Habitat structure may regulate species diversity at local scales, with complex habitats being normally associated with greater species richness than simple ones. We employed a new method of quantifying habitat structure to examine community structure in a stone-dwelling community of mobile macroinvertebrates inhabiting a perennial, upland stream (the Steavenson River) in southeastern Australia. We distinguished between the effects of increasing habitat structure by adding similar physical elements (habitat complexity) from that produced by adding qualitatively different sources of habitat structure (habitat heterogeneity) at spatial scales relevant to the biota. We used a field experiment to ask: (1) Does variation in habitat structure at local scales (i.e., between individual stones) result in variation in species richness (S) and numbers of individuals (N), and if so, are changes in S wrought passively by changes in N, or is there evidence of local regulation of S? (2) Are macroalgae, which are a source of habitat structure for invertebrates, also affected by stone surface structure? (3) What are the effects of habitat structure on faunal composition and body sizes?We used clay bricks as substrata and manipulated three sources of habitat structure in a crossed design: large surface pits and cracks (low density/high density); small pits caused by variation in surface texture (rough/smooth); and the abundance of macroalgae (begun with algae, begun without algae). The bricks were sampled for both fauna and epilithon on days 14 and 28 of colonization, when species richness and densities of individuals were comparable to natural stream stones.Habitat structure altered faunal diversity and abundances, with the majority of common species reaching higher abundances on creviced or rough surfaces. Rough surfaces were additionally associated with shifts in overall faunal composition and markedly smaller body sizes. Each element of habitat structure (large crevices, roughness, and macroalgae) promoted both increased species richness and densities of individuals.Rarefaction indicated that changes in S were disproportionate to changes in N, which suggests that S is regulated by local processes. Overall species richness was highest on day 14 with no difference in S between simple and complex surfaces. By day 28, simple surfaces had lost taxa relative to complex surfaces, suggesting that species richness in this stream community is regulated at a local scale, even though faunal composition changes continually and is contingent upon habitat structure.Habitat structure also affected the epilithon, suggesting that sources of habitat complexity and heterogeneity are interwoven in this system. Furthermore, the epilithon response to surface structure depended on the spatial scale of habitat complexity, with more of the red, filamentous alga Audouinella hermannii being found on rough surfaces than on smooth surfaces, but less on bricks with large crevices than without. These different responses to surface structure at different spati...

Habitat structure may regulate species diversity at local scales, with complex habitats being normally associated with greater species richness than simple ones. We employed a new method of quantifying habitat structure to examine community structure in a stone-dwelling community of mobile macroinvertebrates inhabiting a perennial, upland stream (the Steavenson River) in southeastern Australia. We distinguished between the effects of increasing habitat structure by adding similar physical elements (habitat complexity) from that produced by adding qualitatively different sources of habitat structure (habitat heterogeneity) at spatial scales relevant to the biota. We used a field experiment to ask: (1) Does variation in habitat structure at local scales (i.e., between individual stones) result in variation in species richness (S) and numbers of individuals (N), and if so, are changes in S wrought passively by changes in N, or is there evidence of local regulation of S? (2) Are macroalgae, which are a source of habitat structure for invertebrates, also affected by stone surface structure? (3) What are the effects of habitat structure on faunal composition and body sizes?We used clay bricks as substrata and manipulated three sources of habitat structure in a crossed design: large surface pits and cracks (low density/high density); small pits caused by variation in surface texture (rough/smooth); and the abundance of macroalgae (begun with algae, begun without algae). The bricks were sampled for both fauna and epilithon on days 14 and 28 of colonization, when species richness and densities of individuals were comparable to natural stream stones.Habitat structure altered faunal diversity and abundances, with the majority of common species reaching higher abundances on creviced or rough surfaces. Rough surfaces were additionally associated with shifts in overall faunal composition and markedly smaller body sizes. Each element of habitat structure (large crevices, roughness, and macroalgae) promoted both increased species richness and densities of individuals.Rarefaction indicated that changes in S were disproportionate to changes in N, which suggests that S is regulated by local processes. Overall species richness was highest on day 14 with no difference in S between simple and complex surfaces. By day 28, simple surfaces had lost taxa relative to complex surfaces, suggesting that species richness in this stream community is regulated at a local scale, even though faunal composition changes continually and is contingent upon habitat structure.Habitat structure also affected the epilithon, suggesting that sources of habitat complexity and heterogeneity are interwoven in this system. Furthermore, the epilithon response to surface structure depended on the spatial scale of habitat complexity, with more of the red, filamentous alga Audouinella hermannii being found on rough surfaces than on smooth surfaces, but less on bricks with large crevices than without. These different responses to surface structure at different spati...

Summary 1. Disturbance is an important source of variability in species composition and diversity, but application of disturbance models is contingent upon a very good understanding of the spatial scales and frequencies of disturbance. Such information is particularly lacking from streams. In this study, we measured the disturbance levels of rocks (defined here as the proportion of the original sample disturbed after 6 months) of differing sizes and positions within the stream bed and looked at the variation between and within three upland streams. 2. Rocks were blazed with distinctive marks in situ and mapped using simple trigonometry and permanently marked points on the banks. Forty rocks were selected and marked completely at random, and a further sixty marked from random selections within three size classes (small, medium and large) crossed with two bed‐packing classes (on top of the bed or packed into the bed). This sampling design was used at each of two sites (an upper, order 3 location and a lower, order 4 location separated by ≈ 8–16 km) on each of three rivers and in two periods during the year (a dry summer period and a wetter winter period) for a sample size of 1200 rocks in all. During summer, on‐top rocks were removed from below the study sites and placed at random locations through the riffle after marking, to test whether human‐placed rocks can provide estimates of natural disturbance levels. 3. Rocks were relocated and classified as disturbed (moved or buried) or not disturbed (found at the same location) after ≈ 6 months. Log‐linear modelling revealed that human‐placed rocks moved half as often as on‐top rocks marked in situ. Overall, small rocks disappeared more frequently than medium ones, which disappeared more often than large ones, and rocks lying loosely on top of the bed were disturbed more often than those packed into the bed. There was no interaction between rock size and bed packing in their effects on disturbance and each of these factors affected disturbance levels in the same way at all six sites and in both seasons. During the summer, there were no differences between upper and lower sites, but disturbance was still relatively frequent and patchy in occurrence, with five of six sites showing significant spatial clumping of disturbed rocks. Disturbance levels were higher in the wetter, winter season than during the drier, summer season, but this was caused by a doubling of disturbance rates at all three lower sites, which also showed reduced (but, in two cases, still significant) levels of spatial aggregation. Disturbance levels at upper sites in winter were similar to summer rates, and the level of aggregation of disturbed rocks differed between sites. 4. The results obtained by this study suggest that disturbance levels should not be assessed using methods where rocks are placed in riffles. Disturbance models applied to rocky upland streams may need to heed differences seen at small scales (i.e. between individual rocks), as differences seen at these scales were a crucial source o...

Habitat structure has pervasive effects on community composition and diversity, with physically complex habitats often containing more species than physically simple ones. What factors or mechanism drive this pattern is little understood, but a complicating problem is that different sources of habitat structure can be confounded in both surveys and experiments. In this study, we carried out an experiment in which two sources of habitat structure, attached macroalgae and substrate surface texture, were separately manipulated to discern their joint and separate effects upon the diversity and composition of colonizing macroinvertebrates in a stony, upland stream. Because stream algae vary markedly in abundance in both space and time, we also sampled the epilithon of stream stones at two spatial scales on eight dates over 2 years to gain some preliminary data on how stream algae vary between individual substrata over time. Experimental substrata had either a smooth (siltstones, sandstones, crystal-poor felsic volcanics, plain paving bricks) or rough (granodiorites, crystal-rich felsic volcanics, sand-blasted paving bricks) surface. We allowed these substrata to be colonized naturally by macroalgae, mostly the filamentous red alga Audouinella hermannii. Half of each of the rough and smooth substrata were selected at random and the macroalgae gently sheared off. All substrata were defaunated with a household insecticide with little field persistence, set out randomly through the study riffle, and invertebrates allowed to colonize them for 14 days. Some substrata were sampled immediately to check the efficacy of faunal and algal removals, which proved to be successful. Experimental results showed that both surface texture and macroalgae increase species richness independently of each other. Surface texture had no effect on densities, while macroalgae increased colonization densities, but rarefaction showed that both sources of habitat structure increased species richness above values expected simply on the basis of the numbers of colonists. However, reference stones with high macroalgal cover had the same species richness as those with low cover, suggesting that the effects of macroalgae on species richness are transient relative to those associated with surface texture. Epilithon samples taken at different times suggest that the magnitude of spatial variation in plant growth alters with time. If plants generally recolonize rough surfaces more quickly than smooth, then the effects of habitat structure on macroinvertebrates ought to be strongest after major disturbances during growing seasons of plants.

The density, population size-structure, and fecundity of the exotic snail Potamopyrgus antipodarum are described, for a period of two years, on two shores subject to different levels of disturbance in Lake Purrumbete. The population dynamics of Potamopyrgus antipodarum showed the same seasonal pattern in the numbers of embryos carried per individual female in both years, but seasonal patterns in density and potential reproductive output of the population differed between shores. Densities of Potamopyrgus antipodarum were high, with a trend for highest densities to occur in spring and summer. The population consisted almost entirely of females, which is indicative of a parthenogenic mode of reproduction. Fecundity was high, with the number of embryos carried per female peaking in spring. In summer, the population was dominated by small individuals, but some small snails were present at all times of the year. Potamopyrgus antipodarum was found to be a very successful invader of Lake Purrumbete, with high densities, high fecundity, parthenogenesis and the presence of embryos throughout the year contributing to this success.