Runnelling is a popular method of physical habitat modification employed on saltmarshes to control pest mosquito populations. The runnelling method involves linking the tidal source to isolated mosquito breeding pools via shallow channels that enable slow water movement of low amplitude tides. Increased tidal flushing inhibits mosquito development. The range of organisms which inhabit saltmarsh are likely to be influenced by altered tidal conditions as they exhibit specific physicochemical requirements for feeding, burrowing or growth. The dynamic nature of saltmarsh may mean that changes to the tidal frequency of a particular region of the saltmarsh promotes extension of marine-like conditions. Because runnels increase the frequency of flooding tidal events in specific regions of the saltmarsh this study predicted that resulting changes would be evident in the physical conditions of saltmarsh substrate, in the transport of buoyant vegetative propagules, in the population characteristics of surface grazing snails and in the density and aperture of crab burrows after flooding and non-flooding tidal events. The physical impacts of runnelling were determined at three marshes which appeared similar in terms of topography, substrate and tidal conditions. Soil water content and consolidation were measured using two sampling protocols: a) comparisons between modified and unmodified shores; and, b) comparisons with increasing lateral distance across the shore from the runnel edge. At one marsh, moisture levels were significantly higher at runnelled than at unrunnelled sites when tides filled the runnels, but this pattern was not found at the other marshes. Soil consolidation was greater at higher shore heights, but was not different between runnelled and unrunnelled shores. Measurements at different lateral distances from runnels demonstrated higher moisture levels and lower consolidation up to 5 m from the edge but not further away. Groups of marked Avicennia marina propagules were released at the three runnelled saltmarshes during flooding and non-flooding tidal events. Groups of propagules released within 10 m of a runnel were always transported significantly further from the starting position and further up the saltmarsh shore after both flooding and non-flooding tides than any other groups. In addition, the pattern of stranding on saltmarsh for significantly different groups was closely associated with the path of runnel construction so that propagules were located either in the runnel or in depressions linked to the runnel that had been isolated mosquito-breeding pools prior to runnelling. It is likely that altered physical soil conditions significantly affected the distribution and size structures of Salinator solida and Ophicardelus spp. snails recorded at the three saltmarshes. The interaction of tidal period and the presence of a runnel contributed to patterns with significant differences between runnelled and unrunnelled regions of the marsh. Generally, the runnel population of snails exhibited flood-like features even during non-flood periods. The distribution and size classes of snails did not differ with lateral distance from runnels. The burrow characteristics of the crab Helograpsus haswellianus were compared to increase the accuracy of estimating abundance from burrow counts. Including only those burrows which were obviously maintained by resident crabs significantly increased the confidence limits of estimating crab abundance using only burrow density counts. This method was applied to runnelled and unrunnelled sites to assess any changes in the density of burrows associated with the presence of runnels. Again, it is likely that physical soil conditions resulting from increased tidal frequency at the runnel did influence crab burrowing with fewer small burrows being found at the runnelled site, low on the shore. In addition, mid- and large-sized burrows tended to dominate close to the runnel edge. Site-specific soil characteristics may help to explain the lack of continuity in patterns associated with runnel effects on non-target saltmarsh resources. While the runnel may increase the soil water content of clayey substrates at some sites it could also result in de-watering of porous sandy soils at other shores. This was evident in the structure of the snail population and distribution of crab burrows which appeared to reflect altered soil physical characteristics associated with the runnel. Runnelling does affect non-target organisms in saltmarsh. However, the scale of impact was usually locally restricted (< 10 m from the runnel edge). The fact that patterns were not recorded at all sites suggests that the influence of runnels is variable and limited by substrate and some biological conditions. Given the efficiency and popularity of runnelling as a physical control method for reducing pest vector mosquito habitat, this study found no evidence to suggest that its use should be discontinued on any ecological basis measured.

I estimated patterns of shorebird abundance and species diversity in the Colorado River Delta and Upper Gulf of California wetlands in order to determine the relative contribution of intertidal wetlands and non-tidal anthropogenic wetlands to support shorebird habitat use. Species richness varied from 15 to 26 species among sites and 29 species were detected across sites. Density during the peak migration month was higher at the anthropogenic wetland Cienega de Santa Clara (mean = 168 ind/ha, 95% C.I. 29-367), and the intertidal Golfo de Santa Clara (mean = 153 ind/ha, 95% C.I. 17-323). Anthropogenic wetlands (playa and lagoons) supported high abundance of shorebirds along with intertidal wetlands in the Colorado River Delta (mudflats). In contrast, intertidal wetlands farther south on the Sonoran Coast presented lower abundance but higher diversity of shorebird, likely as a result of the higher diversity of habitats (rocky shore, sandy beach, estuary). I modeled water management scenarios for the Cienega in order to determine the response of the dominant vegetation (southern cattail, Typha domingensis Pers.) and the area of the outflow pool below the marsh to different scenarios of water management. The model indicates that if the inflow rate is reduced below the current 4-5 m³s⁻¹ the vegetated area of the Cienega that supports habitat for marsh birds would decrease in proportion, as would the area of the outflow pool in the Santa Clara Slough identified previously as shorebird habitat. Increases in salinity will also reduce the vegetated area due to the low salt tolerance of T. domingensis. In winter about 90% of inflow water exits the Cienega into the Santa Clara Slough due to low evapotranspiration contributing to inundate areas that are used by wintering and migrating shorebirds. Lastly, I explored the feasibility of using Vegetation Indices (NDVI and EVI) to model Yuma Clapper Rail detections in the Cienega de Santa Clara as well as the effects of adding other habitat variables and the presence of fire events in the performance of linear models based on NDVI. Both NDVI and EVI were positively related to the Yuma Clapper Rail detections. The relationship was weak to moderate, but significant (P<0.001), which suggests other factors besides the vegetation condition play an important role in the bird distribution pattern. A model including all the variability among years was a better predictor of the rails detected per transect, than models for fire and non-fire years. We did not find a significant effect from adding habitat features (water % or vegetation %), and we recommend to include variables at both microhabitat level and landscape level, relevant before and during the breeding season in order to increase the explanatory power of models.