Latitudinal Increase Research Articles

Species richness and canopy productivity of Australian plant communities

The species richness (number of vascular plants per hectare) of Australian plant communities (containing a mosaic of gap, regeneration, maturation and senescent phases) is correlated with the annual biomass productivity of the overstorey canopy. The annual production of leaves and stem in the canopy of the plant community is shown to be limited by the requirements of photosynthesis (particularly light and the availability of water) and the length of the growing season. The species richness of Australian plant communities is the product of the blance between the dominance of the overstorey and the response of the understorey to the shading of the overstorey. For all climatic regions and zones the species richness of the overstorey of the plant community is shown to be exponentially related to the annual shoot growth of the overstorey canopy, until the latitudinal or altitudinal tree line is reached. With latitudinal increase outside the tropics, overstorey canopies of forest communities absorb increasingly more of the incident solar radiation. markedly reducing the species richness of the understorey strata. In contrast, in these latitudes the overstorey of plant communities with widely spaced trees or tall shrubs will absorb far less solar radiation, thus enabling the species richness of the understorey to be maintained.

Latitudinal Clines: A Trade‐Off between Egg Number and Size in Pacific Salmon

The latitudinal variation in clutch size found in many animal species, including Pacific salmon, has been an enigmatic problem in ecology. We analyze egg number and egg size of 17 populations of coho salmon (Oncorhynchus kisutch) distributed over a latitudinal gradient in North America. These populations have a significant latitudinal increase in their egg number. But this increase is accompanied by a significant latitudinal decrease in their egg size. The total biomass of eggs produced also declines with latitude. Thus, the positive latitudinal trend in egg number cannot be explained by latitudinal variation in the total investment in eggs. This suggests that local optima in egg size may result in latitudinal clines in egg number. This reasoning, that egg number evolves around selection for egg size, is in fact predicted by life history theory and may explain the clutch size patterns observed in many organisms.

Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin.

Sunlight has long been recognized as a major provider of vitamin D for humans; radiation in the UVB (290-315 nm) portion of the solar spectrum photolyzes 7-dehydrocholesterol in the skin to previtamin D3, which, in turn, is converted by a thermal process to vitamin D3. Latitude and season affect both the quantity and quality of solar radiation reaching the earth's surface, especially in the UVB region of the spectrum, but little is known about how these influence the ability of sunlight to synthesize vitamin D3 in skin. A model has been developed to evaluate the effect of seasonal and latitudinal changes on the potential of sunlight to initiate cutaneous production of vitamin D3. Human skin or [3 alpha-3H]7-dehydrocholesterol exposed to sunlight on cloudless days in Boston (42.2 degrees N) from November through February produced no previtamin D3. In Edmonton (52 degrees N) this ineffective winter period extended from October through March. Further south (34 degrees N and 18 degrees N), sunlight effectively photoconverted 7-dehydrocholesterol to previtamin D3 in the middle of winter. These results quantify the dramatic influence of changes in solar UVB radiation on cutaneous vitamin D3 synthesis and indicate the latitudinal increase in the length of the "vitamin D winter" during which dietary supplementation of the vitamin may be advisable.

Geographic Variation in Clutch Size in the Northern Flicker (Colaptes auratus): Support for Ashmole's Hypothesis

AbstractThe Northern Flicker (Colaptes auratus), a common hole-nesting North American woodpecker, exhibits a highly significant latitudinal increase in clutch size. Using preserved clutches and information on climatic conditions and breeding densities of confamilial species, I examined the hypothesis that this geographic trend is the result of seasonal fluctuations in resources ("Ashmole's hypothesis"; Ashmole 1961, 1963; Ricklefs 1980). Clutch size is significantly correlated both with the ratio of summer productivity to estimated breeding densities of all woodpecker species and with the ratio of summer to winter productivity, supporting Ashmole's hypothesis. In addition, a substantial portion of the latitudinal gradient can be directly explained by the seasonality of resources, primarily because of an inverse correlation between clutch size and winter productivity. These results provide the first corroboration of Ashmole's hypothesis from an examination of intraspecific clutch-size variation and suggest that geographic trends in fecundity in some cases may be largely attributable to trends in seasonal fluctuations of resources.

Latitude dependence of the solar wind speed: Influence of the coronal magnetic field geometry

The dependence of solar wind speed on latitude as influenced by the magnetic field configuration of the inner corona is studied. It is found that in general, a dipolelike field geometry characteristic of a minimum-type corona tends to produce a solar wind speed distribution which increases with heliographic latitude, in accordance with observations. At very high coronal base densities and temperatures, however, this effect is minimal or even inverted. Physically, the field affects the wind speed through its area divergence, a larger divergence resulting in correspondingly lower speeds. During solar minimum, eclipse photographs suggest that the field divergence increases from pole to equator, a characteristic not apparent during solar maximum. Hence we expect the latitudinal increase in speed to be most pronounced at the minimum phase of solar activity.

Comparative Autecology of the Lizard Cnemidophorus Tigris in Different Parts of Its Georgraphic Range

I studied various aspects of the life history of Cnemidophorus tigris over much of its geographic range, from southern Idaho through southern Arizona and into northern Sonora. In the northern parts of its range, Cnemidophorus usually emerge from hibernation in May, and most adults aestivate during the midsummer months, but in the south the animals are active from April through late August. The seasonal period of activity is therefore considerably shorter in the north. Daily periods of activity are of similar duration from north to south, although the time of emergence tends to be later on northern areas. There is a significant positive correlation between estimated abundance and the total precipitation during the last 5 years, suggesting that the abundance of this species is determined by food supply. There is a latitudinal cline in the mean body temperature of active lizards, with northern populations being active at lower air and body temperature. Whereas termites constitute the major food of southern lizards, beetles and grasshoppers are the primary food of northern lizards. Some seasonal dietary trends are described. The numbers and biomass of predatory lizards, birds, and snakes increase from north to south; correlated with this is a latitudinal increase in the percentage of lizards with broken regenerated tails. Hence there is probably greater predation on southern lizards. The length of fat bodies in C. tigris is not correlated with latitude but shows an inverse correlation with the long—term average annual precipitation. It is suggested that lizards from less productive areas must allow themselves a greater margin of safety due to the more probable occurrence of drought. Northern lizards breed only once during the short northern season but lay significantly larger clutches than southern lizards, which lay at least two clutches annually. Clutch size appears to be flexible in response to feeding conditions; in one study area with a low long—term mean precipitation, females laid larger clutches during the second year, after supranormal rainfall. There is a significant correlation between mean number of eggs per clutch and the deviation of the short term (last 5 years) mean precipitation from the long—term mean precipitation. A number of facts suggest that there was a decided population "crash" on one study area during the period of study. Competition is briefly discussed and it is suggested that if there is any latitudinal trend, it is most likely in the direction of greater competition among southern lizards. In the conclusions, I suggest that the ecological challenges facing the northern populations are primarily physical, largely climatic ones, biotic interactions (particularly predation) assume relatively greater importance to southern lizards.