Delta biodiversity and food webs

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This page is a core topic for River Deltas as developed by the River Delta Adaptive Management Strategy

The following pages are associated with Delta biodiversity and food webs:

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High nutrient availability, continuous moisture, and moderated temperatures create extremely high levels of primary productivity (cite). These productive conditions are further augmented by bioenergetic inputs from river flotsam and debris and longshore drift. Our river deltas are a crossroads between terrestrial, alluvial, nearshore, and marine environments positioned along global bird migration routes. These systems are at the heart of complex food webs. They are a critical element in landscape biodiversity (particularly the sense of Whittaker's concept of beta diversity). Gelfenbaum et al (2006) suggests that the export of biomass into the adjacent nearshore, may make delta wetlands an important resource for surrounding marine waters.

A variety of biota visit deltas over their lifecycle to exploit this productivity. The high productivity and complex habitat structure provides nursery functions for a variety of biota (Hughes et al 2014). Forage opportunities flux daily with the tides, seasonally with pulses of primary productivity, and vary across the landscape as disturbances and delta plain evolution create a variety of serial states.

Our understanding of the precise relationships between delta structure and biodiversity are weak. Current nearshore conservation planning necessarily relies on generalizations developed in other systems, rather than specific observations of Puget Sound delta ecosystems (Greiner 2010). Deltas are the final depositional environment of over 70% of the Puget Sound basin, and so changes in the chemical composition of sediments and waters affect the development of food webs and the resulting biodiversity.

Compared to the effects of tides and rivers, animals play a modest role in defining delta ecosystem architecture. Migratory and resident waterfowl may affect composition or productivity of vegetation, particularly in low marsh (Crandall 2004; Mulder et al 1996). Some observations of beaver use of deltas are available but largely preliminary (Cereghino 2007; Deifenderfer 2008; Hood 2012), and more focused on their modification of habitat for fish, than the ecology of beaver populations in delta settings.

Odum (Cite) provides a broad scale description of ecological differences between freshwater tidal, and more saline wetlands, including higher levels of herbivory, lower sulfate concentration, and greater seasons flux in vegetation in freshwater tidal systems suggesting that there are significant patterns of biodiversity across the salinity gradient.

Because of their role as salmonid prey, invertebrates are frequently studied as an indicator of salmon habitat potential (following Simenstad & Cordell 2000). Due to complex life history, small size, and high abundance, invertebrates may have utility as an indicator of ecosystem condition (Heatwole 2003) but population structure varies over space and time making characterization very difficult (Simenstad & Cordell 2000). Investigators have found somewhat ambiguous differences in salt marsh invertebrate community composition in Puget Sound (Heatwole 2003), and various investigators have ascribed marsh invertebrate variation to plant species composition, vegetation structure, salinity, tidal inundation, or vegetation patchiness. Postulates about invertebrate population structure in Puget Sound deltas are often extrapolated from other ecosystems. While benthic invertebrates are being used to indicate ecosystem state in other systems (Weisberg et al 1997; Borja et al 2003), in the Puget Sound we have no tools to interpret what they are saying.

Long term studies of East coast Spartina marsh following restoration suggest that benthic invertebrates develop to match reference conditions as shallow soils develop—deep soil conditions are not anticipated to strongly affect the shallow domain of benthic invertebrates (Price). However, primary productivity has been observed to be affected by redox (Ewing), and the deep organic soils found in natural marshes may only develop under the natural sequence of delta plain formation (well described by Eilers 1974). Breeding and foraging bird populations appear to follow vegetation patterns with differences between birds benefiting from bare as compared to vegetated tidal flats (Eertman et al 2002). However the recovery of reference levels of biota do not necessarily closely follow vegetation. Warren et al (2002) found restoration sites to have a higher use by ‘generalist’ bird species even 20 years after restoration, and that while fish may be present at reference densities and composition in 5 years, gut content may not reach parity for 15 years. However, overall diverse recovery of biota over a single human generation could be considered fast by most measures of ecological time.

Invasion of recently introduced species has altered delta flora, with unknown ecological effects. Spartina eradication in euryhaline environments has been a focus of introduced species management efforts, however management of narrow leaf cattail in brackish systems, or the range of species that are expanding their range in freshwater systems (e.g. purple loosestrife, reed canarygrass, yellow flag) are only managed haphazardly. The ecological implications of either control or lack of control have not been substantively investigated. As the physical and chemical character of river deltas have been fundamental altered from historical conditions, the ecological drivers and stability of current community composition or the viability of historical community composition is unclear. Modern delta conditions could be sufficiently different than historical conditions that historical vegetation is no longer reproducible under natural competition.

Restoration of freshwater tidal flooding has been observed to increase marsh diversity both in structure and species composition, although keystone introduces species like reed canarygrass are still strongly present (Tanner et al. 2002).

The uncertainties about the structures, processes, and dynamics of food webs and biodiversity are great. Developing a policy for investing in science therefore depends on identifying where lack of knowledge affects restoration effectiveness and policy.