P. S. Lake

Disturbance, patchiness, and diversity in streams

Perturbations in ecosystems consist of a sequence of 2 events: the disturbance, marked by the application of the disturbing forces, and the response shown by the biota to the damage inflicted by the disturbance. The disturbance must be effectively characterized, without confounding it with the response, for progress to be made in the study of the disturbance ecology of streams. A disturbance may take the form of a pulse, a press, or a ramp, and the consequent trajectory of the response may be a pulse, a press, or a ramp. Floods and droughts are the major forms of natural disturbance in flowing waters and, although the effects of floods have been relatively well studied, those of droughts have been largely neglected. Floods accentuate downstream and lateral transport links, often with damaging consequences, whereas droughts fragment the continuity of streams. Both floods and droughts destroy and generate habitat patchiness and patchiness of the biota. During recovery, there are changes in the biotic composition and spatial configuration in patches. Resistance and resilience of the biota to disturbance may be facilitated by the use of refugia. The characterization of flood refugia is much more advanced than that of drought refugia. Recovery from floods is marked by the rapid attainment of relatively constant levels of diversity at the local scale of individual patches. At the regional scale of streams and their catchments, several studies have reported negative correlations between diversity and levels of flood disturbance, whereas other studies have reported unimodal diversity–disturbance curves consistent with patterns expected of the intermediate disturbance hypothesis. Such a unimodal relationship may be generated in several ways that await testing. In flowing waters at the regional scale, disturbance may play a central role in regulating species diversity. A predicted increase in the severity and frequency of disturbances with global climate change requires a comprehensive understanding of the disturbance ecology of running waters.

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? n nWe 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. n nHabitat 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. n nRarefaction 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. n nHabitat 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.

Global Change and the Biodiversity of Freshwater Ecosystems: Impacts on Linkages between Above-Sediment and Sediment Biota

disturbances varying in strength, frequency, predictability, duration, and spatial scale. Such disturbances can deplete the biota, disrupt ecological processes, and redistribute resources (Giller 1996, Lake 2000). Generally, in both lakes and rivers, recovery from the effects of natural disturbance is relatively rapid, although there are exceptions, such as recovery from catastrophes on the scale of the Mount St. Helens eruption (Niemi et al. 1990, Giller 1996). Human activities are now a major force affecting the ecosystems of the earth (Vitousek et al. 1997, Sala et al. 2000). Human enterprises—agriculture, industry, recreation, and international commerce—are the source of disturbances affecting all ecosystems to varying spatial extents and to varying degrees. The disturbances arise from changes in land use, anthropogenic changes in global biogeochemistry, and biotic additions and losses (Vitousek et al. 1997). These three factors are the principal agents of global environmental change. Furthermore, they interact to give rise to the two large-scale phenomena of climate change and loss of biodiversity (Vitousek et al. 1997). Freshwater sediment biota are particularly vulnerable to global change because of direct impact on the sediments and on the water over these sediments, and because of the transmission of impacts from adjacent terrestrial ecosystems. Fresh waters are intimately connected to the terrestrial realm through groundwaters and surface waters. Movement of organic matter, nutrients, and sediment among the terrestrial realm, the water column, and aquatic sediments

Managing Critical Transition Zones

Ecosystems that function as critical transition zones (CTZs) among terrestrial, freshwater, and marine habitats are closely connected to the ecosystems adjacent to them and are characterized by a rapid flux of materials and organisms. CTZs play various roles, including mediating water flows, accumulating sediments and organic matter, processing nutrients, and providing opportunities for recreation. They are particularly difficult to manage because they tend to be small, albeit important, components of large watersheds, and managers may not have control over the entire landscape. Moreover, they are often the focus of intensive human activity. Consequently, CTZs are critically important zones, and their preservation and protection are likely to require unique collaboration among scientists, managers, and stakeholders. Scientists can learn a great deal from the study of these ecosystems, taking advantage of small size and the importance of fluxes, but a good understanding of adaptive management strategies is needed to establish a dialogue with managers and stakeholders on technical and management issues. An understanding of risk analysis is also important to help set meaningful goals and establish logical strategies that include all of the interested parties. Successful restoration of a CTZ is the best test of the quality of knowledge about its structure and function. Much has already been learned about coastal CTZs through restoration projects, and the large number of such projects involving riparian CTZs in particular suggests that there is considerable opportunity for fruitful collaborations between scientists and managers.

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.