53402 What Structures Marine Biodiversity and why does it vary? C. Heip Centre for Estuarine and Coastal Ecology, Netherlands Institute of Ecology, NL-4400 AC Yerseke, The Netherlands corresponding author (e-mail):[email protected] Abstract: Marine ecological biodiversity research is a scientific field with few observational data to support a weak theory largely borrowed from terrestrial ecology and lacking in experimental verification. The relative lack of scientific interest and effort until recently was a consequence of the general feeling that marine biodiversity is far less threatened than terrestrial biodiversity. This view is not sustainable. There is now ample evidence of widespread changes in most coastal habitats in populated areas around the world (coral reefs, mangroves, seagrass fields, intertidal rocky shores and subtidal sediments on the continental shelf and margin) due to exploitation of marine resources, introduction of exotic species and the increased pressure from mariculture and fisheries. The sustainable exploitation of the seas requires development of a sound theoretical framework for marine biodiversity, including genetic, species and habitat diversity and especially the relationships between them. At the present state of knowledge such a general theory is still far from being reality. In this paper an overview is given of the main elements that an ecological theory of species diversity should include and what aspects of human pressure on the biodiversity of marine ecosystems should be given priority attention. Introduction The very rapid and widespread changes due to a far and perhaps an elusive goal. Even the basic human activities in the abundance and distribution job of providing an adequate description of biodi- of biological species that have been documented versity is far from done. On the one hand there is over the last decade have raised the question still a major effort required in scientifically describ- whether these changes in biodiversity have impor- ing the species that inhabit the seas and much con- tant consequences for ecosystem functioning. Eco- cern has been expressed about the shear number of systems perform a series of vital functions for hu- undescribed species and the decline of taxonomic man society, many of which depend on a variety expertise. On the other hand we are at the dawn of of organisms present in a variable environment. unprecedented progress in charting and explaining The relationships between biodiversity and ecosys- genetic diversity in the oceans. For the first time the tem functioning are far from clear and before this prospect of a complete inventory of marine micro- issue can be properly rj::searched a number of more bial populations is no longer utopic. The research basic questions have still to be answered. that will be required to couple knowledge on genetic In current thinking biodiversity equals biologi- structure to the functioning of marine systems will cal variability, from genes to specie~ and habitats take many years and even decades to be accom- within ecosystems. The mech~nisms' that generate plished. The genomes of about 300 species will be and maintain variation at these different levels in characterized in the near future, but only very few the organisation of the biosphere are very differ- of them are relevant to marine ecology; so, although ent. A single coherent theory ofbiodiversity change the start has been taken, the work is only beginning. and maintenance will have as a major challenge the It is clear that many basic facts are missing and linking of these mechanisms over the spatial and that it will require many years of scientific effort to temporal scales at which they operate. This is still obtain them. This task can only be accomplished in From WEFER G, LAMY F, MANTOURA F (eds), 2003, Marine Science Frontiers for Europe. Springer- Verlag Berlin Heidelberg New York Tokyo, pp 251-264 Publication 2891 Netherlands Institute of Ecology (NIOO-KNA W) - - -- - - --- --- - ~ 252 Heip a fruitful way if a coherent and testable theory on which organisms have to adapt without being able marine biodiversity will be constructed over the next to modify them. To include habitats in the biodi- few years. Whether a unified theory on biodiversity versity concept is therefore a far from simple task, should be based on characteristics of individual or- although the debate is part of the marine ecologi- ganisms, including their genetic composition, or on cal literature at least since Sanders (1968) in the species or communities in landscapes remains an frame of the stability-time hypothesis discussed open question. The most easily recognized opera- the difference between biologically and physically tional building block in the biodiversity hierarchy is dominated communities. the individual organism, which is often ignored in Although it would be fascinating to summarize marine ecology. All individual organisms are differ- the processes that structure and change genetic, ent to some extent, even in clonal species. The po- species and habitat variability, and to try and link tential genetic diversity present in populations is them, the task would be far outside the scope of this much large~than the number of individuals in which paper. Even restriction to the classical use of the it is expressed and each generation in a sexually term biodiversity - species diversity - requires reproducing population is a new and unpredictable addressing nearly the whole field of ecology. This sample from a very large gene pool. In a recent paper, is clearly impossible and I will restrict myself to a Pachepsky et al. (2001) present a framework for series of headings indicating what we should con- studying the dynamics of communities which sider to know in order to describe and explain ma- generalises the prevailing species-based approach to rine biodiversity and a short summary of what is one based on individuals that are characterised by known under that heading. I have heavily borrowed their physiological traits. from a number of review papers that appeared At the habitat or ecosystem level as well, there during recent years and that show the explosive is an important random element due to the unique growth of interest in the subject. I also refer to the and unpredictable geological setting in which eco- book ofOrmond et al. (1997) for an important col- logical processes take place. When one discusses lection of papers that cover many aspects of ma- characteristics and changes in habitats, a review rine biodiversity that are ignored here, e.g. phylo- would have to address the. complex physical, genetics and evolution. biogeo-chemical and geological processes and es- pecially their interactions that structure landscapes Is marine biodiversity special? in the sea, including the pelagic environment, and that can operate at very different time and space The theoretical foundations as well as the experi- scales. The newly coined term 'biogeomorphology' mental approach required to understand marine probably comes closest to what kind of questions biodiversity are very poorly developed, in general we have to formulate, at least for the sea floor. The and also when compared to terrestrial ecology. In interactions within biofilms and microbial mats, the fact, the whole literature is so much dominated by biological impact on sediment characteristics such theory developed for terrestrial ecosystems that one as bottom roughness, the structure of the benthic can scarcely speak of a field of marine biodiversity. boundary layer and benthic-pelagic coupling are ex- An important part of this theory has been devel- amples of research areas where a close link between oped by plant ecologists (e.g. Loreau 2000 for an biology and geology exists. Coral reefs in the trop- example) and its application to the oceanic envi- ics are the most prominent case of an environment ronments is of course problematic. One basic ques- nearly completely created by biotic processes, and tion is whether terrestrial and marine systems are several less spectacularreefforming organisms are similar enough to allow theory from one domain active in temperate waters, including maerl forming to be used for the other. Most probably this is not red algae, molluscs and polychaetes, deep water the case. Marine systems have a series of charac- corals and others. At the other extreme we have teristics which distinguish them from terrestrial environments such as sandy beaches which are systems, as explained in the following box (from nearly completely built by physical forces and to Heip et al. 1999). What Structures Marine Biodiversity and why does it vary? 253 Box 1. The distinctive features of marine biodiversity 1. Life has originated in the sea and is much older in the sea than on land. As a consequence, the diversity at higher taxonomic levels is much greater in the sea where there are 14 endemic (unique) animal phyla whereas only 1 phylum is endemic to land. There is also a remarkablediversity of life-history strategies in marine organisms. The sum total of genetic resources in the sea is therefore expected to be much more diverse than on land. 2. The physical environment in the oceans and on land is totally different. Marine organisms live in water, terrestrial organismslive in air. Environmental change in the sea has a much lower frequency than on land, both in time and in space. 3. Marine systems are more openthan terrestrial and dispersal of species may occur over much broader ranges than on land. Although most species in the ocean are benthic and live attached to or buried in a substratum, in coastal seas a very large proportion of them has larvae that remain floating in the water for days to months. These high dispersal capacities are often associated with very high fecundities and this has important consequences for their genetic structure and their evolution. 4. The main marine primary producers are very small and often mobile, whereas on land primary producers are large and static.