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Darren W Johnson

Publications and source records attributed to Darren W Johnson.

13 recordsLinked to original sources
PubMed2006

Predation, habitat complexity, and variation in density-dependent mortality of temperate reef fishes.

Density dependence in demographic rates can strongly affect the dynamics of populations. However, the mechanisms generating density dependence (e.g., predation) are also dynamic processes and may be influenced by local conditions. Understanding the manner in which local habitat features affect the occurrence and/or strength of density dependence will increase our understanding of population dynamics in heterogeneous environments. In this study I conducted two separate field experiments to investigate how local predator density and habitat complexity affect the occurrence and form of density-dependent mortality of juvenile rockfishes (Sebastes spp.). I also used yearly censuses of rockfish populations on nearshore reefs throughout central California to evaluate mortality of juvenile rockfish at large spatial scales. Manipulations of predators (juvenile bocaccio, S. paucispinus) and prey (kelp, gopher, and black-and-yellow [KGB] rockfish, Sebastes spp.) demonstrated that increasing the density of predators altered their functional response and thus altered patterns of density dependence in mortality of their prey. At low densities of predators, the number of prey consumed per predator was a decelerating function, and mortality of prey was inversely density dependent. However, at high densities of predators, the number of prey killed per predator became an accelerating response, and prey mortality was directly density dependent. Results of field experiments and large-scale surveys both indicated that the strength of density-dependent mortality may also be affected by the structural complexity of the habitat. In small-scale field experiments, increased habitat complexity increased the strength of density-dependent mortality. However, at large scales, increasing complexity resulted in a decrease in the strength of density dependence. I suggest that these differences resulted from scale-dependent changes in the predatory response that generated mortality. Whether increased habitat complexity leads to an increase or a decrease in the strength of density-dependent mortality may depend on how specific predatory responses (e.g., functional or aggregative) are altered by habitat complexity. Overall, the findings of this study suggest that rates of demographic density dependence and the resulting dynamics of local populations may largely depend upon attributes of the local habitat.

Animals↗
PubMed2006

Density dependence in marine fish populations revealed at small and large spatial scales.

Experimental manipulation of population density has frequently been used to demonstrate demographic density dependence. However, such studies are usually small scale and typically provide evidence of spatial (within-generation) density dependence. It is often unclear whether small-scale, experimental tests of spatial density dependence will accurately describe temporal (between-generation) density dependence required for population regulation. Understanding the mechanisms generating density dependence may provide a link between spatial experiments and temporal regulation of populations. In this study, I manipulated the density of recently settled kelp rockfish (Sebastes atrovirens) in both the presence and absence of predators to test for density-dependent mortality and whether predation was the mechanism responsible. I also examined mortality of rockfish cohorts within kelp beds throughout central California to evaluate temporal (between-generation) density dependence in mortality. Experiments suggested that short-term behavioral responses of predators and/or a shortage of prey refuges caused spatial density dependence. Temporal density dependence in mortality was also detected at larger spatial scales for several species of rockfish. It is likely that short-term responses of predators generated both spatial and temporal density dependence in mortality. Spatial experiments that describe the causal mechanisms generating density dependence may therefore be valuable in describing temporal density dependence and population regulation.

Animals↗
PubMed2005

Synthesis and characterization of two isomeric, self-assembled arsenic-thiolate macrocycles.

The use of labile As-S bond formation in the self-assembly of discrete supramolecular structures is extended. Macrocyclic structures of chemical formula As2L2Cl2 (H2L=alpha,alpha'-dimercapto-p-xylene) were prepared and characterized. Diastereomeric syn and anti isomers of these macrocycles were selectively crystallized and characterized in the solid state using single-crystal X-ray diffraction. Both the syn and anti macrocycles show close contacts between the arsenic(III) ions and the aromatic carbons, consistent with intramolecular arsenic-pi interactions. The dynamic behavior of the isomers in solution is also investigated. anti-As2L2Cl2.AsCl3 crystallizes in monoclinic space group P2(1)/c (No. 14) with a=10.6194(5) A, b=16.7780(9) A, c=8.5725(4) A, beta=100.6830(10) degrees, and Z=2. syn-As2L2Cl2 crystallizes in orthorhombic space group Pnma (No. 62) with a=10.8881(8) A, b=19.3511(14) A, c=9.9524(7) A, and Z=4.

Journal Article↗
PubMed2002

Metal-switching and self-inclusion of functional cavitands.

Cavitands bearing both eight (5) and two (13) metal-ligating carboxymethylphosphonate groups on their rims were synthesized by Arbuzov reaction of the corresponding bromoacetamido cavitands with trialkyl phosphites. These exist in the vase conformation in CDCl(3) and are stabilized by a cyclic seam of hydrogen bonds. This structure was also found in the solid state for the octabromoacetamide 4a and diphosphonate cavitand 13 by single-crystal X-ray analysis. Cavitands 5 and 13 form caviplexes in CDCl(3), CD(2)Cl(2), and alcohol solutions with adamantane derivatives 15a,b, quinuclidine 15d, ammonium and phosphonium salts 14, and drugs like ibuprofen 15c, all of which are stable on the NMR time scale at 295 K. NMR spectroscopy reveals that at 223 K octaphosphonate 5b exists in two forms: the major C(4)-symmetrical compound is filled with solvent while the minor species shows intramolecular inclusion of a dialkoxyphosphoryl group. In methanol-d(4) 5 and 13 exist in a lower symmetry vase conformation with self-inclusion of one alkyl group. Interaction of these complexes with La(OTf)(3) results in a change in the conformation of the cavitand from vase to kite with concomitant and quantitative release of the encapsulated guests. Two to three equivalents of the lanthanide salt per equivalent of cavitand 5a-d is necessary for the complete decomplexation of the included guest. The kite and the vase conformers equilibrate slowly on the NMR time scale at 295 K. The addition of good ligands for metal cations (nitrate or CMPO calixarene 16) shifts the equilibrium to the vase-shaped caviplex and allows quantitative control of the binding and release of the guest. The lanthanide complexes of octaphosphonates 5 in methanol-d(4) are velcraplex-like dimers held together by four metal cations.

Acetamides↗