The relationship between invertebrate community and the nitrate removal function

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Biodiversity and ecosystem functions

An ecosystem function is an intrinsic ecosystem characteristic related to the matter and energy flows resulting from ecosystem processes. The various ecosystem processes can result from a set of interactions between abiotic elements (e.g. CO2 exchange between water and the atmosphere), between an abiotic element and a living one (e.g. CO2 uptake by primary producers) or between living organisms within the system (e.g. predation) (Harrington et al., 2010).
According to the type of ecosystem, the biodiversity, defined as ‘the variety of life at any hierarchical level, including genes, species, functional groups and ecological diversity across all scales (spatial, temporal and biotic scales of organization) (Naeem, 2002)’ can be more or less important.
Darwin and Wallace (1858) were already concerned with understanding the fundamental mechanisms that mediate the functioning of diverse ecosystems but this theme mainly received more attentions from in the 1990’s. Indeed, a context of consequent biodiversity loss has stimulated functional ecology research focusing on the Biodiversity – Ecosystem Function (BEF) over the past two decades (Ehrlich and Ehrlich., 1981; Walker, 1992; Loreau et al., 2001; Hooper et al., 2005; Duffy et al., 2007; Cardinale et al., 2012; Santos-Martín et al., 2013; Science for Environment Policy, 2015). Hooper et al. (2012) showed that the effects of species loss on two important ecosystem functions (productivity and decomposition) are of comparable magnitude to the effects of many other global environmental changes, according to a suite of meta-analyses of published data. The BEF studies consider the contribution of biodiversity to ecosystem functions, where the main approach is to manipulate the biodiversity (mainly species richness) and investigate its consequences on ecosystem function under controlled conditions. On the other hand, there are also a few field surveys that investigate the biodiversity and ecosystem function simultaneously (rather than manipulate biodiversity) to study the relationship between them (e.g. Huryn and Huryn, 2002; Lecerf et al., 2006). In real world ecosystems, field conditions, both biodiversity and ecosystem functions may co-vary with many abiotic factors in different temporal and spatial patterns, and the variations of interactions between biodiversity and ecosystem functions are still unclear in field conditions (Tylianakis et al., First, regarding the BEF, many studies have demonstrated the fundamental role of biodiversity to regulate ecosystem functions (Díaz et al., 2006; De Bello et al., 2010; Cardinale et al., 2012; Quijas and Jackson, 2012). As biodiversity declines, processes such as primary production, biomass production and nutrient recycling, are reported to be impaired (Cardinale et al., 2012). Meanwhile, biodiversity effects enable these processes to be resistant and resilient in the face of global changes (Cardinale et al., 2012; Loreau and Mazancourt, 2013; Santos-Martín et al., 2013).
In fact, there are several hundreds of papers reporting more than 600 BEF experiments that manipulate more than 500 types of species concerning different ecosystem functions in different ecosystems: forest (e.g. Zhang et al., 2012; Cong et al., 2015), grassland (e.g. Isbell et al., 2011), soil (e.g. Bardgett and Van Der Putten, 2014; Wagg and Bender, 2014), freshwater (e.g. Lecerf and Richardson, 2010, Vaughn, 2010; Cardinale, 2011) and marine systems (e.g. Worm et al., 2006; Gamfeldt et al., 2015). And different designs were used according to (i) laboratory or field experiments (ii) biodiversity manipulation (e.g. species richness, evenness, functional groups) either by experiments changing biodiversity through direct manipulation (substitutive or additive experiments) or using indirect diversity gradients (natural variations or gradients in environmental conditions) (iii) maximum species number (Balvanera et al., 2006).
Three main points were drawn from reviewing these BEF studies:
1) A positive effect of biodiversity on ecosystem functions is generally reported (Loreau et al., 2001; Tilman et al., 2001; Hooper et al., 2005; Balvanera et al., 2006). The increasing biodiversity may enhance and stabilize ecosystem functions, or buffer ecosystems against stresses (Duffy, 2009; Loreau, 2010; Steudel et al., 2012), although negative and no effects of biodiversity on functions still exist.
2) This positive effect on any single ecosystem function is mainly reported to be non-linear and saturating, while the exact models to which BEF corresponds are still debated. Figure I-1 summarizes several models and hypotheses to explore the positive shape of BEF relationships (Boulton et al., 2008). Specifically, in the non-linear relationships, the initial biodiversity loss (X axis from right to left in Figure I-1) in ecosystem with high biodiversity has relative small impacts on ecosystems function, but the increasing loss results to the accelerating rate of change (Cardinale et al., 2012).
3) Concerning the type of biodiversity, in addition to species richness, the importance of species composition and functional properties (e.g. traits) on ecosystem functions has been highlighted. Functional traits can define the role of biological communities, identify the key characteristics and mechanisms by which the organisms interact with the ecosystem properties, and demonstrate the complexity of processes and interactions which occur in ecosystems. Thus this approach is useful to predict the functional consequences of biological changes caused by human activities (De Bello et al., 2010; Menezes et al., 2010).
Despite of these progresses, there are still some shortages and debates concerning the following aspects of BEF knowledge (Balvanera et al., 2006; De Bello et al., 2010; Cardinale et al., 2012):
1) Concerning the ecosystem type, much more studies are on terrestrial ecosystems than on marine and aquatic systems. Balvanera et al. (2006) inventoried, for example, 252 studies in terrestrial and 55 in freshwaters systems between 1954 and 2004.
2) Concerning the type of biodiversity and ecosystem functions, most studies focus on primary producers, especially on higher plants and algae.
3) Concerning the trophic levels, BEF studies are limited to one trophic level (horizontal diversity), only 2% BEF studies considered more than one trophic level (vertical diversity; mostly plants together with pollinators, soil invertebrates or microorganisms) (de Bello et al., 2010). Researches of BEF and trophic ecology have proceeded largely independently, although the incorporation of the vertical diversity into BEF arising from horizontal diversity changes is repeatedly suggested (Duffy et al., 2007; Reiss et al., 2009; Cardinale et al., 2012). Indeed, the loss of diversity across trophic levels is mentioned to influence ecosystem functions stronger than the intra-level loss within trophic levels, since food web interactions are key mediators of ecosystem functions (Bastian et al., 2008; Duffy, 2009; Lecerf and Richardson, 2010).
4) Many studies are designed at small scale and under highly controlled conditions. Their relevance to natural ecosystems and realistic biodiversity’s decline is often unclear (Duffy, 2009). Indeed, in natural ecosystems, where abiotic conditions are less controlled, biodiversity effects on ecosystem functions can be weaker or more difficult to distinguish compared to that in lab, since they could be overridden by the stronger influences of natural abiotic factors (Balvanera et al., 2006). On the other hand, biodiversity effects can also be sometimes stronger, since more niche differences in natural conditions may enhance the biodiversity effects (Zimmerman and Cardinale, 2014).
5) Multisite surveys of BEF relationship sometimes lead to controversy, since biodiversity at any single location, or at any particular time, usually differs from those at other locations and times.
In natural conditions, the biodiversity change, the covariation between biodiversity-ecosystem function-abiotic factors and the different tropic levels involved are the realistic scenarios of research background, which are hard to mimic by BEF experiments (mainly considering random combinations of species, controlled abiotic conditions and single trophic level). Then, recognising the limits of traditional BEF studies, it is worthwhile to conduct simultaneous investigations of biodiversity and ecosystem functions in field conditions, which could provide complementary information compared to the abovementioned BEF studies.
Of course difficulties exist in these fields survey, like that abiotic factors may serve as confounding factors for both biodiversity and ecosystem functions; and hence the causes or the consequences for the change of biodiversity and ecosystem functions is unclear. For example, biodiversity can drive ecosystem functions while ecosystem functions can limit or improve biodiversity development; biodiversity and/or ecosystem functions can change abiotic properties; abiotic changes may cause the variation of biodiversity and/or ecosystem functions (Cardinale and Nelson, 2009; Hooper et al., 2012). Specifically, regarding to the abiotic changes, biodiversity and functions may be both under control of chemical stresses and physical perturbations with different responses (McMahon et al., 2012).
We suggest to integrate traditional BEF studies and in field investigations in studying the relationship between biodiversity and ecosystem functions. Such studies should:
(1) not limit this observation to primary producer and production function but include other biochemical functions such as nutrient cycling; (2) consider more trophic levels (such as in field invertebrate and microorganism community) and expand to large scale experiments; (3) be explored with more efforts in aquatic ecosystems.
Freshwater ecosystems are among the most imperilled, biodiversity losses occurring much faster in freshwater than terrestrial or marine ecosystems (Dudgeon et al., 2006). River ecosystems are suggested as one of the most complex ecological systems to be explored for the studies about the relationship between biodiversity and ecosystem functions, which can integrate more trophic levels (riparian litter producers, aquatic micro-fungi, macro-invertebrates, and fishes) (Lecerf and Richardson, 2010).

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Biodiversity and Ecosystem Services (BES)

After reviewing biodiversity and ecosystem function relationships, this following section is dedicated to ecosystem services (ES), which can link the ecosystem functions to human society. Ecosystem services are the conditions and processes through which natural ecosystems, and the species that make them up, sustain and fulfil human life (Daily, 1997). Consequently, ES contribute to raise awareness of the importance of protecting ecosystems, and can also provide decision makers with quantitative data, enabling them to consider all aspects of the socio-economic-ecological system in which we live (Kremen, 2005; Cardinale et al., 2012).
A large number of studies concerning ecosystem services have been carried out over the last decade and major international search initiatives have formed and rapidly developed. The Millennium Ecosystem Assessment 2005 (MA 2005) firstly brought the concept and classification of ecosystem services into widespread use. Following MA, the Economics of Ecosystems and Biodiversity (TEEB, 2010) centring on economic valuation was launched. Then the Mapping and Assessment of Ecosystems and their Services (MAES) initiative aimed to produce a framework for ecosystem assessment to ensure a harmonised approach across the EU, which uses The Common International Classification of Ecosystem Services (CICES) for more detailed and more comprehensive classification of ES. The Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES) was established in 2012 with the aim to assess ecosystem services on a global level. The number of articles including “Ecosystem services” and the four main categories of ES are shown in Figure I-2 and I-3 respectively.
The complex set of relations that exist between human stress, biodiversity, ecosystem function, ecosystem service and humanity is shown in Figure I-4. On one hand, we need to look at what is the service currently used and expected by the society (socio-cultural and economic sectors) (i.e. demand-side). On the other hand, the knowledge about the capacity of the ecosystem to generate that service (i.e. supply-side) should be considered.
In the supply side, biodiversity can contribute to ecosystem service delivery with direct links, for example: rare species with intrinsic values (this biodiversity gains great concern from the traditional conservation aspect) or cultivated species with direct economic values. The role of these types of biodiversity in the ecosystem services is well stated, and this biodiversity has generally benefited from substantial managements and protection efforts (Mace et al., 2012).
Moreover, biodiversity can act as a regulator of ecosystem functions and then indirectly links to ecosystem services (Mace et al., 2012). For instance, the dynamics of soil nutrient cycles were demonstrated to be governed by the composition of biological communities in the soil, which shows the biodiversity effect on a regulation service (Lavelle et al., 2006). In order to show how biodiversity influences ES, it is required to understand how it influences ecosystem function. The large number of above-mentioned BEF studies (in section I.2) helps to identify what is the biodiversity involved in different ES and to understand how this biodiversity changes the flow of energy and material contributing to ES.

Water purification service

At global scale, although a few provision ecosystem services have been improved (e.g. crop provision), many key ecosystem services are at risk of degradation, mainly regulating services, e.g. 70% of the regulating services are degraded or being used unsustainably (MA, 2005). Particularly, water purification, as a regulating service controlling water quality, is of great importance for the dense populated regions with heavy pressure on water resources, such as Europe (European Water Framework Directive, TEEB 2010). For example, water purification seems to be the most degraded service among all regulating services in Spain (Santos-Martín et al., 2013).
In general, the MA emphasizes the identification and use of indicators for ecosystem services survey and trends assessments (MA, 2005). An ecosystem service indicator is information which communicates the characteristics and trends of ecosystem services, making it possible for policy-makers to understand the conditions of delivery, as well spatial and temporal trends and rate of change in ecosystem services (Layke et al., 2012). A rather broad interpretation of this definition includes datasets and proxy indicators such as land cover and land use (Maes et al., 2016).
Potential indicators used to map (or quantify) water purification service (i.e. biophysical indicator on the supply side) are nutrient retention capacity, denitrification, the area or proposition occupied by riparian forest, the amount of waste processed by ecosystems (volume/mass of water processes) and the naturalness of riverbeds and floodplains (Layke, 2009; Maes et al., 2012; La Notte et al., 2012 a, b; Albert et al., 2015). There are different approaches to conduct the biophysical assessment of water purification service delivery at different scales. For example, the nutrient retention capacity is commonly used in approaches to quantify the water purification capacity in laboratory experiments (microcosm), in situ measurements (e.g. nutrient enrichment experiments) and modelling approaches (e.g. The Soil and Water Assessment Tool (SWAT) models).

Table of contents :

Chapitre I: General introduction
I.1 Résumé du chapitre I
I.2 Biodiversity and ecosystem functions
I.3 Biodiversity and Ecosystem Services (BES)
I.4 Water purification service and nitrate removal
I.5 What are the links between invertebrates and the nitrate removal function?
I.6 Ecosystem functions and nitrate removal under stress in rivers
I.7 Objectives and organization of the thesis
Chapter II: The relationship between invertebrate community and the nitrate removal function
II.1 Résumé du chapitre II
II.2 Part 1: Effect of time and invertebrates on nitrate removal in laboratory experimental conditions
II.3 Part 2: Effects of macroinvertebrate traits on nitrate removal in stream sediments
II.4 Main discussion
Chapter III: The relationship between invertebrate community and the nitrate removal function in the condition of stress
III.1 Résumé du chapitre I III
III.2 Part 1: Effects of meiofauna and macrofauna on nitrate reduction in freshwater
macro-porous sediment under pesticide stress
III.3 Part 2:Biodiversity and ecosystem purification service in an alluvial wetland
III.4 Main discussion
Chapitre IV: General discussion, conclusion and perspectives
IV.1 General discussion
IV.2 Conclusion
IV.3 Perspective
References

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