Summary
1. Biological invasions are numerous in fresh waters around the world. At least hundreds of freshwater species have been moved outside of their native ranges by vectors such as ballast water, canals, deliberate introductions, and releases from aquaria, gardens, and bait buckets. As a result, many bodies of fresh water now contain dozens of alien species.

2. Invasions are highly nonrandom with respect to the taxonomic identity and biological traits of the invaders, the ecological characteristics of the ecosystems that are invaded, and the geographical location of the ecosystems that supply and receive the invaders.

3. Some invaders have had deep and pervasive effects on the ecosystems that they invade. Classes of ecologically important invaders in fresh waters include molluscs that are primary consumers and disrupt the food web from its base, fishes that disrupt the food web from its apex or centre, decapods that act as powerful omnivores, aquatic plants that have strong engineering effects and affect the quality and quantity of primary production, and diseases, which probably have been underestimated as an ecological force.

4. The number of alien species in freshwater ecosystems will increase in the future as new aliens are moved outside of their native ranges by humans, and as established aliens fill their potential ranges. Alien species create “no-analogue” ecosystems that will be difficult to manage in the future. We may be able to reduce future impacts of invaders by making more serious efforts to prevent new invasions and manage existing invaders.

5. Thematic implications: interactions between alien species and other contemporary stressors of freshwater ecosystems are strong and varied. Because disturbance is generally thought to favour invasions, stressed ecosystems may be especially susceptible to invasions, as are highly artificial ecosystems. In turn, alien species can strongly alter the hydrology, biogeochemical cycling, and biotic composition of invaded ecosystems, and thus modulate the effects of other stressors. In general, interactions between alien species and other stressors are poorly studied.

/pdf/alien-species-in-fresh-waters-ecological-effects-5fvv6usa04.pdf

Alien species in fresh waters: ecological effects, interactions with other stressors, and prospects for the future

Wetlands seem to be especially vulnerable to invasions. Even though ≤6% of the earth's land mass is wetland, 24% (8 of 33) of the world's most invasive plants are wetland species. Furthermore, many wetland invaders form monotypes, which alter habitat structure, lower biodiversity (both number and “quality” of species), change nutrient cycling and productivity (often increasing it), and modify food webs. Wetlands are landscape sinks, which accumulate debris, sediments, water, and nutrients, all of which facilitate invasions by creating canopy gaps or accelerating the growth of opportunistic plant species. These and other disturbances to wetlands, such as propagule influx, salt influx, and hydroperiod alteration, create opportunities that are well matched by wetland opportunists. No single hypothesis or plant attribute explains all wetland invasions, but the propensity for wetlands to become dominated by invasive monotypes is arguably an effect of the cumulative impacts associated with landscape sinks, incl...

/pdf/causes-and-consequences-of-invasive-plants-in-wetlands-xksh2hkdyj.pdf

Causes and Consequences of Invasive Plants in Wetlands: Opportunities, Opportunists, and Outcomes

Coastal marshes accrete vertically in response to sea-level rise and subsidence. Inadequate accretion and subsequent conversion of coastal marshes to open water generally is attributed to inadequate mineral sedimentation because mineral sedimentation is widely assumed to control accretion. Using 137Cs dating to determine vertical accretion, mineral sedimentation, and organic matter accumulation, we found that accretion varied with organic accumulation rather than mineral sedimentation across a wide range of conditions in coastal Louisiana, including stable marshes where soil was 80% mineral matter. These results agreed with previous research, but no mechanism had been proposed to explain accretion via vegetative growth. In an exploratory greenhouse experiment, we found that flooding stimulated root growth above the marsh surface. These results indicated the need for additional work to determine if flooding controls accretion in some marshes by stimulating root growth on the marsh surface, rather than by mineral accumulation on the marsh surface. Restoration or management that focus on mineral sedimentation may be ineffective where a relationship between accretion and mineral sedimentation is assumed rather than tested.

http://www.rnr.lsu.edu/people/nyman/pubs/nymECSS2006.pdf

Marsh vertical accretion via vegetative growth

The distribution of tidal saline wetlands (e.g., salt marshes and mangroves) is increasingly impacted by global environmental change, including human alteration of the world’s coasts and sea-level rise (Kennedy et al. 2002; Poff et al. 2002). Rates of salt marsh and mangrove loss appear to be accelerating (Nicholls et al. 1999).A better understanding of wetland accretionary dynamics, controls and constraints, and in particular responses to sea-level rise is required to inform the maintenance and restoration of these systems. Differences in wetland form and function result from a range of continentaland regional-scale processes (Mitsch and Gosselink 2000). Local geomorphology, climate regime, and hydrology result in differences in sediment supply, primary production and decomposition, subsidence, and autocompaction, resulting in variations in elevation among both salt marsh and mangrove sites. Combinations of these controls mean that individual wetland sites show different degrees of vulnerability to current and near-future environmental change. Several vulnerability assessment approaches have been developed during the past two decades, including the Global Vulnerability Assessment, first developed by Hoozemans et al. (1993) and recently refined in DINAS_COAST (McFadden et al. 2003), and the Coastal Vulnerability Index (Gornitz 1991; Hammar-Klose and Thieler 2001). These approaches offer valuable planning tools for coastal resource managers at the broad (i.e., global, continental, macro-regional) scale as they allow the identification of both patterns of relative vulnerability and vulnerability ‘hotspots’. However, the numerical scores that they produce cannot be easily assigned a precise physical meaning and

Coastal Wetland Vulnerability to Relative Sea-Level Rise: Wetland Elevation Trends and Process Controls

Global coastal wetland change under sea-level rise and related stresses: The DIVA Wetland Change Model

Negative connotations of invasive plants worldwide have implicated them as the bearers of unfavorable ecosystem change. We contrasted 5-yr-old and 20-yr-oldPhragmites populations with pre-invasion areas occupied byTypha spp. andPanicum virgatum in an oligohaline tidal marsh of Chesapeake Bay. Peak live biomass was 3 times greater, while standing dead and litter was twice as great in the 20-yr-oldPhragmites. It is this abundance of concentrated litter on the marsh surface of maturePhragmites populations that we implicate as encouraging the trapping of organic and mineral matter. The rate of vertical accretion in 20-yr-oldPhragmites populations is 3–4 mm yr−1 above the adjacent populations. By integrating the constant initial concentration and constant rate of supply models on individual210Pb cores, we estimate thatPhragmites populations require a minimum of 7-yr post-colonization to enhance rates of accretion in this system. In ligh of the considerable loss of marsh habitat from relative sea-level rise, this finding contests the view that invasion creates strictly undesirable change at the ecosystem level.

Increased sediment accretion rates following invasion by Phragmites australis : The role of litter

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Increased sediment accretion rates following invasion by Phragmites australis : The role of litter