Interaction of Threats and Ecosystem Disservices

There is frequently a trade-off between ecological and economic values associated with ecosystem services (see Covich et al., Chapter 3). In this context, the interconnections between services and threats provide an introduction to the concept of disservices. Exploiting one service can negatively affect, or in extreme cases completely eliminate,

Figure 6.2. Conceptual diagram illustrating a disservice for humans, through the potential negative effect of implementation of management on an ecosystem function (EF1) to enhance an ecosystem service (Service A). In this example, the physical alteration of the structure of the ecosystem through the management regime also leads to a reduction or change in function of EF3. This in turn can act as a disservice to EF2, hence causing a negative impact on Service B.

Figure 6.2. Conceptual diagram illustrating a disservice for humans, through the potential negative effect of implementation of management on an ecosystem function (EF1) to enhance an ecosystem service (Service A). In this example, the physical alteration of the structure of the ecosystem through the management regime also leads to a reduction or change in function of EF3. This in turn can act as a disservice to EF2, hence causing a negative impact on Service B.

another service. The potential for disservice is exacerbated when society introduces a management process to enhance one particular ecosystem function, and hence one particular service, that unintentionally leads to a reduction or change in another ecosystem function. The net effect is to degrade a second, non-target, ecosystem service (Figure 6.2).

For example, regulating a river to obtain hydroelectric power has negative consequences on ecosystem services that depend on natural hydrologic regimes and free-flowing water. Natural rivers and streams are heterogeneous systems with flow patterns that vary over time, often in predictable patterns (Poff & Ward 1989). However, in regulated rivers, the purpose of management is to adjust this variation; the reduced seasonal and increased daily dynamics of flow in regulated rivers instead reflect the periods of electricity demand. Reduced spring floods and elevated winter discharge, and even high daytime/weekday flow and low nighttime/weekend flow often result (Malmqvist & Englund 1996). In addition to changing flow regimes, habitat loss, fragmentation, and (in heavily regulated systems) a general change from lotic (running water) to lentic (standing water), or even to dewatered conditions, can occur along some stretches of river. These flow alterations change river habitats and their biota, including sedimentary biota, fundamentally. For example, species richness and abundance of benthic macroinvertebrates are impacted (Ward & Garcia de Jalon 1991) which, given the clear role of such invertebrates in delivery of ecosystem services (see Covich et al., Chapter 3), will lead to degradation of such services (e.g., breakdown ofwastes, support of fisheries, and maintenance of high water quality). Effects on migrations and stocks of anadromous salmonids, in turn, alter various services such as recreation. This is of significant concern, for instance, in Sweden (Malmqvist & Englund 1996; Jansson et al. 2000), where over 70 percent of the rivers are regulated, and also in the northern third of the Earth, where a majority of rivers are heavily influenced by river regulation (Dynesius & Nilsson 1994). Worldwide, the number of "large" dams (greater than 15 m) has increased approximately eightfold over the last 50 years and the total number of dams has risen to nearly 800,000 (Postel & Carpenter 1997; Rosenberg et al. 2000).

The introduction of exotic species further illustrates the concept of the interaction of threats and ecosystem disservices. Exotic species introductions create less visible but ecologically and sometimes economically important problems. Introductions of non-native fish to improve fisheries for sport or to provide protein for human consumption have led to the wholesale collapse of local biological communities and aquatic food webs. The negative impacts on ecosystem functions that rely on intact food webs, such as decomposition, biogeochemical cycling, and overall productivity, will likely affect water quality, other fisheries, and hence a number of other services. The introduction of a predatory cichlid into Gatun Lake in the Panama Canal (Paine & Zaret 1973) and the introduction of Nile Perch into the African rift valley lakes (Kitchell et al. 1997) are two cautionary cases. The zebra mussel (Dreissena polymorpha) is perhaps the most famous and pervasive invading freshwater exotic. Originally native to the Black and Caspian Seas, it spread throughout European waters during the 19th century, but it has only recently reached Ireland (Minchin et al. 2003). Discovered in 1988 in Lake St. Clair in the Great Lakes of the United States, it rapidly spread throughout the Great Lakes and Mississippi river basin (Ludyanskiy et al. 1993). There is a range of abiotic, biotic, direct, and indirect impacts of a zebra mussel introduction (MacIsaac 1996), but one of the greatest is the impact of biofouling (undesirable accumulation on artificial surfaces) associated with the settlement of the mussels. Water intake structures for municipal, industrial, and hydroelectric plants are extremely vulnerable to fouling if they draw intake water from an infested water body. Reaching densities of up to 334,000 mussels per square meter, the exotic mussels have significant ecological effects such as competitive exclusion of native unionid mussel species (Ludyanskiy et al.1993; Fitzsimons et al. 1995) and removal of phytoplankton, which results in the disruption of food webs (Karatayev et al. 1997). Theoretical estimates suggest that the native mussel population in Lake Erie can filter up to 14 times the entire lake and remove phytoplank-

ton equivalent to 25 percent of primary production each day (Bronmark & Hansson 1998). On the other hand, this trait can be utilized in management of eutrophication problems as seen in the Netherlands. By efficiently filtering phytoplankton, these mussels are able to alter nutrient cycling patterns, transfer carbon and nutrients from the pelagic to the benthic zone through the build up of mussel biomass, reduce the concentration of suspended solids, and hence improve water clarity and increase macro-phyte growth.

Given the various threats to ecosystem functioning in freshwater systems, one might ask why the problem is not worse. After all, large population centers are still supported by surface waters that continue to provide a number of ecosystem services. The answer lies in part with the level of technology and infrastructure we are able to bring to bear, such as wastewater processing, but also in the ability of the freshwater ecosystems and biological communities either to cope with a certain level of disturbance or to recover rapidly and restore function—that is, an innate resistance and/or resilience of the ecosystem. The question remains: What role do benthic biota play in reducing the impact of a threat? Specifically, how does benthic biodiversity influence ecosystem processes in freshwater systems and are there key taxa involved, the loss of which will have devastating consequences on ecosystem services? Overall, key taxa do influence ecosystem functioning; the presence of certain species can substantially influence the system (e.g., on decomposition) (Giller et al. 2004). Benthic studies, however, have generally focused on relatively low levels of species richness, and experimental studies have been at local scales (Covich et al. 1999; Covich et al. 2004). More research is needed to provide a comprehensive understanding of the various roles played by different benthic species and functional groups of species.

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