Jill Lancaster

Assembly rules within a contingent ecology

Ecologists have searched for general principles, or assembly rules, which determine how species combine to form communities. Two major strands of research highlight the interaction of assembly rules operating at different levels of community organisation. and within uncertain local and historical contingencies. Here, we review the aterature on community assembly within the context of three principal determinants: dispersal constraints, environmental constraints and internal dynamics. This classification separates external factors (dispersal and environmental constraints) from mternal processes (internal dynamics). We assert that assembly rules are general, deterministic and mechanistic, and operate on internal dynamies within the constraints imposed by the local environment and the history of species invasions. Several assembly rules have been proposed to operate within and across trophic levels, as well as on whole systems. The rules act through resource dynamics and spatial dynamies to select species or subsets of species that can coexist, and may lead to patterns of increasing biomass and non-random spatial distributions. Environmental constraints restriet species establishment and mediate interactions among successful colonists; a change in environmental constraints, either exogenous or endogenous. may drive community change. Even in systems at equilibrium, environmental constraints influence the outcome of assembly rules. Dispersal constraints determine the pool of potential colonists available at a particular time and place, and are a complex amalgam of species-specific traits, storage effects, landscape ecology, and history. The order and timing of species invasions (dispersal constraints) interact with assembly rules to produce priority effects. The directionality of community assembly may be lost if either the rate of species invasions or the rate of non-selective species extinctions exceeds the rate at which species are selected by assembly rules. The falling apart, or disassembly, of a community through breakdown of species interactions may follow different rules or a different assembly trajectory.

Flow Refugia and the Microdistribution of Lotic Macroinvertebrates

Changes in the microdistribution of benthic macroinvertebrates were observed, between patches identified as refugia from high flow and the remaining benthic habitat, over a range of discharges in a small stream with prominent flow refugia. Total invertebrate abundances were similar across all patch types at baseflow, but local densities were higher in flow refugia after periods of high and fluctuating flow. Different species- and size-specific responses were observed: microdistributions of larval Chironomidae and small nymphs of the stonefly Leuctra nigra did not change with discharge. Conversely, the stonefly Nemurella pictetii and large nymphs of L. nigra were most abundant in refugia, relative to the remaining benthic habitat, during periods of high and fluctuating discharge. The interactive effects of leaf litter and local flow conditions on species microdistributions were investigated. In some seasons, some species may respond indirectly to hydraulic conditions through their direct response to litter mass. Observed microdistribution changes probably do not reflect very short-term reactions to individual flow events, but perhaps a longer-term response to seasonal flow conditions. These are some of the first field data implicating the role of in-stream flow refugia in macroinvertebrate community structure.

Nested Hierarchies and Scale-Dependence of Mechanisms of Flow Refugium Use

We outline a hierarchical framework of classes of mechanisms, operating at different spatial and temporal scales, by which populations may exploit environmental heterogeneity to ensure persistence in environments subject to physical disturbance. Examples are provided for stream systems subject to high flow disturbances. Genotypic heterogeneity, encompassing morphological and physiological adaptations, operates at evolutionary and biogeographic scales, and ultimately constrains mechanisms at smaller scales. On ecological scales, organisms may exploit temporal or spatial heterogeneity, or both, to maintain population sizes larger than possible by morphological adaptations alone. For 2 classes of mechanism operating at large scales (>1 generation and >1 habitat patch), individuals do not survive disturbances and population persistence depends on recruitment from external sources. Where generations overlap and populations exploit temporal refugia in complex life cycles, dispersal occurs between habitat patches of different types. Alternatively, dispersal occurs between habitat patches of similar type. For 2 other classes of mechanism, operating at smaller scales (<1 generation and ≤1 habitat patch), individuals survive disturbances by moving between microhabitat patches of different types, or by changes in habitude which reduce the negative effects of disturbance. A simple mathematical model is used to explore the efficacy of 4 mechanisms of refugium use operating through microhabitat heterogeneity within a single habitat patch. Simulations of population changes over a series of disturbance events showed that each of 4 mechanisms could maintain a viable population under certain conditions. Total refugium area and proportion of the population lost at each disturbance event had a strong effect on final population size for all mechanisms. Doubling disturbance frequency had little effect on final population size if movements of individuals into refugia were facilitated by small accessible refugium patches, or if individuals remained in refugia during benign inter-disturbance periods. Empirical evidence supports the efficacy of these mechanisms: organisms with short life cycles and, perhaps, poor mobility may be lost from erosive patches but persist in refugia; macroinvertebrates may accumulate in refugia during disturbance events and redistribute throughout the stream after the disturbance. More empirical data on the spatial attributes of refugium patches and the mobility of stream organisms are required to test and to add realism to these models.

Scaling the effects of predation and disturbance in a patchy environment

The effects of hydraulic disturbances on the impact of two predatory benthic invertebrates on their prey were examined in a stream at two distinct spatial scales. At the scale of small habitat patches (0.0625 m2), hydraulic patch type was an important determinant of the microdistribution of prey and predators. Prey abundances were similar across all patch types at baseflow, but local densities were higher in patches identified as low-flow refugia after periods of high and fluctuating flow. The microdistribution pattern of predatory larvae of a caddisfly, Plectrocnemia conspersa, was similar to that of its prey, whereas predatory larvae of an alderfly, Sialis fuliginosa, did not shift their microdistribution significantly with discharge and were always most abundant in lowflow refugia. There was little evidence of an aggregative response of predators with prey, even though both predators and prey are mobile. Both predator species showed similar patch-specific patterns of per capita consumption rates: uniform consumption rates across hydraulic patch types at low and moderate flows, but highest in flow refugia during high flows. Species-specific patterns, however, were apparent in the magnitude and direction of differences between consumption rates during disturbance events, and in comparable patches at base flow: At high flow, consumption rates for P. conspersa were exaggerated (3.9 times higher) in flow refugia but “at par” in other patches; for S. fuliginosa they were “at par” in flow refugia but reduced in other patches (up to 3.3. times lower). These differences may be related to species-specific foraging behaviours (search vs ambush predators) and the influence of prey movements on feeding success. Using the patch-scale results only, it is difficult to predict the effects of physical disturbance on predation intensity at the larger scales of whole habitats, populations or communities. At the large scale (>200 m2), net predator impacts were estimated over the stream reach, using a spatially explicit model that accounts, in an additive way, for habitat heterogeneity and patch-specific responses of predators and prey. The relationship between predator impact over the whole reach and hydraulic disturbance differed for the two predators. The predator impact of S. fuliginosa decreased with increasing hydraulic disturbance, as predicted by the harsh-benign hypothesis. There was no directional trend for P. conspersa, however, and maximum predator impact may occur at intermediate disturbance levels. For the prey community in this stream, predation pressure from S. fuliginosa appears to fluctuate directly with the discharge hydrograph, whereas predation from P. conspersa may be more persistent. Flow refugia may play a dual role in the sructure of stream communities by preventing catastrophic mortality of animals (predators and prey) from physical forces during disturbances, and by maintaining (or perhaps increasing) predation pressure. Summing the effects of species interactions in small habitat patches to the larger scale of a whole stream reach indicates that the scale of approach influences the observed patterns and their implied underlying process.

INVERTEBRATE PREDATION ON PATCHY AND MOBILE PREY IN STREAMS

SUMMARY (1) A series of field experiments with predatory net-spinning larvae of the caddisfly Plectrocnemia conspersa (Curtis) were carried out in a small fishless stream to determine any impact of this predator on the abundance of its prey. Densities of fifth instar caddisfly larvae were manipulated in enclosures permeable to most other invertebrates in the stream. (2) Three experiments were carried out (spring, summer and winter), each with a 2-week preliminary colonization period to establish prey assemblages inside enclosures. Then zero, one or four predators were introduced per cage (low, ambient or high predator densities). Significant predator impacts were observed in winter only, and results varied with prey taxon. These observations could be attributed either to largely trivial effects of predation, or to technical difficulties in its detection. (3) Two subsequent experiments (A and B) were designed to assess how the detection of predator impact in enclosures is influenced by the patchy microdistribution of lotic invertebrates, and by the continuous exchange of mobile prey with the benthos. (4) In experiment A, prey exchange rates were manipulated by wrapping some cages with fine mesh after the initial colonization period. The evidence that exchange rate influenced the detection of impact by predators was equivocal and may have been obscured by the natural, spatial variability of lotic invertebrates. (5) In experiment B, exchange rates were again altered by manipulating the mesh size of enclosures but, in addition, initial variability in the number of prey per cage was reduced by stocking rather than by allowing colonization. The abundance of total prey and of individual prey groups was reduced significantly in predator treatments compared with controls. Predator impacts were most pronounced at low exchange rates, and were less distinct at high exchange. The effect of mesh size on prey exchange varied among taxa in relation to species-specific morphology and dispersal behaviour. The experimental detection of predator impacts therefore appears to be highly dependent on both prey mobility and spatial heterogeneity.

Stream flow and predation effects on the spatial dynamics of benthic invertebrates

Field experiments were carried out to determine the influence of predation and prey movements on the accumulation of prey in enclosures. Experimental enclosures permitted exchange of prey with the benthos, but not of the large, predatory larvae of the caddisfly, Plectrocnemia conspersa (Curtis). Unseasonally heavy rainfalls during the experiment resulted in high flows and enabled us to examine the effects of a major, abiotic disturbance on invertebrate spatial dynamics. Prey colonization rates of cages without predators were determined in nine 24 h periods. Colonization rates increased exponentially with flow and were species-specific, depending on dispersal behaviour. Prey accumulation and predator impacts were measured in cages, with and without P. conspersa larvae, placed in the stream for 1, 2 or 3 weeks. Prey densities in cages increased with exposure time, but increases were not gradual and depended on flow regime. Flow was reduced within cages and they accumulated large numbers of invertebrates during high discharge. Analogous, naturally occurring refugia in the stream channel could be important for the recovery of lotic communities after major disturbances. Overall, prey densities were lowest in cages with predators. For fast colonizers, predation effects were detectable early in the experiment, but quickly obscured thereafter by continuous exchange of prey. For slow colonists, predation effects were detectable later, but persisted longer. Consumption rates for P. conspersa varied with prey density and flow regime. We suggest that the spatial dynamics of benthic invertebrates, especially as they are influenced by stochastic events, are important in understanding and detecting predation effects in stream communities.

Predation and drift of lotic macroinvertebrates during colonization

SummaryA field experiment was carried out to determine the effect of an invertebrate predator on the colonization and drift of benthic macroinvertebrates in experimental stream channels. Lotic invertebrates colonized four replicate channels: two controls with no predators, and two channels with low densities (2.8 m−2) of predatory stonefly nymphs, Doroneuria baumanni (Perlidae). Immigration rates were measured at the inflow of two other channels. Drift rates of invertebrates immigrating to and emigrating from channels were measured daily, and benthic samples were collected every five days. Over a 25-day colonization period, benthic densities of Baetis nymphs and larval Chironomidae were reduced by D. baumanni. Colonization curves were fit with a power function and significantly different colonization rates were indicated for both Baetis and chironomids in predation and control channels. A predator-induced drift response was exhibited by Baetis only and this response was size-dependent. In the presence of D. baumanni, large Baetis drifted more frequently than small nymphs and, correspondingly, small nymphs were more frequent in the benthos. Net predator impacts on invertebrate densities in channel substrates were partitioned into predator-induced drift and prey consumption. These estimates suggest that predator avoidance by Baetis is a prominent mechanism causing density reductions in the presence of predators. Reductions in the density of Chironomidae, however, were attributed to prey consumption only. A rainstorm during the experiment demonstrated that stream flow disruptions can override the influence of predators on benthic invertebrates, at least temporarily, and re-set benthic densities.

The ridiculous notion of assessing ecological health and identifying the useful concepts underneath

Abstract The notion that the ecological health of the environment can be assessed is a ridiculous notion in a scientific context because there can be no objective definition of ‘health’ or method for defining degrees of health. Ecological health is a value judgement. A potentially useful concept embedded within the notion of ecological health is that environmental monitoring programs need to adopt a more holistic, ecosystems approach than has been used hitherto. In this paper, I outline a framework for ecosystem monitoring and discuss some of the variables that might be incorporated to assess ecosystem structure and processes. The interpretation of the data produced by ecosystem monitoring programs is problematic. Defining control or reference sites is virtually impossible; data can only be assessed with reference to extant ecosystems or to past situations.

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.

Hydraulic habitat and the assemblage structure of stream benthic microcrustacea

The relationship between microcrustacean assemblage structure and flow habitat was investigated in 9 streams with differing hydraulic characteristics. A series of partial canonical correspondence analyses was used to relate species abundances of microcrustacea to geographical, chemical, hydraulic, and seasonal variables. Once the influence of geography (latitude) on species distributions was removed, the environmental variables accounted for almost 38% of the variation in the combined (all seasons) data set. Circumneutral streams had greater species richness and higher abundance of cosmopolitan species than more acidic streams. The microcrustacean assemblages were distinguished further on the basis of stream hydraulic character. Previous research had categorised the 9 streams into Type I, II, or III depending on the fraction of stream bed retaining low shear stress at high discharge (declining from Types I to III). Taxa that are largely epibenthic in habit tended to be most abundant in Type I streams. These taxa may use as habitat the extensive low shear stress areas present in these streams, enabling their survival even at high discharge. Conversely, taxa that are largely interstitial in habit tended to be most abundant in Type II and III streams. The distribution of interstitial taxa may reflect conditions within the substratum rather than at the surface. The differences in microcrustacean assemblage structure were largely species-specific; the Harpacticoida were the only group to show a consistent pattern in relation to stream hydraulics.