James G. March

Effects of a low-head dam and water abstraction on migratory tropical stream biota

Migration of large-bodied “macroconsumers” (e.g., fishes, shrimps, and snails) is an important functional linkage between many tropical rivers and their estuaries. Increasingly, this linkage is being severed by dams and water abstraction. The ecological impacts of these activities are poorly understood and are largely being ignored by dam operators. We investigated the direct effects of a water intake and low-head dam on the migration of amphidromous freshwater shrimps between the headwater streams and estuary of the Rio Espiritu Santo, Puerto Rico, USA. Both downstream migratory drift of larvae and upstream migration of postlarvae had strong diel patterns, with most activity occurring at night. Unlike large dams on the island, this low-head dam did not act as a complete barrier to the upstream migration of metamorphosed postlarvae. However, the dam did cause large numbers of postlarval shrimps to accumulate directly downstream of the structure. Mortality of drifting first-stage larvae by entrainment into...

Damming Tropical Island Streams: Problems, Solutions, and Alternatives

Abstract The combination of human population growth, increased water usage, and limited groundwater resources often leads to extensive damming of rivers and streams on tropical islands. Ecological effects of dams on tropical islands can be dramatic, because the vast majority of native stream faunas (fishes, shrimps, and snails) migrate between freshwater and saltwater during their lives. Dams and associated water withdrawals have been shown to extirpate native faunas from upstream reaches and increase mortality of downstream-drifting larvae. A better understanding of the effects of dams and the behavior of tropical island stream faunas is providing insights into how managers can mitigate the negative effects of existing dams and develop alternatives to dam construction while still providing freshwater for human use. We review the ecological effects of dams on tropical island streams, explore means to mitigate some of these effects, describe alternatives to dam construction, and recommend research priorities.

Testing isosource : stable isotope analysis of a tropical fishery with diverse organic matter sources

We sampled consumers and organic matter sources (mangrove litter, freshwater swamp-forest litter, seagrasses, seagrass epiphytes, and marine particulate organic matter [MPOM]) from four estuaries on Kosrae, Federated States of Micronesia for stable isotope (sigma13C and sigma34S) analysis. Unique mixing solutions cannot be calculated in a dual-isotope, five-endmember scenario, so we tested IsoSource, a recently developed statistical procedure that calculates ranges in source contributions (i.e., minimum and maximum possible). Relatively high minimum contributions indicate significant sources, while low maxima indicate otherwise. Litter from the two forest types was isotopically distinguishable but had low average minimum contributions (0-8% for mangrove litter and 0% for swamp-forest litter among estuaries). Minimum contribution of MPOM was also low, averaging 0-13% among estuaries. Instead, local marine sources dominated contributions to consumers. Minimum contributions of seagrasses averaged 8-47% among estuaries (range 0-88% among species). Minimum contributions of seagrass epiphytes averaged 5-27% among estuaries (range 0-69% among species). IsoSource enabled inclusion of five organic matter sources in our dual-isotope analysis, ranking trophic importance as follows: seagrasses > seagrass epiphytes > MPOM > mangrove forest > freshwater swamp-forest. IsoSource is thus a useful step toward understanding which of multiple organic matter sources support food webs; more detailed work is necessary to identify unique solutions.

LINKING SPECIES AND ECOSYSTEMS: DIFFERENT BIOTIC ASSEMBLAGES CAUSE INTERSTREAM DIFFERENCES IN ORGANIC MATTER

Here we test the hypothesis that differences in macrobiotic assemblages can lead to differences in the quantity and quality of organic matter in benthic depositional environments among streams in montane Puerto Rico. We experimentally manipulated biota over a 30–40 d period in two streams with distinctly different macrobiotic assemblages: one characterized by high densities of omnivorous shrimps (Decapoda: Atyidae and Xiphocarididae) and no predaceous fishes, and one characterized by low densities of shrimps and the presence of predaceous fishes. To incorporate the natural hydrologic regime and to avoid confounding artifacts associated with cage enclosures/exclosures (e.g., high sedimentation), we used electricity as a mechanism for experimental exclusion, in situ. In each stream, shrimps and/or fishes were excluded from specific areas of rock substrata in four pools using electric “fences” attached to solar-powered fence chargers. In the stream lacking predaceous fishes (Sonadora), the unelectrified control treatment was almost exclusively dominated by high densities of omnivorous shrimps that constantly ingested fine particulate material from rock surfaces. Consequently, the control had significantly lower levels of inorganic sediments, organic material, carbon, and nitrogen than the exclusion treatment, as well as less variability in these parameters. Tenfold more organic material (as ash-free dry mass, AFDM) and fivefold more nitrogen accrued in shrimp exclosures (10.6 g AFDM/m2, 0.2 g N/m2) than in controls (1.1 g AFDM/m2, 0.04 g N/m2). By reducing the quantity of fine particulate organic material and associated nitrogen in benthic environments, omnivorous shrimps potentially affect the supply of this important resource to other trophic levels. The small amount of fine particulate organic matter (FPOM) that remained in control treatments (composed of sparse algal cells) was of higher quality than that in shrimp exclosures. This is evidenced by the significantly lower carbon-to-nitrogen (C/N) ratio (an indicator of food quality, with relatively low C/N indicating higher food quality) in the control relative to the shrimp exclosure treatment. In contrast, the stream with predaceous fishes (Bisley) was characterized by very low numbers of shrimps, and macrobiota had no significant effect on benthic sediments, organic matter, C, N, and C/N. All parameters were highly variable through time, with levels and ranges in variability similar to the shrimp exclusion treatment in the Sonadora. Our experimental results are consistent with findings of an independent survey of six streams in four different drainages. Four streams that had an abundance of omnivorous shrimps, but lacked predaceous fishes, had extremely low levels of fine benthic organic and inorganic material. In contrast, two streams that had low densities of shrimps and contained predaceous fishes had significantly higher levels. Results show a strong linkage between species and ecosystem characteristics: interstream differences in the quantity and quality of fine benthic organic matter resources were determined by the nature of the macrobiotic assemblage. Furthermore, patterns in the distribution of shrimp assemblages reflected landscape patterns in the benthic depositional environment among streams.

Benthic Community Structure and Invertebrate Drift in a Pacific Island Stream, Kosrae, Micronesia1

Tropical Pacific island streams have poorly understood communities that deserve scientific attention. We examined benthic macroinvertebrates and fishes of the Inem River on Kosrae, Federated States of Micronesia. Larval chironomids, lepidopterans, odonates, and freshwater shrimps dominated the benthos and drift. Diel periodicity in drift was not evident. Nine fishes, two shrimps, and one snail species were identified. Kosrae’s stream fauna appears even more depauperate than other Pacific high islands, possible due to its extreme isolation.

Biodiversity and ecosystem function in species-poor communities: community structure and leaf litter breakdown in a Pacific island stream

Abstract Pacific island stream communities are species-poor because of the effects of extreme geographic isolation on colonization rates of taxa common to continental regions. The effects of such low species richness on stream ecosystem function are not well understood. Here, we provide data on community structure and leaf litter breakdown rate in a virtually pristine stream on a remote island in the eastern Caroline Islands of Micronesia. The Yela River catchment on Kosrae, Federated States of Micronesia, is uninhabited, completely forested, and not traversed by any road. At each of 5 sampling stations along the Yela River we measured physical and chemical variables, characterized the benthic invertebrate community, and estimated the relative abundance of macroconsumers (fishes, shrimps, and snails) using snorkeling surveys and trapping. Benthic invertebrate biomass decreased with stream size and was dominated by macroconsumers (i.e., decapods and gastropods). Benthic insect species richness and density were extremely low, characterized by the complete absence of Ephemeroptera, Trichoptera, and Plecoptera, with nonshredding larval Chironomidae making up ∼85% of the exceptionally low insect biomass (∼0.6 mg dry mass/m2). Six species each of fishes and shrimps and 4 snail species were found. We examined the functional role of these noninsect macroconsumers in organic matter processing by comparing litter breakdown rates inside and outside electrified macroconsumer-excluding quadrats at a single site in the lower Yela River. Breakdown rates were slow relative to those reported in most tropical litter decay studies (k = 0.004–0.018 among quadrats). Despite differences in macroconsumer density (p = 0.01), the leaf breakdown rate was not significantly different between reference and electrified treatments (p = 0.77). Thus, macroconsumers at this site had neither a direct role in shredding nor an indirect effect on leaf breakdown through their interactions with any nonexcluded taxa. In the absence of a significant role played by insect or macroconsumer shredders, leaf litter breakdown in streams of remote Pacific oceanic islands might be driven purely by physical and microbial processes. Our results suggest that biogeographic processes have placed some Pacific island streams at the extreme low end of a continuum of shredder biodiversity and biomass, restricting the ecosystem function of their species-poor communities.

Factors influencing the distribution and abundance of the endangered winged mapleleaf mussel Quadrula fragosa in the St. Croix River, Minnesota and Wisconsin

-We examined physical and biological factors that may influence the distribution of the endangered winged mapleleaf mussel Quadrula fragosa (Conrad, 1835). Quantitative sampling of the mussel community was undertaken at two sites in the St. Croix River known to harbor Q fragosa. Additional searches were conducted specifically for Q. fragosa individuals. For each quantitative sample of mussels, substrate composition, water velocity and depth were assessed and mussels were identified and measured. In general, Q. fragosa does not have habitat requirements different from the rest of the mussel community, although Q. fragosa occurred in shallower areas with lower bottom current velocity compared to the overall mussel community. There was no difference in substrate composition in areas with and without Q fragosa. Mussel community density and richness were higher in areas where Q fragosa was found. The mussel community associated with Q. fragosa was not significantly different from the general mussel community in the area. However three species (Truncilla truncata, Truncilla donaciformis and Quadrula metanevra) were significantly associated with Q fragosa. Due to its association with dense and diverse mussel communities, management that benefits the entire mussel community should be effective in protecting this endangered species.

Effects of urbanization on the population structure of freshwater turtles across the United States: Urbanization Effects on Turtles

Landscape-scale alterations that accompany urbanization may negatively affect the population structure of wildlife species such as freshwater turtles. Changes to nesting sites and higher mortality rates due to vehicular collisions and increased predator populations may particularly affect immature turtles and mature female turtles. We hypothesized that the proportions of adult female and immature turtles in a population will negatively correlate with landscape urbanization. As a collaborative effort of the Ecological Research as Education Network (EREN), we sampled freshwater turtle populations in 11 states across the central and eastern United States. Contrary to expectations, we found a significant positive relationship between proportions of mature female painted turtles (Chrysemys picta) and urbanization. We did not detect a relationship between urbanization and proportions of immature turtles. Urbanization may alter the thermal environment of nesting sites such that more females are produced as urbanization increases. Our approach of creating a collaborative network of scientists and students at undergraduate institutions proved valuable in terms of testing our hypothesis over a large spatial scale while also allowing students to gain hands-on experience in conservation science.