Functional biodiversity of macroinvertebrate assemblages along major ecological gradients of boreal headwater streams

1. Biodiversity–environment relationships are increasingly well‐understood in the context of species richness and species composition, whereas other aspects of biodiversity, including variability in functional diversity (FD), have received rather little rigorous attention. For streams, most studies to date have examined either taxonomic assemblage patterns or have experimentally addressed the importance of species richness for ecosystem functioning. 2. I examined the relationships of the functional biodiversity of stream macroinvertebrates to major environmental and spatial gradients across 111 boreal headwater streams in Finland. Functional biodiversity encompassed functional richness (FR – the number of functional groups derived from a combination of functional feeding groups and habit trait groups), FD – the number of functional groups and division of individuals among these groups, and functional evenness (FE – the division of individuals among functional groups). Furthermore, functional structure (FS) comprised the composition and abundance of functional groups at each site. 3. FR increased with increasing pH, with additional variation related to moss cover, total nitrogen, water colour and substratum particle size. FD similarly increased with increasing pH and decreased with increasing canopy cover. FE decreased with increasing canopy cover and water colour. Significant variation in FS was attributable to pH, stream width, moss cover, substratum particle size, nitrogen, water colour with the dominant pattern in FS being related to the increase of shredder‐sprawlers and the decrease of scraper‐swimmers in acidic conditions. 4. In regression analysis and redundancy analysis, variation in functional biodiversity was not only related to local environmental factors, but a considerable proportion of variability was also attributable to spatial patterning of environmental variables and pure spatial gradients. For FR, 23.4% was related to pure environmental effects, 15.0% to shared environmental and spatial effects and 8.0% to spatial trends. For FD, 13.8% was attributable to environmental effects, 15.2% to shared environmental and spatial effects and 5% to spatial trends. For FE, 9.0% was related to environmental variables, 12.7% to shared effects of environmental and spatial variables and 4.5% to spatial variables. For FS, 13.5% was related to environmental effects, 16.9% to shared environmental and spatial effects and 15.4% to spatial trends. 5. Given that functional biodiversity should portray variability in ecosystem functioning, one might expect to find functionally rather differing ecosystems at the opposite ends of major environmental gradients (e.g. acidity, stream size). However, the degree to which variation in the functional biodiversity of stream macroinvertebrates truly portrays variability in ecosystem functioning is difficult to judge because species traits, such as feeding roles and habit traits, are themselves strongly affected by the habitat template. 6. If functional characteristics show strong responses to natural environmental gradients, they also are likely to do so to anthropogenic environmental changes, including changes in habitat structure, organic inputs and acidifying elements. However, given the considerable degree of spatial structure in functional biodiversity, one should not expect that only the local environment and anthropogenic changes therein are responsible for this variability. Rather, the spatial context, as well as natural variability along environmental gradients, should also be explicitly considered in applied research.     

Leaf Phenology and Leaf Damage of Saplings in the Luquillo Experimental Forest,Puerto Rico1

Changes in light or water availability can result in synchronous leaf production, concentrating food availability for herbivores of young leaves to only a few months. To determine the importance of food availability on herbivory, leaf phenology and leaf damage were studied in the Luquillo Experimental Forest (LEF) of Puerto Rico. We studied 20 individuals of eight species for two years. Every month, new leaves were marked; the following month, leaf area and area of damage were measured. Over two years, comparison of leaf production and percent herbivory were performed for each species, and for all species taken together. More than 30 percent of the annual leaf production occurred in May and June. Leaf production was associated with an increase in PFD (photon flux density) and was not related to the patterns of rainfall. Although leaf production was synchronous, there were no differences in herbivory between the peak and non‐peak periods of leaf production. Possible explanations for the constant levels of herbivory throughout the year are the presence of a generalist herbivore community, the ability of herbivores to track changes in food availability, or high densities of herbivore predators that control herbivore populations.     

Parallels and contrasts in the effects of drought on stream macroinvertebrate assemblages

• 1 It is axiomatic that unusually long dry periods (droughts) adversely affect aquatic biota. Recovery after drought is rapid by macroinvertebrates that possess strategies to survive drying or are highly mobile but other taxa take longer to recolonise depending on the timing, intensity, and duration of the dry phase.
• 2 Although drought acts as a sustained ‘ramp’ disturbance, impacts may be disproportionately severe when certain critical thresholds are exceeded. For example, ecological changes may be gradual while a riffle dries but cessation of flow causes abrupt loss of a specific habitat, alteration of physicochemical conditions in pools downstream, and fragmentation of the river ecosystem. Many ecological responses to drought within these habitats apparently depend on the timing and rapidity of hydrological transitions across these thresholds, exhibiting a ‘stepped’ response alternating between gradual change while a threshold is approached followed by a swift transition when a habitat disappears or is fragmented.
• 3 In two Australian intermittent streams, drought conditions eliminated or decimated several groups of macroinvertebrates, including atyid shrimps, stoneflies and free‐living caddisflies. These taxa persisted during the early stages of the drought but did not recruit successfully the following year, despite a return to higher‐than‐baseflow conditions. This ‘lag effect’ in response to drought emphasises the value of long‐term survey data. Although changes in faunal composition were inconsistent among sites, marked shifts in taxa richness, abundance and trophic organisation after the riffle habitat dried provide evidence for a stepped response.
• 4 Responses by macroinvertebrate assemblages to droughts of differing severity in English chalk streams were variable. The prolonged 1988–92 drought had a greater impact than shorter droughts in the early 1970s but recovery over the next 3 years was swift. Effects of the 1995 summer drought were buffered by sustained groundwater discharge from the previous winter. These droughts tended to reduce available riverine habitats, especially via siltation, but few taxa were eliminated because they could recolonise from perennial sections of the chalk streams.
• 5 In the contrasting environments of the intermittent streams studied in England and Australia, there are parallels in the rapid rates of recolonisation. However, recruitment by taxa that lack desiccation‐resistant stages or have limited mobility is delayed. Currently, long‐term data on these systems may be insufficient to indicate persistent effects of droughts or predict the impacts of excessive surface or groundwater abstraction or the increased frequency and duration of droughts expected with global climate change.

     

Leaf litter decomposition and macroinvertebrate communities in headwater streams draining pine and hardwood catchments

Benthic invertebrates, litter decomposition, andlitterbag invertebrates were examined in streamsdraining pine monoculture and undisturbed hardwoodcatchments at the Coweeta Hydrologic Laboratory in thesouthern Appalachian Mountains, USA. Bimonthlybenthic samples were collected from a stream draininga pine catchment at Coweeta during 1992, and comparedto previously collected (1989–1990) benthic data froma stream draining an adjacent hardwood catchment. Litter decomposition and litterbag invertebrates wereexamined by placing litterbags filled with pine ormaple litter in streams draining pine catchments andhardwood catchments during 1992–1993 and 1993–1994. Total benthic invertebrate abundance and biomass inthe pine stream was ca. 57% and 74% that of thehardwood stream, respectively. Shredder biomass wasalso lower in the pine stream but, as a result ofhigher Leuctra spp. abundance, shredderabundance was higher in the pine stream than thehardwood stream. Decomposition rates of both pine andred maple litter were significantly faster in pinestreams than adjacent hardwood streams (p<0.05). Total shredder abundance, biomass, and production weresimilar in maple bags from pine and hardwood streams. However, trichopteran shredder abundance and biomass,and production of some trichopteran taxa such asLepidostoma spp., were significantly higher in maplelitterbags from pine streams than hardwood streams(p<0.05). In contrast, plecopteran shredders(mainly Tallaperla sp.) were more important inmaple litterbags from hardwood streams. Shredderswere well represented in pine litterbags from pinestreams, but low shredder values were obtained frompine litterbags in hardwood streams. Resultssuggest conversion of hardwood forest to pinemonoculture influences taxonomic composition of streaminvertebrates and litter decomposition dynamics. Although the impact of this landscape-leveldisturbance on invertebrate shredder communitiesappeared somewhat subtle, significant differences indecomposition dynamics indicate vital ecosystem-levelprocesses are altered in streams draining pinecatchments.     

Barriers to Forest Regeneration in an Abandoned Pasture in Puerto Rico

Sources of forest regeneration (soil seed bank, seed rain) and barriers to seedling establishment were examined in a recently abandoned pasture in eastern Puerto Rico. Few woody species were found in the soil seed bank or in the seed rain. The number of seeds and species in the seed rain and soil seed bank declined with distance from the adjacent secondary forest. Nine species naturally dispersed and colonized plots during the study, with the wind‐dispersed tree Tabebuia heterophylla being the predominant colonizer (91% of all seedlings). Barriers to seedling establishment were determined using a blocked field experiment with eleven woody species representative of a variety of life histories. Each species was planted under the pasture vegetation (control) or in areas where all vegetation was removed (removal). Germination was enhanced for four species in the control treatment, five species were not affected, and two species did not germinate under either treatment. Survival to 6 months was higher in the removal treatment for two species. Seedling biomass was greater in the removal treatment at 12 months for one species. Seed mass was a good predictor of germination success and final shoot biomass, but not survival. This study demonstrates that seeding recently abandoned pastures with a mix of known pioneer species may accelerate the rate of secondary succession, but some species will have to be planted in later successional stages in order to overcome strong barriers to establishment.     

Replicability of physicochemistry and macroinvertebrate assemblages in stream mesocosms: implications for experimental research

1. The replicability of mesocosms is assessed in a case study of artificial streams (4 m²) flanking a lowland chalk river in southern U.K. Among‐mesocosm comparisons of physicochemistry and macroinvertebrate assemblage composition (in drift and benthos) were made during a 2‐year survey of 12 outdoor once‐through linear channels. 2. Mesocosm physicochemistry was highly replicable, with statistically significant congruence in temporal variation across the mesocosm series, low spatial variation and no significant blocking or positional effects arising from mesocosm layout. Parallel physicochemistry was attributed to the outdoor stream‐side location and short water residence times. 3. Mesocosms were colonized by 127 macroinvertebrate taxa from 15 taxonomic orders. Both benthic and drifting assemblages were comparable among mesocosms, with no significant effect of mesocosm layout. Small differences in taxonomic composition were, however, evident among particular mesocosms, with higher (e.g. Tinodes, Limnius and Elmis) or lower (e.g. Pisidium and Valvata) abundances observed for a minority (5%) of taxa. We conclude that large (4 m²) outdoor flow – through mesocosms can be replicable when located near to the source system and allowed to establish naturally.     

Spatial variability in macroinvertebrate assemblages along and among neighbouring equatorial glacier‐fed streams

1. During the past two decades, understanding of the structure and function of glacier‐fed stream ecosystems at temperate latitudes has increased substantially. In contrast, information on their tropical counterparts is very limited. We studied three neighbouring glacier‐fed streams in the tropical Andes of Ecuador. Our main goals were (i) to determine overall longitudinal patterns in density, taxon richness and the composition of macroinvertebrate assemblages and driving factors in equatorial glacial streams and (ii) to examine variability among replicate streams in faunal metrics and assemblages, and stream‐specific effects of supposed environmental key factors. 2. We measured four geographical and 17 environmental factors and collected five Surber samples (500 cm²) of macroinvertebrates at each of nine sites, three sites along three streams. The streams were located 1–5 km apart. In each stream, the three sites were placed at comparable distances from the glacier and were grouped as ‘upper’ (50–200 m), ‘middle’ (1.5 km) and ‘lower’ sites (3.5–5.6 km). 3. In total, 2200 individuals (64% chironomids) were collected and 47 taxa (30 dipterans, 18 of these Chironomidae) identified. Density ranged from 176 to 372 ind. m^?2, and the number of taxa ranged from 2 to 6 at the upper sites and 868–3044 ind. m^?2 and 21–27 taxa at the lower sites. Density, number of taxa, rarefied richness and axis‐1 coordinates from a MDS ordination increased logarithmically with distance from the glacier. These faunal metrics were equally related to altitude and glacier per cent of catchment and correlated with maximum conductivity, mean temperature, mean daily maximum temperature and a channel stability index. As expected, the mean difference in distance decay in similarity was higher at the upper (47% km^?1) than at the lower reaches (20% km^?1) of the streams. 4. The number of taxa varied among sites within the upper and middle groups, but not among the lower sites. In contrast, but in accordance with our expectation, assemblage composition did not differ among upper sites but did so at middle and lower sites, following a supposed decrease in environmental harshness along the streams. Relationships between faunal metrics and the four environmental variables mean temperature, the stability index, chlorophyll a and coarse particulate organic matter also varied among the three streams. Generalised linear model analyses revealed that temperature interacted with stream on macroinvertebrate density, while chlorophyll a had a significant effect on the number of taxa in interaction with stream and stability. 5. The basic predictions of the Milner et al. (2001a) , model regarding longitudinal faunal patterns and temperature and stability as main driving factors were met by our three replicate equatorial glacial streams. Qualitative departures from the model were mainly because of zoogeographical differences. We demonstrated that variability in assemblages between comparable sites in closely situated streams was considerable, and the effect of key environmental factors varied among streams and interacted with other factors. Quantifying spatial variation in benthic assemblages may help us foresee possible consequences for biodiversity as a result of glacial retreat.     

Multiple facets of stream macroinvertebrate alpha diversity are driven by different ecological factors across an extensive altitudinal gradient

Environmental filtering and spatial structuring are important ecological processes for the generation and maintenance of biodiversity. However, the relative importance of these ecological drivers for multiple facets of diversity is still poorly understood in highland streams. Here, we examined the responses of three facets of stream macroinvertebrate alpha diversity to local environmental, landscape‐climate and spatial factors in a near‐pristine highland riverine ecosystem. Taxonomic (species richness, Shannon diversity, and evenness), functional (functional richness, evenness, divergence, and Rao's Quadratic entropy), and a proxy of phylogenetic alpha diversity (taxonomic distinctness and variation in taxonomic distinctness) were calculated for macroinvertebrate assemblages in 55 stream sites. Then Pearson correlation coefficient was used to explore congruence of indices within and across the three diversity facets. Finally, multiple linear regression models and variation partitioning were employed to identify the relative importance of different ecological drivers of biodiversity. We found most correlations between the diversity indices within the same facet, and between functional richness and species richness were relatively strong. The two phylogenetic diversity indices were quite independent from taxonomic diversity but correlated with functional diversity indices to some extent. Taxonomic and functional diversity were more strongly determined by environmental variables, while phylogenetic diversity was better explained by spatial factors. In terms of environmental variables, habitat‐scale variables describing habitat complexity and water physical features played the primary role in determining the diversity patterns of all three facets, whereas landscape factors appeared less influential. Our findings indicated that both environmental and spatial factors are important ecological drivers for biodiversity patterns of macroinvertebrates in Tibetan streams, although their relative importance was contingent on different facets of diversity. Such findings verified the complementary roles of taxonomic, functional and phylogenetic diversity, and highlighted the importance of comprehensively considering multiple ecological drivers for different facets of diversity in biodiversity assessment.     

Deterministic tropical tree community turnover: evidence from patterns of functional beta diversity along an elevational gradient

Explaining the mechanisms that produce the enormous diversity within and between tropical tree communities is a pressing challenge for plant community ecologists. Mechanistic hypotheses range from niche-based deterministic to dispersal-based stochastic models. Strong tests of these hypotheses require detailed information regarding the functional strategies of species. A few tropical studies to date have examined trait dispersion within individual forest plots using species trait means in order to ask whether coexisting species tend to be more or less functionally similar than expected given a null model. The present work takes an alternative approach by: (i) explicitly incorporating population-level trait variability; and (ii) quantifying the functional beta diversity in a series of 15 tropical forest plots arrayed along an elevational gradient. The results show a strong pattern of decay in community functional similarity with elevation. These observed patterns of functional beta diversity are shown to be highly non-random and support a deterministic model of tropical tree community assembly and turnover.     

Eucalypt leaf decomposition in an intermittent stream in south-eastern Australia

Eucalypt leaf packs were placed at two sites in an intermittent stream during summer to examine the hypothesis that terrestrially-exposed leaf litter accumulates a richer microbial flora than submerged leaves — a phenomenon observed in Canadian temporary vernal pools. This did not occur; during the experiment, microbial biomass (as ATP) rose steadily on submerged leaves but remained low on terrestrially-exposed leaves. Densities of most functional feeding groups on the submerged leaves increased with time. Scrapers appeared to be more important than shredders in eucalypt leaf breakdown at both sites.     

Leaf Litter Decomposition and Substrate Chemistry of Early Successional Species on Landslides in Puerto Rico1

Decomposition is a critical process for nutrient release and accumulation of soil organic matter in disturbed soils, such as those found on landslides. I conducted a decomposition experiment on five landslides in the Luquillo Mountains of Puerto Rico as part of an investigation of the successional roles of two of the most common plant colonists to landslides, Cecropia schreberiana Miq. (Cecropiaceae) a pioneer tree species, and Cyathea arborea (L.) Sm. (Cyatheaceae) a pioneer tree fern. I compared leaf litter decomposition over one year and the initial and 1‐yr chemistry for both species. Initial litter chemistry differed between the two species, as Cecropia had slightly higher nitrogen (9.2 mg/g) than Cyathea (8.2 mg/g) and higher lignin (28.6%) than Cyathea (26.0%), but water‐soluble carbon and nonpolar extractable carbon (fats and oils, waxes, chlorophylls) were higher in Cyathea than Cecropia. Total carbon, acid‐soluble carbon, total phosphorus, and pH did not differ significantly between leaf litter species. Across all five landslides, Cyathea (k= 0.93 ± 0.06) leaves decomposed significantly faster than Cecropia (k= 0.68 ± 0.06). The differences in these species leaf litter decomposition rates and chemical composition could potentially influence organic matter dynamics and nutrient cycling rates in these early successional systems.     

Spatial and temporal trends in stream macroinvertebrate communities: the influence of catchment disturbance

Macroinvertebrate communities of five headwater streams in catchments disturbed by wildfire were compared with five similar streams with no catchment disturbance. Over the five years of observation, communities in disturbed streams were more similar to one another than they were to reference streams. Communities in disturbed streams exhibited more year-to-year variation than reference streams, although some indication of decreasing variation was evident through time, and species richness was greater in reference streams than disturbed streams. No increasing trend in richness over time was observed in disturbed streams. Stability of the relative abundance structure and persistence of dominant taxa through time may be characteristic of temperate streams over moderate time intervals. Local effects of catchment-wide disturbance have persistent effects that alter these trends.     

Effects of light,moisture, temperature,and litter on the regeneration of five tree species in the tropical montane wet forest of Puerto Rico

Field experiments were conducted to investigate the effects of light, moisture, temperature, and litter on the regeneration of two early-, one mid-, and two late-successional tropical tree species. High light and litter seem to be universally good cues for regeneration, increasing seed/seedling survival for all species except for Cecropia (an early-successional species) whose small seeds may not be able to penetrate the litter layer. In addition, the high temperature environment in both artificially shaded and nonshaded areas of a natural gap exhibits less seed loss, an increase in the percent and rate of germination, and an increase in seedling survival for Dacryodes (a late-successional species), than the lower temperature environment under an intact canopy. Low soil water is also a good cue for Dacryodes germination as it is for Prestoea and Cecropia. Finally, the lower temperature environment found under the forest canopy (compared to the natural gap) leads to less seed loss and more germination for Guarea (a mid-successional species). Our results suggest that a good patch for regeneration of many species in this forest, early- as well as late-successional species, would have high light and a litter layer that moderates temperature and moisture extremes. The substantial variation in suitability among regeneration filters and species could: (1) contribute to low establishment success, i.e., most dispersed propagules do not become trees, (2) make it difficult to group species into germination strategies, and (3) make it hard to generalize about a net effect of any specific environmental variable on establishment. We suggest that tropical disturbances should be viewed in terms of their impact on a variety of environmental cues, which may signal germination and impact subsequent growth and survival.     

Effect of native and exotic leaf litter on macroinvertebrate communities and decomposition in a western Montana stream

Invasion by exotic trees into riparian areas has the potential to impact terrestrial and aquatic systems. To test the effect of different streamside tree species on the aquatic food web in a stream in Montana, we compared decomposition rates of leaf litter and invertebrate assemblages associated with the leaf litter of the exotic Acer platanoides and the dominant native Populus trichocarpa trees. Macroinvertebrate family richness, evenness, and diversity increased with days of aquatic processing; however, there was no effect of leaf species. Leaves of the A. platanoides were associated with 70% greater density of macroinvertebrates of the family Nemouridae. This family consists primarily of detritivores and had the greatest overall density and frequency of occurrence relative to other macroinvertebrate families. The density of a family of predatory macroinvertebrates (Rhyacophilidae) was also generally (73%) greater in association with A. platanoides than P. trichocarpa leaves. The density of Ephemerellidae and Rhyacophilidae increased over time. In contrast to studies comparing leaves of exotic vs. native trees, we observed no difference in leaf decomposition rates; however, the amount of leaf inputs are likely to differ between native and invaded forests. The results indicate that replacement of native riparian trees with exotics affected the most common family of macroinvertebrates and possibly a common family of predatory macroinvertebrates (Rhyacophilidae), which may affect the detrital food web.     

Lentic macroinvertebrate assemblage structure along gradients in spatial heterogeneity, habitat size and water chemistry

Littoral zones of small water bodies are spatially heterogeneous habitats, harbouring diverse biotic communities. Despite this apparent heterogeneity, many studies have stressed the importance of water chemistry in determining the structure of littoral macroinvertebrate assemblages. The purpose of this study was to consider the relative importance of several spatial and water chemistry variables in explaining the patterns in the structure of macroinvertebrate assemblages in 21 lentic water bodies in northeastern Finland. Water bodies were selected to represent various habitat conditions ranging from small permanent bog ponds to small forest lakes. According to canonical correspondence analysis (CCA), the most important environmental factors related to assemblage composition were water body area, moss cover, total nitrogen and water hardness. In general, species composition in small bog ponds tended to differ from that in larger lakes with forested shoreline. Total species richness was best explained by a composite variable (PCA) describing physical habitat heterogeneity, species richness being lowest in small bog lakes with simple bottom structure and low amount of aquatic plants. Species numbers in dominant functional feeding groups were related to different environmental factors. Shredder species richness was best explained by a regression model incorporating total nitrogen and the amount of organic matter, both of which were negatively related to the number of shredder species. The number of gatherer species increased with mean substratum particle size. Scraper species richness was negatively affected by the abundance of detritus and positively affected by depth, and a model including both variables explained most of the variation. Variation in the number of predatory species was best explained by a regression model including moss cover and lake area.     

外来植物凋落物分解对底栖动物多样性及其摄食功能群的影响

在亚热带地区一条2级溪流中比较了本地植物(红锥和薏米)和外来植物(托里桉和马缨丹)凋落物的分解速率及底栖动物定殖及其摄食功能群的组成。结果显示,本地植物凋落物的分解速率为:0.020 d-1(红锥)和0.056 d-1(薏米),外来树叶的分解速率则为:0.038 d-1(托里桉)和0.041 d-1(马缨丹),它们均属于快速分解组。定殖在外来和本地树叶凋落物上的大型底栖动物种类分别为21种和24种。在外来和本地树叶上定殖的底栖动物平均密度差异不显著(P0.05)。从大型底栖动物摄食功能群的比例来看,定殖在本地凋落物上的集食者的比例最高(73.5%),其次是撕食者(10.9%)和捕食者(8.7%),刮食者(6.8%)的比例最低,在外来凋落物上,摄食功能群的比例分别为集食者(67.6%)、撕食者(13.9%)、刮食者(12.1%)和捕食者(6.1%),两者相比,底栖动物的摄食功能群均以集食者和撕食者为主,在组成比例上并无显著差异。结果表明2种外来植物凋落物分解对底栖动物多样性及其摄食功能群的影响不明显。     

An assessment of mesh bags and plastic leaf traps as tools for studying macroinvertebrate assemblages in natural leaf packs

Aggregation of leat litter formed against small mesh obstacles — placebo traps — were studied in four streams differing in natural retentiveness and pH. In three of the streams, natural benthic accumulations of leaf litter were available for comparison, and in these the fauna in the plastic traps and the natural accumulations was similar. In two of the streams comparisons were made, in terms of percent composition, between the fauna of the stony benthos and that colonizing plastic traps and leaf-filled mesh bags. In an acid, naturally retentive stream, the fauna of the three treatments was similar, although shredders were relatively more abundant in plastic traps and mesh bags. In a circumneutral, non-retentive stream diversity of taxa was reduced in plastic traps compared with the stony benthos, and in mesh bags compared with plastic traps. Numbers of animals per g of leaf litter were similar in plastic traps and mesh bags in the retentive stream. In the non-retentive stream, however, there were fewer animals in mesh bags than in the plastic traps. For many purposes, the plastic traps produce leaf packs which closely mimic natural packs, but the results from mesh bags depend on the background retentiveness of the streams in question.