Barbara J. Downes

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.

Changes in male mate choice in a sexually cannibalistic orb-web spider (Araneae: Araneidae)

In theory, male mate choice should occur when the costs of copulation, in terms of future mating opportunities, are high. The criteria males use to choose mates may change depending upon male mating history and the potential for future matings. We examine male mate choice in the St. Andrews Cross Spider (Argiope keyserlingi Araneae: Araneidae). Laboratory experiments revealed that death and injury caused by female sexual cannibalism limits males to a maximum of two copulations. We assessed the mate choices of virgin and mated males for females of different reproductive status. We used field and laboratory choice bioassays involving airborne and web-based pheromones. In field experiments, wild males were strongly attracted to webs built by laboratory-raised virgin females. Webs from mated females did not attract males. Male mate choice was affected by male reproductive status: while virgin males strongly preferred penultimate and virgin females to mated females, mated males were apparently indifferent to females of different mating status. Such post-copulatory changes in male mate choice have not been previously documented, and may reflect a decreased potential for future mating.

A new method for characterizing surface roughness and available space in biological systems

1. Surface roughness and available space for organisms inhabiting heterogeneous habitats have important effects on measurements of species diversity, population abundance and dispersal. Quantifying habitat complexity is important but very difficult to achieve, especially at small scales. 2. A method of quantifying topographic roughness is described, which produces estimates of space potentially available to organisms in different size classes rather than giving general descriptors. The method uses samples of profiles across the surface of interest that are taken over any spatial scale and that are then analysed by a low-cost, image-processing technique. 3. A case study shows that the method allows quantification of the effects of surface texture on abundance of stream algae. Stream stones differing in geological rock type differed in roughness at very small spatial scales (crevices < 0.4 mm in size). Chlorophyll a concentrations were correlated positively with roughness measured at the smallest spatial scale (0.2 mm). A colonization experiment, in which bricks were sandblasted to produce rough surfaces, showed that rough substrata attain higher abundances of stream algae than smooth. The imaging technique permitted the quantification of the exact differences in texture between smooth and rough surfaces while demonstrating that both rough and smooth bricks still fell within the natural range of texture shown by stream stones

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.

THE EFFECT OF HYDROLOGICAL DISTURBANCE ON THE IMPACT OF A BENTHIC INVERTEBRATE PREDATOR

The harsh-benign model of community dynamics predicts that the impact of predation will decline as abiotic conditions become more stressful to biota. Experiments were conducted to determine whether hydrological disturbance altered the impact of an invertebrate predator in stream benthic communities. The impact of a predatory stonefly, Cosmioperla kuna, on its mayfly prey was measured in experimental stream channels re- ceiving constant or variable flow (flooding) regimes over a one-week period. Contrary to predictions of the harsh-benign hypothesis, the impact of Cosmioperla on its two major prey taxa was either unchanged or increased by artificial floods, despite increased predator emigration from variable-flow channels. Predator impacts in variable-flow treatments were apparently strongly influenced by predator-induced prey emigration during floods. The results of this study show that nonlethal predator effects may be important during abiotic disturbance, and that it may not be reasonable to predict the impact of predation solely on the basis of the relative tolerances of predators and prey to prevailing abiotic conditions.

Flow-related disturbance in streams: an experimental test of the role of rock movement in reducing macroinvertebrate population densities

Densities of hydropsychids (Trichoptera) on different-sized stones were compared before and after a winter flood, and the effects of rock movement (a likely form of disturbance during floods) on these organisms were tested. Before the flood, the density of hydropsychids was an order of magnitude higher on large than on small stones; after the flood, densities on larger stones had fallen to levels similar to those on small stones. In a four-week colonization experiment over the period during which the flood occurred, densities of hydropsychids were compared on bricks of two sizes, which were either fixed to the bed of the stream so that they could not move, or unfixed and able to move with changes in flow. Disturbance rates generally differed between small and large unfixed bricks, but all unfixed bricks moved during a large flood, regardless of size. The densities of hydropsychids on fixed and unfixed bricks were similar, and they reflected densities on natural stones after the flood, suggesting that even substrata that do not move during floods may fail to provide a refuge from the effects of high flows.

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.

Environmental constraints on oviposition limit egg supply of a stream insect at multiple scales

Species with complex life cycles pose challenges for understanding what processes regulate population densities, especially if some life stages disperse. Most studies of such animals that are thought to be recruitment limited focus on the idea that juvenile mortality limits the density of recruits (and hence population density), fewer consider the possibility that egg supply may be important. For species that oviposit on specific substrata, environmental constraints on oviposition sites may limit egg supply. Female mayflies in the genus Baetis lay egg masses on the underside of stream rocks that emerge above the water’s surface. We tested the hypothesis that egg mass densities are constrained by emergent rock densities within and between streams, by counting egg masses on emergent rocks. All emergent rocks were counted along 1-km lengths of four streams, revealing significant variation in emergent rock density within streams and a more than three-fold difference between streams. In each stream, egg mass density increased with the density of emergent rocks in 30-m stretches. We used regression equations describing these small-scale relationships, coupled with the large-scale spatial variation of emergent rocks, to estimate egg mass densities for each 1-km stream length, a scale relevant to population processes. Scaled estimates were positively associated with emergent rock density and provided better estimates than methods that ignored environmental variation. Egg mass crowding was inversely related to emergent rock density at the stream scale, a pattern consistent with the idea that oviposition substrata were in short supply in streams with few emergent rocks, but crowding did not compensate entirely for differences in emergent rock densities. The notion that egg supply, not larval mortality, may limit population density is an unusual perspective for stream insects. Environmental constraints on egg supply may be widespread among other species with specialised oviposition behaviours.

Does dispersal control population densities in advection-dominated systems? A fresh look at critical assumptions and a direct test

1. In advection-dominated systems (both freshwater and marine), population dynamics are usually presumed to be dominated by the effects of migrants dispersing by advection, especially over the small spatial scales at which populations can be studied, but few studies have tested this presumption. We tested the hypothesis that benthic densities are controlled by densities of dispersers for two aquatic insects in upland streams. 2. Our study animals were two species of caddisflies (Hydropsychidae), which become sedentary filter-feeders following settlement onto substrata. Densities of dispersers in the drift (advective dispersal) were quantified using nets placed along the upstream edges of riffles, where the latter abruptly abutted a slower, upstream run. Settlement was estimated at each site using brick pavers, half of which had been fenced to prevent colonization of their top surfaces by walking hydropsychids, thus allowing us to distinguish also the mode of movement during settlement. 3. First through fifth instars of two species, Smicrophylax sp. AV2 and Asmicridea sp. AV1, were abundant and showed disparate results. Drift and settlement were relatively strongly related for Smicrophylax. The best fit lines were shown by second and third instars settling on plain bricks, suggesting that drift played a strong role in settlement, but that some drifters dropped to the bottom and located substrata by walking. Quantile regression suggested that drift sets limits to settlement in this species and that settlement success was highly variable. In contrast, settlement by Asmicridea was poorly related to drift; settlers were mainly individuals re-dispersing within sites. 4. Smicrophylax densities appear to be controlled by dispersal from upstream, but benthic density of Asmicridea is more likely linked to local demography. Our data demonstrate the dangers of assuming that supposedly drift-prone species can all be modelled in the same way. Alternative models emphasizing little or different kinds of movement should be considered. Variability in oviposition coupled with weak dispersal, for example, is a viable alternative hypothesis to explain variation in benthic density along channels. Moreover, the constraints on settlement of Smicrophylax show that immigrants into sites can be in short supply, an hypothesis rarely considered in stream research.