Abstract
Anthropogenic impacts, including urbanization, deforestation, farming, and livestock grazing have altered riparian margins worldwide. One effect of changes to riparian vegetation is that the ground-level light, temperature, and humidity environment has also been altered. Galaxias maculatus, one of the most widely distributed fishes of the southern hemisphere, lays eggs almost exclusively beneath riparian vegetation in tidally influenced reaches of rivers. We hypothesized that the survival of these eggs is greatly affected by the micro-environment afforded by vegetation, particularly relating to temperature, humidity and UVB radiation. We experimentally reduced riparian vegetation height and altered shading characteristics, tracked egg survival, and used small ground-level temperature, humidity and UVB sensors to relate survival to ground-level effects around egg masses. The ground-level physical environment was markedly different from the surrounding ambient conditions. Tall dense riparian vegetation modified ambient conditions to produce a buffered temperature regime with constant high relative humidity, generally above 90%, and negligible UVB radiation at ground-level. Where vegetation height was reduced, frequent high temperatures, low humidity, and high UVB irradiances reduced egg survival by up to 95%. Temperature effects on egg survival were probably indirect, through reduced humidity, because developing eggs are known to survive in a wide range of temperatures. In this study, it was remarkable how such small variations in relatively small sites could have such a large effect on egg survival. It appears that modifications to riparian vegetation and the associated changes in the physical conditions of egg laying sites are major mechanisms affecting egg survival. The impacts associated with vegetational changes through human-induced disturbances are complex yet potentially devastating. These effects are particularly important because they affect a very small portion of habitat that is required to complete the life history of a species, despite the wide distribution of adults and juveniles across aquatic and marine environments.
Highlights
Alterations to the riparian zone on coastal plains have occurred worldwide as urbanization, coastal development and flood control measures increase [1]
Physical Environment The ground-level physical environment in all of the experimental plots was markedly different from the surrounding ambient conditions, and there was a gradient of temperature reduction from ambient across the experimental vegetation heights (Fig. 1)
Our results clearly show that the environment of the micro-site around G. maculatus eggs is of critical importance to survival, and that there is considerable buffering of the ambient conditions provided by the characteristics of the surrounding vegetation
Summary
Alterations to the riparian zone on coastal plains have occurred worldwide as urbanization, coastal development and flood control measures increase [1]. The impacts producing change include channelization of streams and rivers, increased water flow and scouring, deposition of sediments from catchments, changes to vegetation, and alteration of the light, temperature and humidity environment. Taken together, these have resulted in often disastrous consequences to the structure of habitats and the species that rely on them for all or part of their life histories. These have resulted in often disastrous consequences to the structure of habitats and the species that rely on them for all or part of their life histories This is so for diadromous fishes during the transition between freshwater and marine habitats. But important in a cultural and fisheries context, are impacts to smaller diadromous fishes such as Galaxias maculatus (Jenyns, 1842), one of the most widely distributed species of the southern hemisphere [3]
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