Showing papers in "Ecological Monographs in 1975"

Experimental Evaluation of Ecological Dominance in a Rocky Intertidal Algal Community

Abstract: The mechanisms by which various species exert influence disproportionate to their abundance or mass on the structure of a lower intertidal algal community were evaluated experimentally. These functional roles were evaluated experimentally. These functional roles were evaluated by various controlled manipulations at seven stations along the Washington coastline ranked according to an exposure/desiccation gradient. The algae were divided into three categories: canopy species, which grow above the other species and apparently succeed in competitively dominating the light resources as demonstrated by algal blooms following their removal; obligate understory species, which die after the canopy species are removed; and fugitive species, which are quick to colonize new space. Ecological dominance was exerted in areas of moderate wave exposure by Hedophyllum sessile, which competitively displaces a large number of fugitive algal species and which furnishes a protected habitat for many obligate understory algae that die or defoliate after the removal of Hedophyllum. Hedophyllum loses this dominance in the most exposed areas, although such sites apparently represent its physiologically optimal habitat, because in these areas it is out—competed by Laminaria setchellii and Lessoniopsis littoralis. In these wave exposed habitats Lessoniopsis was demonstrated to exert a strong competitive dominance over all the other species in the association. The molluscan herbivores were not observed to express any measurable effects on the recruitment or survival of the algae. However, the echinoid Strongylocentrotus purpuratus often overexploits its prey and has a pronounced influence on most of the algal species. In this respect S. purpuratus enjoys an important community role singular among the many herbivores. Similarly, Pycnopodia helianthoides and Anthopleura xanthogrammica are disproportionately important carnivores, because their predation on Strongylocentrotus, clearing large areas of urchins, results in patches in which algal succession follows. The rate of algal succession following removal of the dominant algal species or of Strongylocentrotus is proportional to the degree of wave exposure. The Hedophyllum canopy recovery at the Eagle Point area of San Juan Island, a site exposed to relatively little wave action and thus high levels of desiccation, was relatively slow, with only 10%—26% cover reestablished after 3 yr. In contrast, Hedophyllum canopy developed up to 66% cover in only 1 yr in the exposed area of Waadah Island; it then quickly lost its dominance to Laminaria and Lessoniopsis. Algal succession in deeper Portage Head tidepools was found to be relatively slow with no clear dominance expressed after 5 yr.

A Phytosociological Analysis of Species‐Rich Tropical Forest on Barro Colorado Island, Panama

Abstract: Data on tree species composition and population structure are used as a source of ecological information on the species—rich forest of Barro Colorado Island, Panama Canal Zone (BCI). Thirteen stands, representing both the young and old forest on the Island, were sampled using 10 m ° 20 m quadrats for all individuals °2.5 cm dbh. Over 300 species were encountered and most were identified. The data are evaluated for interpreting late secondary succession (>60 yr), detecting soil—vegetation patterns, and for yielding autecological information. Successional status was estimated by placing species with similar population structure patterns into groups, and then calculating the abundance of species groups that seemed indicative of successional status. Five population structure patterns were recognized. The results suggest that the older BCI forest is not climax after at least 130 yr of succession. Both principal components analysis and a Bray—Curtis type ordination were used to determine whether ecological patterns on BCI are reflected by species composition. Forest age was best reflected by an ordination of the larger tree species. Species found only in the young forest or only in the older forest are identified. Stands with a distinctive gley soil were not segregated on the ordination. Species diversity increases most rapidly during the first 15 yr of succession, but continues to increase slowly after 65 yr. The Shannon—Wiener diversity measure averaged 4.8 (log base 2). Some data suggest that wind—caused canopy gaps are important for the persistence of several species in the older forest, e.g., Cecropia sp.

The Colonization and Formation of Equilibrium Plant Species Associations on Badger Disturbances in a Tall-Grass Prairie

Abstract: Badger disturbances in a tall-grass prairie were used to study colonization patterns and the formation of equilibrium plant species associations in a complex mainland community. Colonization processes were described from field observations over a 4-yr period. A qualitative colonization model was developed to predict noninteractive species equilibria. Predicted colonization rates were based upon relative immigration rates determined by interactions among propagule production rates, dispersal capacities, and the source-site distances of the species involved. The immigration rates both between groups with different life history characteristics (mature prairie, prairie fugitive, and ruderal species) and within groups (K-type, intermediate, and r-type fugitive species) were predicted. Manipulation of important variables enabled different conditions affecting relative immigration rates to be simulated. Species with intermediate life history characteristics (propagule production and dispersal capacity) and located at intermediate distances from the colonization site were predicted to have the highest immigration rates. Thus, prairie fugitive forbs were predicted to have higher rates than either mature prairie or ruderal species. Immigration rates of different fugitive species onto a site were predicted to depend upon the frequency and distribution of previously colonized disturbances in the vicinity of the colonization site. In general, model predictions were consistent with field observations. The model predicted the noninteractive species equilibrium among seedlings of mature prairie, fugitive, and ruderal forbs, but not the interactive species equilibrium among these groups of forbs. Within the fugitive species, both noninteractive and interactive species equilibria were predicted. The noninteractive colonization model was constructed for prediction of colonization patterns on local disturbances in complex plant communities, but since it utilized general life history characteristics to predict immigration rates it should also be applicable to other colonization processes. Equilibrium plant species associations were studied on badger disturbances in virgin prairie and in a less complex tract of overgrazed prairie. Peak standing crop biomass was not different on and off disturbances in either community, but biomass production occurred earlier in the year on disturbances. Although the species present differed, species diversity, equitability, and the distribution of biomass among species were similar on and off disturbances in virgin prairie and on disturbances in overgrazed prairie. The dominant species comprised 20%-26% of the biomass. In contrast, off disturbances in overgrazed prairie were different. The species diversity and equitability were low, and the biomass was concentrated in the dominant species (60% ). Thus, in overgrazed prairie local disturbances depressed dominance and resulted in increased complexity of the plant species association formed. In virgin prairie alternate states of equivalent complexity resulted from local disturbances, and thus spatial heterogeneity was increased. The fugitive species comprise a guild using a common resource (disturbance sites). Direct competition among the species is reduced by spatial and temporal separation of the species. Overlap of species persistent on badger disturbances is reduced because of colonization specificity, site contingencies that determine germination and growth, and suppression of later colonists. The species guild of fugitive plants thus is characterized by diffuse competitive interactions. The reduction of direct competition enhances the formation of persistent species associations on badger disturbances, enhances coexistence, and enables species packing to occur within fugitive species guild.

Effects of Fire on Factors Controlling Plant Growth in Adenostoma Chaparral

Abstract: In the first few years following fire, burned chaparral areas are typified by a flush of seed germination and seedling growth not seen in unburned chaparral The effects of fire on factors of potential importance to this plant response were examined in recently burned and long unburned chaparral, dominated by Adenostoma fasciculatum, in the Santa Ynez Mountains near Santa Barbara, California Measurement of soil moisture, soil texture, and light indicated that changes in these factors following fire had little direct effect on postfire germination and growth Bacteria and fungi were more abundant in burned than in unburned chaparral soil Thus, the hypothesis that microbial depletion of soil oxygen in unburned chaparral inhibits seed germination appears untenable Changes in organic matter content, pH, nitrogen, phosphorus, sulfur, potassium, calcium, and magnesium were examined in the upper soil layers of both burned and unburned chaparral during the first 18 mo following fire Mineral addition as ash—fall was also evaluated Burning increased levels of most mineral elements significantly Furthermore, a large reservoir of readily available organic nutrients was added in the ash Causes of low nutrient levels in unburned chaparral were also investigated Additions of nutrients to unburned chaparral soil in the greenhouse and field resulted in increased growth of species common in burned chaparral areas Bioassays of aqueous A fasciculatum leaf washings were performed against 10 plant species common in burned chaparral areas Significant depression of germination and growth was observed Rain throughfall collected under the shrubs was also inhibitory Efforts to isolate and identify the toxins are described Numbers of small herbivorous mammals were greatly reduced in recently burned chaparral This factor is shown to have a marked effect on seedling survival Applications of various heat treatments to unburned chaparral soil and to fresh seeds of species common to burned chaparral indicated that the seed of several species are released from dormancy by heat Insolational heating of soil in cleared, but unburned, chaparral is sufficient to stimulate germination

Presence and Absence of Habitat Shift in Some Widespread Lizard Species

Abstract: This study analyzes the circumstances under which certain lizards shift and fail to shift their habitats. At each of 20 localities, I measured the structural habitats utilized by all the diurnal arboreal lizard species as well as the availability of those habitats. I selected localities so as to include for four widespread species (Anolis grahami, A. sagrei, A. carolinensis, A. distichus) nearly all of the species—combinations in which they occur. Data were fitted to equations that (1) adjust for locality—specific differences in vegetation, and (2) estimate the direction and intensity of apparent interaction between sympatric forms. Shift was valuated both for species and separately for age and sex classes within species. Female—sized individuals shift more frequently than do adult ♂ ♂. Linear equations that evaluate sympatric forms one at a time showed the strongest apparent competitors for a widespread from to be (1) adult ♂ ♂ rather than female—sized individuals, especially when adult ♂ ♂ represent the widespread species; (2) species of similar climatic habitat; (3) classes of similar size (especially against female—sized individuals of widespread species; and (4) classes of large size (especially against adult ♂ ♂). The most abundant classes are the strongest apparent competitors for A. distichus but not for the other widespread species. These results are unchanged or strengthened when different habitat categories or nonlinear equations are used. Combining all sympatric forms into locality—specific linear equations supports Results 1 and 2 but is inconclusive for Results 3 and 4. The parameter proportional to the size of a refugium from interference is estimated for some cases to be significantly greater for female—size individuals than for adult ♂ ♂. In general, results imply that animals similar to widespread forms in some niche dimension other than structural habitat are those most likely to cause shift in structural habitat. In addition, they suggest existence of a competition function with respect to size: in such a function intensity of competition is uniquely determined by the direction and amount of size difference, regardless of the competitors' absolute sizes. Competition intensity appears to (1) decrease overall with increasing difference in size, (2) be greater for a given size difference if the competitor is larger than if it is smaller, and (3) decrease at nonconstant rates, such that near complete size similarity there is a more rapid decline in intensity for smaller than larger competitors. Morphological differences between populations and short—term field observations suggest that both evolutionary and behavioral mechanisms regulate habitat shift.

Structure and function of successional vascular plant communities in central new york

Abstract: Succession of vascular plant communities was studied on fields in central New York abandoned for the previous 4 to 36 yr. Changes within fields as they aged were con- sistent with the overall pattern of change between fields. Over the period covered, Phleum pratense was replaced successively as the major species by Solidago altissima, Picris hieraci- oides, and Cornus racemosa. Succession was accompanied by increased biological diversity, reduced dominance, decreased energy flow per unit biomass, and increased stability to a pulse perturbation of inorganic nutrients. Although soil phosphorus is an important factor influenc- ing community function on the fields studied, the fertilization experiment indicated a fundamental reorganization of community relationships during succession which reduces community responsiveness to fluctuations in soil nutrients. Increased diversity resulted from the partitioning of constant density and net productivity among increasing numbers of species. Accompanying the increase in diversity was decreasing seasonal synchrony of resource demands among the constituent species pool. Although successional stages could be arbitrarily recog- nized by the taxonomic identification and growth forms of dominant species, there was a gradual and continuous change in structural and functional properties of the plant community, and no objectively definable stages could be delimited. These studies indicate that a failure to determine productivities of individual species may seriously distort the evaluation of com- munity properties.

Feeding Relationships among Species of Notropis (Pisces: Cyprinidae) in a Wisconsin Stream

Abstract: The relationship between spatial preference and diet in four species of the genus Notropis (Pisces: Cyprinidae) was investigated in Roxbury Creek, a small stream in southern Wisconsin. Characteristic spatial distributions for each species were determined from frequency of catch in unbaited minnow traps set at varying heights and locations in the pools of the stream. Data for the year 1966—67 indicate that two species, Notropis atherinoides and N. spilopterus are common in midwater. Notropis dorsalis is most frequently found near the bottom and shows a strong preference for the upstream edge of pools Notropis stramineus is also found most often near the bottom but within the pools themselves. The spatial distributions of aquatic invertebrates, the major prey of Notropis in the pools of Roxbury Creek, were also investigated. Data gathered from drift and bottom samples taken in eight sampling periods during 1970—71 suggest a separation between the relatively sedentary bottom fauna, composed of oligochaetes, isopods, several genera of chironomids, tipulids, and trichopterans; and the drift fauna, composed of copepods, chironomids of the subfamily Orthocladiinae, ephemeropterans, and terrestrial anthropods. Certain genera, including the ubiquitous Gammarus were found commonly in both drift and benthic samples. Indices of similarity between pool benthos and drift ranged between 7% and 59%. Data on food habits of Notropis gathered from stomach content analysis of fishes collected over the same period indicate that 73% of the invertebrate genera present are utilized by Notropis species in Roxbury Creek. Considerable overlap exists in the invertebrate genera chosen by the four species (69%—84%), and strong overall correlations between species are found in the importance of various orders of aquatic invertebrates in the diet. Differences in food habits relate principally to differences in space utilization. Midwater species tend to feed on drifting chironomids, copepods, terrestrials, and other animals found in the water column. Bottom—dwelling fishes feed on benthic genera of chironomids, tipulids, oligochaetes, and other organisms found commonly in pool substrates. It is concluded that spatial rather than taxonomic considerations are the chief determinants of prey selection in these four species, and that this pattern of resource subdivision acts to reduce competition in sympatric, syntopic populations of these fishes. Data from other Wisconsin Streams, from aquarium observations, and from literature review suggest that multispecific assemblages of mutually responsive species like the ones in Roxbury Creek are common in Notropis.

Sublittoral, Benthic Marine Algae of Southern Cape Cod and Adjacent Island: Seasonal Periodicity, Associations, Diversity, and Floristic Composition

Abstract: The periodicity, associations, species diversity, and composition of the sublittoral, attached algal vegetation were studied at four ecologically distinct stations in southern Cape Cod and adjacent islands. In situ collections and observations of 142 species of attached sublittoral algae (Chlorophyta, Phaeophyta, and Rhodophyta) were made with the aid of diving equipment. These were categorized into four groups with respect to seasonal periodicity (seasonal annuals, aseasonal annuals, pseudoperennials, and perennials) based on thallus longevity, plant form during their adverse season, and the period in which each species population was present in the community. Using these criteria we recognize the influence of environmental change on algal development, and the plants' temporal influence on the community. Based on qualitative and quantitative sampling, 10 sublittoral alga associations have been recognized from 2 bottom substrata types. The distribution of sublittoral attached algae at our four stations was more complex than can be accounted for by the three sublittoral algal associations as described in studies of other geographic areas. This may be due to the different levels at which associations have been recognized, to the type and degree of substratum stability, and to whether consideration is given to algal seasonal periodicity. The concept of a universal tripartite zonation of sublittoral algae is unacceptable when these factors are considered in vegetational analyses. An in—habitat, seasonal comparison of species diversity and composition was based on adjacent quantitative collections from a uniform shell bottom at 6 and 12 m. Species diversity remained seasonally stable at these depths; species composition, however, changed dramatically throughout the year. At 6 m, brown algae dominated in winter and were replaced by red algae during summer. At 12 m, the seasonal floristic changes were less pronounced, and red algae dominated year—round. A between—habitat comparison of species numbers and composition was based on year—round qualitative collections and observations made at four regularly visited stations (5—22 m depth). Species numbers decreased with increased station depth from 5 to 22 m except for one heavily silted station. The seasonal change of floristic composition was most striking at West Chop (5—8 m) where short— and long—lived species were about equally represented. Seasonal differences in species composition between summer and winter were striking at a 5—8 m station but were less pronounced with increased station depth where long—lived species made up an increasingly larger percentage of the total vegetation. Floristic composition also differed among stations. About equal numbers of red and brown species occurred at 5—8 m in contrast to the occurrence of about 4 times more red than brown algal species at 22 m. The crust vegetation also made up an increasingly larger percentage of the deep vegetation when compared with the vegetation of shallower areas sampled. Species diversity at 20—22 m was much higher in southern Cape Cod than reported for similar depths to the north (Cape Ann, Massachusetts, New Hampshire; Halifax, Nova Scotia); this may be due in part to the intensive collecting of crustose forms during the present study.

Influence of Effluents from a Zinc Factory on Lichens

Abstract: Lichen species richness and abundance are reduced by approximately 90% in lichen communities near a zinc smelter at Lehigh Water Gap in comparison with the lichen communities of Delaware Water Gap. The principal cause of the impoverished lichen flora in the Lehigh Water Gap area is prob- ably high concentrations of zinc. Of the non-pollution and pollution factors considered, only abnormal soil concentrations of zinc and cadmium extend beyond the limits of the lichen impoverishment zone. Because Zn is present in concentrations generally 100 times higher than that of Cd and because Cd is experimentally shown to be no more toxic to lichens than is Zn, it is probable that Zn is the more important, detrimental factor to lichens in the Palmerton area. Near the smelters, Zn, Cd and sulfur dioxide are all present in sufficiently high concentra- tions to be detrimental to lichen growth and survival. However, at the perimeter of the lichen- impoverished zone, only Zn is present in high enough concentrations to be phytotoxic. Although microclimatic alteration in forested areas near Palmerton may be sufficiently great to be a stress for a few lichen species that are adapted to shaded conditions, no microclimatic differences were demonstrable between Delaware and Lehigh Water Gaps for open habitats where most lichen species normally occur in this area. Variations in such factors as lichen geographical range, climate, substrate composition and abundance, and fire history are probably of negligible importance in explaining the reduction in lichen numbers or abundance in Lehigh Water Gap.

Classification, Description, and Dynamics of Upland Plant Communities within a Minnesota Wilderness Area

Abstract: A bstract. The major upland plant community types of the Boundary Waters Canoe Area (BWCA) of northeastern Minnesota, identified by multivariate analyses (clustering and canonical and discriminant analysis) of 68 stands disturbed by logging and 106 stands undisturbed by logging, include the following: lichen, jack pine-oak, red pine, jack pineblack spruce, jack pine-fir, black spruce-feather moss, maple-oak, aspen-birch, aspenbirch-white pine, maple-aspen-birch, maple-aspen-birch-fir, fir-birch, and white cedar. Each of these types is based on a complex of 53 common species, though the name may incorrectly imply that one or two dominant overstory species are indicative of the type. Other forest stands from the BWCA are quantitatively related to the regional vegetation through discriminant analysis. Succession on the uplands in the area, without disturbance, leads to fir-birch and ultimately to the white-cedar community type. Whitetail deer may have had an impact on restricting the occurrence and reproduction of the white cedar type.

Edaphic Climax Pattern of the Pygmy Forest Region of California

Abstract: The dwarfed conifer forests occurring on the coastal terraces of Mendocino County, California, are oligotrophic communities containing edaphic endemics as dominants. Species distributions and community characteristics are traced along a gradient in podzolization of the soils from these pygmy conifer stands on highly weathered terrace spodosols, to redwood stands on weakly developed slope spodsols. Vegetation stature, canopy closure, species richness, litter biomass and pH generally decrease along the gradient as podzolization and nutrient impoverishment increase, whereas soil organic matter and available water capacity tend to be highest at the gradient extremes. Analyses of pygmy forest soils show low levels of macro— and micro—nutrients, and high levels of exchangeable aluminum. When ordinations of stands based on vegetational data are compared with rankings based on environmental data, changes in pH of the soil A horizon are found to correlate most highly with vegetation changes. Nutrient loss and acidification, initiated by varying rates of soil weathering in the pygmy forest region, may be aggravated, in the case of pygmy forest soils, by a series of feedback effects, including solubilization of possibly toxic amounts of aluminum by low pH, which may contribute to the observed plant stunting and shrinking pools of nutrients in the biomass. A ranking of stands along a soil nutrient gradient, based on field observations and presence data for species, correlated successively less strongly with ordinations computed by similarity—projection, Bray—Curtis method, importance—projection, principal components analysis, and factor analysis. Although these ordinations were expected to produce distortions in the interstand relations due to the modal and even polymodal nature of species response curves and to the presence of species not spanning the range of the data set, additional distortions due to variability in sample data were also highlighted.

The Dynamics of Aquatic Systems: A Comparative Study of the Structure of Four Zooplankton Communities

Abstract: This study compares the structure of the limnetic zooplankton communities of four very different lakes ranging from an oligotrophic Great Lake to a small dystrophic reservoir. Only filter feeding cladocerans and calanoid copepods are considered. Using Levins' Theory of the Niche analysis, I measured habitat overlap values for each species pair to determine to what extent habitat selection can explain the number of limnetic zooplankton species coexisting in these lakes and to what extent other factors, such as resource allocation or differential predation, must be invoked to explain the observed diversity. A substantial portion of the ecological overlap of zooplankton populations can be alleviated by habitat selection. Zooplankton communities are dominated by two to three species, in both numerical abun- dance and grazing intensity. In Lake Michigan (large, oligotrophic), habitat selection was the major mechanism of competitive coexistence and it was achieved by extensive vertical migrations (mean a = .47). In Gull Lake (small, mesotrophic), habitat selection via vertical migration also predominated (mean a = .37) as the major mechanism of competitive coexis- tence. Three species, Diaptomus sp., Daphnia retrocurva, and Daphnia galeata exhibited high habitat overlap coefficients. In Cranberry Lake, a dystrophic reservoir, most species possessed high overlap coefficients (mean a = .62) and minimal migration amplitudes. Diaptomus spp. and Daphnia catawba predominated. In Lake George, a medium-sized oligo- trophic lake, mean habitat overlap was also high (mean a = .65). The three dominant species were Diaptomus sp., Diaphanosoma leuchtenbergianum and Daphina galeata. For the latter two communities, which possessed similar high and mean overlap values, the basic equation was weighted by the standing crop and turnover rate of phytoplankton. The spatial-temporal positioning of the Lake George populations were correlated with algal standing crop and turnover patterns while the Cranberry Lake populations were not.

Dynamics and Energetics of Three Deposit‐Feeding Benthic Invertebrate Populations in Puget Sound, Washington

Abstract: The dynamics and energetics of subpopulations of a numerically dominant deposit—feeding polychaete species, Pectinaria californiensis Hartman, were studied and compared with crude determinations of the same for the larger but rarer coexisting species of the same feeding mode, the heart urchin Brisaster latifrons (Agassiz) and the sea cucumber Molpadia intermedia (Ludwig). Monthly samples, taken for 1 yr at five stations in Puget Sound representing different habitats, were used in conjunction with laboratory measurements of respiration to assess the effects of seasonal and spatial variation in growth, mortality, and respiration on estimates of energy flow through these species populations. Pectinaria larval settlement (2,900—24,000 animals/m2) occurred at all locations in June 1970. Two of three age classes or cohorts were present simultaneously. Pectinaria represented 4%—26% of macrofaunal (> 1 mm) biomass, and 9%—47% of numbers at the five locations, based on the mean of four seasonal estimates. At the t...

Production ecology of grassland plant communities in western north dakota

Abstract: Net primary productivity and distribution of mixed grassland plant communities in western North Dakota were studied. The semiarid continental climate supports several grass- land types, delimited on the bases of topography and substratum. The dominant species and herbage (shoot) production (mean of 2 yr) of stands located on what were classified as silty range sites included, on rolling upland, (1) Stipa viridula (128 g m-2), (2) Stipa comata (128 g m-2), and (3) Bouteloua gracilis (110 g m-2); on ravine slopes, (4) Stipa spartea- forbs (135 g m-2) and (5) Andropogon spp. (126 g m-2). On sandy range sites were found, in shallower ravine bottoms and at bases of slopes, (6) Sporobolus heterolepis-Stipa spartea (160 g m-2) and (7) Sporobolus heterolepis (185 g m-2); on slopes with very sandy soils, (8) A. scoparius-Stipa spartea (132 g m-2), (9) A. scoparius-forbs (107 g m-2), and (10) A. scoparius (119 g m-2). The highest annual herbage production was measured in Stand 7 (200 g m-2) and the lowest in Stand 3 (77 g m-2). Root mass ranged from 6230 g m-2 (Stand 7) to 3030 g m-2 (Stand 9). Annual root production could not be measured but was esti- mated to be equivalent to herbage production. Variation in herbage production between sites and between years was attributed to differences in soil moisture and fertility (especially the latter in sites with sandy soils). Fresh mulch was highest in the Stipa-dominated sites (288- 320 g m-2) and lowest in Andropogon- and Bouteloua-dominated stands (212-234 g m-2). The highest fresh mulch decay constant and shortest equilibrium time were determined to occur in sites dominated by Sporobolus heterolepis, and the converse for Stipa-dominated sites. Chlorophyll values ranged from 78 mg m-2 (Stand 3) to 542 mg m-2 (Stand 7); variation depended on plant morphology and phenology as well as biomass. Maximum efficiency in herbage production occurred later in the year for stands with a large warm-season component (Sporobolus heterolepis and Andropogon spp.) than in sites dominated by cool-season grasses (Stipa spp.). On an annual basis, Stand 7 was most efficient (0.12%) and Stand 3 least (0.06%). The interrelationships between drought tolerance, phenology (cool- vs. warm-season development), physiology, and production are discussed.

A Population of Great Basin Pocket Mice, Perognathus parvus, in the Shrub‐Steppe of South‐Central Washington

Abstract: A population of Great Basin pocket mice, Perognathus parvus, was live-trapped in 1967-72 on a 2.7-ha study area in south-central Washington to determine their density, reproductive performance, longevity, movements, and home range, as well as their functional role in northern shrub-steppe habitat. During 41,310 trap-nights we captured 15,386 mice representing seven species: Perognathus parvus, Peromyscus maniculatus, Onychomys leuco- gaster, Reithrodontomys megalotis, Lagurus curtatus, Microtus montanus, and Sylvilagus nut- tallii. Pocket mice accounted for 91%, deer mice 8%, and the remaining species only 1% of the captures. Most pocket mice remained inactive underground between December and February. Adult e became trappable at an average date of 26 March; 9 9 emerged about 18 April. Once active, adults remained trappable for an average of 60 days in years of average-to-excellent food production, and for 90 days in years of poor production. Sex ratios of trappable animals differed significantly from 1:1 in about half the sampling periods, and usually favored adult . Males had scrotal testes for an average of 4 mo, with the highest proportion in April. The first signs of estrus were observed in April, the first palpable pregnancies in May, and the last pregnant 9 9 usually in July. In years of high food availability adult 9 9 averaged 2 litters; in average years they had 1.1 litters; and in poor years, only one 9 in three became pregnant. The average size of three litters found in live-traps was 3.7. In 1969, a year of excellent food production, juveniles from early litters bred during late summer. Subadults were first trapped in June for an average of 40-70 days. Survival from weaning to the following breeding season ranged between 56%-58% (1967, 1970) and 80% (1968); 17%-19% survived to the 3rd yr; and 2%-3% survived to the 4th yr. The highest overwintering success was in juveniles born during the year of lowest precipitation, poorest food supply, and reduced reproduction. Differential survival affected the age distribu- tion of animals during the breeding season. One-year-olds dominated the breeding populations in 1968 and 1970; but in 1969 only 55% of the breeders were born the preceding year. The age distribution had little apparent effect on the reproductive success of the population. Perognathus parvus ceased aboveground activities between late summer and autumn de- pending upon environmental conditions, sex, and age. Population size was estimated by means of the Jolly-Seber stochastic model which helped interpret population responses when components of the population were not trappable. The peak autumnal population varied about a mean of 320 animals with only a twofold variation between the estimates for the lowest (206, 1968) and the highest (437, 1969) years. Within years the greatest variation was a fivefold increase in numbers observed between the early breeding season and fall population peak of 1969. The estimated summer population size showed a high degree of correlation with the amount of precipitation falling between October and April, precipitation that largely stimulated growth and reproduction in vegetation, which in turn affected the small-mammal population. Adult 8 8 had larger home ranges (1,560-4,005 m2) than 9 9 (508-2,301 m2); adults had larger home ranges than subadults. Home ranges were largest in years of average-to-excellent food production and moderate population densities; they were smallest either in years of poor resources and moderate population densities or in years of average resources but high densities.

Energy and carbon balances in cheatgrass: an essay in autecology

Abstract: Biotic and abiotic pathways of incident energy are rarely considered together in comparable detail, so little is known about whether energy or carbon relations remain constant in contrasting energy environments. An experiment to determine the fates of energy and carbon in cheatgrass (Bromus tectorum L.) was carried out on steep (40/sup 0/) north- and south-facing slopes on a small earth mound, using many small lysimeters to emulate swards of cheatgrass. Meteorological conditions and energy fluxes that were measured included air and soil temperatures, relative humidity, wind speed, incoming and reflected shortwave radiation, net all-wave radiation, heat flux to the soil, and evaporation and transpiration separately. The fate of photosynthetically fixed carbon during spring growth (31 March to 31 May) was determined by separation of the plant tissues into mineral nutrients, ash, crude protein, crude fat, crude fiber, and nitrogen-free extract (NFE) (the proximate analysis scheme routinely used for feed analysis) for roots, shoots, and seeds separately. Root and seed production were both about 30 percent greater on the north exposure than on the south, but shoot production was not different. Heats of combustion (caloric content) differed between roots, shoots, and seeds, but not between exposures. Total production and total energymore » fixation were not statistically different on the two exposures, because the greater biomass and variability of the shoots overwhelmed the statistically significant differences in roots and seeds. Incident shortwave radiation was very much greater on the south exposure than on the north, so the south exposure was much less efficient in converting solar energy into chemical energy stored in plant tissues.« less