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).