Alena Glaister

HABITAT STRUCTURE AND REGULATION OF LOCAL SPECIES DIVERSITY IN A STONY, UPLAND STREAM

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 spatial scales demonstrate the importance of quantifying and manipulating substrate complexity at scales that are comparable with natural surfaces.

Habitat structure, resources and diversity: the separate effects of surface roughness and macroalgae on stream invertebrates

Abstract 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.

Spatial variation in the force required to initiate rock movement in 4 upland streams: implications for estimating disturbance frequencies

Lotic models of disturbance generated by floods and spates suffer from 2 main short-comings: a lack of knowledge regarding the appropriate spatial scale at which to apply models and a poor understanding of the relationship between discharge sizes and actual disturbance frequencies and intensities. Here, we examine the spatial variability in the forces needed to shift rocks and the utility of hydraulic equations that predict critical shear stresses (τ c), which are sometimes used to infer disturbance frequencies in streams. We used spring balances to measure directly the forces needed (Fc) to move rocks up and out of the stream bed in 4 upland streams (Acheron River, Taggerty/Steavenson rivers, Connelly Creek, and Little River) in southeastern Australia. We measured 25 rocks at each of 32 sites overall, with sites distributed in a nested design: sites were paired in 2nd, 3rd, upper 4th, and lower 4th orders on each river. For each rock, we determined whether it was wedged into place by surrounding rocks, estimated percentage burial in fine sediments, and measured rock size and ambient water velocity and depth. Nested analyses of variance indicated that Fc and its correlates varied most between rivers and greatly between rocks within individual sites; the spatial scales of stream order and site contributed little explanatory power. Hierarchical, log-linear modelling showed that both rock size and bed packing varied systematically between rivers, with the Little and Taggerty/Steavenson rivers having relatively large rocks that were often packed into the bed, whereas Connelly Creek and the Acheron River had many relatively-small rocks lying loosely on top of the bed. A river-by-river analysis showed that values of Fc were related highly to rock sizes but that the nature of the relationships differed greatly between packed-in rocks and those lying on top of the bed and also varied between rivers. The Little and Taggerty/Steavenson rivers were similar to each other but both differed from the Acheron River and Connelly Creek, which differed from each other. Our estimates of Fc suggest that an oft used approximation, which equates τ c directly with rock sizes in mm, and the commonly-used equations from which the approximation is derived, are likely to produce poor estimates of τ c; these poor estimates would cause equally poor estimates of likely disturbance frequencies. The application by ecologists of reach-level hydraulic equations to estimate shear stresses and the sizes and numbers of rocks moved by floods and spates could be flawed by a focus on inappropriate spatial scales. Our data suggest that variation in likely disturbance frequencies between rocks within individual sites might be of a similar magnitude to variability between different rivers. We argue that spatial variation in stream systems need not be organized in the top-to-bottom hierarchical models that have been recently promoted for rivers.

How do we know about resilience? An analysis of empirical research on resilience, and implications for interdisciplinary praxis

We sought to understand how knowledge about resilience is produced. We examined empirical research into resilience from the social and natural sciences, randomly selected a sample of these studies and analysed their methods using common criteria to enable comparison. We found that studies of resilience from social scientists largely focus on the response of individuals to human-induced change events, while those from natural scientists largely focus on the response of ecological communities and populations to both environmental and human-induced change events. Most studies were of change over short time periods and focused on small spatial scales. Social science studies were dominated by one-off surveys, whereas natural science studies used a diversity of study designs to draw inferences about cause-and-effect. Whilst these differences typically reflect epistemological and methodological traditions, they also imply quite different understandings of resilience. We suggest that there are significant methodological barriers to producing empirical evidence about interactions between complex social and ecological systems.

Interacting environmental gradients, trade-offs and reversals in the abundance-environment relationships of stream insects: when flow is unimportant

Flow is often presumed to determine the distribution of stream invertebrates across stream beds. When temperatures are high, however, dissolved oxygen (DO) and its interactions with other environmental gradients may be more important. Field surveys were carried out in summer at two sites in a sand-bed stream in south-east Australia. Using quantile regression, we quantified the abundance–environment relationships of a caenid mayfly and an ecnomid caddisfly, and determined whether DO, fine detritus or velocity was the dominant limiting variable, and to gain insight into the causal mechanisms. Local densities of caenids were driven by food resources (detritus) at a site with a short DO gradient. The relationship was completely reversed where long DO and detritus gradients interacted, and here DO appeared to limit density. Densities of ecnomids were limited by prey-rich detritus patches at both sites. The velocity gradient did not explain the distribution patterns in either species. Ecnomid diet altered with changes in the spatial distribution of caenids between sites; caenids were the dominant prey at one site, but proportionately fewer were consumed where there was a negative spatial overlap of predators and prey. These results show that invertebrate responses to environmental gradients can be complex and that flow may be unimportant.

Plastic and unpredictable responses of stream invertebrates to leaf pack patches across sandy-bottomed streams

Detrital inputs to ecosystems provide potential food sources and can produce trophic cascades, but this effect is influenced by whether species specialise in consuming or inhabiting accumulations of detritus. To test whether species are differentially associated with leaves or sand, we compared densities of stream invertebrate species in patches of leaves and bare sand in two sandy-bed creeks in south-eastern Australia, in summer and spring. We also assessed the quality of information on diet and substrate association in the literature. Most species showed no density differences between leaf and sand patches (‘microhabitat generalists’), but categorisation as generalists, leaf or sand species differed between datasets. We developed a method for identifying important effect sizes; power analyses showed that many species were true generalists, but many non-significant results were potentially Type II errors. The literature provided information that was broadly consistent with our data, but few studies publish reliable information about either diet or patch use. Our results support a contention that few Australian stream invertebrates are obligate shredders, and this may also be true for streams elsewhere. Predicting and detecting the responses of such generalist taxa to detrital inputs will be very challenging.

Egg masses of some stream-dwelling caddisflies (Trichoptera: Hydrobiosidae) from Victoria, Australia: Egg masses of some Hydrobiosidae

Eggs are a largely neglected life stage in most ecological studies of aquatic insects, despite the importance of oviposition behaviour and fecundity estimates for many research questions. Incorporating egg stages into ecological research requires that ecologists can identify and quantify eggs, but descriptions of eggs and egg masses are scarce for many groups and particularly for Australian taxa. This paper focusses on stream‐dwelling caddisflies in the family Hydrobiosidae and provides species‐level identifications, morphological descriptions and images of egg masses of some species that commonly occur in south‐eastern Australia. All the species we identified laid plaque‐shaped egg masses attached to the underside of river rocks that protrude above the water surface. These egg masses can be identified in the field with the naked eye or the aid of a low‐magnification hand lens. Interspecific variations in egg mass morphology were primarily thickness and firmness of the spumaline layer, egg mass size and arrangement of eggs within the mass. We also provide some ecological information on clutch sizes and the physical characteristics of oviposition sites, and we discuss various ways in which these taxa could be exploited as model systems for ecological research.