During flooding, surface flow may recharge groundwater aquifers and spill out over the floodplains, eroding or depositing sediment in accordance with the energy dynamics of water interacting with geomorphic features. During dry periods, flow in the channel may be maintained by groundwater draining alluvial and karstic aquifers. Thus, rivers are not merely conduits for runoff from headwaters to the oceans. Rather, rivers are dynamic multidimensional pathways along which aquatic-terrestrial linkages vary spatially in three spatial dimensions and temporally often considered as a fourth dimension; Ward Anthropogenic influences contribute greatly to this variation, as river valleys have been foci for human settlements and commerce for millennia.
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Riparian and riverine plants and animals are variously adapted, often uniquely so, to exploit the dynamic nature of river systems Junk et al. For example, cottonwood Populus spp. Hence, seed release has to coincide precisely with the flood recession so as not to wash the seeds away or dry the substratum so quickly that seedling roots cannot grow fast enough to stay in contact with the capillary fringe of the water table. Additionally, healthy rivers and their associated riparian zones are complex interconnected corridors that allow biota to disperse and adapt to particular conditions at particular locations.
Fish and other aquatic and semiaquatic vertebrates and their prey, along with plants, microbes, and organic detritus, compose complex food webs within the habitat complex of the stream network, both above and below ground.
Populations cluster in favored locations where resources support enough reproduction to sustain them, with gene flow maintained by immigration and emigration. Dispersal is a natural feature of all populations in the struggle for living space and in the acquisition of resources needed to complete life cycles.
River networks are ideal corridors for dispersal of individuals or propagules. Some organisms have life stages that are spatially dispersed along the river corridor. For example, migrating birds use riparia as navigational aids, stopover sites, and brood-rearing habitat. Additionally, riparia offer unique habitat for many species, including the adult stages of numerous invertebrates, amphibians, reptiles, birds, and mammals that spend much of their life in water. One well-known example is beaver Castor canadensis that not only use riparia as habitat but also shape its community composition and the spatial-temporal dynamics of the vegetation Naiman and Rogers Finally, plants adapted to flooding grow on the banks and on floodplains in complex vegetative arrays associated with variation in soils and local hydrology.
It is also crucial to consider riparia as systems where conservation and development need to be integrated, particularly where riparian resources and biodiversity are essential for livelihoods Salafsky and Wollenberg But this is not an easy task; there are major deficiencies in linking ecosystems and management institutions and in rules governing the use of riparian areas Berkes and Folke Often, management does not reflect the complexity and multiple functions of riparia.
New institutions with adaptive comanagement approaches are necessary for a successful integration of conservation and development, as convincingly suggested in protected areas in the Ganges River Floodplain in Nepal Brown There, the protection of emblematic, rare, and endangered species such as the Bengal tiger Pantera tigris and the Asian one-horned rhinoceros Rhinoceros unicornis is balanced against human needs. Many variations on this general theme occur as organisms exploit the spatially and temporally dynamic mosaic of habitats within the interconnected pathways of rivers.
In tropical regions, such as in South America and in Africa, fish life histories are tuned to the predictable flooding that provides access to floodplain lakes and riverine wetlands where food resources are seasonally abundant Welcomme Indeed, the floodplains produce many times more fish biomass than the main river channels.
This biomass production in turn supports a wide variety of higher consumers, including humans. Aboriginal populations focused on floodplains, locating villages in strategic locations for exploiting floodplain fisheries and other biotic resources, particularly edible plants as well as rushes and trees for building shelters.
Habitat for riverine and riparian organisms is a constantly changing mosaic, biophysically dynamic in space and time, and the biota are uniquely adapted to the dynamics of the system Salo et al. Traditionally, ecologists have focused research on either purely terrestrial or aquatic attributes and processes, often attempting to segregate physical and biological attributes. Today it is well recognized that the key to understanding riverine and riparian networks is to integrate functional processes driving linkages between terrestrial and aquatic components across multiple biophysical gradients, from watershed divides to the oceans.
This is riparian ecology. How important are riparia in a catchment context and across biomes? Some studies provide preliminary support for the generality of riparian controls on river ecosystem structure and function, thus integrating landscape and food web ecology Polis et al. Insights have been provided, for example, on marine nutrients from salmon Oncorhynchus spp. Key issues concerning riparia include their potential role as keystone units of catchment ecosystems, which include acting as nodes of ecological diversity and providing clean water and flood control.
A crucial issue is knowing how to integrate the complex multidimensionality into management decisions about riparian systems, especially when most are already culturally modified. Riparia form dendritic networks and, as such, may be the dominant structuring attribute that organizes catchments and landscapes.
Riparia : Ecology, Conservation, and Management of Streamside Communities
For example, riparian vegetation may act as buffer zones along rivers in various ways. Riparia minimize downriver flooding by physically slowing the water, absorbing it or increasing the rates of evapotranspiration. Finally, riparia constitute habitat for rare or uncommon species, and these species may move along the unique dendritic networks of riparian vegetation. Throughout the book we examine this potentially unifying theme.
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Landscape ecology, the study of interactions between spatial patterns and ecological processes in the context of spatial heterogeneity, holds the potential for developing a truly holistic perspective of riparian systems, one that rigorously integrates structure, dynamics, and function in a catchment context see Sidebar 1.
Several decades have passed since it was first fully acknowledged that the character of the catchment basin, including riparian areas, fundamentally influences biotic patterns and processes in streams and rivers Hynes Nowadays, river corridors—inclusive of riparia—are considered major components of viable landscapes Malanson , Forman A thorough analysis of riparian ecology in a landscape context may be attained in several ways, but using a hierarchical patch dynamics perspective has proven to be most useful Townsend A landscape perspective of riparian systems is frequently advocated in the professional literature, even if the meaning of such a perspective may differ between authors.
This perspective is often an ecological one: Riparian systems are viewed as multiscaled nested hierarchies of interactive terrestrial and aquatic elements—that is, homogenous units or patches observable within a landscape at a given spatial scale Poole According to Forman , land mosaics along rivers appear as corridors where the interactions between water table, land surface, soil type, and slope determine the richness of vegetation and habitat.
Observed patterns result from hydrologic flows, particle flows, animal activities, and human activities. Such a perspective allows one to answer questions such as: How do patterns composed of patches and boundaries influence ecological processes?
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How, in turn, do ecological processes influence spatial organization? What are the causes and consequences of spatial heterogeneity at various scales? Importance is given to the effect of spatial configuration on ecological processes, and the areas investigated are larger than those traditionally studied in ecology.
Another aspect is the consideration given to humans and society, particularly as landscapes are comprised of both nature and culture—objective and subjective representations of the environment. Spatial configuration influences the relationships developed by living beings between themselves and their environment, requiring one to understand how spatial organization of the environment shapes processes that drive the dynamics of populations, communities and ecosystems Turner et al. Nature cannot be divorced from man and society, requiring one to be open to other disciplines often better qualified to study spatial organizations and humans such as geography, history, anthropology, economy, and sociology.
This also requires one to incorporate symbolic and aesthetic values and to remember that every landscape has witnessed a culture and therefore has a memory as well as an environmental savoir faire created and recreated with time Nassauer Our philosophy is that a landscape perspective can aid one enormously in understanding the causes and consequences of the current transformation of riparia, so long as it is inserted into a plurality of approaches where ecologists share their principles with landscape architects and designers and with the society who participates in the creation and the cultural representation of riparia.
A hierarchical patch dynamics perspective addresses the fundamental attributes of riparia, particularly the dynamics of heterogeneity in space and time, visualizing interactions between structure and function at scales ranging from microhabitats to landscapes. It also provides a framework for linking riparian ecology to key concepts underpinning river ecology, namely the river continuum, serial discontinuity, flood pulse, and hyporheic corridor concepts.
This framework suggests a complex, dynamic, and nonlinear functioning for riparia involving a full range of interactions between the biophysical components, and thereby shaping the emergent ecosystem-scale characteristics. In general, vegetation—whether upland or riparian—is the key moderator of cut-and-fill alluviation. Forests, shrub lands, and grasslands intercept and retain runoff and increase infiltration. However, evapotranspiration by vegetation is a primary feedback to the atmosphere that can deplete soil moisture, tap near-surface aquifers, and even withdraw significant amounts of stream flow from the channel.
Vegetation moderates soil conditions as leaf litter is decomposed by soil microfauna, changing uptake trajectories of nutrients used by plants for growth.
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Nutrient cycles in the soil-vegetation complex of uplands determine the ion contents of runoff and thereby influence the production dynamics of riparian forests. Riparian forests, in turn, create microclimates through shading and transpiration and thereby influence stream and floodplain temperature patterns as well as nutrient cycles. Moreover, riparian trees and other vegetation eroded into river channels vastly alter water and material flow paths.
Hence, both living vegetation and wood debris deposited in the riparian corridor change the ability of the water to transport sediments, and this changes channel shape over time, especially in expansive floodplain reaches that are heavily forested Naiman et al. In any case, riparian-derived wood strongly interacts with the bed sediment characteristics—and the load, the water volume, and the slope of the channel—to determine channel geomorphology over multiple time scales.
Fires, drought, mass wasting, wind throw, herbivory, and other natural disturbances, coupled with human interventions such as logging, urbanization, farming, and damming, alter vegetative patterns and soil—plant nutrient exchange at a variety of scales. This has direct consequences for ecological processes in rivers—such as productivity, biodiversity, sediment transport, and live and dead wood recruitment—as well as for riparia, which also are influenced by interactions with upland vegetation e.
As any landscape, riparia are both natural and cultural. People see them differently according to the social group they belong to, as well as according to where they are from. And the way people see riparia may change with time. As Han Lorzing reminds us, a landscape is not merely a place of the real world; it is also a creation of the human mind in Rodieck Riparia are at the same time factual landscapes that we know , man-made landscapes that we make , perceived landscapes that we see, or we hear, smell, or feel , and emotional landscapes that we believe.
This book is about riparian ecology. Nevertheless, as authors, we are conscious that ecology does not tell the whole story and that history, for example, may be more reliable than theory when people make decisions Jackson This is not to expect historical knowledge to provide recipes or strategies for ecological management, conservation, or restoration. This is to acknowledge that over centuries cultural habits have formed which have done something with nature other than merely work it to death, that help for our ills can come from within, rather than outside, our shared mental world Schama Such a shared mental world changes in time and space.
Riparia: Ecology, Conservation, and Management of Streamside Communities
In presently developed countries, reading books and looking at drawings, paintings, photographs, or films influence our mental world. Everywhere, what people think should be a natural riparian landscape is strongly influenced by their cultural history, which differs between social groups and countries.
As eloquently suggested by Joan Nassauer , landscapes more apt to be protected are those that are appreciated—in other words, those that satisfy our cultural or aesthetic aspirations.
By incorporating principles that refer to ecological health to the cultural or aesthetic aspirations, we can obtain culturally sustainable landscapes. Such landscapes require sustained attention to the dynamics of ecological functions. They also require recognition of the limits and uncertainties of knowledge, leading, for example, to the protection of remnants of ecosystems even if we ignore why it may be interesting or useful to protect them. In addition, sustained attention to change must be remarkable in the sense that it must indicate an intention to care for riparia for the long term.
Thus, a riparian landscape has a better chance of being culturally sustainable if its ecological functions are known and if signs of intention for long-term care are apparent. Change probably characterizes the best examples of riparia: ecologically, culturally, and scientifically. Ecologically, riparian landscapes change because they are highly dynamic ecological systems, independent of those who care for them. Culturally, the perception of riparian landscapes changes continuously in time and space because social groups evolve to view them differently.
Scientifically, the perception of riparian landscapes is also changing because knowledge of their structure and function is improving—at a particularly high rate during the last two decades. These characteristics make the study of riparia a fascinating topic in a period of accelerated environmental and societal change.