Quadrat covariance analysis and the scales of interspecific association during primary succession

Abstract. Three term covariance analysis is used to investigate interspecific association at a range of spatial scales, using both density and presence/absence data to evaluate the strength of species interactions. This procedure also investigates the relationship between the spatial patterns of species. These methods are applied to the vegetation on a series of glacial moraines near Mt. Robson, British Columbia, Canada, in order to test whether the plants of different species and their patterns become more independent and more random during succession. Another hypothesis tested is that if a species exerts a positive influence on others by increasing soil nutrients, this effect decreases with surface age. The vegetation analysed supports both these hypotheses to the extent that most species pairs have maximum covariance intensity on medium aged surfaces. The covariance-blocksize graphs derived from density data and from presence/absence data were qualitatively similar in their trends; however, the results are sufficiently different to suggest that the relationships among species can be subtle, affecting density rather than presence. The results also show that the affect of scale on species associations is more complex than simple models predict.     

Impaired locomotor learning and altered cerebellar synaptic plasticity in pep-19/PCP4-null mice

PEP-19/PCP4 maps within the Down syndrome critical region and encodes a small, predominantly neuronal, IQ motif protein. Pep-19 binds calmodulin and inhibits calmodulin-dependent signaling, which is critical for synaptic function, and therefore alterations in Pep-19 levels may affect synaptic plasticity and behavior. To investigate its possible role, we generated and characterized pep-19/pcp4-null mice. Synaptic plasticity at excitatory synapses of cerebellar Purkinje cells, which express the highest levels of Pep-19, was dramatically altered in pep-19/pcp4-null mice. Instead of long-term depression, pep-19/pcp4-null mice exhibited long-term potentiation at parallel fiber-Purkinje cell synapses. The mutant mice have a marked deficit in their ability to learn a locomotor task, as measured by improved performance upon repeated testing on an accelerating rotarod. Thus, our data indicate that pep-19/pcp4 is a critical determinant of synaptic plasticity in cerebellum and locomotor learning.     

Multiple species effects and spatial autocorrelation in detecting species associations

Abstract. The traditional approach to the analysis of species association within a community, based upon co-occurrence in sampling units such as quadrats, has been to test all pairs of species, using a 2 × 2 contingency table for each pair. It has long been recognised that all these tests are not independent of each other, but there is an additional problem in that the association between any particular pair may depend on the combination of the other species that are present or on the environmental factors that determine that combination. We use a 2^k contingency table to examine this problem and find that pairwise associations are not independent of the other species. The second problem that we consider is the effect of spatial autocorrelation in the data which makes the statistical tests too liberal. In the absence of a derived solution for a deflation factor to correct the test statistic calculated from a 2^k table, we describe a Monte Carlo approach that provides an approximate solution to this problem. In our data the amount of deflation that is necessary for a 2^k table is small compared to the amount required for the 2 × 2 tables used to test pairwise association.     

Long‐term growth‐increment chronologies reveal diverse influences of climate forcing on freshwater and forest biota in the Pacific Northwest

Analyses of how organisms are likely to respond to a changing climate have focused largely on the direct effects of warming temperatures, though changes in other variables may also be important, particularly the amount and timing of precipitation. Here, we develop a network of eight growth‐increment width chronologies for freshwater mussel species in the Pacific Northwest, United States and integrate them with tree‐ring data to evaluate how terrestrial and aquatic indicators respond to hydroclimatic variability, including river discharge and precipitation. Annual discharge averaged across water years (October 1–September 30) was highly synchronous among river systems and imparted a coherent pattern among mussel chronologies. The leading principal component of the five longest mussel chronologies (1982–2003; PC1_mussel) accounted for 47% of the dataset variability and negatively correlated with the leading principal component of river discharge (PC1_discharge; r = ?0.88; P < 0.0001). PC1_mussel and PC1_discharge were closely linked to regional wintertime precipitation patterns across the Pacific Northwest, the season in which the vast majority of annual precipitation arrives. Mussel growth was also indirectly related to tree radial growth, though the nature of the relationships varied across the landscape. Negative correlations occurred in forests where tree growth tends to be limited by drought while positive correlations occurred in forests where tree growth tends to be limited by deep or lingering snowpack. Overall, this diverse assemblage of chronologies illustrates the importance of winter precipitation to terrestrial and freshwater ecosystems and suggests that a complexity of climate responses must be considered when estimating the biological impacts of climate variability and change.     

Morphology and muscle stress of chelae of temperate and tropical stone crabs Menippe mercenaria

Chela morphology and muscle stress were compared between temperate and tropical populations of stone crabs Menippe mercenuriu (Say) to test whether environmental differences might result in greater crushing strength in the tropics. Such differences include increased crab diversity in the tropics (which might lead to greater fighting among congeners), increased prey exoskeleton calcification in the tropics, and year round chela use in the tropics as opposed to seasonal chela use in the temperate population.
No latitudinal differences were found in any aspect of chela morphology, including relative chela size, mechanical advantage, apodeme surface area, and angle of muscle fibre pinnation. Summer measurements of crusher chela muscle stress were also similar between the two populations.
The maximum muscle stress determined for M. mercenariu was 220 N ∼r n -∼, much higher than stress levels previously reported for crustaceans. Other researchers have typically measured forces either from autotomized chelae or by measuring forces required to open a closed chelae. I have determined muscle stress using a force transducer that measures active gripping strength in live crabs.     

Effects of temperature and thermal history on neuromuscular properties of two crustacean species

Summary The effect of temperature on the ability of neuromuscular junctions and muscle fibers to contract, release neurotransmitter, and maintein postsynaptic membrane properties, was measured in vivo and in vitro in claw closer muscles in stone crabsMenippe mercenaria (Say) and blues crabsCallinectes sapidus (Rathbun).In vivo muscle stress (defined as force generated per unit of muscle cross-sectional area) was measured in crabs of both species collected from northern populations (annual temperature range 2–30°C) and southern populations (annual temperature range 15–30°C). Muscle stress was compared between (1) crabs of both species maintained in the laboratory at 30°C (laboratory warmed); (2) crabs given a brief acclimation period (4 weeks for blue crabs; 7 weeks for stone crabs) at 8°C in the laboratory (laboratory colled), and (3) stone crabs that had been naturally acclimated from summer (30°C) to winter (8°C) temperatures over a 6 month period in the field (naturally cooled). No differences were found in the abilities of the northern and southern populations of either species to generate muscle stress when tested at summer temperatures (30°C) common to both populations. Northern and southern blue crabs produced similar levels of muscle stress whether laboratory warmed (30°C) or laboratory cooled (8°C). Conversely, northern and southern stone crabs showed significantly reduced muscle stress in laboratory cooled crabs compared with laboratory warmed crabs. Stone crabs from both populations generated the same amount of muscle stress after having been naturally cooled to 8°C as they had generated the previous summer (30°C).In vitro neuromuscular properties (i.e. (1) muscle stress as a measure of contractile ability; (2) excitatory junction potential (EJP) amplitude as a measure of neurotransmitter release; (3) specific membrane resistance (R_m) as a measure of postsynaptic membrane properties) were compared at 8, 20, and 30°C between northern cold acclimated (naturally cooled stone crabs and laboratory cooled blue crabs) and non-cold acclimated (laboratory cooled stone crabs. Muscle fibers in claws of stone crabs and blue crabs showing cold acclimation had higher R_m at 8°C than non-cold acclimated crabs. This higher R_m resulted in a broadening of EJP's which enhanced EJP summation, muscle fiber depolarization, and muscle stress.Abbreviations EJP excitatory junction potential - E _r resting membrane potential - F _e lacilitation - R _m specific membrane resistance     

Juvenile Anadromous Atlantic Salmon of Three Lakes of Newfoundland

Anadromous Atlantic salmon parr (Salmo salar) were captured from three Newfoundland lakes. Yearling and 2 year parr were captured most often in shallow water <2 m deep around lake perimeters and were not abundant at greater depths. The 3+ and 4+ parr age groups were rarely found inshore (<2 m) but were captured in deeper offshore areas. Though the parr population in one deep lake was too small to be estimated by mark-recapture, estimates of the yearlings plus 2+ age groups in two shallower lakes were 55 and 63 parr ha⁻¹. respectively. No under yearlings were found in lake habitats. Parr density in the three study lakes varied inversely with mean lake depth. Lacustrine parr growth rate was greater than or equal to that of stream dwelling parr. It is concluded that some shallow lakes of Newfoundland provide major rearing areas for juvenile anadromous Atlantic salmon.     

Quadrat variance analysis and pattern development during primary succession

Abstract. The interpretation of Hill's ‘Two Term Local Quadrat Variance’ analysis to detect the scale of spatial pattern in vegetation is improved by an equation that relates the block size of a variance peak to the scale of the pattern that gave rise to it. (Contrary to previous belief, the two are not the same, especially for large block sizes.) Deviations of pattern from a perfectly regular alternation of equally sized gaps and patches of uniform density cause changes in the variance. To aid in the interpretation of these changes, two indices of pattern regularity are proposed, one based on density and one based onpresence/absence, in orderte distinguish the effects of irregularities of patch density from irregularities of patch size and position. These methods are applied to a study of primary succession on glacial moraines near Mt. Robson, British Columbia, Canada, in order to evaluate certain hypotheses about the development of pattern. Other researchers have proposed that during succession, the pattern at first intensifies at the scales initially observed, then as succession proceeds some scales of pattern are lost due to coalescence of patches and eventually the intensity of those that remain decreases as the patterns become more and more irregular. The vegetation on the Mt. Robson moraines confirms this sequence of changes in vegetation pattern, only to the extent that patterns intensify initially in the chronosequence; the number of scales of pattern in the vegetation remains about the same throughout and there is no evidence that the patterns become more irregular. The variance-block size graphs derived from presence / absence data matched those from density data well, indicating that the simpler data, in this case, are almost as informative about pattern as the more detailed data.