid competition, also
affect community functionality. Hydrological disturbances can affect the
relative role that functional traits within the resistance/resilience
and environment use axes have on organism distribution. An increased
environmental disturbance tends to reduce the importance that
environmental drivers and competitive interactions have on the
functional diversity of communities in a given environment (Lopez et al.
2016). If environmental requirements become less relevant when
environmental disturbance is high, aquatic communities will tend to be
structured according to traits related to resistance and resilience. In
such circumstances, we can expect organisms with different attributes to
have different responses to hydrological disturbances. On the other
hand, differences in the intensity and frequency of disturbances would
select different traits in the communities, leading to different
trajectories in the structure of the communities over time.
Under this perspective, we conducted a manipulative hydrological
experiment in streams of the Cerrado hotspot to test whether modifying
stream flow alter the rate of occurrence and colonization of aquatic
insects with different functional traits. We tested two general
hypotheses: i) an increased disturbance intensity would reduce the
persistence of organisms in the substrates; and ii) higher disturbance
frequency would negatively affect organisms that maximize the permanence
in the environments. To test these two hypotheses, we categorized the
aquatic insects into nine groups with similar functional traits, both
morphological and ecological. For each group we established hypotheses
related to the permanence and colonization parameters. We designed an
experiment in a space-state design (Pedersen et al. 2011), changing both
intensity and frequency of water flow, and evaluated the occurrence of
the genera. We also estimated how these changes influenced the
probabilities of permanence and colonization of each functional group
using a Bayesian model. We showed that the frequency of the disturbance
was more relevant in changing the probabilities of persistence and
colonization, and that this effect is dependent on the functional group.