Discussion
The increase of hydrological disturbances is expected to impact aquatic
biodiversity worldwide in the coming years (IPCC 2014, Fischer and
Knutti 2015). Previous studies have shown that high water flow intensity
and frequency can drive rapid declines in species richness and abundance
of aquatic biodiversity (Resh et al. 1988, Melo et al. 2003). However,
functional responses of tropical biodiversity, particularly aquatic
insects, remains poorly understood. Here, we showed different responses
of aquatic insect to changes in the water flow frequency and intensity
in streams of the Cerrado biome, a biodiversity hotspot expected to
experience strong hydrological changes related to land use and climate
change (Bowman 2016). In general, we observed that a greater water flow
intensity tends to affect the permanence of almost all insect groups;
however, this effect is softened over time. On the other hand, less
frequent disturbances, regardless of intensity, tend to reduce the
permanence of most groups of aquatic insects over time.
Our study adds evidence that a generalization like “water flow
intensity affected the permanence of…” is valid mainly for the
period immediately after the disturbance, because the effects of
disturbance lose its intensity over time on macroinvertebrates. This
pattern may be related to the colonization capacity of
macroinvertebrates, since many recolonize substrates subject to
disturbances in few hours or days (Townsend and Hildrew 1976, Brooks and
Boulton 1991, Landeiro et al. 2010, Godoy et al. 2016). In addition,
Poff & Zimmerman (2010) reviewed the response of aquatic organisms to
changes in water flow and showed that the abundance and diversity of
macroinvertebrates have mixed responses to changes in intensity of the
flow regime. We did not assess the abundance and diversity of aquatic
insects. Still, our results also indicate that the occurrence of insects
in relation to disturbance intensity depends on time elapsed
(immediately or days after the disturbance).
Frequency affected the permanence in an opposite direction than we had
hypothesized (second hypothesis) based on classical studies on
disturbance in aquatic systems (Connell 1978, Resh et al. 1988, Ward
1989, Townsend et al. 1997b). Most groups that showed this reduced
permanence when under less frequent disturbances (depressed bodied
beetles, cylindrical beetles, Ceratopogonidae, depressed body insects,
larvae with anal claws and shelter-building Trichoptera, Figure 4A, B,
D, G, H and I) had, in general, a greater baseline probability of
colonization than of permanence. The combination of these results shows
that the effect of disturbance frequency does not imprint a general
pattern in all groups, but rather depends on the functional
characteristics of the organisms.
Disturbance may be an external element that reduces the effects of
interactions between the organisms inhabiting the artificial substrates
(Menge and Sutherland 1987). With a higher frequency of disturbances,
habitats become more susceptible to colonization, because frequent
disturbances remove organisms from artificial substrates (Burton et al.
2020), loosening competition. In this scenario, groups with selected
characteristics that favor displacement and colonization in detriment of
mechanisms of resistance to disturbances may quickly colonize
open-habitats, or even increase their population sizes in a short period
of time (e.g., Chironomidae), thus increasing the likelihood of groups
permanence. On the other hand, less frequent disturbances can favor
populations with high growth rates, increasing the effect of
intraspecific competition (Huston 1979, Magurran and Huston 1995). This
may explain, for example, moderate probabilities of colonization and
permanence to Empididae and Simulidae, groups with strategies to remain
fixed on the stream substrate and fast life cycle (Carvalho and Uieda
2004, 2006, Landeiro et al. 2010). Such reasoning is congruent with part
of the assumptions of the intermediate disturbance hypothesis, in which
the frequency of disturbances is directly related to the relaxation of
interactions among species (Wilkinson 1999).