Ecology

Short, Robert A.; Canton, Steven P.; Ward, James V.

doi: 10.2307/1936741pmid: N/A

Investigation of leaf litter processing in a Colorado mountain stream revealed that processing of alder (Alnus tenuifolia), willow (Salix bebbiana), and aspen (Populus tremuloides) occurred at rapid rates despite water temperatures at or near 0°C. Pine (Pinus ponderosa) was processed much more slowly. The loss rate coefficient (k) ranged from .0038 for pine to .0308 for alder. The high biomass of shredders per gram of leaf pack is a likely explanation for the rapid processing. Whereas a few large shredders per leaf pack are generally characteristic of eastern woodland streams, a large number of small—sized shredders (Capnia and Zapada) colonized leaf packs in the Colorado mountain stream. The numbers and biomass of collectors per leaf pack were directly related (P < .01) to the ash—free dry mass of fine organic particles (75—1000 @mm) indicating the importance of fine particulate organic matter in determining the suitability of leaf litter as habitat for fine particle—feeding detritivores.

Westman, Walter E.

doi: 10.2307/1936742pmid: N/A

In seeking to identify environmental factors controlling the distribution of individual species, ecologists have most commonly used techniques which assume a linear relationship between an environmental factor value and species response (e.g., canonical correlation analysis, multiple linear regression analysis, discriminant analysis). Many species assume nonmonotonic, curvilinear response curves along a variety of environmental axes, however. A method is presented to help identify environmental factors controlling species distributions when the latter species response occurs. The technique builds on theoretical and empirical observations indicating that the distribution of species importance values is most often approximately bell—shaped when arrayed along an axis representing linear change in an environment factor strongly influencing the growth and survival of the species. The method consists of identifying those environmental factor axes along which a species distribution curve best fits a Gaussian form. The method is illustrated using coastal sage scrub vegetation and habitat data from southern California. The "Gaussian analysis" technique described here will not identify all of the important predictors of species response, for reasons arising largely from the multifactorial nature of influences on species performance. The technique does, however, permit identification of some important environmental predictors which are missed by traditional techniques that assume linear response curves.

Jenkins, Stephen H.

doi: 10.2307/1936743pmid: N/A

The relationship between sizes of trees cut by beavers and distances from the borders of their ponds was examined at three sites in central Massachusetts. For most tree genera, the beavers cut a smaller range of sizes far from shore than close to shore, and relatively more small trees and fewer large trees at greater distances. The second of these results differs from the pattern of preference found in other studies in which predators were much larger than their prey, unlike this case of beavers feeding on trees. Both kinds of results are consistent with an optimal foraging model of size—distance relations in which pursuit or provisioning time depends on size of prey as well as distance (Schoener 1979).

Swain, Albert M.

doi: 10.2307/1936744pmid: N/A

Paleoecological studies of the past 400 yr indicate that the mosaic of vegetation patterns in the Boundary Waters Canoe Area in northeastern Minnesota USA appears to be related to topography and presence of large lakes, and to the ways in which these features influence periodic forest fires. Pollen and charcoal analyses from a site (Hug Lake) located in primarily deciduous forest were compared to those from Lake of the Clouds which is located in a region dominated by coniferous forest. The results showed that the average interval between fires at both sites was similar (70 yr), but the absence of conifers in the area surrounding Hug Lake probably was related to the lack of effective firebreaks such as steep topography and large lakes that are essential for the survival of conifers when regional fire frequency is high. The deciduous character of the forests around Hug Lake has existed for at least 400 yr and was not the result of several large fires that occurred in the area during the 1800's.

Dodson, Stanley I.; Egger, David L.

doi: 10.2307/1936745pmid: N/A

Food preferences and feeding rate coefficients were estimated for five Red Phaloropes (Phalaropus fulicarius) eating zooplankton. Birds were captured in the Barrow, Alaska, area and put into enclosures containing a known amount of zooplankton. Predation preference and intensity was calculated from the rate of disappearance of zooplankton from the cages. Results indicated phalaropes are size—selective predators on zooplankton, and reaffirm the size—selection predation model previously applied to fish and salamander populations.

DeAngelis, D. L.

doi: 10.2307/1936746pmid: N/A

The resilience, defined here as the speed with which a system returns to equilibrium state following a perturbation, is investigated for both food web energy models and nutrient cycling models. Previous simulation studies of food web energy models have shown that resilience increases as the flux of energy through the food web per unit amount of energy in the steady state web increases. Studies of nutrient cycling models have shown that resilience increases as the mean number of cycles that nutrient (or other mineral) atoms make before leaving the system decreases. In the present study these conclusions are verified analytically for general ecosystem models. The behavior of resilience in food web energy models and nutrient cycling models is a reflection of the time that a given unit, whether of energy or matter, spends in the steady state system. The shorter this residence time is, the more resilient the system is.

Warner, Robert R.; Hoffman, Steven G.

doi: 10.2307/1936747pmid: N/A

We propose that in some species, local population density can strongly affect the economic defendability of a mating territory. This is so because the numbers of females and potentially interfering males determine allocations of time and energy to reproduction and defense. At low densities, allocations to defense should be small and territorial mating success should initially rise with local density, reflecting the supply of females. If defense takes priority over mating, higher population densities can create a situation in which the time or energy devoted to defense against other males detracts from allocations to reproduction. Thus a point is reached where territorial mating success declines with increasing density, as a function of the number of nonterritorial males. We investigated these hypotheses by recording changes in the daily mating success of territorial males of the bluehead wrasse (Thalassoma bifasciatum) following experimental manipulations of local population size and composition. On large reefs, where mating population densities are already high, territorial mating success varied inversely with changes in overall population density and with changes in nonterritorial male numbers only; changes in female numbers had little effect. Thus at higher densities the demands of defense appear to be more important in determining mating success than the supply of available mates. Territorial mating success varied directly with population density changes only on the smallest experimental reff, where there were few nonterritorial males. The reduction of mating success at higher densities was correlated with a decreases in both the time spent in courtship and the efficiency of courtship itself. The effect of population density should be particularly important in species with short breeding periods and where the male contributes relatively little time or energy to each mating.

Schlesinger, William H.; Gill, David S.

doi: 10.2307/1936748pmid: N/A

Pure stands of the evergeen shrub, Ceanothus megacarpus, attain a mean of 4871 g/m2 in aboveground live biomass in 21 yr of growth after fire in the Santa Ynez Mountains of California. Mean net aboveground primary productivity is 850 g°m—2°y—1. In 21—yr—old stands there is also 990 g/m2 of attached dead wood as lower branches on the living shrubs and 476 g/m2 of standing dead shrubs that have died during the thinning of these stands due to intraspecific competition. During stand development, leaf—area—index increases to 1.6 m2/m2 and available light below the canopy decreases linearly with increasing leaf biomass. Foliar mineral analyses suggest no foliar deficiencies of minerals in stands up to 20 yr old. Many years are required for the development of extensive root systems which are able to decrease the water stress experienced by C. megacarpus during summer drought. Water potential measurements show that moisture is a more limited resource in young (6—yr—old) stands that in older, well—developed stands, though the latter have considerably more leaf area for transpiration. Density—dependent competition for moisture may drive the self—thinning process seen in stands of C. megacarpus during an interval from 5 to 15 yr after fire, but decreasing available light may determine steady—state levels of vertical structure and biomass in this evergreen community.

Fraser, Douglas F.; Sise, Thomas E.

doi: 10.2307/1936749pmid: N/A

The seasonal habitat use and movement patterns of juvenile and adult Semotilus atromaculatus and Rhinichthys atratulus, two stream—dwelling cyprinids, were studied for a period of 2 yr in a small headwater stream. The responses of the fish to the patchy habitat were used to test a prediction of the Fretwell—Lucas Theory of Habitat Distribution, namely that populations tend to become more uniformly distributed over habitat patches as population size increases. Marked individuals executed complex movement patterns which were species, age, and season dependent. Although all age classes of both species were able to move among all habitat patches, adult S. atromaculatus were clumped at high densities, while adults of R. atratulus and juveniles of both species conformed to the prediction of the theory. The clumping behavior observed at low population densities is discussed as a possible adaptation to ambush predation.

Brubaker, Linda B.

doi: 10.2307/1936750pmid: N/A

Regional patterns of ring width anomalies in Washington, Oregon, and Idaho, USA forest are examined using eigenvector (principal component) techniques. The first two eigenvectors, accounting for nearly 50% of the total variance, represent large—scale spatial patterns. Eigenvector I represents a pattern in which growth anomalies are positively correlated among all sites, and Eigenvector II one in which growth anomalies are negatively correlated between sites located on opposite sides of the Cascade Mountain crest. These patterns most likely result from two types of tree growth responses to climate variations that extend uniformly across the study area. Common responses to spring—summer rainfall probably cause the positive, region—wide correlations identified by Eigenvector I, and opposite east—west responses to summer temperature and winter precipitation may account for the negative correlations identified by Eigenvector II. No evidence supports the idea that the tree growth eigenvectors directly reflect two distinctive patterns of climate anomalies. The spatial patterns of these tree growth anomalies have remained essentially constant during the past 300 (possibly 400) yr.