Functional Ecology ( IF 4.6 ) Pub Date : 2022-06-24 , DOI: 10.1111/1365-2435.14122 Romy Wild 1, 2 , Björn Gücker 3 , Markus Weitere 1 , Mario Brauns 1
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1 INTRODUCTION
Agricultural land use has been identified as one of the most severe threats to freshwater ecosystems and biodiversity (Dudgeon et al., 2006; Tilman et al., 2001) and often involves deforestation and riparian clear-cutting, which results in decreased habitat heterogeneity, higher water temperatures and light penetration due to the loss or reduction of riparian shading (Young & Huryn, 1999). Furthermore, terrestrial particulate organic matter (POM) inputs and the temporal availability of resources are altered in agricultural streams (Wild et al., 2019). Excess inputs of nutrients (Weijters et al., 2009; Woodward et al., 2012) increased sedimentation, channelization, and altered hydrology degrade the natural properties of stream ecosystems (Allan, 2004; Gücker et al., 2009).
The biological effects of agricultural land use on stream ecosystems are commonly evaluated in terms of community structure, for example, species composition and diversity metrics (Lammert & Allan, 1999; Sponseller et al., 2001). However, structural metrics alone are insufficient to fully understand the functional consequences of anthropogenic perturbations. Thus, studies increasingly investigate the effects of land-use alterations also on a functional level, using functional trait categories (Doledec et al., 1999; Menezes et al., 2010) such as functional feeding groups (FFGs) (Doledec et al., 2006; García et al., 2017) and ecosystem processes such as leaf litter decomposition (Gessner & Chauvet, 2002) or macroinvertebrate secondary production (MSP) (Benke & Huryn, 2010; Gücker et al., 2009).
MSP is an ecosystem process reflecting the functional performance and fitness of a population by integrating information on population dynamics, biomass, growth rates and mortality (Benke & Huryn, 2007; Dolbeth et al., 2012). Several studies have experimentally linked aggregate (Shieh et al., 2002) and individual agricultural stressors to MSP, showing a decrease in productivity following the application of pesticides (Lugthart & Wallace, 1992 ) or removal of terrestrial POM from forested streams (Wallace et al., 2015). Nutrient enrichment and higher nutrient quality, in contrast, considerably increased MSP in forested headwaters (Cross et al., 2006, 2007) and urban streams (Gücker et al., 2011). Agricultural stressors can affect the productivity of benthic communities in two ways. Either, environmental conditions in agricultural streams exceed community-wide environmental tolerance levels, leading to an overall decrease in productivity. Or alternatively, agricultural stressors change benthic community composition toward more tolerant and generalist species, profiting from higher nutrient concentrations and higher food quality, leading to increased productivity. Although no study has compared MSP of pristine headwater sites with that of agricultural sites, most research that addressed the aggregate effects of agriculture on MSP showed an overall increase in productivity (Finlay, 2011; Shieh et al., 2002; Statzner & Lévêque, 2007).
The ecological theory relevant to explain biological mechanisms behind patterns of productivity has been developed within two major ecological concepts, the biodiversity-ecosystem functioning theory (BEF) (Naeem, 2002) and the productivity–diversity relationship (PDR) (Currie, 1991). Within the PDR, nutrient and resource supply have been hypothesized to control species richness within a hump-shaped relationship. Species richness is highest at intermediate levels of resource supply and decreases when resource levels are too low for species to coexist or when resource supply is too high, which releases certain species from bottom-up control, which then outcompete less competitive species. Within the BEF framework, species richness is controlling, rather than responding to, the productivity of a community based on mechanisms such as complementary resource use, niche partitioning and identity effects (Ives, 1995). Cardinale et al. (2009) achieved a unifying theory, the multivariate productivity-diversity (MPD) hypothesis explaining how PDR and BEF operate concurrently. The MPD hypothesis arguments that there is (a) a direct influence of resource supply on the biomass/productivity of primary producers (PDR), (b) the producer biomass is directly influenced by the richness of species locally competing for resources (BEF), and as an interaction of the first two: (c) the resource supply rate indirectly affects producer biomass as it mediates the selection of a set of co-occurring species from a local colonist pool.
However, the question how abiotic and biotic mechanisms are interrelated in driving secondary production of headwater streams has been poorly addressed.
This study contributes to filling this gap by identifying environmental and biological mechanisms associated with alterations of MSP in streams of contrasting land-use types and exploring the interrelation between these drivers. Here, we estimated macroinvertebrate density, biomass and MSP in two forested reference and two agricultural headwater streams for 1 year. Moreover, we quantified environmental and resource characteristics of streams as well as interaction strengths between consumers and resources to address the following hypotheses:
Hypothesis 1.Environmental mechanisms
The shift from allochthonous to autochthonous primary resources is expected to increase the quantity and quality of resources, leading to higher MSP and biomass in agricultural than forested streams.
We also expect lower ingestion rates and higher interaction strengths of shredders and higher ingestion rates and lower interaction strengths of collector/gatherers and grazers in agricultural than in forested streams.
Hypothesis 2.Biological mechanisms
In accordance with predictions from PDR and mechanisms of environmental sorting, we expect that environmental stressors and high resource supply override positive biodiversity-ecosystem functioning effects leading to lower species richness in agricultural streams.
We expect lower evenness and higher dominance in agricultural streams than forested streams and a positive relationship between dominance and MSP due to the homogenization of habitat and resource conditions in agricultural streams.
Furthermore, we expect that lower fish biomass in agricultural streams, due to unfavourable habitat conditions, will lead to lower predation pressure and lower top-down control of MSP in these streams compared with forested streams.
中文翻译：
资源供应和生物优势与温带农业溪流中的高次生产量有关
1 简介
农业土地利用已被确定为对淡水生态系统和生物多样性的最严重威胁之一（Dudgeon 等人， 2006 年；Tilman 等人， 2001 年），并且经常涉及森林砍伐和河岸清除，导致栖息地异质性降低，由于河岸阴影的损失或减少，更高的水温和透光率（Young & Huryn, 1999）。此外，陆地颗粒有机物 (POM) 输入和资源的时间可用性在农业流中发生了变化（Wild 等人， 2019 年）。营养物质的过量输入（Weijters 等人， 2009 年；Woodward 等人， 2012 年) 增加的沉积、渠道化和改变的水文会降低河流生态系统的自然特性（Allan， 2004 年；Gücker 等人，2009 年）。
农业用地对河流生态系统的生物影响通常根据群落结构进行评估，例如物种组成和多样性指标（Lammert & Allan, 1999 ; Sponseller et al., 2001）。然而，仅凭结构指标不足以完全理解人为扰动的功能后果。因此，越来越多的研究使用功能性状类别 (Doledec et al., 1999 ; Menezes et al., 2010 )，例如功能性摄食组 (FFG) (Doledec et al., 2010)，研究土地利用变化对功能水平的影响。 , 2006 年；加西亚等人， 2017 年) 和生态系统过程，例如落叶分解 (Gessner & Chauvet, 2002 ) 或大型无脊椎动物二次生产 (MSP) (Benke & Huryn, 2010 ; Gücker et al., 2009 )。
MSP 是一个生态系统过程，通过整合有关种群动态、生物量、增长率和死亡率的信息，反映种群的功能表现和适应性（Benke & Huryn， 2007 年；Dolbeth 等人， 2012 年）。一些研究通过实验将聚集体（Shieh 等人， 2002 年）和个体农业压力源与 MSP 联系起来，表明在施用杀虫剂（Lugthart & Wallace， 1992 年）或从森林溪流中去除陆地 POM（Wallace 等人）后生产力下降., 2015 年）。相比之下，养分富集和更高的养分质量显着增加了森林源头中的 MSP（Cross 等， 2006 年，2007 年）) 和城市溪流 (Gücker et al., 2011 )。农业压力源可以通过两种方式影响底栖生物群落的生产力。要么，农业溪流中的环境条件超过了整个社区的环境耐受水平，导致生产力整体下降。或者，农业压力源将底栖群落组成改变为更宽容和更普遍的物种，从更高的营养浓度和更高的食品质量中获利，从而提高生产力。虽然没有研究将原始源头地点的 MSP 与农业地点的 MSP 进行比较，但大多数针对农业对 MSP 的总体影响的研究表明生产力总体提高（Finlay， 2011 年；Shieh 等人， 2002 年）; Statzner 和 Lévêque， 2007 年）。
与解释生产力模式背后的生物学机制相关的生态理论已经在两个主要生态概念中发展起来，即生物多样性-生态系统功能理论 (BEF) (Naeem, 2002 ) 和生产力-多样性关系 (PDR) (Currie, 1991 )）。在 PDR 中，养分和资源供应被假设为以驼峰形关系控制物种丰富度。物种丰富度在资源供应的中间水平最高，当资源水平太低而物种无法共存或资源供应太高时，物种丰富度会降低，这会使某些物种从自下而上的控制中释放出来，从而超过竞争力较弱的物种。在 BEF 框架内，物种丰富度控制而不是响应基于互补资源利用、生态位划分和身份效应等机制的群落生产力（Ives， 1995 年）。卡迪纳尔等人。( 2009) 实现了一个统一的理论，即解释 PDR 和 BEF 如何同时运作的多元生产力多样性 (MPD) 假设。MPD 假设认为 (a) 资源供应对初级生产者的生物量/生产力 (PDR) 有直接影响，(b) 生产者生物量直接受到当地竞争资源的物种丰富度 (BEF) 的影响，并且作为前两者的相互作用：（c）资源供应率间接影响生产者生物量，因为它介导从当地殖民者池中选择一组共生物种。
然而，非生物和生物机制如何在驱动源头水流的二次生产中相互关联的问题一直没有得到很好的解决。
本研究通过在不同的土地利用类型中识别与 MSP 变化相关的环境和生物机制，并探索这些驱动因素之间的相互关系，有助于填补这一空白。在这里，我们在两个森林参考和两个农业源头流中估计了 1 年的大型无脊椎动物密度、生物量和 MSP。此外，我们量化了流的环境和资源特征以及消费者和资源之间的交互强度，以解决以下假设：
假设 1.环境机制
预计从异地原生资源向本土原生资源的转变将增加资源的数量和质量，导致农业中的 MSP 和生物量高于森林溪流。
我们还预计，与森林溪流相比，在农业中，碎纸机的摄取率和相互作用强度更高，而采集/采集者和食草动物的摄取率和相互作用强度更高。
假设 2.生物学机制
根据 PDR 的预测和环境分类机制，我们预计环境压力和高资源供应会覆盖积极的生物多样性 - 生态系统功能效应，从而导致农业河流中物种丰富度降低。
由于农业溪流中栖息地和资源条件的同质化，我们预计农业溪流的均匀度低于森林溪流，优势度高于森林溪流，并且优势度与 MSP 之间存在正相关关系。
此外，我们预计，由于不利的栖息地条件，农业溪流中较低的鱼类生物量将导致与森林溪流相比，这些溪流中较低的捕食压力和较低的 MSP 自上而下控制。
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