Introduction
Habitat loss and fragmentation, over-exploitation of natural resources, climate change, and introduction of invasive species are listed among the main causes of the current environmental crisis, characterized by an impressive extinction rate (Dirzo, Ceballos, & Ehrlich, 2022). Among the various strategies implemented to contrast this negative trend, the increase and extension of protected areas are considered one of the most useful and immediate (Lovejoy, 2006; Acreman, Hughes, Arthington, Tickner, & Dueñas, 2020). Unfortunately, however, while the creation of natural parks can be effective in protecting and conserving terrestrial ecosystems and species, the difficulties are often much greater regarding lotic environments and organisms living within them. In fact, because of their longitudinal and dendritic shape, streams are subject to stressors that affect their structure and functioning over large spatial scales. Usually, protected areas cover only a part of the catchment through which a river flows, so the pressures exerted in the downstream and especially in the upstream section can have dramatic repercussions on the integrity of the protected area. For these reasons, despite being among the most threatened environments (Harrison et al., 2018), stream ecosystems paradoxically are among the most difficult to protect, unless the protected area is located at the head of the drainage area.
Alpine and mountain low-order lotic systems are incomparable hotspots of biodiversity, rich in highly sensitive and stenothermic taxa strongly adapted to harsh conditions such as low temperatures, seasonal high flows, and physical instability of channels (Petts, Gurnell, & Milner, 2000; Smith, Hannah, Gurnell, & Petts, 2001). Unfortunately, these environments are increasingly threatened by factors acting on a global scale, such as temperature growth, changes in precipitation amounts, patterns, and seasonality, and at a local scale, in particular human derived alterations of the hydro-morphological characteristics (Piano et al., 2020; Doretto et al., 2021). Even in relatively pristine areas, the hydro-morphology of Alpine rivers is often altered by human interventions such as bank reinforcement, water abstractions, presence of dams and weirs. These interventions are becoming more and more frequent in recent decades due to the growing need for water and modification in the natural hydrological cycle induced by climate change, representing a major threat to Alpine lotic ecosystems and related aquatic communities.
About 90% of all alpine water courses are used and capitalized on for producing electricity through hydropower (CIPRA, 2014), also in protected areas. Regardless of the plant size and typology, water storage and abstraction imply morphological alterations of river bottoms and banks which can threaten aquatic biodiversity. In recent years, the estimation of local and species contributions to ß-diversity is receiving increasing scientific interest (Vilmi, Karjalainen, & Heino, 2017; Bo, Doretto, Levrino, & Fenoglio, 2020). Indeed, ß-diversity plays a key role in conservation and bioassessment viewpoints because it allows the identification of spatial trends and sites that most contribute to regional diversity and may address specific restoration programs. This can be achieved by considering species and local contribution to ß-diversity in addition to more traditional metrics widely used in environmental bioassessment, such as biotic or diversity indices.
In 2005 we carried out a study that was the first evaluation of high-altitude river morphological alterations in an Alpine protected area, the Gran Paradiso National Park, the oldest National Park in Italy, and their effect on the benthic (macroinvertebrates and diatoms) communities included in the EU Water Directive Framework 2000/60 (Bona, Falasco, Fenoglio, Iorio, & Badino, 2008, in this journal).
In this study, we sought to assess if the presence of the protected area prevented significant local alteration of the stream habitat and if the aquatic biodiversity and ecological quality of the sampled stretches were comparable after 15 years or underwent significant changes, measured as biotic indices and ß-diversity-related metrics. With this aim, we repeated the same samplings, on the same sites and with the same team and methods.