LAND USE EFFECT ON URBAN STREAM

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LAND USE EFFECT ON URBAN STREAM

CHAPTER ONE

INTRODUCTION

1.1 Background of the study

Profound and far-reaching impacts of humans on the environment are mediated through land use changes which destroy, degrade, and fragment habitat, and constitute the primary cause of losses in biodiversity worldwide (Sala et al., 2000; Chapin et al., 2001). Human impacts on landscapes often diminish the capabilities for ecosystems to provide essential services for people, including clean air and water and natural products (Foley et al., 2005). Land use changes have had a large effect on terrestrial and aquatic environments in mediterranean climate regions (medregions), that is the Mediterranean Basin, CaliforniaBaja California, southwestern Australia, southwestern South Africa, and central Chile, because of their long history with human activity, rapid urban and agricultural development, and large climatic and topographic variation (Underwood et al., 2009; Sirami et al., 2010). Because med-regions contain many endemic species, human impacts on these regions have a disproportionate effect on global biodiversity and make mediterranean ecosystems (med-ecosystems) among the ‘‘most imperiled’’ in the world (Myers et al., 2000; Horwitz et al., 2008; Underwood et al., 2009). Although considerable research has been conducted on the effects of land use change on terrestrial environments in med-regions, studies of land use impacts on aquatic habitats are more limited. Because water is seasonally scarce in med-regions, many rivers and streams have been dammed or diverted to provide water supplies for human activities (Gasith & Resh, 1999). Given the critical importance of water quality and quantity in med-regions and concerns about the impacts of human environmental alterations on aquatic systems and species, there is a need for more data and knowledge to guide management and policy decisions balancing human and environmental needs for sustainable water resources. Research in other temperate regions of the world has shown that land use changes denude native vegetation, increase runoff and erosion, alter stream geomorphology and substrata characteristics, modify flow regimes, and enhance the transport of nutrients, sediment, and contaminants from catchments to receiving waters, all with numerous implications for the stream and river biota (Paul & Meyer, 2001; Allan, 2004; Walsh et al., 2005; Johnson & Horst, 2010). Human land use impacts on med-ecosystems are predicted to be heightened by climate change which will result in warmer, drier, and more variable weather (Klausmeyer & Shaw, 2009).

The growth of the wildland–urban interface in many med-regions has increased the intensity and frequency of wildfires and promoted the introduction of exotic species (Verkaik et al., 2012). Land use changes and fires have similar short-term effects on the physical, chemical, and biological characteristics of streams, but with fires constituting a pulsed perturbation, with rapid ecosystem recovery (Verkaik et al., 2012), and land use changes constituting a press perturbation with sustained, long-term impacts on ecosystems. Frequent clearance, burning, or intense grazing of med-vegetation, however, can produce a vegetation type conversion from forest or shrubland to degraded scrub or grassland (Keeley, 2002; Van de Wouw et al., 2011), with persistent effects on both terrestrial and aquatic ecosystems. Other human effects on med-rivers include straightening and building levees along river channels, decoupling rivers from their floodplains and destroying riparian vegetation as floodplains are converted to grazing, farming, industrial, or urban uses. For example, riparian vegetation cover in the middle section of the River Ebro declined from 40% in the 1950s to 4.5% today (Ollero, 2007). Although land use changes have diminished riparian vegetation in many med-ecosystems, canopy cover by riparian vegetation can be as high or higher in human-altered basins as in undeveloped basins because of the protection or restoration of riparian bufferstrips, planting of ornamental trees, and increased return flows of irrigation or sewage water in some catchments. In addition, the construction of dams on rivers reduces floods in downstream areas, which may allow the encroachment of riparian vegetation; however, the net effects of dams on downstream riparian vegetation depend on the effects of dams on seasonal river flows (i.e., increased vs. decreased dry season flows).