11 research outputs found
Disturbance and diversity at two spatial scales
The spatial scale of disturbance is a factor potentially influencing the relationship between disturbance and diversity. There has been discussion on whether disturbances that affect local communities and create a mosaic of patches in different successional stages have the same effect on diversity as regional disturbances that affect the whole landscape. In a microcosm experiment with metacommunities of aquatic protists, we compared the effect of local and regional disturbances on the disturbance–diversity relationship. Local disturbances destroyed entire local communities of the metacommunity and required reimmigration from neighboring communities, while regional disturbances affected the whole metacommunity but left part of each local community intact. Both disturbance types led to a negative relationship between disturbance intensity and Shannon diversity. With strong local disturbance, this decrease in diversity was due to species loss, while strong regional disturbance had no effect on species richness but reduced the evenness of the community. Growth rate appeared to be the most important trait for survival after strong local disturbance and dominance after strong regional disturbance. The pattern of the disturbance–diversity relationship was similar for both local and regional diversity. Although local disturbances at least temporally increased beta diversity by creating a mosaic of differently disturbed patches, this high dissimilarity did not result in regional diversity being increased relative to local diversity. The disturbance–diversity relationship was negative for both scales of diversity. The flat competitive hierarchy and absence of a trade-off between competition and colonization ability are a likely explanation for this pattern
Stochastic Species Turnover and Stable Coexistence in a Species-Rich, Fire-Prone Plant Community
Understanding the mechanisms that maintain diversity is important for managing ecosystems for species persistence. Here we used a long-term data set to understand mechanisms of coexistence at the local and regional scales in the Cape Floristic Region, a global hotspot of plant diversity. We used a dataset comprising 81 monitoring sites, sampled in 1966 and again in 1996, and containing 422 species for which growth form, regeneration mode, dispersal distance and abundances at both the local (site) and meta-community scales are known. We found that species presence and abundance were stable at the meta-community scale over the 30 year period but highly unstable at the local scale, and were not influenced by species' biological attributes. Moreover, rare species were no more likely to go extinct at the local scale than common species, and that alpha diversity in local communities was strongly influenced by habitat. We conclude that stochastic environmental fluctuations associated with recurrent fire buffer populations from extinction, thereby ensuring stable coexistence at the meta-community scale by creating a “neutral-like” pattern maintained by niche-differentiation
Nuclear Spin Crossover in Dense Molecular Hydrogen
The laws of quantum mechanics are often tested against the behaviour of the
lightest element in the periodic table, hydrogen. One of the most striking
properties of molecular hydrogen is the coupling between molecular rotational
properties and nuclear spin orientations, giving rise to the spin isomers
ortho- and para-hydrogen. At high pressure, as intermolecular interactions
increase significantly, the free rotation of H2 molecules is increasingly
hindered, and consequently a modification of the coupling between molecular
rotational properties and the nuclear spin system can be anticipated. To date,
high-pressure experimental methods have not been able to observe nuclear spin
states at pressures approaching 100 GPa and consequently the effect of high
pressure on the nuclear spin statistics could not be directly measured. Here,
we present in-situ high-pressure nuclear magnetic resonance data on molecular
hydrogen in its hexagonal phase I up to 123 GPa at room temperature. While our
measurements confirm the presence of I=1 ortho-hydrogen at low pressures, above
70 GPa, where inter- and intramolecular distances become comparable, we observe
a crossover in the nuclear spin statistics from a spin-1 quadrupolar to a
spin-1/2 dipolar system, evidencing the loss of spin isomer distinction. These
observations represent a unique case of a nuclear spin crossover phenomenon in
quantum solids