Effects of land-use on structure and function of aquatic invertebrate communities : a thesis presented in partial fuflillment of the requirements for the degree of Master of Science in Ecology at Massey University
Before the arrival in New Zealand of the first Polynesian settlers native forest covered 75% of the country's land area (Fahey and Rowe, 1992). Following two waves (Polynesian and European) of immigration and exploitation forest land has been reduced to 29% of the country's land area and consists of 6.4 million hectares of native forest and 1.4 million hectares of exotic production forest, of which 90% is Pinus radiata (New Zealand Official Yearbook, 1996). During this time 36% of New Zealand's land area has been developed as improved pasture (Rutherford et. al., 1987). A similar process has been evident in many countries worldwide (Townsend et. al., 1997). Current thinking, stressing the importance of a catchment's landscape and vegetation to the characteristics of the stream draining it was aptly summarised by Hynes (1975) when he argued that "in every respect the valley rules the stream". Because of the dependence of a stream on it's catchment, land-use changes such as native forest conversion to Pinus radiata forest and pasture have resulted in alterations to stream light, temperature and flow regimes, reduced water quality though sedimentation and eutrophication, changes to water pH and conductivity and changes in stream morphology (Cowie, 1985; Fahey and Rowe, 1992; Prat and Ward, 1994; Townsend et. al., 1997). Predictably, these disturbances have had a profound impact on the aquatic insect communities inhabiting the streams affected. A number of studies have reported land-use effects on measures of community structure such as taxonomic richness, diversity, abundance and composition (Allen, 1959; Winterbourn, 1986; Quinn and Hickey, 1990; Reed et. al., 1994; Richards and Host, 1994; Townsend et. al., 1997). Although land-use impacts on aquatic invertebrate community structure are well documented how these changes in structure impact on community function is less clear. Community function includes such community processes and attributes as food processing, colonisation, nutrient cycling and resilience.