▪ 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

Cascading trophic interactions are important in many freshwater pelagic food webs, but their importance in more complex, omnivore-rich littoral-zone food webs is less well known. We tested the existence of a trophic cascade involving omnivorous crayfish (Orconectes rusticus), macroinvertebrates, periphyton, and macrophytes using 9-M2 cages in the littoral zone of Plum Lake, Wisconsin, USA. Treatments in the replicated (N = 4) experiment were crayfish enclosures, crayfish exclosures, and cageless references. During June-September, we measured macrophyte shoot numbers, macroinvertebrate numbers, and periphyton (on plastic strips) chlorophyll a, and dry mass (DM). We expected that crayfish foraging would directly reduce abundance and change species composition of macrophytes and macroinvertebrates and would indirectly enhance periphyton abundance by reducing the abundance of grazing snails. In enclosures, macrophyte and snail (but not nonsnail macroinvertebrate) densities declined significantly throughout the experiment, whereas densities of macrophytes, snails, and nonsnail macroinvertebrates increased in exclosures and cageless references. Some of the reduction in macrophytes resulted from nonconsumptive fragmentation of macrophytes by crayfish. Consistent with the cascading trophic interactions model, periphyton chloro- phyll a per unit surface area increased in enclosures, but declined in exclosures. Periphyton quality (as indexed by chlorophyll a/DM) also increased in enclosures relative to exclosures and cageless references. However, because of large reductions in macrophyte surface area (which periphyton colonizes) in enclosures, total amount of periphyton chlorophyll a in enclosures (relative to exclosures) probably declined while periphyton quantity per unit surface area and periphyton quality increased. Thus, the impacts of crayfish omnivory on periphyton, expressed in two conflicting indirect effects, confirm the possibility that om- nivory can complicate cascading trophic predictions. Overall, results support the existence of strong trophic interactions in the littoral zone, in which omnivorous crayfish control abundance of macrophytes, snails, and periphyton.

Effects of an omnivorous crayfish (Orconectes rusticus) on a freshwater littoral food web

Do top-down (e.g., trophic interactions) or bottomup (e.g., nutrients) effects control communities? In recent years, an increasing number of ecologists have argued that, as with most ecological controversies (Schoener 1987), this dichotomy is artificial and counterproductive. Both top-down and bottom-up factors can have important effects on community structure (e.g., Carpenter 1988, Hunter and Price 1992, Power 1992). Thus, it seems more appropriate to ask "how do bottom-up and top-down effects interact and influence each other," and "what are the mechanisms underlying variation in each?" These rather simple questions still mask enormously complex issues. However, in contrast to earlier, "single-factor" approaches, recent work dealing directly with the multi-factorial nature of natural communities has offered important insights. Among these are that bottom-up processes can underlie variations in community structure that can have important consequences for the influence of top-down factors (e.g., Carpenter and Kitchell 1984, 1987, Oksanen 1988, Persson et al. 1988, Power 1990). For instance, the level of primary production may determine the number of trophic levels, which in turn could have major effects on community structure (Oksanen et al. 1981, Fretwell 1987, Persson et al. 1988). It is important to define what is meant by "control." As used here, control means having a major quantitative and/or qualitative effect on community structure. Quantitative effects are changes in abundance (numbers, cover, biomass), while qualitative effects are changes in community composition and/or the nature of interactions among community components. For example, increased production might increase the abundance of herbivores without changing species composition (i.e., a quantitative but not qualitative change). On the other hand, predation might reduce prey abundance, allowing both increases in abundance of competitors already present (a quantitative change)

https://www.jstor.org/stable/pdfplus/1940155.pdf

Community Regulation: Under What Conditions Are Bottom-Up Factors Important on Rocky Shores?

Climate change effects on trematodiases, with emphasis on zoonotic fascioliasis and schistosomiasis.

Alien species (those carried outside their original ranges by human activities) have strongly affected the distribution and abundance of mollusks in many North American fresh waters The best known of these aliens in North America is the zebra mussel (Dreissena polymorpha), which has nearly extirpated native unionid clams from infested lakes and rivers by fouling their shells and outcompeting them for food Zebra mussels also have reduced populations of native sphaeriid clams, and both increased and reduced populations of snails The effects of the other well-known alien bivalve in North America, Corbicula fluminea, are surprisingly poorly known Corbicula probably caused some populations of native bivalves to decline, but other native populations seem to coexist with Corbicula Several plausible mechanisms of interaction between Corbicula and the native biota have been proposed, but not demonstrated Other aliens, including the recently arrived snail Potamopyrgus antipodarum, probably compete strongly wi

D M Lodge

Papers

Distribution of Fresh water Snails - Spatial Scale and the Relative Importance of Physicochemical and Biotic Factors