Not all human impacts are simple or direct. The occurrence of toxic blooms of dinoflagellates (e.g. red tides) appears to have increased substantially over the past couple of decades (Smayda 1989; World Resources Institute 1992; Hallegraeff 1993; Anderson 1994). The toxins impact shellfish and humans, potentially causing extensive mortality. The increase in frequency of red tides has been tied to increased nutrient inputs to coastal systems. The blooms may be primarily coastal in nature, but this may be an artifact of observation. Limited monitoring of dynamic events in the open ocean leaves the question of impact of toxic algal blooms very much as open one.
A decrease in the earth's ozone protection will allow more biologically damaging UV-B radiation to reach sea-level. Atmospheric UV-B radiation can penetrate through tens of meters of water in most marine environments (Smith el al. 1992). UV B radiation at levels found today in some oceans is known to be detrimental to many forms of life in marine ecosystems at all levels (Hunter el al. 1981; Hardy and Gucinski 1989; Hader et al. 1989; Karentz et al. 1991; Behrenfeld el al. 1993a, 1993 b; Herndl et al. 1993; Bothwell et al. 1994). Historic alterations to the atmosphere point to planktonic species being the most sensitive species (McKinney 1987). suggesting that they would suffer first from current changes to the atmosphere.
Climate change will have a significant, but largely unpredictable, impact on the open ocean. Changes in sea-surface temperature can alter wind patterns and thus oceanic currents. Because hydrodynamics affects the ecological and evolutionary spatial and temporal scale of so much of oceanic-life. any change to ocean currents will percolate throughout the pelagic food-web and down to the benthos. Past climatic changes are thought to have been the principal driving forces behind shifts in the abundance of the dominant fishes in many different systems (Alheit and Berna) 1993; Bas 1993; Kuznetsov et ul. 1993; Tang 1993). Oscillations in the relationship between oceanic and atmospheric conditions in the Southern Ocean periodically produce meteorological events known as the El Niño phenomenon. Off the Pacific coast of South America, El Niño years result in significant shifts in water masses of different temperature, with accompanying shifts in productivity and diversity. Coastal-based seabirds and marine mammals may die by the tens to hundreds of thousands during particularly strong El Nino years. Although initially thought to be a localized phenomenon, the El Niño Southern Oscillation is now known to have global effects with largely unpredictable results. It is likely that anthropogenic alterations to the world's atmosphere or oceans will modify El Niño events, and thus the distribution of productivity and biodiversity, although the direction and magnitude of this change in uncertain.
Open-ocean biomes are generally not dominated by biogenic structure, such as angiosperms, that humans can alter. Reports at the close of the 19th century talk of "seas of weeds" in the ccntcrs of many of the major ocean basins, but there has not been a single report of this phenomenon in recent years (J.T. Carlton, personal communication 1994). The reasons why this pelagic biogenic superstructure has disappeared are unclear. In the Sargasso Sea, very large rafts of floating seaweeds support a complex community complete with pelagic morphotypes of Ascophyllum and Fucus, and an endemic isopod, I ¡Jo tea me tallica. The disappearance of the seaweeds will have an impact on forms of life that depend on it.
Physical structure is important to the functioning of the deep benthos, primarily because of the tunneling and mixing of the sediments by specific biota, which alter biogeochemicai cycling of nutrients and oxygen availability. Much of the benthic environment is very distant from most human activities, although mining and the collection of benthic organisms by dragging sleds or nets may impact significant members of the benthic community (Watling and Langton 1994). Because of the importance of bioturbation in modulating the quality of the sediments, a reduction in the diversity of these taxa will have a significant impact on the rest of the community that can be supported in the benthos.
Removal of manganese nodules by mining can eliminate one important type of surface structure exploited by a distinct assemblage of epifauna (Thiel 1992). Several decades may be required for the re-establishment of these communities. Whether this activity has altered in any manner the biodiversity living in these environments is unknown, but the possibility for impact exists through alteration of physical structure, chemical or radiological contamination, and alteration of nutrient inputs from surface waters.
16.4 BIODIVERSITY AND ECOSYSTEM PROCESSES: GENERAL THOUGHTS
Whether species arc important to the "functioning" of ecosystems processes has often been phrased as choosing between the species as "rivets" hypothesis (Ehrlich and Ehrlich 1981), and the species as "passengers" hypothesis (Walker 1992). In the latter case, one, or a very few. species are key to the operation of the ecosystem process; the other species are accessories that play no additional role in the system, in effect, passengers on a bus driven by a few key species. Under the rivet hypothesis, each species has a small yet important role in the dynamics of the system, and although the removal of one species may not lead to any perceptible loss in the functioning of the ecosystem, loss of several species will lead to a gradual reduction in the ability of the ecosystem to "sustain" itself. A continuum exists between these two hypotheses; at one extreme, variance in the contribution of each species to ecosystem processes is high, and at the other extreme, all species arc equal contributors and variance is minimized.
One must consider at least one other factor when assessing the role of diversity in ecosystem processes. Ecosystems are dynamic entities, structured by processes that operate at multiple scales, and the dynamics of any one patch is generally unpredictable in either time or space (Williamson 1988; Bell 1992; Holling 1992; Bell et al. 1993). Some processes occur in a more or less predictable cycle, especially when there is some endogenous biological feature to the process (e.g. fire, spruce budworm outbreaks, hare-lynx population cycles). Other processes are less predictable (i.e. rare colonization events, volcanic eruptions), and are independent of the organisms that form part of the ecosystem. With respect to identifying the relative importance of species to ecosystem processes, an understanding of the predictable dynamics that occur in ecosystems can help identify the species important in the succession of communities that maintain sustainability or system resilience over scales that matter to the functioning of whole ecosystems. What about the less predictable dynamics that occur, such as the "accidental" introduction of alien species (chestnut blight, rabbits, cattle egrets), or other dramatic changes to the landscape? In these cases, ccosystems are less likely to be pre-adapted to these specific perturbations, and stability may depend on unforeseen properties of the species assembly. When stochastic effects play a large role in ecosystem dynamics, our ability to predict the importance of specific biodiversity in ecosystem processes decreases as temporal and spatial scales increase.
It would be difficult to come up with a list of the most indispensable species for even the best-studied ecosystem, let alone one such as the open ocean, about which so little is known. The four areas of ecosystem dynamics that we know least well, and for which ranking species for importance is most difficult, are:
• identifying the alternate states of each stage of succession, and the paths between successional stages;
• identifying the facilitator or mediator species that help direct the rate and direction of succession;
• identifying the species that buffer the ecosystem process against the less predictable disturbances;
• identifying the biotic interactions that create threshold effects within the system.
We will proceed by briefly examining the nature of structuring processes in open-ocean systems and the stability of ocean ecosystems. We then discuss the general nature of functional groups in pelagic and benthic ecosystems.
16.4.1 Structure and stability of open-ocean ecosystems
Holling alluded to the overwhelming importance of the size-scaling of biogenic habitats, referring to levels of habitable volume (as defined by pine needles, canopies, and so on up to the landscape scale) as "nuggets" or "lumps" in terrestrial ecosystems (Holling 1992). There is no evidence as yet for such a structure in open-ocean communities (Steele 1985; Holling et al. 1994). Consequently, there is also little reason to expect the existence of discrete, alternate, stable configurations linked by disturbance or successional processes, such as exist in terrestrial and coastal marine benthic communities (Sutherland 1974; Margalef 1978; Holling 1992; Holling et al. 1994). Holling et al. (1994) stated that "terrestrial systems are functionally more localized than marine systems". The potential consequences of these differences arc intriguing. Are pelagic communities devoid of ecosystem resilience (in the classical sense of Holling 1973), i.e. does the community shift frequently and without much resistance from one "stable" state to another? At what state are biotic interactions important? Is there no inter-annual succession in pelagic assemblages? Are cyclical shifts in species dominance entirely stochastic?
In the open ocean, the dynamics of epipelagic life vary greatly over even short time periods (Steele 1991). The populations themselves may be susceptible to short-term perturbations (i.e. over-harvesting, physical fluxes, nutrient pulses), but the community is flexible and adaptable to these new conditions and shows resilience over short time-scales. This is because pelagic species have planktonic larvae, and because of the physical mixing that dominates oceanic ecosystems. At moderate time-scales, changes in the ocean's physical characters, or over-harvesting, can lead to a breakdown of community resilience and a change of state. Empirical evidence for this comes from the frequent "flips" in the dominant pelagic fish (e.g. anchovy to sardine, these changes appear to be a natural switch between two different "stable" states (Soutar and Isaacs 1974; Alheit and Bernal 1993). Such changes to alternate states in community structure appear regularly (Sherman and Alexander 1986, 1989; Bas 1993; Blindheim and Skojdal 1993; Kuznetsov et al. 1993; Tang i993). Many of these changes have been driven by over-fishing, and not all previously abundant fish populations have rebounded from depressed populations. These changes may be truly permanent, or they be oscillating in a cycle with a a longer period than we have been monitoring.
Steele (1991) compared the response of large oceanic fish to the response of trees in terrestrial systems and concluded that terrestrial organisms are less able to respond/adapt to short-term disturbances. This trend needs to be confirmed in a more rigorous manner, but the pattern is interesting. Primary producers in the open oceans (i.e. plankton) do respond more quickly to environmental disturbances than their terrestrial counterparts, the trees. In general, pelagic organisms may disperse more rapidly in space and not very much in time, whereas terrestrial organisms and benthic marine taxa disperse in both, but in neither very quickly. With the demise of the ocean's large apex consumers as a caveat, it may not be unreasonable to assume that species which live in the pclagic open ocean are more resilient to small-scale perturbations (Steele 1991) because of their dynamic metapopulation structure and the physical mixing of oceanic systems.
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