The potential relationship between riparian arachnids and aquatic insect productivity was assessed in forest streams throughout the central South Island of New Zealand. Initially, a survey was conducted of thirty seven, first-third order forest streams. Streams were selected to represent a range of benthic invertebrate standing crops (as a surrogate measure of "productivity") from Banks Peninsula streams with relatively high benthic invertebrate densities to acid mine drainage streams near Reefton that were almost devoid of aquatic life. At each site benthic invertebrate densities and biomass were measured in riffle habitats and adjacent gravel bars were sampled for terrestrial invertebrates. At a sub-set of 16 sites, a 20 metre longitudinal web-building spider survey was conducted along each bank of the stream. As an additional component, a 20 metre transect starting at the stream margin and running perpendicularly into the forest was used to survey the density of web-building spiders with increasing distance from the stream. Results from the survey of in-situ stream insect biomass and gravel bar invertebrates showed a strong relationship between aquatic insect biomass and the biomass of riparian arachnids (R2 = 0.42, P < 0.001) having accounted for potentially confounding factors such as stream size, elevation, substrate and disturbance. The 20 metre longitudinal survey showed that streams with the highest in-situ insect biomass had significantly higher densities of web-building spiders along their banks (R2 = 0.28, P < 0.05), having accounted for potential confounding variables of elevation, habitat architecture and stream and channel width. The stream to forest survey showed a strong exponential decay in web-building spider densities with increasing distance from the stream (R2 = 0.96, P < 0.0001). Regardless of stream productivity web-building spiders were most abundant at the stream margins and rapidly declined to very low densities 20 metres from the stream. In order to further test the relationship between riparian web-building spider densities and stream insect productivity, a stream fertilization experiment was conducted on six first-second order streams in the Maimai experimental catchment, Reefton. Three streams were enriched by the addition of a fertiliser solution mainly consisting of sodium nitrate for seven months, and the other three streams were used as controls. Water chemistry, benthic invertebrate communities, emerging aquatic adults, and the densities of web-building spiders along the stream corridor and in the forest were monitored in three seasons (spring, summer and autumn) over the course of the nutrient-addition. By the end of the experiment, conductivity was significantly higher in nutrient-addition streams than in the control streams (F = 80.5, P < 0.001), but chlorophyll concentrations showed no significant differences between treatments. Both benthic mayfly densities (F = 6.15, P < 0.05) and the biomass of adult aquatic dipterans (Chironomidae, Simuliidae) (F = 9.25, P < 0.01) were significantly higher in nutrient-addition streams in the last sampling round. Spiders recorded from intercept traps indicated that by the end of the experiment spider activity was significantly higher within 2.5 metres of the nutrient-addition streams (F = 5.70, P < 0.01). However, seasonal densities of web-building spiders along the stream margin and in the forest decreased with no significant differences observed between nutrient-addition and control streams. The results from these studies indicate that adult insects emerging from streams represent an important source of prey that could influence the biomass and abundance of riparian arachnids. Additionally, the results imply that stream productivity and size could mediate the strength of the interaction between riparian and stream habitats. Moreover, feedback mechanisms present in both systems could have implications for such interactions. The elevated densities of web-building spiders observed at the stream margin led to the proposal of the "Highway Robber" hypothesis. This hypothesis suggests that such higher densities of spiders are the result of increased insect activity along the stream corridor: the emergence of adult aquatic insects was predicted to vary less over temporal and spatial scales than that of terrestrial insects due to the poorly synchronized life histories in many New Zealand stream insects. I conclude by suggesting that there are numerous anthropocentric perturbations such as loss of heterogeneity, introduced species, pollution and habitat degradation that could undermine and decouple the intimate linkages between aquatic and terrestrial ecosystems.

The potential relationship between riparian arachnids and aquatic insect productivity was assessed in forest streams throughout the central South Island of New Zealand. Initially, a survey was conducted of thirty seven, first-third order forest streams. Streams were selected to represent a range of benthic invertebrate standing crops (as a surrogate measure of "productivity") from Banks Peninsula streams with relatively high benthic invertebrate densities to acid mine drainage streams near Reefton that were almost devoid of aquatic life. At each site benthic invertebrate densities and biomass were measured in riffle habitats and adjacent gravel bars were sampled for terrestrial invertebrates. At a sub-set of 16 sites, a 20 metre longitudinal web-building spider survey was conducted along each bank of the stream. As an additional component, a 20 metre transect starting at the stream margin and running perpendicularly into the forest was used to survey the density of web-building spiders with increasing distance from the stream. Results from the survey of in-situ stream insect biomass and gravel bar invertebrates showed a strong relationship between aquatic insect biomass and the biomass of riparian arachnids (R2 = 0.42, P < 0.001) having accounted for potentially confounding factors such as stream size, elevation, substrate and disturbance. The 20 metre longitudinal survey showed that streams with the highest in-situ insect biomass had significantly higher densities of web-building spiders along their banks (R2 = 0.28, P < 0.05), having accounted for potential confounding variables of elevation, habitat architecture and stream and channel width. The stream to forest survey showed a strong exponential decay in web-building spider densities with increasing distance from the stream (R2 = 0.96, P < 0.0001). Regardless of stream productivity web-building spiders were most abundant at the stream margins and rapidly declined to very low densities 20 metres from the stream. In order to further test the relationship between riparian web-building spider densities and stream insect productivity, a stream fertilization experiment was conducted on six first-second order streams in the Maimai experimental catchment, Reefton. Three streams were enriched by the addition of a fertiliser solution mainly consisting of sodium nitrate for seven months, and the other three streams were used as controls. Water chemistry, benthic invertebrate communities, emerging aquatic adults, and the densities of web-building spiders along the stream corridor and in the forest were monitored in three seasons (spring, summer and autumn) over the course of the nutrient-addition. By the end of the experiment, conductivity was significantly higher in nutrient-addition streams than in the control streams (F = 80.5, P < 0.001), but chlorophyll concentrations showed no significant differences between treatments. Both benthic mayfly densities (F = 6.15, P < 0.05) and the biomass of adult aquatic dipterans (Chironomidae, Simuliidae) (F = 9.25, P < 0.01) were significantly higher in nutrient-addition streams in the last sampling round. Spiders recorded from intercept traps indicated that by the end of the experiment spider activity was significantly higher within 2.5 metres of the nutrient-addition streams (F = 5.70, P < 0.01). However, seasonal densities of web-building spiders along the stream margin and in the forest decreased with no significant differences observed between nutrient-addition and control streams. The results from these studies indicate that adult insects emerging from streams represent an important source of prey that could influence the biomass and abundance of riparian arachnids. Additionally, the results imply that stream productivity and size could mediate the strength of the interaction between riparian and stream habitats. Moreover, feedback mechanisms present in both systems could have implications for such interactions. The elevated densities of web-building spiders observed at the stream margin led to the proposal of the "Highway Robber" hypothesis. This hypothesis suggests that such higher densities of spiders are the result of increased insect activity along the stream corridor: the emergence of adult aquatic insects was predicted to vary less over temporal and spatial scales than that of terrestrial insects due to the poorly synchronized life histories in many New Zealand stream insects. I conclude by suggesting that there are numerous anthropocentric perturbations such as loss of heterogeneity, introduced species, pollution and habitat degradation that could undermine and decouple the intimate linkages between aquatic and terrestrial ecosystems.

Although it is well documented how introduced species can negatively affect native species, we only poorly understand how they may alter ecosystem functions. We investigated how an invasive fish affected the flux of aquatic insects to terrestrial food webs using mesocosms in a desert spring ecosystem. We compared aquatic insect emergence between alternative community states with monocultures and polycultures of two native species of fish, least chub (Iotichthys phlegethontis) and Utah chub (Gila atraria) plus, introduced western mosquitofish (Gambusia affinis). We tested three hypotheses: (1) aquatic insect biomass will be greater than terrestrial insect biomass and thus, constitute a vital source of energy for terrestrial consumers (2) invasive mosquitofish will negatively impact the biomass of emerging aquatic insects, and (3) terrestrial consumers will negatively respond to decreased emerging aquatic insect biomass. Aquatic insects represented 79% of the flying insect community, and treatments with mosquitofish significantly reduced emergent aquatic insect biomass by 60% relative to the control without mosquitofish. Behavioral traits of invasive species are important, because mosquitofish most heavily affected insects that emerged during the day. Also, spiders that build horizontal webs were negatively correlated with decreasing aquatic insect biomass. Invasive mosquitofish can achieve very dense populations because of their high intrinsic rate of population increase, which can significantly disrupt the flow of energy between aquatic and terrestrial ecosystems, thereby reducing the energy available for terrestrial consumers.

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