The assembly of ecological communities inferred from taxonomic and functional composition.

Among-site variation in metacommunities (beta diversity) is typically correlated with the distance separating the sites (spatial lag). This distance decay in similarity pattern has been linked to both niche-based and dispersal-based community assembly hypotheses. Here we show that beta diversity patterns in community composition, when supplemented with functional-trait information, can be used to diagnose assembly processes. First, using simulated data, we show how the relationship between distance decay patterns in taxonomic and functional measures of community composition can be used to predict the influence of a given trait on community assembly. We then use the patterns generated by the simulation as a template to show that the sorting of benthic macroinvertebrate metacommunities in headwater streams is likely influenced by different sets of functional traits at regional and local scales. We suggest that functional-trait databases and spatially referenced taxonomic surveys can be used to predict the spatial scales at which different aspects of interspecific functional variation are involved in niche-based community assembly while accounting for the influence of dispersal-based community assembly processes.

Spatial scale and structure of complex life cycle trematode parasite communities in streams.

By considering the role of site-level factors and dispersal, metacommunity concepts have advanced our understanding of the processes that structure ecological communities. In dendritic systems, like streams and rivers, these processes may be impacted by network connectivity and unidirectional current. Streams and rivers are central to the dispersal of many pathogens, including parasites with complex, multi-host life cycles. Patterns in parasite distribution and diversity are often driven by host dispersal. We conducted two studies at different spatial scales (within and across stream networks) to investigate the importance of local and regional processes that structure trematode (parasitic flatworms) communities in streams. First, we examined trematode communities in first-intermediate host snails (Elimia proxima) in a survey of Appalachian headwater streams within the Upper New River Basin to assess regional turnover in community structure. We analyzed trematode communities based on both morphotype (visual identification) and haplotype (molecular identification), as cryptic diversity in larval trematodes could mask important community-level variation. Second, we examined communities at multiple sites (headwaters and main stem) within a stream network to assess potential roles of network position and downstream drift. Across stream networks, we found a broad scale spatial pattern in morphotype- and haplotype-defined communities due to regional turnover in the dominant parasite type. This pattern was correlated with elevation, but not with any other environmental factors. Additionally, we found evidence of multiple species within morphotypes, and greater genetic diversity in parasites with hosts limited to in-stream dispersal. Within network parasite prevalence, for at least some parasite taxa, was related to several site-level factors (elevation, snail density and stream depth), and total prevalence decreased from headwaters to main stem. Variation in the distribution and diversity of parasites at the regional scale may reflect differences in the abilities of hosts to disperse across the landscape. Within a stream network, species-environment relationships may counter the effects of downstream dispersal on community structure.

Variation in ecosystem function in Appalachian streams along an acidity gradient.

Acidification is a widespread phenomenon that damages aquatic systems, and it has been the focus of intensive management efforts. While most management has focused on community structure as an endpoint, ecosystem function is also sensitive to acidification and important in stream health. We examined how a key ecosystem function in streams, leaf breakdown, varied along a gradient of pH resulting from acid deposition, natural conditions, and liming. We also measured how invertebrate and microbial assemblage structure and microbial function were related to altered leaf breakdown rates. Leaf breakdown rates declined more than threefold along a gradient of stream acidity from pH 6.8 to 4.9. The diversity of leaf-shredding invertebrates, bacteria, and fungi showed little response to variation in pH. The abundance of one acid-sensitive caddisfly, Lepidostoma, declined with acidification, and Lepidostoma abundance explained 37% of the variation in leaf breakdown rates among sites. Microbial respiration was suppressed along the acidity gradient, although the pattern was weaker than that for breakdown rate. In short-term laboratory incubations, microbes at acidic and circumneutral sites demonstrated adaptation to ambient pH. The activity of microbial extracellular enzymes was strongly influenced by pH. In particular, the pattern of activity of phosphatase indicated increasing P limitation of microbes with increasing acidification. Our results show that leaf breakdown is a sensitive tool for examining the response of stream function to acidification and also for defining the mechanisms that drive functional response. Future management efforts should focus on key taxa that are particularly sensitive and effective at shredding leaves and also the role of shifting acidity in mediating the availability of phosphorus to microbial films that are important for stream function.

The land-cover cascade: relationships coupling land and water.

We introduce the land-cover cascade (LCC) as a conceptual framework to quantify the transfer of land-cover-disturbance effects to stream biota. We hypothesize that disturbance is propagated through multivariate systems through key variables that transform a disturbance and pass a reorganized disturbance effect to the next hierarchical level where the process repeats until ultimately affecting biota. We measured 31 hydrologic, geomorphic, erosional, and substrate variables and 26 biotic responses that have been associated with land-use disturbance in third- and fourth-order streams in the Blue Ridge physiographic province in western North Carolina (USA). Regression analyses reduced this set of variables to include only those that responded to land cover and/or affected biota. From this reduced variable set, hypotheses were generated that predicted the disturbance pathways affecting each biotic response following the land-cover-cascade design. Cascade pathways began with land cover and ended with biotic responses, passing through at least one intermediate ecosystem abiotic component. Cascade models were tested for predictive ability and goodness-of-fit using path analysis. Biota were influenced by near-stream urban, agricultural, and forest land cover as propagated by hydrologic (e.g., discharge), geomorphic (e.g., stream bank height), erosional (e.g., suspended sediments), and depositional streambed (e.g., substrate size) features occurring along LCC pathways, reflecting abiotic mechanisms mediating land-cover disturbance. Our results suggest that communities are influenced by land-cover change indirectly through a hierarchy of associated abiotic components that propagate disturbance to biota. More generally, the land-cover cascade concept and experimental framework demonstrate an organized approach to the generic study of cascades and the complex relationships between landscapes and streams.

Seasonal and Annual Variation in Trematode Infection of Stream Snail Elimia proxima in the Southern Appalachian Mountains of Virginia.

Understanding temporal variation of host-pathogen dynamics can be important for predicting disease risks and anticipating how disease systems may change in response to natural or human disturbances. Seasonal changes in weather, especially those associated with changes in temperature or precipitation, are often a key component of temporal changes in infection risk and can have important impacts on disease systems. However, these patterns can be difficult to track due to interannual variation and the need for longer term, multi-year surveillance efforts. We assessed seasonal and annual changes in the trematode component community of first-intermediate host stream snail Elimia (= Oxytrema = Goniobasis) proxima across 5 streams in the southern Appalachian Mountains. Over 3 yr, we found no evidence of consistent seasonal peaks of trematode infection in E. proxima. There was some across-site consistency in infection prevalence over 4 yr, because high prevalence sites tended to maintain higher prevalence from year to year, relative to lower prevalence sites. In addition, we examined the relationship between prevalence of first-intermediate host infection, weather variables, and site-level factors, including snail density and water quality metrics. Trematode prevalence was negatively related to total precipitation, which may have been due to the movement of infectious parasite stages and hosts downstream during high flows. We found no strong relationships between trematode prevalence and snail density or any of the water quality metrics examined in this study, indicating that snail infection may be driven primarily by definitive host activity.

Fate of Bacillus sphaericus and Bacillus thuringiensis serovar israelensis in the aquatic environment.

Bacillus sphaericus spores were suspended in bottles of filtered (0.45 microns) freshwater and seawater under various conditions of temperature, pH and salinity. Heat resistant culturable counts (spores) slowly decreased with time. Spores suspended in dialysis bags submerged in a freshwater pond or in flowing seawater underwent a more rapid drop in heat resistant spore counts than did spores held in bottles. Thus, laboratory studies may overestimate spore longevity in the environment. Spore settling rate was related to the nature of particulate material in the water column. Paraspores (or perhaps spores and toxin) of B. thuringiensis serovar israelensis (B.t.i.) had a greater tendency to adhere to and settle with suspended sediment and fine particulates than did paraspores of B. sphaericus. These observations may at least partially explain the greater persistence of B. sphaericus larvicidal activity in field tests than that of B.t.i..

Metagonimoides oregonensis (Heterophyidae: Digenea) infection in Pleurocerid snails and Desmognathus quadramaculatus salamander larvae in Southern Appalachian streams.

Metagonimoides oregonensis (Heterophyidae) is a little-known digenetic trematode that uses raccoons and possibly mink as definitive hosts, and stream snails and amphibians as intermediate hosts. Some variation in the life cycle and adult morphology in western and eastern populations has been previously noted. In the southern Appalachians, Pleurocera snails and stream salamanders, e.g., Desmognathus spp., are used as intermediate hosts in the life cycle. We completed a series of studies in this system examining some aspects of larval trematode morphology and first and second intermediate host use. Molecular sequencing of the 28S rDNA of cercariae in our survey placed them clearly within the heterophyid family. However, light and scanning electron microscopy revealed both lateral and dorso-ventral finfolds on the cercariae in our region, whereas original descriptions of M. oregonensis cercariae from the west coast indicate only a dorso-ventral finfold, so further work on the systematics of this group may be warranted. A survey of first intermediate host, Pleurocera proxima, from 7 streams in the region identified only M. oregonensis, virgulate-type cercariae, and cotylomicrocercous-type cercariae in the streams, with M. oregonensis having the highest prevalence, and the only type present that use amphibians as second intermediate hosts. Based on clearing and staining of 6 Desmognathus quadramaculatus salamander larvae, we found that individual salamanders could have over 600 metacercariae, which form between muscle fibers throughout the body. Histological observations suggest that the metacercariae do not cause excessive tissue damage or inflammation, and likely persist through metamorphosis, thereby transmitting potentially large numbers of worms to definitive host raccoons foraging along streams.

Bacillus sphaericus mosquito pathogens in the aquatic environment.

The fate of Bacillus sphaericus spores in the aquatic environment was investigated by suspending spores in dialysis bags in fresh and seawater. Spore viability was lost more rapidly in seawater. Neither B. sphaericus nor B. thuringiensis israelensis (B.t.i.) spores mixed with pond sediment appeared to attach to the sediment. However, rapid decrease in B.t.i. toxicity suggested attachment of parasporal bodies to sediment. B. sphaericus toxin settled more slowly and less completely. B. sphaericus spores fed to larvae of four aquatic invertebrates were mostly eliminated from the animal gut in less than one week. An exception was the cranefly (Tipula abdominalis) where spores persisted in the posterior gut for up to five weeks.

Fate of Bacillus sphaericus 2362 spores following ingestion by nontarget invertebrates.

Elimination of Bacillus sphaericus spores ingested by midge larvae, snails, and oysters was most rapid among midge larvae. Spores remained in oysters up to 21 days and in snails up to 49 days. Viable spores were recovered in snail and oyster feces for these same periods. There was no indication of actively growing B. sphaericus in the animals. Passage through oyster gut detoxified the B. sphaericus mosquito larval toxin, but there was a 33% retention of toxicity following snail gut passage. Midge larvae reared to adults in spore-containing water carried spores in/on the adult body. This suggests that these animals could carry the bacteria to sites beyond the application area.

Stream biodiversity: the ghost of land use past.

The influence of past land use on the present-day diversity of stream invertebrates and fish was investigated by comparing watersheds with different land-use history. Whole watershed land use in the 1950s was the best predictor of present-day diversity, whereas riparian land use and watershed land use in the 1990s were comparatively poor indicators. Our findings indicate that past land-use activity, particularly agriculture, may result in long-term modifications to and reductions in aquatic diversity, regardless of reforestation of riparian zones. Preservation of habitat fragments may not be sufficient to maintain natural diversity in streams, and maintenance of such biodiversity may require conservation of much or all of the watershed.