Nutrient pollution and reduced grazing each can stimulate algal blooms as shown by numerous experiments. But because experiments rarely incorporate natural variation in environmental factors and biodiversity, conditions determining the relative strength of bottom-up and top-down forcing remain unresolved. We factorially added nutrients and reduced grazing at 15 sites across the range of the marine foundation species eelgrass (Zostera marina) to quantify how top-down and bottom-up control interact with natural gradients in biodiversity and environmental forcing. Experiments confirmed modest top-down control of algae, whereas fertilisation had no general effect. Unexpectedly, grazer and algal biomass were better predicted by cross-site variation in grazer and eelgrass diversity than by global environmental gradients. Moreover, these large-scale patterns corresponded strikingly with prior small-scale experiments. Our results link global and local evidence that biodiversity and top-down control strongly influence functioning of threatened seagrass ecosystems, and suggest that biodiversity is comparably important to global change stressors.

Dinucleoside polyphosphates act as agonists on purinergic P2Y receptors to mediate a variety of cellular processes. Symmetrical, naturally occurring purine dinucleotides are found in most living cells and their actions are generally known. Unsymmetrical purine dinucleotides and all pyrimidine containing dinucleotides, however, are not as common and therefore their actions are not well understood. To carry out a thorough examination of the activities and specificities of these dinucleotides, a robust method of synthesis was developed to allow manipulation of either nucleoside of the dinucleotide as well as the phosphate chain lengths. Adenosine containing dinucleotides exhibit some level of activity on P2Y 1 while uridine containing dinucleotides have some level of agonist response on P2Y 2 and P2Y 6 . The length of the linking phosphate chain determines a different specificity; diphosphates are most accurately mimicked by dinucleoside triphosphates and triphosphates most resemble dinucleoside tetraphosphates. The pharmacological activities and relative metabolic stabilities of these dinucleotides are reported with their potential therapeutic applications being discussed.

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Interacting changes in predator and prey diversity likely influence ecosystem properties but have rarely been experimentally tested. We manipulated the species richness of herbivores and predators in an experimental benthic marine community and measured their effects on predator, herbivore and primary producer performance. Predator composition and richness strongly affected several community and population responses, mostly via sampling effects. However, some predators survived better in polycultures than in monocultures, suggesting complementarity due to stronger intra- than interspecific interactions. Predator effects also differed between additive and substitutive designs, emphasizing that the relationship between diversity and abundance in an assemblage can strongly influence whether and how diversity effects are realized. Changing herbivore richness and predator richness interacted to influence both total herbivore abundance and predatory crab growth, but these interactive diversity effects were weak. Overall, the presence and richness of predators dominated biotic effects on community and ecosystem properties.

Extracellular diadenosine polyphosphates play important signaling functions in a number of physiological responses. Here we show that diadenosine polyphosphates are normal constituents of tear fluid and are potent stimulators of tear secretion through their interaction with P2Y receptors. Diadenosine tetraphosphate (Ap 4 A) and Ap 5 A were found in rabbit tears under basal conditions at concentrations of 2.92 and 0.58 M, respectively. Single applications of UTP, ATP, and Ap 4 A increased tear secretion to 160 Ϯ 8% (n ϭ 16) (P Ͻ 0.001), 131 Ϯ 6% (P Ͻ 0.05), and 162 Ϯ 11% (P Ͻ 0.05) of placebo values, respectively. Ap 4 A, Ap 5 A, and Ap 6 A, but not Ap 2 A and Ap 3 A, were able to stimulate tear secretion in a dose-dependent manner. Concentration-response studies produced pD 2 values of 5.56 Ϯ 0.03, 5.75 Ϯ 0.12, and 5.50 Ϯ 0.09 for Ap 4 A, Ap 5 A, and Ap 6 A, respectively, with Ap 4 A showing the greatest efficacy. Diadenosine polyphosphates also stimulated P2Y 1 and P2Y 2 receptors expressed in 1321N1 cells with no apparent effect on the other P2Y receptors tested. Nonselective P2 antagonists did not modify the tear secretion induced by UTP or Ap 4 A in rabbit eyes in vivo or in cloned receptors, except for a weak but significant reduction in stimulated tear secretion by reactive blue 2. These results suggest that diadenosine polyphosphates stimulate tear secretion via a P2Y receptormediated mechanism. Comparing the effects of diadenosine polyphosphates applied to the rabbit eye and to cloned P2Y receptors, it appears that the P2Y 2 receptor subtype is responsible for the prosecretory effects of these compounds.