The influence of climate change scenarios on populations of the mayfly Cloeon dipterum

Populations of the mayfly Cloeon dipterum from 48 ponds (3000 l fibre-glass tanks of 1 m depth) were monitored over the course of 1 year. To simulate possible patterns of climatic change, the ponds were subject to three temperature treatments: continuous heating to 3 °C above ambient; heating to 3 °C above ambient during the summer only; and no heating. Further experimental complexity included enhanced nutrient input into the ponds and the presence or absence of fish, giving a factorial combination of 3 temperature regimes × 2 nutrient levels × presence/absence of fish predation.Few nymphs were found in the presence of fish. Where fish were absent, the temperature treatments did not significantly affect nymph abundances, and only marginally influenced mean nymph body-lengths. In contrast, the nutrient treatment had significant effects on both nymph abundance and size, with greater numbers of generally larger nymphs occurring in those fish-free ponds receiving additional nutrients. Adult emergence began earlier in the year from the heated ponds, particularly those ponds receiving additional nutrients. Adult body-length differed between temperature treatments, but consistent patterns were difficult to ascertain because of interactions with nutrient treatment and seasonal effects.Our results show that during the short term at least, elevated temperature as a simulation of climate change does not have an overwhelming influence on either mayfly abundance or size. The influence of temperature is subtle and subject to complex interaction with other habitat variables. We therefore suggest that the direct consequences of small changes in temperature will likely be of little significance to C. dipterum, relative to indirect effects operating through interactions with predation and nutrient input.     

Resistance and resilience of macroinvertebrates to irrigation water withdrawals

1. Water withdrawal for irrigated agriculture is one of the leading uses of freshwater resources in the world; however, effects of low flow disturbances on lotic ecosystems are poorly understood. We studied an intensively managed agricultural catchment to determine: (i) how macroinvertebrate assemblages and environmental variables respond to water withdrawals of varying magnitude and duration; (ii) what environmental variables are associated with macroinvertebrate responses and (iii) the resiliency of macroinvertebrate communities to irrigation water withdrawals. 2. We sampled above and below four irrigation diversions that create a gradient of increasing water withdrawal from upstream to downstream (i.e. 0%, 22%, 87%, 90% and 97% water withdrawn) along a 36 km river section. Three reaches were sampled above and below each point of diversion from June to September 2004 and 2005, which represented average and drought water conditions respectively. 3. Irrigation water withdrawals were associated with both direct and indirect changes to the physicochemical environment. Direct effects (e.g. decreased velocity, depth and wetted habitat) were approximately proportional to the amount of water withdrawn, while indirect effects (e.g. increased conductivity and temperature) occurred when water withdrawals exceeded 85% of ambient levels. 4. Changes in macroinvertebrate communities were more strongly related to indirect than direct effects of irrigation water withdrawals. In an average water year, community changes were associated with interacting thresholds of reduced discharge and increased conductivity. During a drought year, community changes were related to the interacting thresholds of reduced discharge and increased temperature. 5. Between years, macroinvertebrate responses differed with the magnitude and duration of low flow conditions. In 2004, high‐intensity, relatively short‐duration water withdrawals (<2 months) and alterations to the physicochemical environment changed the relative abundance of macroinvertebrate communities, while macroinvertebrate indices and proportional abundances of functional feeding groups remained unchanged. In contrast, discharge reductions exceeding 90% of ambient levels and temperatures above 30 °C from July to September 2005 were associated with shifts in community composition from a dominance of collector‐gatherer and filterer Ephemeroptera, Plecoptera and Trichoptera taxa to predatory insects, non‐insect taxa and scraping elmid beetles. 6. On an annual basis macroinvertebrate communities appeared resilient to the impacts of water withdrawals following winter high flows. In contrast, recovery was not observed after discharge and physicochemical variables returned to predisturbance conditions for only one month. 7. Irrigation water withdrawals appear to impact macroinvertebrates through indirect effects that intensify with the magnitude and duration of water withdrawals and annual water availability. Preserving environmental conditions within natural ranges of variability, especially during low water years, appears critical to mitigating adverse biological responses to water withdrawals.     

The impact of physical disturbance on stream communities: lessons from road culverts

This study examined the impact of physical disturbance from long-established road culverts on stream macroinvertebrate communities. Three streams within a 6 km section of highway on the Avalon Peninsula, Newfoundland, Canada, were sampled. Streams had the entire upstream watershed and at least 100 m downstream of the road with natural boreal forest/barren vegetation and all had, within the sampled reaches, similar physical streambed characteristics. The fauna on stones from riffles was sampled at two upstream and three downstream sites, i.e., from 50 m above to about 100 m below the road in each stream. A total of 33 taxa were identified among the streams, with differences limited to a few rare taxa. The sample site communities did not significantly differ from each other with respect to the taxa present. Total macroinvertebrate abundance by site, for combined data of all streams, indicated the site at the exit of culvert plunge pool (site 3) had significantly elevated abundances. Analysis of individual taxa showed this was primarily due to very high numbers of Simulium spp. The other most notable changes were a decrease in numbers of Hydropsyche spp. and Elmidae below the road. The abundances of the remaining taxa were more variable among all sites. The study indicated that long-standing point source physical disturbance primarily impacted taxa abundance rather than community present/absent data, which will recolonize the disturbed zone by downstream drift. The differences in abundance are probably the result of the cleaning of substrate by abrasion, movement of substrate and reduction of detritus during each spate. Handling editor: D. Dudgeon     

Macroinvertebrates associated with two submerged macrophytes,Lagarosiphon ilicifolius and Vallisneria aethiopica,in the Sanyati Basin,Lake Kariba,Zimbabwe: effect of plant morphological complexity

Vallisneria aethiopica and Lagarosiphon ilicifolius are common and abundant submerged macrophytes in Lake Kariba, Zimbabwe. The two species have distinct structural morphologies, with Vallisneria consisting of long ribbon-like leaves, while Lagarosiphon has filiform stems with numerous small alternate leaves. This study investigated the effect of these architectural differences between the two plant species on their epiphytic macroinvertebrate assemblages in the shallow inshore waters of Lake Kariba. Ten sites were sampled on three occasions between May and July 2005. A total of 56 macroinvertebrate taxa was collected, 48 from Lagarosiphon and 45 from Vallisneria. Generally, the two plant species were associated with similar macroinvertebrate communities, but the average abundances of most taxa, and thus the overall macroinvertebrate abundances, were significantly greater on Lagarosiphon. The main macroinvertebrate functional feeding groups found on both plant species were collector-gatherers, grazers and predators, all of which were significantly more abundant on Lagarosiphon. Although the macroinvertebrate assemblages associated with Vallisneria and Lagarosiphon generally consisted of the same taxa, there were distinct and significant differences between them, probably due to the architectural differences between the two submerged macrophytes.     

The extant Chaoborus assemblage can be assessed using subfossil mandibles

1. Assemblages of Chaoborus were examined in 80 thermally stratified southern central Canadian Shield lakes to explore whether subfossil mandibles could be useful in assemblage‐level studies of Chaoborus. 2. Chaoborus (Sayomyia) (probably Chaoborus punctipennis in this study region) and Chaoborus flavicans were the most common taxa recorded, while Chaoborus trivittatus was rarer. Chaoborus americanus was not recorded in subfossil assemblages, because no fishless lakes were included in this study. Chaoborus flavicans had higher relative abundances (%) in lakes with higher dissolved organic carbon (DOC), probably because of reduced fish predation in less transparent water. 3. Results from logistic regression indicate that patterns of presence/absence for Chaoborus mandibles in the study lakes were influenced primarily by hypolimnetic oxygen concentration, probably because of the presence or extent of a hypolimnetic refugium from fish predation. 4. Chaoborus species richness in lakes, derived from subfossil assemblages, did not differ significantly from species richness estimates derived from plankton sampling with a net. Patterns of dominance and coexistence [e.g. the widespread co‐occurrence of C. flavicans and C. (Sayomyia)] determined from subfossil assemblages agreed with previous studies of the contemporary living assemblage. 5. These results suggest that subfossil assemblages may be used as an alternative to nocturnal plankton sampling to carry out research on the community ecology of Chaoborus. 6. We propose a hierarchical conceptual model of assemblage‐level patterns of Chaoborus in temperate lakes. Chaoborus americanus dominates in fishless lakes, whereas in lakes with fish Chaoborus is typically absent where there is no anoxic hyplimnion. In lakes with anoxic strata, C. trivittatus tends to dominate in lakes with few fish; in the remaining lakes, C. flavicans and C. (Sayomyia) dominate, although C. flavicans is more relatively abundant in lakes with lower water clarity (higher DOC).     

Predation and refugia: implications for Chaoborus abundance and species composition

1. Previous studies have suggested that the occurrence of larval Chaoborus in lakes may be affected by fish predation, pH, elevation, temperature, nutrient level, water transparency and interspecific competition, but so far, a detailed statistical evaluation of these findings has not been performed. 2. The aim of this study was to apply regression and ordination techniques to a large data set of 56 lakes in order to test which variables related to lake morphology, water chemistry, and fish predation determine (1) the abundance of individual Chaoborus species and (2) their species composition. 3. Individual Chaoborus species were influenced by very different sets of environmental factors. Nutrient levels positively affected the largest species, Chaoborus americanus, which was restricted to fishless lakes. Abundance of the smallest and most transparent species, C. punctipennis, seemed to be controlled more by the larger Chaoborus species than by fish. Larger chaoborids required low water clarity in order to co‐exist with fish, probably to increase refuge availability. Generally, small lakes (for C. flavicans/C. trivittatus) and shallow lakes (for C. punctipennis) supported higher abundances of Chaoborus.     

Larval chaoborid mandibles in surface sediments of small shallow lakes in Finland: implications for palaeolimnology

The aim of this study was to assess the value of phantom midge Chaoborus (Diptera: Chaoboridae) larvae as a palaeoenvironmental proxy in northern Europe. The presence of Chaoborus was examined in 80 small shallow lakes across Finland (60–70°N), based on their subfossil mandibles preserved in lake sediments. Chaoborus flavicans was present in 33 lakes, but was the most abundant midge taxon in only one of the lakes. Chaoborus crystallinus/obscurripes mandibles were present in two lakes. Statistical analyses showed that the distribution of Chaoborus was significantly correlated with lake depth and mean July air temperature. Chaoborus was absent in the northern study lakes, whereas the results indicated that C. flavicans was frequently found in small shallow lakes in southern Finland. Chaoborus flavicans was particularly abundant in fish-free, macrophyte-rich, dystrophic lakes, but absent in oligotrophic clear-water lakes having fish. The results of this study indicate that the subfossil mandibles of Chaoborus can provide valuable information in multiproxy palaeolimnological studies, especially when investigating past changes in water level and temperature in small shallow lakes in northern Europe. Guest editors: K. Buczkó, J. Korponai, J. Padisák & S. W. Starratt Palaeolimnological Proxies as Tools of Environmental Reconstruction in Fresh Water     

Zooplankton community structure driven by vertebrate and invertebrate predators

Summary A zooplankton community was established in outdoor experimental ponds, into which a vertebrate predator (topmouth gudgeon: Pseudorasbora parva) and/or an invertebrate predator (phantom midge larva: Chaoborus flavicans) were introduced and their predation effects on the zooplankton community structure were evaluated. In the ponds which had Chaoborus but not fish, small- and medium-sized cladocerans and calanoid copepods were eliminated while rotifers became abundant. A large-sized cladoceran Daphnia longispina, whose juveniles had high helmets and long tailspines as anti-predator devices, escaped from Chaoborus predation and increased. In the ponds which had fish but not Chaoborus, the large-sized Daphnia was selectively predated by the fish while small-and medium-sized cladocerans and calanoid copepods predominated. In the ponds containing both Chaoborus and fish, the fish reduced the late instar larvae (III and IV) of Chaoborus but increased the early instar larvae (I and II). Small- and large-sized cladocerans were scarcely found. The former might have been eliminated by predation of the early instar larvae of Chaoborus, while the latter was probably predated by fish. Consequently, the medium-sized cladocerans, which may have succeeded in escaping from both types of predator, appeared abundantly. The results suggest that various combinations of vertebrate and invertebrate predators are able to drive various kinds of zooplankton community structure.     

Impact of Holoshestes heterodon Eigenmann (Pisces: Characidae) on the plankton community of a subtropical reservoir: the importance of predation by Chaoborus larvae

We analyzed the effects of planktivorous Holeshestes heterodon Eigenmann (Characidae) predation on the plankton community of a small subtropical reservoir, using four enclosures (volume about 17.5 m³), open to the sediment, established in the littoral zone. Two enclosures were stocked with fish (mean TL 5.7 cm), at a density of about 4–5 fish m^–3 (approx. 8 g m^–3), whereas two remained fishless. The experiment lasted a little longer than one month. In the fish enclosures, most Crustacea and Chaoborus larvae remained scarce, probably as a result of visually selective fish predation. In both fishless enclosures, Chaoborus larvae became abundant. However, in only one of these did large individuals become relatively numerous; this discrepancy in the demographic structure of the Chaoborus populations between the two fishless enclosures is unexplained. Only in the fishless enclosure without appreciable numbers of large Chaoborus did densities of Crustacea increase greatly. It is suggested that in the enclosure containing large Chaoborus individuals, crustacean populations were prevented from developing due to predation pressure, while the small Chaoborus larvae of the other enclosure could not readily consume these prey. Rotifers were low in abundance in the absence of fish, probably as a consequence of Chaoborus predation. Phytoplankton density increased in all four enclosures, due probably to the lack of water flow. Only in the fishless enclosure with high densities of crustaceans did phytoplankton abundance decrease markedly at the end of the experiment, perhaps because of grazing losses.     

The effect of turbidity state and microhabitat on macroinvertebrate assemblages: a pilot study of six shallow lakes

Shallow lakes can occur in two alternative stable states, a clear-water state and a turbid state. This is associated with separate assemblages of fish, zooplankton and plants. Little is known about whether macroinvertebrate assemblages differ across both stable states. This study investigated this in a connected set of three turbid and three clear-water shallow lakes. To overcome confounding effects of differences in spatial structure of macrophytes in turbid and clear-water lakes, we sampled three microhabitats that occurred in both alternative stable states: open water, sago pondweed (Potamogeton pectinatus) and reed (Phragmites australis). Univariate analyses indicated no differences in the number of organisms, taxon richness or diversity between turbid and clear-water lakes. Multivariate analysis, however, showed significant differences in the macroinvertebrate community structure of both stable states. Nine taxa explained a significant amount of the variation between both lake types, of which seven preferred the clear-water lakes. The number of organisms and the taxon richness were higher in reed than in the other microhabitats, but diversity and evenness did not differ among the microhabitats. Multivariate analyses could separate all three microhabitats. Eight taxa, mainly detritus feeders and collector–gatherers, explained most of the variation in the data and preferred the reed microhabitat. The effects of stable state (6.8% explained variance) and microhabitat (13.1% explained variance) on the macroinvertebrate assemblages were largely independent from each other (1.5% shared variance). Although macroinvertebrates are not implemented in the initial theory of stable states, our results show clearly different assemblages across both stable states.     

Can lake restoration by fish removal improve the status of profundal macroinvertebrate assemblages?

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Temperature-dependent energetics of Chaoborus populations: hypothesis for anomalous distributions in the great lakes of East Africa

Chaoborus, the phantom midge (Insecta, Diptera, Chaoboridae), has a widespread distribution, commonly occurring in lakes and ponds all over the world. In the great lakes region of East Africa Chaoborus is present in Lakes Victoria, Albert, Edward, Malawi and George, but absent from Lakes Tanganyika, Kivu and Turkana. Tropical lakes typically have water temperatures in the range of 22–26 °C year round. Lakes Tanganyika and Kivu have only 20% of their bottom sediments oxygenated during full circulation, contrary to 95–100% in all of the other lakes, excluding Lake Malawi (45%) (Hecky & Kling, 1987). Planktivorous fish are present in all lakes (Lehman, 1995). We hypothesized that the absence of Chaoborus larvae from some lakes of East Africa may be the result of interaction among high temperatures, low oxygen levels, and fish predation.We developed a model to estimate energetic costs for Chaoborus larvae at temperatures greater than 14 °C. We hoped to shed light on the bioenergetics of Chaoborus populations, and the possibility that extant distributions of Chaoborus larvae are the result of energetic constraints.We found that relative respiratory and growth costs of Chaoborus larvae are highest in the early stages of development. We estimated that non-feeding instar I larvae living in 25 °C water will starve to death in less than one day. It is possible that Chaoborus populations are prevented from establishing in certain areas because high energetic costs condemn young larvae to death by either predation or starvation.     

Chaoborus and fish-mediated influences on Daphnia longispina population structure,dynamics and life history strategies

Summary This study examined the long term effects of predation by larvae of the midge Chaoborus and simulated fish predation on experimental Daphnia longispina populations. Chaoborus predation, relative to fish predation, led to populations composed of larger individuals as a whole, larger egg-bearing individuals, and a larger primiparous instar. Daphnia retained helmets beyond the first instar in response to the presence of Chaoborus. Both types of predation, relative to predator-free controls, reduced prey population size and rates of increase, but increased population death rates. The reduction in population size due to predation led to increased resource availability for individuals remaining in the populations and increased individual fecundity in the predation treatments. The differences noted between the Chaoborus, fish, and control treatments increased with predation intensity.     

Effects of predator-released chemicals on some life history parameters of Daphnia pulex

The effects of chemicals released by fish and Chaoborus larvae on some life history traits in Daphnia pulex were studied in an in situ enclosure experiment. The mean size of Daphnia individuals was smaller in the presence of fish-released cues. Also the minimal size of an egg bearing female in the presence of fish exudates was smaller than in the population exposed to the chemicals released by Chaoborus larvae as well as in the control population. Fish-released chemicals caused the increase in clutch size in Daphnia. There were no statistically significant differences between the studied life history parameters of the control and Chaoborus treatments. The results are discussed in reference to recent laboratory research.     

Factors affecting the distribution of a predatory mite on greenhouse sweet pepper

The predatory mite Neoseiulus cucumeris is used for biological control of phytophagous mites and thrips on greenhouse cucumber and sweet pepper. In a previous study, N. cucumeris provided effective control of broad mite but was only rarely found on the sampled leaves, raising questions about the factors affecting N. cucumeris distribution. To determine the distribution of N. cucumeris, leaves of pepper plants were sampled three times per day: just after sunrise, at noon and just before sunset for two years and throughout a 24 h period in one year. The presence of other mites and insects was recorded. Biotic (pollen) and abiotic (temperature, humidity) factors were monitored from the three plant levels. The effect of direct and indirect sunlight on the mites was assessed. N. cucumeris was found primarily in flowers; however, the mite’s distribution was affected by other predators (intraguild predation); in the presence of the predatory bug Orius laevigatus virtually no mites occurred in the flowers. Whereas temperature and humidity varied from the top to the lower level of the plants, apparently neither these factors nor the presence of pollen outside the flowers influenced mite distribution. N. cucumeris was found to be negatively phototropic; therefore N. cucumeris were pre-conditioned to light by rearing under light conditions for 4 months before being released. The light-reared mites were initially more numerous during the noon sampling period, however, rearing conditions caused only a temporary and non-significant change in distribution.     

Diversity assessment of benthic macroinvertebrate communities in Banco National Park (Banco Stream,Côte d'Ivoire)

In the present study, a first inventory of benthic macroinvertebrates in the Banco Stream, Côte d'Ivoire, and the correlations between environmental variables and taxonomic richness were analysed. Seven stations were sampled monthly over a 1‐year period, using a hand net (10 × 10 cm, 250 μm mesh, 50 cm length). One hundred and thirty‐two macroinvertebrate taxa were recorded. These taxa were distributed among 74 families and 15 orders belonging to Insecta (118 taxa; 89% of total richness), Oligochaeta (seven taxa), Crustacea (five taxa) and Mollusca (two taxa). Kruskal–Wallis test revealed significant difference (at least P < 0.05) in macroinvertebrate richness between upstream stations (S1 and S2) and stations S4, S5 and S6. Chironominae and Tanypodinae (Insecta) were the two very frequent taxa in all the stations. Lumbriculidae (Oligochaeta), Desmocaris trispinosa (Crustacea) and Eurymetra sp. (Insecta) were frequently found in the samples. Hierarchical cluster analysis revealed three groups of sampling stations according to taxonomic similarity. Taxonomic richness was significantly and negatively correlated with conductivity, while it was significantly and positively correlated with substrate types (woody debris and gravel). Due to the fact that Banco stream is the locality type of an endemic shrimp species (Macrobrachium thysi), this basin is of high conservation priority.     

Short‐term effects of low intensity fire on soil macroinvertebrate assemblages in different vegetation patch types in an Australian tropical savanna

Abstract Early dry season fires are a common land management regime employed across the tropical savannas of northern Australia. The rationale is that this reduces fuel loads and so reduces fire risk in the latter part of the dry season. Despite the acceptance of fire as a major management tool the ecological effects of fire remain uncertain. Vegetation patches and their associated macroinvertebrates play a critical role in the capture and recycling of water and nutrients. The aim of this study was to examine the responses of soil macroinvertebrates, within different types of vegetation patches, to early dry season fires in tropical savanna woodland in northern Australia. The abundance of major macroinvertebrate taxa and functional groups, and taxon richness were quantified in three vegetation patch types 2 weeks before and 2 weeks after burning. Termites dominated the soil macroinvertebrate assemblage sampled. Fire led to significant decreases in ant and spider abundances and overall taxon richness. Functional group analyses showed significant overall declines in the abundances of macropredators and litter transformers. There were also interactions between fire and patch type; in tree patches, fire significantly reduced total macroinvertebrate abundance, as well as the abundance of termites and ecosystem engineers. These changes in soil macroinvertebrates will potentially influence patch functionality, with important implications for soil processes and landscape health.     

Ecology of a saline stream: community responses to spatial gradients of environmental conditions

Spatial changes in structural and functional characteristics of fish and macroinvertebrate communities in eastern Kentucky were investigated in a drainage system chronically exposed to high levels of chloride salts from nearby oilfield operations. Salinity levels at biological monitoring stations ranged from 0.12–31.3‰. Lotic regions with salinities greater than 10‰ were dominated by larvae of the dipterans Ephydra and Culicoides. In regions with salinities less than 10‰ species richness increased more or less linearly with decreasing levels of chloride salts. Ephemeropterans appeared to be one of the major invertebrate groups least tolerant of elevated NaCl levels and were absent in regions with salinities greater than 2‰ Availability of food resources, such as periphyton and particulate organic matter, did not appear to be grossly altered in disturbed regions, and it is suggested that the observed distribution of macroinvertebrate fauna was largely in response to taxonomic differences in salt tolerance. Fish seemed to be more tolerant of highly saline conditions, and several species were observed in regions experiencing salinities as high as 15‰. Accordingly, assemblages of fish taxa along the salinity gradient may have been influenced by trophic factors, such as spatial limitations in availability of invertebrate prey.     

Ecological distributions of <Emphasis Type="Italic">Chaoborus</Emphasis> species in small,shallow lakes from the Canadian Boreal Shield ecozone

Small, shallow lakes and ponds are often the dominant landscape features in many regions, but are comparably less studied than larger lakes. Shallow lakes are more likely to lack fish populations; however, it is often difficult to ascertain whether these sites were naturally fishless or lost their fish populations due to anthropogenic or natural stressors. We examined the distributional abundances of four Chaoborus species by identifying and enumerating their larval mandibles in the surface sediments of 146 randomly selected lakes from northwestern Ontario, Canada, to determine the key environmental gradients influencing chaoborid distributions. Chaoborus mandibles were encountered at 110 lakes and, in 65% of those lakes, total counts were ≥10. Direct gradient analyses were then used to show that lakewater total aluminum concentrations (negatively correlated with pH), lakewater sodium concentrations, lake surface area, and maximum water depth were significant predictors of the distributional abundances of Chaoborus. Generalized linear models indicated that Chaoborus species varied in their responses to significant environmental factors. C. (Sayomyia) was not significantly associated with any environmental variable and the abundances of larger chaoborids may be an important biotic factor affecting this taxon. Chaoborus americanus, an indicator of fishless lakes, was significantly correlated with all five key variables and demonstrated a clear threshold of occurrence in relatively small lakes (i.e., <10 ha in surface area). Furthermore, based on the occurrence and abundance of C. americanus, we estimated that 20% of the lakes we surveyed are currently fishless. These lakes significantly differ in several geomorphic and water-chemistry measures compared to the other study lakes.     

The effect of plant density on epiphytic macroinvertebrates associated with a submerged macrophyte,Lagarosiphon ilicifolius Obermeyer,in Lake Kariba,Zimbabwe

The effect of variations in the density of a submerged macrophyte, Lagarosiphon ilicifolius, on epiphytic macroinvertebrate community structure in the shallow waters of a sheltered bay of Lake Kariba were investigated. The body size class distributions of a mayfly, Cloeon (Ephemeroptera: Baetidae), and the damselfly family, Coenagrionidae, were also assessed with respect to variation in vegetation density. Macroinvertebrates were sampled from low-, moderate- and high-density beds of L. ilicifolius. There were no significant differences in individual taxon and total macroinvertebrate abundances, macroinvertebrate richness and diversity with respect to vegetation density. In all three density categories the functional feeding group (FFG) composition was dominated by collector-gatherers and collector-filterers. The abundance of the two FFGs did not change significantly within each, as well as among, the three vegetation-density categories (ANOVA, p > 0.05). The largest size class of Cloeon occurred only in high-density beds, whereas the largest coenagrionid individuals were obtained from low- and moderate-density beds and were absent from high-density beds. The results suggest that variation in the density of Lagarosiphon does not affect epiphytic macroinvertebrate community structure, but does affect body-size distributions of macroinvertebrate taxa, probably by affecting predator-prey interactions.