Abstract
Water quality and riparian communities are among the most affected stream components by agriculture. However, little is known about the effects of riparian management for both aquatic and terrestrial taxa at different spatial scales. Here, we surveyed aquatic (diatoms) and terrestrial taxa (bryophytes, vascular plants, litter-dwelling snails, and ground and volant arthropods), to compare the abundance and richness of riparian taxa and chemical quality between reference and exposed sites in two stream reaches each of c. 3.5 km in northwestern Spain. Impacts in exposed sites were mainly due to traditional farming practices (TFPs), which included traditional meadow management, weirs built for now-unused water mills and sporadic timber harvesting. Therefore, we measured ten covariates and predictors related to the intensification of TFPs at local and within-stream scales and explored associations with riparian and water-quality measures to study the potential effects of TFPs in more detail. Reference and exposed sites did not differ significantly in water properties (diatom-biotic indices, conductivity, total organic carbon and nitrates), but exposed sites had less concentrations of soil metals Cd, Cu, Ni and Zn and less cover and richness of riparian trees, as inferred by the index QBR. Exposed sites had more volant insect decomposers and reference sites a greater abundance or richness of snails, ground predators and decomposers. Bryophyte richness was greater in reference sites. Our inferences may inform the joint cumulative downstream effects of weirs, meadow uses and riparian alterations but were generally consistent with most riparian taxa benefiting from having larger forested areas. Given the contrasting responses among taxa, we argue that land snails, terrestrial flies, and centipedes may be valuable additions to current riparian assessments mostly based on plants, beetles and spiders as indicator taxa. Our study also suggests caution when inferring farming impacts on streams from the surface area of pastoral land.
Highlights
The use of traditional farming practices (TFPs) is becoming rare due to the movement of people from rural areas to cities and from the expansion of intensive farming (MacDonald et al, 2000; Johnson and Lichter, 2019)
In the LMM for diatoms (R2 = 0.81), we found that CEE scores were positively associated with understory plant height but were negatively with the number of upstream weirs and the riparian forest width (Table 3)
Our study suggests that the conservation of traditional meadow management in two stream reaches each of c. 3.5 km in northwestern Spain may have greater impacts on stream hydromorphology and riparian communities than on in-stream water quality
Summary
The use of traditional farming practices (TFPs) is becoming rare due to the movement of people from rural areas to cities and from the expansion of intensive farming (MacDonald et al, 2000; Johnson and Lichter, 2019). There is a risk of losing the cultural heritage of human populations and the allied biodiversity that has developed in these humanmodified landscapes (Gavin et al, 2015; Merçon et al, 2019) This socio-ecological situation may be reversed through direct payments to small farms to ensure the farms' continuity and ecological benefits (Guth et al, 2020). The design of funding schemes for empowering rural areas, such as the EU's Common Agricultural Policy (Pe'er et al, 2020; De Castro et al, 2020), may benefit from studies relating different intensities of TFP to changes in biodiversity and water quality in streams, which are among the most adversely affected ecosystems by agriculture (Allan, 2004; EEA, 2012; Flávio et al, 2017). The ecological effects of TFPs on streams should be studied both at local and broader spatial scales because many seemingly mild upstream effects may manifest as severe downstream effects (Peterson et al, 2001; Allan, 2004; EEA, 2012)
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