▪ Abstract Lentic freshwater habitats in temperate regions exist along a gradient from small ephemeral ponds to large permanent lakes. This environmental continuum is a useful axis for understanding how attributes of individuals ultimately generate structure at the level of the community. Community structure across the gradient is determined by both (a) physical factors, such as pond drying and winter anoxia, that limit the potential breadth of species distributions, and (b) biotic effects mediated by ecological interactions, principally predation, that determine the realized success of species. Fitness tradeoffs associated with a few critical traits of individuals often form the basis for species turnover along the gradient. Among species that inhabit temporary ponds, distributions are often constrained because traits that enhance developmental rate and competitive ability also increase susceptibility to predators. In permanent ponds, changes in the composition of major predators over the gradient limit ...

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Mechanisms creating community structure across a freshwater habitat gradient

The link between species invasions and the extinction of natives is widely accepted by scientists as well as conservationists, but available data supporting invasion as a cause of extinctions are, in many cases, anecdotal, speculative and based upon limited observation. We pose the question, are aliens generally responsible for widespread extinctions? Our goal is to prompt a more critical synthesis and evaluation of the available data, and to suggest ways to take a more scientific, evidence-based approach to understanding the impact of invasive species on extinctions. Greater clarity in our understanding of these patterns will help us to focus on the most effective ways to reduce or mitigate extinction threats from invasive species.

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Are invasive species a major cause of extinctions

Estimation of invertebrate biomass is a critical step in addressing many ecological ques- tions in aquatic environments. Length-dry mass regressions are the most widely used approach for estimating benthic invertebrate biomass because they are faster and more precise than other methods. A compilation and analysis of length-mass regressions using the power model, M (mass) = a L (length)b, are presented from 30 y of data collected by the authors, primarily from the southeastern USA, along with published regressions from the rest of North America. A total of 442 new and published regressions are presented, mostly for genus or species, based on total body length or other linear measurements. The regressions include 64 families of aquatic insects and 12 families of other invertebrate groups (mostly molluscs and crustaceans). Regressions were obtained for 134 insect genera (155 species) and 153 total invertebrate genera (184 species). Regressions are provided for both body length and head width for some taxa. In some cases, regressions are provided from multiple localities for single taxa. When using body length in the equations, there were no significant differences in the mean value of the exponent b among 8 insect orders or Amphipoda. The mean value of b for insects was 2.79, ranging from only 2.69 to 2.91 among orders. The mean value of b for Decapoda (3.63), however, was significantly higher than all insects orders and amphipods. Mean values of a were not significantly different among the 8 insect orders and Amphipoda, reflecting considerable variability within orders. Reasons for potential differences in b among taxa are explained with hypothetical examples showing how b responds to changes in linear dimensions and specific gravity. When using head width as the linear dimension in the power model, the mean value of b was higher (3.11) than for body length and more variable among orders (2.8-3.3). Values of b for Ephemeroptera (3.3) were significantly higher than those for Odonata, Megaloptera, and Diptera. For those equations in which ash-free dry mass was used, % ash varied considerably among functional feeding groups (3.3-12.4%). Percent ash varied from 4.0% to 8.5% among major insect orders, but was 18.9% for snails (without shells). Family-level regressions also are presented so that they can be used when generic equations are unavailable or when organisms are only identified to the family level. It is our intention that these regressions be used by others in estimating mass from linear dimensions, but potential errors must be recognized.

Length-mass relationships for freshwater macroinvertebrates in North America with particular reference to the southeastern United States

We describe here the ecological and evolutionary processes that modulate the effects of invasive species over time, and argue that such processes are so widespread and important that ecologists should adopt a long-term perspective on the effects of invasive species. These processes (including evolution, shifts in species composition, accumulation of materials and interactions with abiotic variables) can increase, decrease, or qualitatively change the impacts of an invader through time. However, most studies of the effects of invasive species have been brief and lack a temporal context; 40% of recent studies did not even state the amount of time that had passed since the invasion. Ecologists need theory and empirical data to enable prediction, understanding and management of the acute and chronic effects of species invasions.

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Understanding the long-term effects of species invasions.

Foreword Preface Contributors Participating Institutions Acknowledgements 1. Biodiversity: Our Precious Heritage 2. Discovering Life in America: Tools and Techniques of Biodiversity Inventory 3. A Remarkable Array: Species Diversity in the United States 4. Conservation Status of U.S. Species 5. State of the States: Geographic Patterns of Diversity, Rarity, and Endemism 6. The Geography of Imperilment: Targeting Conservation towards Critical Biodiversity Areas 7. More than the Sum of the Parts: Diversity and Status of Ecological Systems 8. Leading Threats to U.S. Biodiversity: What's Threatening Imperiled Species 9. Strategies for Biodiversity Protection 10. Owning Up to Our Responsibilities: Who Owns Lands Important for Biodiversity? 11. Safeguarding Our Precious Heritage Appendix A: Extinct and missing species of the United States Appendix B: State Diversity, Endemism, and Rarity Appendix C: Kuchler Potential Natural Vegetation Types Appendix D: Principal Sources for the Natural Heritage Central Databases Literature Cited

Wisdom and Wonder@@@2000. Precious Heritage: The Status of Biodiversity in the United States

Nos etudes de laboratoire sur les effets physiologiques de l'invasion des moules zebrees (Dreissena polymorpha) sur les mulettes Actinonaias ligamentina et Amblema plicata (Unionidae) montrent que (i) l'infestation par la moule zebree cause du stress et des symptomes d'inanition chez les unionides, (ii) les especes d'unionides sont affectees de facon inegale et (iii) les symptomes d'inanition sont plus prononces quand l'etat de depart est faible. Le stress alimentaire chez les mollusques unionides affectes se traduit par une baisse des taux metaboliques, un metabolisme davantage base sur les proteines (rapports O:N plus bas) et une augmentation compensatoire des taux de broutage. L'inanition peut resulter d'une rarefaction locale des ressources alimentaires et (ou) d'un accroissement du cout metabolique. Actinonaias ligamentina (sous-famille Lampsilinae) etait plus sensible a l'infestation que Amblema plicata (sous-famille Ambleminae), ce que revelent particulierement les changements dans le taux d'assimilation de l'oxygene et letaux de broutage. Les effets de l'infestation etaient plus manifestes chez les mollusques dont l'etat etait faible au depart. Nos resultats indiquent que la baisse de la diversite chez les mollusques unionides depuis l'introduction des moules zebrees est due a des taux d'inanition specifiques aux especes.

Acute physiological effects of zebra mussel (Dreissena polymorpha) infestation on two unionid mussels, Actiononaias ligamentina and Amblema plicata

It has been hypothesized that variation in shell shape and sculpture in freshwater mussels from streams is related to hydrologic variability. Using quantitative sampling, we found smooth-shelled species were more common in upstream portions of the St. Croix River, Minnesota and Wisconsin, which has coarser substrate while species with sculptured shells were more common downstream and in finer substrate. For four species distributed throughout the length of the river, individuals tended to have shells that increase in obesity (width/length) and relative height (height/length) downstream. These results support suggestions that mussels with sculptured or obese shells may have an anchoring advantage in areas of high discharge while smooth-shelled or slim forms may reburrow more readily after dislodgement in hydraulically flashy environments. One species that showed increased obesity downstream had juveniles with similar shell shape at all locations suggesting phenotypic plasticity in shell shape. The juvenile and adult valve shape for the other three species examined varied among locations making it is less clear whether the differences noted are genetically fixed or phenotypically plastic. Understanding the complex interplay between shell shape and sculpture and disturbance regimes in rivers may provide useful information when developing conservation plans for these vulnerable animals. While both shell ornamentation and shape vary along river length, and may be correlated with substrate anchoring and stream flow, other factors such as behavior and physiology also contribute to mussels' success in maintaining their location in a river.

https://bioone.org/journals/The-American-Midland-Naturalist/volume-164/issue-1/0003-0031-164.1.22/Variation-in-Freshwater-Mussel-Shell-Sculpture-and-Shape-Along-a/10.1674/0003-0031-164.1.22.pdf

Variation in Freshwater Mussel Shell Sculpture and Shape Along a River Gradient

We sampled the mussel assemblages at 4 locations in the St. Croix River, Minnesota and Wisconsin, to examine sampling bias between quantitative (sediment excavation) and qualitative (timed searches) methods. Both methods provided similar species richness, diversity, and evenness values among locations examined. At all locations, each sampling method resulted in similar relationships between the number of mussels collected and the number of species found. However, assemblage composition and the mean length of individuals collected differed significantly between methods. Quantitative samples included smaller mussels than qualitative samples; individuals 1000 individuals) and was probably related to differences in substrate composition among locations.

A Comparison of a Qualitative and a Quantitative Collection Method for Examining Freshwater Mussel Assemblages

The aim of this study was to evaluate the recovery and survival of four species of unionid mussles [pimpleback, Quadrula pustulosa pustulosa (I. Lea, 1831); spike, Elliptio dilatata (Rafinesque, 1820); Higgins eye, Lampsilis higginsii (I. Lea, 1857); and pocketbook, Lampsilis cardium (Rafinesque, 1820)] that were experimentally relocated to in situ refugia in the St Croix River of Minnesota and Wisconsin, USA. In 1996, 150 mussels of each of the first three species (450 total) were relocated to three 5 � 5 m study grids (Site A), one near Lakeland, Minnesota, which served as a source-site control, and two in the experimental refuge 48 km upstream, near Franconia, Minnesota. In a second relocation in 1997, L. cardium was substituted for L. higginsii and 150 mussels of this and each of the other two species (450 total), were relocated to two study grids (Site B). The source site control was near Sunrise, Minnesota and the experimental refuge was 14 km downstream near Almelund, Minnesota. Mussel recovery, survival and substratum characteristics were evaluated annually at Site A for 2 years and for 3 years at Site B. Mean annual recovery of all three species ranged from 90 to 100% at Site A, and from 34 to 70% at site B. The mean annual survival of recaptured mussels ranged from 85 to 100% at Site A, and from 88 to 100% at Site B. The textural characteristics of the substratum differed significantly between the control and the two refuge locations at the beginning of the study, but did not differ from this initial status among subsequent years at Site A. At Site B, there was a significant shift in textural characteristics from large to smaller fractions over the four years. The relatively high survival of mussels during this study demonstrates the importance of proper handling and transport protocols when relocating mussels and the selection of suitable relocation habitat with stable substratum. When established correctly, in situ refugia may be a viable tool for preserving unionid mussels.

Evaluation of relocation of unionid mussels to in situ refugia

Muskrat (Ondatra zibethicus) predation has been hypothesized to influence the structure of freshwater mussel assemblages. We compared the species composition and sizes of mussels collected from middens and in situ at 3 locations in the St. Croix River and 1 location in the Mississippi River. Species diversity was greater for mussels collected from the rivers compared to those collected from middens. Logistic regression revealed mussel species composition and mussel size differed significantly between the rivers and middens, which suggested that muskrats were both size-selective and species-specific feeders. A significant interaction between species and size may result from muskrats taking large individuals from small species and small individuals from large species. There was selection for the small species, Truncilla truncata, at the 2 sites where it was the dominant species, and avoidance at the other 2 sites. This result suggested that muskrats may be using a strategy to maximize energy by choosing pre...

Daniel J. Hornbach

Papers

Acute physiological effects of zebra mussel (Dreissena polymorpha) infestation on two unionid mussels, Actiononaias ligamentina and Amblema plicata

Variation in Freshwater Mussel Shell Sculpture and Shape Along a River Gradient

A Comparison of a Qualitative and a Quantitative Collection Method for Examining Freshwater Mussel Assemblages

Evaluation of relocation of unionid mussels to in situ refugia

Selective Predation by Muskrats on Freshwater Mussels in 2 Minnesota Rivers