Introduction: biodiversity as an integrative concept
Whereas species diversity is a community attribute, the newer concept of biodiversity encompasses all levels of organization while integrating biotic and abiotic patterns and processes across scales (Noss, 1990; Johnson et al., 1996; Mooney et al., 1996; Naeem & Li, 1997; Peterson, Allen & Holling, 1998; Wetzel, 1999). Because of its broad scope and multidimensional nature, biodiversity has the potential to serve a unifying role (i) by linking ecology with evolution, genetics and biogeography, (ii) by elucidating how interactions between disturbance regimes and habitat heterogeneity influence niche diversification and resource partitioning, (iii) by integrating functional processes with spatio-temporal heterogeneity, (iv) by promulgating a hierarchical perspective of ecosystems, and (v) by providing a basis for effective management and restoration initiatives.
The primary goal of this paper is to develop a holistic view of biodiversity as a unifying theme for investigating river ecosystems. We believe that such an approach will lead to a better understanding of the complex behaviour of both natural and altered rivers, as well as elucidating critical interactions between the environment and the biota. However, it is not feasible here to treat all aspects of such a broad topic. Some major aspects, such as genetic diversity, are not considered. The goal is not to present a comprehensive review, but to develop some thoughts on biodiversity in river systems from a largely conceptual perspective, with selected examples drawn from our research on European rivers. In cases where empirical data are presented, the reader is referred to published works for details on sampling programmes and analytical procedures. The following focuses on habitat and landscape spatial scales and the ecological time scale, emphasizing the major role of natural disturbance in riverine flood plains, thereby serving as a logical extension of Ward, Tockner & Schiemer (1999c).
Biodiversity in a riverine context
Lotic ecology is a young discipline that developed after virtually all European river systems had been severely engineered for purposes of navigation, flood control and human use of the river corridor (Whitton, 1984; Petts, Moller & Roux, 1989). Therefore, the conceptual foundations of the discipline were somewhat constrained by a limited appreciation of the crucial roles of spatial complexity and fluvial dynamics in natural river ecosystems. This is especially apparent for floodplain reaches, in which main channels have been straightened, dredged and confined by levées. In addition, the flood plains themselves underwent major modifications to accommodate agricultural production and human habitation. Until quite recently, and with few exceptions (Antipa, 1912), river ecologists perceived river courses as stable, single-thread channels with little or no consideration of flood plains or contiguous groundwater aquifers. Such a perception produces a distorted picture of structural and functional attributes, including biodiversity (Ward & Stanford, 1995).
Natural alluvial rivers are characterized by shifting multiple channels and a remarkable degree of environmental heterogeneity engendered by complex interactions and transitions between surface waters, subsurface waters and riparian systems (Fig. 1), all of which are integral components of river ecosystems (Ward et al., 1998). It is only a slight exaggeration to state that the conceptual foundations of stream ecology were derived largely from studies confined to lotic surface waters, although such an approach may only be valid for certain headwater streams and canyon-constrained riverine reaches, or for highly managed rivers that have been isolated from floodplain–aquifer systems. For example, Hynes (1975), while promulgating a catchment approach, largely ignored the influence of the flood plain. The River Continuum Concept (RCC, Vannote et al., 1980), arguably the most influential paper in river ecology over the past two decades, also lacked a floodplain perspective. It required studies from tropical rivers which had escaped severe anthropogenic impacts (e.g. Welcomme, 1979; Junk, Bayley & Sparks, 1989) and historical investigations of temperate rivers (e.g. Sedell & Frogatt, 1984; Triska, 1984; Petts et al., 1989) to make lotic ecologists recognize the extent to which regulated rivers deviate from pristine conditions (Dynesius & Nilsson, 1994). As stated by Welcomme (1995), evidence ‘indicates that temperate rivers in their pristine condition acted in a similar manner to tropical systems’. In this paper, we present examples from our research on European rivers to illustrate the remarkable degree of spatio-temporal heterogeneity that may be attained where natural processes operate on a large scale.
In this article, riverine biodiversity is discussed under the headings structural (spatial) diversity, functional (process) diversity and species diversity, although a clear separation between them is not always possible. Under Structural diversity, we consider the types and spatial array of habitat/landscape elements, including environmental gradients, as part of the landscape mosaic in river corridors. Within the subsection Landscape dynamics, we present examples to illustrate links between spatial heterogeneity and hydrologic processes at the landscape scale in river flood plains. Functional diversity from a riverine context includes myriad phenomena such as disturbance processes, nutrient cycling, energy flow and biotic interactions. In this section, the focus is on the interactions between hydrological and successional processes in flood plains. We argue that fluvial dynamics, greatly suppressed by human intervention, is a major component of functional diversity in natural river ecosystems. The section on Species diversity begins with a theoretical consideration of the determinants of species diversity, presents plots of species richness along a floodplain connectivity gradient, and ends with a hierarchical model for assessing alpha, beta and gamma diversity.