For unicellular organisms, a lack of effects of local species richness on ecosystem function has been proposed due to their locally high species richness and their ubiquitous distribution. High dispersal ability and high individual numbers may enable unicellular taxa to occur everywhere. Using our own and published data sets on uni- and multicellular organisms, we conducted thorough statistical analyses to test whether (1) unicellular taxa show higher relative local species richness compared to multicellular taxa, (2) unicellular taxa show lower slopes of the species:area relationships and species:individuals relationships, and (3) the species composition of unicellular taxa is less influenced by geographic distance compared to multicellular taxa. We found higher local species richness compared to the global species pool for unicellular organisms than for metazoan taxa. The difference was significant if global species richness was conservatively estimated but not if extrapolated, and therefore higher richness estimates were used. Both microalgae and protozoans showed lower slopes between species richness and sample size (area or individuals) compared to macrozoobenthos, also indicating higher local species richness for unicellular taxa. The similarity of species composition of both benthic diatoms and ciliates decreased with increasing geographic distance. This indicated restricted dispersal ability of protists and the absence of ubiquity. However, a steeper slope between similarity and distance was found for polychaetes and corals, suggesting a stronger effect of distance on the dispersal of metazoans compared to unicellular taxa. In conclusion, we found partly different species richness patterns among uni- and multicellular eukaryotes, but no strict ubiquity of unicellular taxa. Therefore, the effect of local unicellular species richness on ecosystem function has to be reanalyzed. Macroecological patterns suggested for multicellular organisms may differ in unicellular communities.
The recent introduction of Gracilaria vermiculophylla (Rhodophyta) to the Kiel Fjord area was a reason for concern, since this red macroalga perfoms best under mesohaline conditions and thus appears well adapted to thrive and spread in the Baltic Sea environment. A systematic survey on a coastal range of 500 km in 2006 and 2007 indicated considerable multiplication and spreading of G. vermiculophylla within Kiel Fjord, but provided little evidence of long-distance transport. Nonetheless, flow-through growth experiments conducted at a range of salinities under ambient light showed that G. vermiculophylla should be able to grow in most of the Baltic Sea. Growth declined only below a salinity of 5.5. High water temperatures in summer seem to reduce resistance against low salinity. Growth of G. vermiculophylla in the SW Baltic is limited by light and is only possible during summer and above a depth of 3 m. Drifting fragments are dispersed by currents. Either they sink to deeper waters, where they degrade, or they accumulate in shallow and sheltered waters, where they form perennial mats. These overgrow not only soft bottom sediments, but also stones, which are an important habitat to Fucus vesiculosus, the main native perennial alga in the Baltic Sea. As compared to F. vesiculosus, G. vermiculophylla seems to represent a preferred refuge for mesograzers and other invertebrates, particularly in winter. Nonetheless, feeding trials showed that potential grazers avoided G. vermiculophylla relative to F. vesiculosus. Daily biomass uptake by grazers associated with G. vermiculophylla in nature did not exceed 2 g kg -1 and is <11% of average daily net growth (18.5 g kg -1 ) in the first 2 m below sea level. Consequently, feeding may not be sufficient to control the spread of G. vermiculophylla in the SW Baltic. Our study suggests that absence of feeding enemies and adaptation to brackish water may allow G. vermiculophylla to invade most shallow coastal waters of the inner Baltic Sea despite light limitation.
Multiple-choice feeding experiments were performed with the isopod Idotea granulosa and the amphipod Gammarus locusta as consumers. In a first experiment, 2 different types of tissues of the brown seaweed Fucus vesiculosus and its main macroepiphytes, Ulva lactuca and Elachista fucicola, were offered. I. granulosa rejected apices of F. vesiculosus and preferred E. fucicola, while G. locusta clearly preferred F. vesiculosus tissue, especially the meristematic apices. In a second experiment, F. vesiculosus tissue with and without E. fucicola was offered together. For I. granulosa, the consumption of F. vesiculosus was enhanced by the presence of the epiphyte, while for G. locusta there was no difference in consumed F. vesiculosus mass. G. locusta, however, showed behavioural rejection of E. fucicola, and thus, the epiphyte acted as 'protective coating'. We conclude that host (F. vesiculosus) tissue could be 'co-consumed' by mesograzers ( I. granulosa) that were attracted by the presence of epiphytes and that these epiphytes therefore may have a 2-fold negative effect on the host (i.e. competion for light, nutrients etc. and attraction of consumers). 'Co-consumption' and 'protective coating' add 2 more facets to the very variable and case-dependent interrelationships of mesograzer-epiphyte-host systems; their relevance in nature, however, remains to be demonstrated.
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