Ecosystem structure and function of afrotropical streams with contrasting land use

Abstract

Land use changes can strongly influence stream ecosystems, yet these effects remain poorly documented in many hotspots of deforestation such as tropical Africa. The few studies conducted in this region have mostly focused on structural aspects of stream integrity; sparse data are available for key ecosystem processes such as ecosystem metabolism and litter decomposition. To address this knowledge gap, we measured multiple ecosystem functions in rainforest and nearby agricultural (farm) streams located in an afrotropical deforestation hotspot. In two farm and forest stream pairs, we quantified the standing stock biomass of basal resources and invertebrate functional feeding groups, measured rates of litter decomposition (microbial and shredder‐mediated) and modelled invertebrate secondary production. We also calculated whole‐stream metabolic parameters over multiple weeks. To test the regional generality of our results, we then reanalysed a published dataset and assessed the functional composition of invertebrate communities from many local streams. We found that farm streams had greater algal standing stocks, gross primary production (GPP) and production‐to‐respiration ratios than forest streams, although they remained strongly heterotrophic with relatively low GPP values. Agricultural land use also led to the almost complete extirpation of invertebrate shredders, resulting in much slower litter decomposition rates at farm than forest sites despite comparable rates of microbial decomposition. Finally, farm streams had lower invertebrate biomass despite abundant basal resources, leading to a 41%–66% reduction in estimated invertebrate production. We speculate that the functional response of our streams to agricultural land use (i.e., a modest increase in GPP but large declines in invertebrate production and shredder‐mediated litter decomposition rates) could be common across the afrotropics due to low fertiliser usage in this region and/or features of the local fauna. Although our study focuses a small number of streams, the large effect sizes that we report suggest that afrotropical streams could be strongly affected by rapid land use changes in the region, highlighting the need for additional research, monitoring, and conservation efforts in this poorly studied area of the world that is under increasing human pressures.

Land Use Change Influences Ecosystem Function in Headwater Streams of the Lowland Amazon Basin

“…Our GPP values were similar to other studies in the neighboring Cerrado region (<0.1-0.818 g m −2 d −1 ; [95,96]), the Peruvian Amazon (0.07-0.189 g m −2 d −1 ; [97]), and forested headwater tropical streams more broadly, which typically have very low or undetectable rates of GPP [9,92,93,98,99]. Rates of ER ranged from 0.1-11.9 g m −2 d −1 , which was similar to studies from the Cerrado (0.12-19.58 g m −2 d −1 ; [95,96,99], Amazon (1.8-15. Although we found no land use or seasonal differences in rates of GPP, it was more frequently measurable and, thus, higher in cropland streams than forested streams (Tables S7 and S8, Figure 5), similar to studies showing greater GPP in agricultural streams [92,100,101]. We measured lower rates of GPP than reported in streams where riparian forest cover was highly disturbed or absent, e.g., [92,93].…”

“…We found no significant effect of land use on ER. Other studies in tropical headwater streams have generally found ER to be higher in agricultural watersheds than forested watersheds [92,93,[95][96][97] but not always [99,100]. The major difference we observed was that ER did not change seasonally in forested streams but was~2.8 times greater in the wet season compared to the dry season in cropland streams.…”

Editorial: Advances in Biomonitoring for the Sustainability of Vulnerable African Riverine Ecosystems

“…Although data is limited, the status of African freshwater resources is one of a general decline (Darwall et al, 2011;UNEP-WCMC, 2016). Major concerns have been raised on the effects of pollution, urbanization, landuse change, overexploitation of biological resources, agriculture and invasive species on water quality and quantity, aquatic biodiversity, nutrient cycling, and energy sources supporting food webs (e.g., Masese et al, 2017;Fugère et al, 2018;Sayer et al, 2018;Matomela et al, 2021). These impacts are reducing ecosystem services offered by rivers and their floodplains, and undermining human well-being across the continent (IPBES, 2018).…”

No Difference in Instream Decomposition Among Upland Agricultural and Forested Streams in Kenya

“…This approach has been extended to leaf litter decomposition rates (Gessner and Chauvet, 2002;Young and Matthaei, 2008). Several studies have investigated the influence of agriculture on the composition of macroinvertebrates functional feeding groups and leaf litter decomposition in tropical streams (Jinggut et al, 2012;Boyero et al, 2015;Fugère et al, 2018;Cornejo et al, 2020). Compared to temperate streams, higher mean water temperature and low diversity of shredders in tropical streams have been suggested as the main reasons why litter decomposition may be driven mainly by microorganisms (Irons et al, 1994;Boyero et al, 2011;Tiegs et al, 2019).…”