On Gibbs sampling for state space models

We show how to use the Gibbs sampler to carry out Bayesian inferenceon a linear state space model with errors that are a mixtureof normals and coefficients that can switch over time. Our approachsimultaneously generates the whole of the state vector giventhe mixture and coefficient indicator variables and simultaneouslygenerates all the indicator variables conditional on the statevectors. The states are generated efficiently using the Kalmanfilter. We illustrate our approach by several examples and empiricallycompare its performance to another Gibbs sampler where the statesare generated one at a time. The empirical results suggest thatour approach is both practical to implement and dominates theGibbs sampler that generates the states one at a time.     

Mating System and Population Fluctuations of the Prairie Vole, Microtus ochrogaster

The basic mating system of the prairie vole, Microtus ochrogaster,consists of monogamous breeding units comprised of one reproductivefemale and male and their offspring. Most surviving young remainat the natal nest; those that do so normally remain non-reproductive.Of 281 total breeding units monitored in free-living populations50.2% were monogamous. Twenty-seven percent consisted of a singlereproductive female that apparently has lost her mate. Theremaining22.8% of the breeding units included two or more unrelated adultsmales and/or females (=‘complex’ breeding units).In general, monogamy was the predominant type of breeding unitat the highest (>100 adults/ha) as well as at low (>20adults/ha) population densities. There was no consistent relationshipbetween the number of residents of a breeding unit and populationdensity. Significantly more young females remaining at the natalnest became reproductive at high rather than at low populationdensities (77.1 and 17.6%, respectively). Monogamy was prevalentduring the winter as well as during the other seasons; however,there were significantly more complex breeding units duringthe winter than during other seasons. Breeding unit size wassignificantly larger during the winter than at other seasons.There was no difference in the proportions of breeding unittypes or the number of residents of a breeding unit during breedingand nonbreeding seasons. Increased breeding unit size appearsrelated to low temperatures and associated physiological stressesduring the winter rather than to population density.     

Leaf Abscission Zones in Molinia caerulea (L.) Moench, the Purple Moor Grass

Leaf separation and loss in the grass Molinia caerulea (L.)Moench was investigated using scanning electron microscopy.Leaf senescence and subsequent shedding of leaves was precededby the formation of a leaf abscission zone. The zone containscells which have more than doubled their wall thickness to greaterthan 0-4 µm. The line of fracture associated with thezone principally followed the middle lamellae, leaving intactcells on the fracture faces. Molinia should prove an interestingmodel in which to study abscission processes in the Gramineae. Molinia caerulea, leaf abscission zone, electron microscopy