Oviposition by lotic macroinvertebrates in sand-slugged streams: a case study from the Granite Creeks of north-central Victoria

Explaining the stability and persistence of populations is one of the most difficult challenges confronting twentieth-century ecologists. Organisms may possess a range of life history stages that are controlled genetically and are influenced by an array of environmental variables. Each life history stage has the potential to affect the density, distribution and abundance of organisms within a population, therefore influencing population dynamics. Ultimately, recognising the importance of each life history stage is essential when attempting to describe patterns of distribution and abundance of individuals within populations. There is great diversity in how and where female invertebrates lay their eggs, termed oviposition, which in turn has a range of potential consequences for populations and their dynamics. There are extensive empirical and theoretical investigations suggesting that invertebrate females respond to physical cues, operating at a variety of spatial scales, when distinguishing between potential oviposition habitats. Therefore, understanding how adults determine the distribution of propagules throughout the environment and the subsequent supply of recruits to local and regional populations plays an important role in population dynamics. Invertebrate populations may be heavily dictated by the availability of `suitable' oviposition sites. However, several anthropogenic activities reduce habitat availability which may have an array of consequences for insect populations. Deficiencies in oviposition habitat may cause reductions in the supply of recruits, population growth and regulation. This study in north-central Victoria was focussed on three creeks that rise in the Strathbogie Ranges and flow out onto lowland plains, where they contain sand slugs. Sand slugs result from a range of anthropogenic activities and are discrete volumes of coarse sediment that move through a stream system slowly, burying woody debris and reducing the geomorphic complexity of the bedform. The study comprised an observational survey undertaken at 7 sites and an experiment carried out at 2 sites to gain information surrounding oviposition by lotic invertebrates on woody debris within sandslugged sections of Creightons Creek, Castle Creek and Pranjip - Nine Mile Creek. The study showed that the physical attributes of potential oviposition substrata varied spatially between sites within rivers, but not between rivers. Also, egg mass abundance for 2 out of the 3 taxa studied, Polypedilum sp. (Diptera: Chironomidae) and Baetidae Genus 1 MV sp. 5 (Ephemeroptera: Baetidae), varied significantly between rivers, and between sites within rivers for Polypedilum sp. egg masses. Pearson's correlation coefficients suggested that physical cues were being employed by chironomid and baetid adults, but that females were responding individualistically to different physical variables. The results of the experiment further confirmed that egg mass abundance varied between sites within rivers and that baetid females were potentially responding to velocity as a signal of prospective oviposition substrata. The results of the study strongly suggest that there is significant spatial variation in the physical characteristics of potential oviposition substrata, egg mass abundance and in the response of females to physical cues. The study provides fresh insight into oviposition behaviour by lotic invertebrates in degraded systems. The results present evidence to suggest that in sand-slugged creeks, relatively homogenised by the movement of coarse sediment, some females employ specific site selection cues during oviposition. The results also suggest that it is inappropriate to generalise about ecosystem processes across spatial, taxonomic and developmental boundaries. Selective oviposition behaviour facilitates dispersal between populations, maximises connectivity, and also acts as a regulating mechanism by supplying recruits to local and regional populations through immigration and emigration. The findings of the study reiterate the significance of recognising the importance of each life history stage when attempting to explain patterns of distribution and abundance of individuals within populations. ii