Biotic Interactions, Biodiversity, and Community ...

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Biotic Interactions, Biodiversity, and Community Productivity Richard Michalet and Blaise Touzard

Contents 4.1 Introduction..................................................................................................... 59 4.2 Biotic Interactions and Biodiversity................................................................60 4.2.1 Competition and Biodiversity..............................................................60 4.2.2 Facilitation and Biodiversity................................................................ 65 4.3 Biodiversity and Ecosystem Function: The Role of Biotic Interactions.......... 68 4.3.1 Niche Complementarity and Selection Effect..................................... 69 4.3.2 Facilitation........................................................................................... 69 4.4 Reconciling Diversity Experiments and Natural Patterns............................... 70 4.4.1 The Debate........................................................................................... 70 4.4.2 Constrained Environments and Facilitation........................................ 71 4.4.3 Productive Environments and Niche Complementarity...................... 71 References................................................................................................................. 75

4.1 Introduction The global decline in biodiversity induced by human activities has induced scientists to investigate the potential value of biodiversity for ecosystem services (Chapin et al. 2000; Schröter et al. 2005; Diaz et al. 2007). A large number of experiments have been conducted in the last decade, in particular in herbaceous communities, in order to assess the potential roles of different components of biodiversity for community productivity, stability, and invasibility (see Hooper et al. 2005). Because most results of these experiments were considered to be opposite to the natural patterns of biodiversity along productivity gradients (Loreau et al. 2001), this induced substantial debate about the underlying mechanisms of the effect of biodiversity on ecosystem functions (e.g., Huston et al. 2000). Furthermore, this created a new interest in the ecological drivers of biodiversity in natural environments, a topic of research that had interested plant ecologists in the past (Whittaker 1972; Grime 1973; Connell 1978; Huston 1979). In theoretical ecology, biotic interactions are considered to be important drivers of community composition and richness, together with chance biogeographical events (e.g., dispersal) and local environmental factors (Lortie et al. 2004; see 59

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also Figure 6.2 in Chapter 6 of this book). All hypotheses addressing the role of biodiversity for ecosystem functioning have also emphasized the crucial importance of biotic interactions. In this chapter we focus on (a) the role of negative and positive interactions (competition and facilitation) in driving natural patterns of biodiversity (Section 4.2) and (b) the effect of biodiversity for ecosystem functions and in particular productivity (Section 4.3). We will detail the main theoretical models proposed in the literature, while other chapters of this book will present more detailed results on both aspects of the relationship between diversity and productivity. In Section 4.4, we focus on the debate about the discrepancy between natural patterns of biodiversity and the results of diversity experiments. We will propose alternative reconciliations to those existing in the literature, in particular two hypotheses emphasizing the roles of facilitation and niche complementarity for both biodiversity and productivity along natural environmental gradients and in diversity experiments.

4.2 Biotic Interactions and Biodiversity 4.2.1 Competition and Biodiversity One important initial step for understanding the role of biotic interactions for biodiversity was done through the development and refining of the niche concept and in particular with the proposition of Hutchinson (1959) to distinguish the fundamental from the realized niche (Bruno, Stachowicz, and Bertness 2003; see Figure  4.1a). At this time, only competition was thought to affect the spatial distribution of species along environmental gradients and in particular to restrict the realized niche (or habitat, see Whittaker 1972) of poorly competitive species to the harshest part of their fundamental or ecophysiological niche (Ellenberg 1956; Austin and Smith 1989). Following the principle of “competitive exclusion” of Gause (1934), competition was thought to strongly affect species richness in the most benign environments, as demonstrated by a number of gradient analyses (e.g., Whittaker 1956).

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Figure 4.1  Inclusion of positive interactions (b) within the niche concept (a). Adapted from Bruno, Stachowicz, and Bertness (2003). With permission.

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Figure 4.2  The humped-back relationship between species richness and standing crop: (a) along a gradient of increasing management intensity and (b) along a gradient of increasing environmental stress. Adapted from Grime (1973). With permission.

Grime (1973) was the first to propose a conceptual model shaping variation in both species richness and biotic interactions along environmental gradients for herbaceous communities. He initially distinguished two main ecological gradients: a gradient of increasing disturbance (“increasing intensity of management,” Figure  4.2a) for fertile environments and a gradient of increasing environmental stress and decreasing productivity (Figure 4.2b). Using data from British communities, he obtained a humped-back or unimodal relationship between species diversity and environmental conditions along both gradients, with the highest species richness occurring at intermediate position along the gradients. The decrease in species richness occurring from sites with intermediate levels of stress or disturbance to very stressed or disturbed sites was proposed to be only driven by the species’ physiological tolerances to either environmental stress or disturbance but not by biotic processes (right part of the gradients, Figures 4.2a and 4.2b). In sharp contrast, competitive exclusion was thought to regulate diversity in benign environmental conditions (left part of the gradients, Figures 4.2a and 4.2b) due to the niche-shrinking process affecting ruderals and stress-tolerant species (Grime 1974), whereas the effect of the abiotic environment was thought to be minimal. A number of authors have gathered both humped-back relationships of Grime (1973) within a single graphical model diversely known as the “intermediate disturbance” or “compensatory mortality” hypothesis (Menge and Sutherland 1976;

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Figure 4.3  Hypothesized relationships between (a) diversity-productivity patterns driven by environmental conditions across sites (natural patterns) and (b) the local effect of diversity on productivity (experiments). Adapted from Loreau et al. (2001). With permission.

Connell 1978; Huston 1979). In this synthetic model, physical disturbance, predation, and physical stress are thought to similarly enhance diversity at intermediate position along a single gradient of mortality. In most textbooks and recent papers on the effect of diversity on community productivity, this relationship was currently named the unimodal or humped-back diversity-productivity relationship (e.g., Loreau et al. 2001; see Figure 4.3a). However, in a meta-analysis on the relationship between species richness and productivity, Mittelbach et al. (2001) have stressed that humped-back-shaped curves were especially common (65%) in studies of plant diversity that used plant biomass as a measure of productivity. It should be noted that the humped-back shape proposed by Grime (1973) for British communities was elaborated from results of studies that have also used plant biomass as a surrogate of productivity (Al-Mufti et al. 1977). Patterns of species richness are also highly dependent on the scale at which they are measured. Mittelbach et al. (2001) have shown that the humped-back shape observed at local and regional scales commonly disappears at a scale larger than a continent (see also Kikvidze et al. 2005). Furthermore, Huston (1999) argued that species interactions are likely to play a strong role in determining richness only at the local scale, whereas other mechanisms (including speciation and extinction) are more likely to affect species richness at larger scales. The effect of seed limitation has in particular been strongly emphasized by a number of authors (e.g., Tilman 1997; Zobel et al. 2000), and there is an increasing number of modeling studies that have shown that, at intermediate regional scale, stochastic factors may produce patterns of species richness that are similar to those recurrently described at local scale (Rajaniemi et al. 2006). For example, Loreau, Mouquet, and Gonzalez (2003) built a model showing that landscape-driven variation in dispersal rate drives both species diversity and ecosystem productivity. Similar results were found by Matthiessen and Hillebrand (2006) on benthic microalgal metacommunities. Helm, Hanski, and Pärtel (2006) emphasized the role of past

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landscape structure and thus the role of past species pools on community richness. Rajaniemi et al. (2006) showed that species pools (and climate variability) shape biodiversity at regional scale for sand dune communities in Israel, although other processes (competition and the abiotic environment) affect diversity at local scale. Similarly, Freestone and Harrisson (2006) showed that regional richness drives local richness in wetlands, although local factors may also operate. Alternatively, Zeiter, Stampfli, and Newbery (2006) showed that seed limitation was important for biodiversity only in intermediate-productivity sites but not in (a) stressed sites due to the effect of the abiotic conditions (see also Wilsey and Polley 2003) or (b) very productive ones because of the dominant role of competition. Foster and Dickson (2004) also demonstrated that biodiversity and productivity were regulated in grasslands from Kansas by both the biodiversity at the level of the propagule pool and fluctuations of resources, i.e., the abiotic environment, with strong interactive effects. Grime (1979) also considered that large-scale differences in species pools might also explain why grasslands from calcareous areas are in general more species-rich than grasslands from siliceous areas, independent of the competitive abilities of both species types. He argued that calcareous areas are more spatially represented in southern latitudes and siliceous areas in northern latitudes, where the higher abiotic constraints might have decreased the latter species pool relative to the former during Quaternary species migrations. However, Michalet et al. (2002) have shown that at local scale the lower species richness of subalpine communities from acidic soils (as compared with communities from calcareous soils) may be explained by differences in species strategies and competitive exclusion processes. Between 2000 and 2100 m of elevation in the French Alps, they analyzed differences in species composition between communities of both soil types using ordination techniques. They found a humped-back relationship between species richness and relevés scores along the first axis of a CA analysis (Figure 4.4), which axis was strongly and significantly correlated to an index of “aboveground space occupancy” (I = vegetation cover × vegetation height; r = 0.58, p < .001). The highest species richness was found at intermediate position along the gradient on deep and moderately acidic soils localized on calcareous rocks. From this intermediate position, species richness (a) strongly decreased with increasing CA axis 1 relevés scores and aboveground space occupancy on acidic soils localized on siliceous rocks and (b) moderately decreased with decreasing CA axis 1 relevés scores and aboveground space occupancy on dry calcareous soils. Additionally, Choler, Michalet, and Callaway (2001) experimentally measured competitive responses for five species in both a species-poor acidic community and a species-rich calcareous community. They found strong negative interactions for all five species from the acidic community and only one weakly significant competitive response in the calcareous community (Figure 4.5). Michalet et al. (2002) concluded that the lower species richness of acidic soils was very likely to be explained by the competitive exclusion of stress-tolerant species from calcareous soils by the competitive species from acidic soils. This increase in community biomass and competitive abilities of species along the gradient was due to the higher water availability of siliceous soils as compared to calcareous soils, which argument was supported by soil-water measurements.

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