Showing papers in "Ecology in 1988"
TL;DR: An overview of micrometeorological theory and the different micromETeorological techniques available to make flux measurements is provided.
Abstract: Ecologists are expected to play an important role in future studies of the biosphere/atmosphere exchange of materials associated with the major biogeochemical cycles and climate. Most studies of material exchange reported in the ecological literature have relied on chamber techniques. Micrometeorological techniques provide an alternative means of measuring these exchange rates and are expected to be used more often in future ecological studies, since they have many advantages over the chamber techniques. In this article we will provide an overview of micrometeorological theory and the different micrometeorological techniques available to make flux measurements.
1,258 citations
TL;DR: In this paper, an analysis of data collected at 9500 sites throughout the central United States confirmed the overwhelming importance of water availability as a control on production of aboveground primary production.
Abstract: Aboveground net primary production of grasslands is strongly influenced by the amount and distribution of annual precipitation. Analysis of data collected at 9500 sites throughout the central United States confirmed the overwhelming importance of water availability as a control on production. The regional spatial pattern of production reflected the east-west gradient in annual precipitation. Lowest values of aboveground net primary production were observed in the west and highest values in the east. This spatial pattern was shifted eastward during unfavorable years and westward during favorable years. Vari- ability in production among years was maximum in northern New Mexico and southwestern Kansas and decreased towards the north and south. The regional pattern of production was largely accounted for by annual precipitation. Production at the site level was explained by annual precipitation, soil water-holding capacity, and an interaction term. Our results support the inverse texture hypothesis. When precipitation is 370 mm/yr.
1,235 citations
TL;DR: It is argued that generalist natural enemies of herbivorous insects provide a major selection pressure for restricted host plant range and the significance of plant chemistry is discussed in terms of regulating behavior.
Abstract: We argue that generalist natural enemies of herbivorous insects provide a major selection pressure for restricted host plant range. The significance of plant chemistry is discussed in terms of regulating behavior, while the chemical coevolutionary theories are considered to be of limited value.
1,117 citations
TL;DR: It is surmised that endophytes may be as common among plants as are mycorrhyzae, and inducible mutualists grow rapidly and produce toxins against herbivores when damaged host tissues provide new sites for infection.
Abstract: Endophytes are fungi that form inapparent infections within leaves and stems of healthy plants Closely related to virulent pathogens but with limited, if any, pathogenic effects themselves, many endophytes protect host plants from natural enemies Animal herbivores and, in some cases, pathogenic microbes are poisoned by the mycotoxins produced by endophytes "Constitutive mutualism" is the relatively faithful association, usually with grasses, of endophytes that infect host ovules and are propagated in host seed; substantial fungal biomass with probable high metabolic cost develops throughout the aerial parts of the host plant "Inducible mutualist" endophytes are not involved with host seed and disseminate independently through air or in water Infecting only vegetative parts of the host and remaining metabolically inactive for long periods with relatively little fungal biomass, inducible mutualists grow rapidly and produce toxins against herbivores when damaged host tissues provide new sites for infection I surmise that endophytes may be as common among plants as are mycorrhyzae
1,006 citations
TL;DR: It is demonstrated that risk of predation by largemouth bass was 40-80 times as great for small bluegills in the open water as for those in the vegetation, and that the pelagic zone is the more profitable habitat for all size classes of the bluegill.
Abstract: The bluegill sunfish (Lepomis macrochirus) undergoes several habitat shifts between the littoral and pelagic zones of small lakes during its life history. After hatching in the littoral zone, bluegill fry migrate to the pelagic zone to feed on zooplankton. In this study the fry then returned to the littoral zone in four different lakes at a relatively constant size of 12.5 mm standard length. Several years are spent feeding in the littoral zone vegetation before the bluegill again shifts to feeding on zooplankton, first in the water column above littoral vegetation and subsequently in the true pelagic zone. This shift to feeding on zooplankton occurred at a specific body size within a lake, but the size ranged from 50 to 83 mm among five different lakes. The size at which the shift occurred was directly correlated with the density of the major predator of the bluegill in these lakes, the largemouth bass (Micropterus salmoides). We demonstrate that the bluegill is faced with a growth (or feeding) rate-predation risk trade-off in making these habitat shifts. Analyses of stomach contents, the growth of small fish caged in the pelagic zone, and predictions of foraging rates in both habitats all indicate that the pelagic zone is the more profitable habitat for all size classes of the bluegill. Through a series of pond experiments we further show that risk of predation by largemouth bass was 40-80 times as great for small bluegills in the open water as for those in the vegetation. We interpret the patterns of habitat use by the bluegill in terms of a model that explicitly weights the costs (predation) and benefits (growth) of making a size-specific habitat shift to the pelagic. Finally, we discuss evidence that the bluegill can facultatively respond to changes in feeding rates and predation risk, and the consequences of such ontogenetic habitat shifts to community dynamics.
883 citations
TL;DR: Results demonstrate a direct relationship between phenotypic variation generated in the larval stage and adult traits closely associated with an individual's fitness.
Abstract: The relationships among timing of metamorphosis, size at metamorphosis, and traits related to adult fitness were studied for 8 yr in the salamander Ambystoma talpoideum at a temporary pond. Among years, the modal time of metamorphosis and mean body size at metamorphosis were positively correlated with the date the pond dried. In years that the pond dried late, one group of larvae metamorphosed well before the pond dried, whereas the other group metamorphosed just before pond drying. Mean body size of late—metamorphosing individuals was not greater than that of individuals metamorphosing early. Early—metamorphosing males and females were larger at first and second reproduction than were late—metamorphosing individuals. Independent of timing of metamorphosis, larger juveniles at metamorphosis were also larger adults at first reproduction. Age at first reproduction for males was not associated with timing of or size at metamorphosis but large early—metamorphosing females reproduced at a younger age than did small early—metamorphosing females. Neither time of metamorphosis nor size at metamorphosis was associated with survival to first reproduction. These results demonstrate a direct relationship between phenotypic variation generated in the larval stage and adult traits closely associated with an individual's fitness.
871 citations
TL;DR: Experiments demonstrate that plant growth and seed production can be increased by infection of systemic fungal endophytes in grasses, suggesting a defensive mutualism in which the fungi defend their hosts against herbivory, thereby defending their own resources.
Abstract: Many grasses are infected by systemic fungal endophytes (family Clavicipitaceae, Ascomycetes) that produce pysiologically active alkaloids in the tissues of their hosts. Infection makes grasses toxic to domestic mammals and increases resistance to insect herbivores. Some grasses are sterilized by endophyte infection while remaining vegetatively vigorous; other infected grasses remain completely fertile. Experiments demonstrate that plant growth and seed production can be increased by infection. This symbiotic association may be a defensive mutualism in which the fungi defend their hosts against herbivory, thereby defending their own resources. Recent studies suggest that defensive mutualism of endophytes with grasses may be widespread.
800 citations
TL;DR: The results of the study indicate that a steady—state condition in CWD may not be reached for >1000 yr, and that the nature and timing of disturbance play a key role in the dynamics of CWD in the region.
Abstract: Amounts and structural characteristics of coarse woody debris (CWD) were examined in relation to stand age and site moisture condition in 196 Pseudotsuga menziesii stands in western Oregon and Washington. Stands ranged from 40 to 900 yr old, and most if not all, originated after fire. In a chronosequence from the Cascade range, the amount of CWD followed a U—shaped pattern for stands 200 yr old) among site moisture classes. Dry sites averaged 72 Mg/ha moderate sites 137 Mg/ha, and moist sites 174 Mg/ha. The dynamics of CWD were modeled for three fire histories, each beginning with an initial fire in an old—growth stand but differing in number and severity of subsequent fires. All three models exhibited low values of CWD between 80 and 200 yr. The lowest and most prolonged minimum in CWD between 80 and 200 yr. The lowest and most prolonged minimum in CWD during succession occurred when additional fires burned early in succession, which probably happened preceding many stands in the southern Coast Range. The results of the study indicate that a steady—state condition in CWD may not be reached for >1000 yr, and that the nature and timing of disturbance play a key role in the dynamics of CWD in the dynamics of CWD in the region.
625 citations
TL;DR: The magnitude of the response of these two species to small canopy gaps is correlated with their degree of plasticity in patterns of branching and leaf display.
Abstract: Patterns of aboveground growth, branching, and leaf display were examined in saplings of Acer saccharum Marsh. (sugar maple) and Fagus grandifolia Ehrh. (beech) to determine (1) the responses of aboveground growth rates to variation in forest light regimes and (2) the importance of branching and leaf display patterns to the ability of saplings of these two species to respond to changes in forest light regimes produced by canopy gaps. For both species, rates of height growth, lateral growth, and the production of new shoots in even the low gap light levels created by small canopy gaps (15-75 M2) were as much as an order of magnitude greater than growth rates of saplings beneath closed canopies. However, saplings of both species showed little response to further increases in gap light levels. The strong response of maple saplings to low gap light levels was correlated with an increase in sapling leaf area index and the efficiency of leaf display (measured as leaf area per unit length or surface area of branches). In beech, the more modest response to low light levels in small gaps and the higher growth rates than maple beneath a closed canopy were paralleled by a lack of significant increase in beech leaf area indices in small gaps and a higher efficiency of leaf display beneath a closed canopy than in small gaps. Thus, the magnitude of the response of these two species to small canopy gaps is correlated with their degree of plasticity in patterns of branching and leaf display. Both species can be considered small-gap specialists in the sense that their combinations of shade tolerance, growth responses, and canopy architecture make them particularly successful at exploiting small canopy gaps. However, the two species differ in their placement on a gradient in the degree to which woody plants respond to canopy disturbances.
582 citations
TL;DR: Highly variable and unpredictable flow regimes appear to be a high—frequency disturbance that effects fish differently depending on the way they use stream habitat and acts to reduce community complexity.
Abstract: Many regulated streams are characterized by high variable and unpredictable flow regimes. Since changes in streamflow directly modify physical habitat, streams with such highly variable flows provide highly unstable aquatic habitats. We evaluated the effect of artificial streamflow fluctuations on stream fish communities by comparing fish densities, in species and habitat groups, between two rivers differing in daily flow regime: on with with a natural flow, and one with highly regulated flows. We developed a simple model describing the relationship between available streams habitat and its use by 15 species or size classes of fish in the natural river. Species and size classes that used a specific set of microhabitat conditions were identified by comparing habitat characteristics for samples with and without each type of fish; for fish that used a particular type of microhabitat, we grouped species and size classes according to similarity in microhabitat use. Next, we categorized stream habitat samples in both the natural and regulated rivers into groups on the basis of fish habitat use criteria. Fish densities for each fish and habitat group were ten individually compared between the two rivers. An abundant (>90% of all fish) and diverse (nine species) group of small—fish species and size classes were restricted to microhabitat that was characterized as shallow in depth, slow in current velocity, and concentrated along stream margins. This group of fish was reduced in abundance in the regulated river and absent at the study site with the greatest flow fluctuation. Another fish group included species and size classes that used either a broad range of habitat or a microhabitat that was deep, fast, or both, and was concentrated in midstream areas. The density of fish in this group was higher in the regulated river and peaked at the sites with the greatest fluctuations in flow. Highly variable and unpredictable flow regimes appear to be a high—frequency disturbance that effects fish differently depending on the way they use stream habitat and acts to reduce community complexity.
531 citations
TL;DR: In this paper, the authors examined the pattern of self-thinning in a 0.25-ha even, pure stand of jack pine in the boreal forest near Elk Lake, Ontario, Canada.
Abstract: Spatial statistics were used to examine the pattern of self—thinning in a 0.25—ha even—aged (65 yr old), pure stand of jack pine in the boreal forest near Elk Lake, Ontario, Canada. The positions of 459 living and 916 dead trees were recorded and refined nearest neighbor analysis [G(W)] and second—order spatial statistics [L(t)] were used to examine distributional deviations from both the Poisson expectation and the hypothesis of random mortality. The results indicate that the initial (live + dead, n = 1375) distribution was locally random. By contrast, the distribution of live trees was locally highly regular, while the dead trees were significantly more clumped than random mortality would dictate. It is suggested that the development of a strong regular pattern in the survivors is attributable to differential mortality involving two distinct competitive phases: an early scramble phase involving two—sided competition for soil resources, and a lager contest phase involving one—sided competition for light. Analysis of L(t) for the live trees indicated a mean "area of influence" for each individual of an °3.5 m radius, suggesting that trees may compete directly only with their immediate neighbor.
TL;DR: Natural selection rather than phylogeny best explains interspecific variation in antipredator defen- ses and members of at least two orders and four families of amphibians use chemical cues to reduce predation risk from predatory fish.
Abstract: Communities of larval amphibians in fishless habitats often differ strikingly from those in habitats with fish. We surveyed larvae of 15 amphibian species to determine if presence or absence of specific defenses against fish was correlated with breeding habitat. Each species was tested for two key defenses: unpalatability and chemically mediated predator avoidance. In eight of nine cases, larvae of species that often encounter fish had at least one of these defenses. In contrast, larvae of seven species that breed in fishless pools consistently lacked defenses against fish. Lack of appropriate defenses appears to be a primary reason why temporary pool species cannot successfully coexist with predatory fishes in permanent habitats. Palatability and responses to chemical cues from fish often differed among closely related taxa and were correlated strongly with frequency of encounter with fish. Thus, natural selection rather than phylogeny best explains interspecific variation in antipredator defen- ses. Our data show that members of at least two orders and four families of amphibians use chemical cues to reduce predation risk from predatory fish.
TL;DR: Press perturbation experiments, in which individuals of a particular species are continually added to or removed from an ecosystem and the responses of other species' densities are observed, are discussed in the context of whole systems.
Abstract: Press perturbation experiments, in which individuals of a particular species are continually added to or removed from an ecosystem and the responses of other species' densities are observed, are discussed in the context of whole systems. When the sizes of direct interactions are determined to within an order of magnitude, the long—term outcomes of press pertubations are highly indeterminate, in terms both of whether species densities increase or decrease and of which interactions have the largest effects. It is emphasized that short—term observations of systems subject to press perturbations (e.g., toxin additions to ecosystems, or experimental manipulations of species using exclosures) are not useful for estimating long—term impacts. The difference between long— and short—term responses is detailed, and a rule of thumb is given for what constitutes "long—term" in this context.
TL;DR: In this paper, a regression analysis reveals that the northern boundary metabolic rate (NBMR) is 2.45 times the basal metabolic rate, and that the NBMR to BMR ratio shows little interspecific variation.
Abstract: A long-standing hypothesis within ecology states that external environmental factors, such as extreme temperatures, are the primary forces shaping species' biogeographic ranges. On a continentwide scale, the correlative evidence presented in this study strongly indicates that the locations of winter distribution and abundance patterns of several avian species are directly linked with their physiological demands. I calculated the metabolic rate at the northern boundary of the distribution (NBMR) for 14 of 5 1 passerines known to have range boundaries associated with a particular average minimum January temperature isotherm. Based on physiological measures taken from the literature, a regression analysis reveals that the northern boundary metabolic rate (NBMR) is 2.45 times the basal metabolic rate (BMR). The NBMR to BMR ratio shows little interspecific variation; the mean of this ratio is 2.49 with a standard error of ?0.07. Thus, the scalar multiple of BMR ranges from 2.42 to 2.56. This strong association between NBMR and BMR implies that the winter ranges of these 14 birds are restricted to areas where the energy needed to compensate for a colder environment is not greater than 2.5 times the BMR. Body masses of 36 of the remaining 37 birds, those with their range limits associated with isotherms and without physiological measurements, were used to estimate the various physiological parameters. Even when using such crude estimates of these parameters, the approximation of the metabolic rate of individuals at the northern boundaries of their distributions was 2.64 times their BMR. Furthermore, high-density populations of seven of the 14 species with accurately measured physiological parameters were limited to regions where homeostasis requires energy output of no more than 2.13 times their BMR. The range of this scalar extends from 2.08 to 2.34. These findings strongly suggest that on a broad scale the winter ranges of a large number of passerines are limited by energy expen- ditures necessary to compensate for colder ambient temperatures.
TL;DR: Neither gap size, microhabitat within gaps, nor gap age have measurable effects on nutrient loss, nor do they appear to affect plant density, plant estab- lishment, or plant mortality.
Abstract: In a 1 -ha plot of old-growth tierra firme forest near San Carlos de Rio Negro, Venezuela (northwest Amazon Basin), 88 trees - 10 cm dbh out of 786 died over a 10-yr period. Most deaths resulted in the formation of small (5-100 m2) canopy openings (gaps). Occasionally, large gaps are formed in this region when strong winds topple many trees together. In five small (single-treefall) gaps and one large (multiple-treefall) gap we studied changes in soil fertility and nutrient leaching, and also plant establishment, mortality, and growth during the first 4 yr following gap formation. Gaps were divided into four zones, or mi- crohabitats: a trunk zone, an open zone (between bole and forest edge), a crown zone, and a root-pit zone. Sampling was conducted in each microhabitat. Soil nutrient levels in single-treefall gaps did not differ in a predictable fashion in response to microhabitat within gap or gap age. Moreover, except for a small, short-term increase in NO3-N, leaching losses from single-treefall gaps did not differ from forest levels. Advance regeneration plays a dominant role in treefall gap succession at San Carlos. Under closed forest, advance regeneration has a mean annual survivorship of - 80%; height growth is only a few centimetres a year, and leaf retention times frequently exceed 4 yr. Four years after gap formation, advance regeneration accounted for 97% of all trees ?-1 m tall in the single-treefall gaps and 83% of all trees in the multiple-treefall gap. Almost all trees in both gap types were of primary forest species; pioneer trees comprised only a small fraction of the regrowth. In general, microhabitat within gaps did not influence plant density, or plant estab- lishment and mortality patterns. However, mortality was much higher for individuals that germinated after gap formation than for individuals present as advance regeneration. Plant growth within treefall gaps was influenced by gap size, gap microhabitat type, gap age, and plant size. Seedlings and saplings of forest trees in the multiple-treefall gap grew three times as fast as those in the single-treefall gaps. Within single-treefall gaps, height growth was greater in the trunk and open zones than in the crown zone, and trees generally grew more slowly as gaps aged. Furthermore, tree growth was positively correlated with tree size, causing the size differential between trees of different heights to expand as gaps aged. At San Carlos, tree seedlings do not grow to the canopy during a single gap event; rather, canopy closure occurs by growth of larger pole-sized trees that survived treefalls or by lateral expansion of bordering subcanopy trees. Our results indicate that neither gap size, microhabitat within gaps, nor gap age have measurable effects on nutrient loss, nor do they appear to affect plant density, plant estab- lishment, or plant mortality. Size, microhabitat, and temporal affects are minimized, in large part, because of the great importance of advance regeneration in gap succession.
TL;DR: Variation in patch favorableness must prevent populations from reaching a stable spatial distribution and from reach a stable age distribution in any given patch, according to the rank order of quadrat favorability for seedlings.
Abstract: To investigate some of the factors that determine the fate of a seedling, censuses of seedlings of two native grasses, Aristida longiseta and Bouteloua rigidiseta, were made at frequent intervals in a Texas grassland for 2 yr. Seedlings of both species with neighboring seedlings or juvenile plants of either species within 2 cm had higher rates of survival and growth than those without such neighbors, implying that the effects of aggregation in favorable microsites outweighed the effects of competition among these plants. Neighboring adults of these species, and neighboring juveniles of other species, had positive effects in some instances, negative in others. Litter, even in amounts too small to cover the surface, reduced the survival rates of seedlings of both species, while surface rocks may have increased them. Overall, seedlings that germinated earlier in the autumn were more successful. The rank order of quadrat favorableness for seedlings varied in a complex fashion between species, between years, among census intervals, and among germination cohorts. This variation in patch favorableness must prevent these populations from reaching a stable spatial distribution and from reaching a stable age distribution in any given patch.
TL;DR: In this paper, the spatial variability of N mineralization, nitrification, and denitrification at a resolution of 1 m over a 0.5-ha portion of an old field in southeast Michigan was examined.
Abstract: We examined the spatial variability of N mineralization, nitrification, and denitrification at a resolution of 1 m over a 0.5-ha portion of an old field in southeast Michigan. Net mineralization and nitrification rates were estimated from changes in am- monium and nitrate during 45-d laboratory incubations of soil from >300 individual sample locations. Denitrification was estimated from nitrous oxide accumulation rates during 24-h incubations of intact cores (n = 252) under acetylene atmospheres at a pressure of 10 kPa. We used geostatistical procedures to characterize the spatial distributions of these and other soil variates. Semivariograms for all three N transformations showed a high degree of spatial de- pendence among points sampled within 1-40 m of one another. Nugget variances were 27-37% of structural variances, indicating that most of the variation within the sample populations for these rates could be attributed to spatial autocorrelation at a scale > 1 m. Isopleths calculated using punctual kriging algorithms show a nonuniform distribution of these transformations across the field. High rates of all processes occurred in swales on the northern edge of the sample area, but also occurred elsewhere in the field on drier, more level sites. These results indicate that spatial characteristics of the measured nitrogen transfor- mations in this old field are complex, and that only some of this complexity is associated with surface topography. Whether spatial complexity affects or mainly reflects plant com- munity structure is not known, but this small-scale heterogeneity may influence existing plant and microbial population dynamics and should be considered by those attempting to understand community dynamics or to quantify ecosystem-level nutrient fluxes.
TL;DR: It is concluded that two mechanisms operate in the stream subtidal to give rise to the greater fish abundance at marsh surface sites adjacent to depositional areas where (1) the availability of benthic invertebrate prey is greatest and (2) predator pressure is less.
Abstract: Fishes moving onto the surface of a tidal freshwater marsh from an adjacent stream were sampled with flume nets in spring through autumn for 2 yr. Significantly higher numbers were found at sites adjacent to shallow-sloped depositional banks than at sites adjacent to deeper, steeper sloped erosional banks of the stream. Marsh surface features appeared similar, so explanations for this apparent habitat selection were sought in the subtidal portion of the stream. To determine whether the relative abundance of benthic invertebrate prey differed in the two environments, litter bags were placed in the subtidal for 4 wk. Despite higher numbers of invertebrates in erosional zones, both the wet biomass per sample and the mean wet biomass per organism were significantly greater in depositional sites. To test the hypothesis that invertebrate food is more available to fishes in the depositional subtidal, banded killifish (Fundulus diaphanus) were placed in feeding enclosures in both depositional and erosional subtidal habitats in two streams. These fish obtained significantly more food in the shallow depositional subtidal. To test the hypothesis that predation pressure differs with subtidal geomorphology, mummichogs (Fundulus heteroclitus) were tethered on the first half of the rising tide in both depositional and erosional environments in three creeks. Significantly more mum- michogs disappeared from tethers and were presumed taken on erosional banks. We conclude that two mechanisms operate in the stream subtidal to give rise to the greater fish abundance at marsh surface sites adjacent to depositional areas. At low tide, when small fishes are confined to creek channels, they select shallow depositional habitats where (1) the availability of benthic invertebrate prey is greatest and (2) predator pressure is less. As the tide rises and inundates the marsh surface, these small fishes seek shelter on the marsh surface adjacent to their preferred low-tide refuge.
TL;DR: Experimental investigation of colonization of exposed soil in treefall gaps indicated that buried seeds give rise to many more tree seedlings than seeds dispersed into recent gaps, and many of the saplings in gaps started life as epiphytic seedlings in the crowns of the trees that fell.
Abstract: The patterns and processes of canopy tree death and replacement were studied in the elfin forest of Monteverde, Costa Rica. Natural treefalls and limbfalls in a 5.2-ha study area opened 0.8, 1.4, and 1.0% of the area in three consecutive years with about four gaps* ha-' yr-I larger than 4 M2. Forty-one percent of the gaps were formed by uprooted trees, 39% by snapped trees, and the remainder by limbfall, the collapse of epiphyte masses, and dead standing trees killed by lightning. Gaps were found to be spatially aggregated, with more gaps occurring within 17-20 m of one another than expected by chance. Variation among gaps was complex; the first principal component of the variation in eight important gap characteristics among 88 gaps contrasted measures of gap size with the way the gapmaker broke and the position of the gap on the slope, but accounted for only 56% of the total variation. In gaps < 8 mo old, the leaf area index was 1.6, and only 8% of the area was not covered by living plants. Leaf area index increased logarithmically with time since gap formation and with gap area; 50% of the mature-forest leaf area index of 5.1 was recovered in 3 yr in gaps of 10 m2 and in 1.5 yr in gaps of 40 M2. Saplings of both shade-tolerant and shade- intolerant canopy tree species were more abundant in gaps than in the understory of mature forest. Sapling density increased with time since gap formation, but, given the effect of time, shade-tolerant sapling density decreased with gap area, while shade-intolerant sapling density increased. Saplings of eight species were concentrated on nurse logs, while those of one other species were concentrated on the mineral soil disturbed by uprooting trees. Experimental investigation of colonization of exposed soil in treefall gaps indicated that buried seeds give rise to many more tree seedlings than seeds dispersed into recent gaps. Many of the saplings in gaps, however, started life as epiphytic seedlings in the crowns of the trees that fell. The dynamics of this lower montane rain forest resemble many lowland forests in regards to the importance of gap-phase regeneration but differ in the sources of canopy gap colonists and in the importance of different substrates for seedling establish- ment.
TL;DR: Testing a hypothesis of exploitative competition between juvenile sunfishes indicates that the behavioral avoidance of predators by small fish can have significant effects on invertebrate size and abundance in littoral habitats.
Abstract: In small lakes, juvenile sunfishes (Centrarchidae) commonly occupy vege- tated habitats, reducing the risk of predation by piscivorous fish. Eight species of sunfish coexist in Lawrence Lake, Michigan, and the bluegill greatly dominates the fish fauna in terms of numbers and biomass. I hypothesized that the bluegill's use of the vegetation as a predation refuge could have a significant negative effect on the growth rates of other species occupying this habitat. A "target-neighbor" experimental design was used to test this hypothesis. Twelve 3-_M2 cages were arrayed linearly along the vegetated littoral zone of Lawrence Lake in 1986. Two juvenile pumpkinseeds (the "target" species) were placed into each cage, together with either 0, 2, 4, 6, 8, or 10 juvenile bluegills. Bluegill densities were randomly assigned to cages; average bluegill density in Lawrence Lake is equivalent to - 5 fish per cage. Growth in mass over a 50-d period was used as a measure of competitive effect. Benthic invertebrates were also sampled from each cage to determine whether fish density significantly affected invertebrate size and abundance and whether invertebrate availability influenced sunfish growth. Growth of both pumpkinseeds and bluegills declined linearly as a function of final bluegill density in the cages, indicating that the species were competing while occupying the vegetation refuge. Growth was positively related to the density of large invertebrate prey remaining at the end of the experiment. Using an optimal foraging model, I estimated the net energy return available to bluegills in each cage. Predicted net energy gains (in joules per second of foraging time) were also positively related to bluegill growth, indicating that prey availability directly influenced growth rates. Mean invertebrate size decreased signif- icantly as fish density increased, owing to a reduction in the number of large invertebrates; the number of small prey showed no relationship to fish density. These results are consistent with a hypothesis of exploitative competition between juvenile sunfishes and indicate that the behavioral avoidance of predators by small fish can have significant effects on inver- tebrate size and abundance in littoral habitats.
TL;DR: In this article, the responses of decomposition and primary production to nitrogen supply were investigated in a shortgrass prairie, a mountain meadow, and a lodgepole pine forest.
Abstract: The responses of decomposition and primary production to nitrogen supply were investigated in a shortgrass prairie, a mountain meadow, and a lodgepole pine forest. Nitrogen (N) supply was increased by applying ammonium nitrate, or decreased by applying sucrose. The litterbag technique was used to follow decomposition of leaves of the dominant plants: blue grama (Bouteloua gracilis) from the prairie, western wheatgrass (Agropyron smithii) from the meadow, and lodgepole pine (Pinus contorta) from the forest. Soil from beneath the litterbags was sampled at the time of litterbag retrieval in order to detect interactions between decomposition and properties of the underlying soil. There was no consistent effect of soil properties on decomposition rate, but there was a significant effect of litter type on N mineralization in the underlying soil. Decomposition was fastest in the forest, intermediate in the prairie, and slowest in the meadow. Blue grama decomposed faster than the other litters. Each litter type decomposed faster than expected when placed in its ecosystem of origin. This interaction suggests that decomposers in an ecosystem are adapted to the most prevalent types of litter. Nitrogen supply had a small but significant effect on decomposition rate. Within an ecosystem, there was a positive association between decomposition and accumulation of N within the litter, but this relationship was reversed when comparing across ecosystems, possibly because of the overriding effects of differences among ecosystems in abiotic factors. Aboveground net primary production was estimated in the grasslands by a single harvest at the end of the growing season, and growth increment of boles was measured in the forest. These indices of primary production showed a greater relative response to N fertilization than did decomposition, suggesting that primary production is the more N—limited process.
TL;DR: Compared lizard thermal microclimate use and rates of lizard move- ments, feeding, and social displays with estimates of the relative availability of thermal microclimates near 32.20C to test the effects of the thermal environment on lizard activity, S. merriami timed the initiation, peak, and cessation of overall and microclimate-specific activity to coincide with the overall andMicro climate-specific avail- ability of favorable operative temperatures.
Abstract: We investigated the thermal biology of the lizard Sceloporus merriami in a hot, arid locality in west Texas. The effects of the thermal environment on lizard activity were hypothesized to be extensive because S. merriami body temperatures average lower than the body temperatures of any other North American desert iguanid (32.20C). To test this hypothesis, we compared lizard thermal microclimate use and rates of lizard move- ments, feeding, and social displays with estimates of the relative availability of thermal microclimates near 32.20. These comparisons were made throughout the lizards' morning activity period. The thermal environment had a major impact on S. merriami and restricted individual activities (movement rates, feeding strikes, and social displays) to a 2-h period beginning around local sunrise (1000) and to a brief period in late afternoon. In the morning, rates of activities were maximal within 1 h after local sunrise. This activity maximum coincided with the time that the average available environmental temperature approximated the mean body temperature for S. merriami. Also at this time, lizards occupied thermal mi- croclimates randomly with respect to microclimate availability on their home ranges, whereas when activity rates were lower, lizard microclimate use was nonrandom. Thus, when thermally favorable microclimates were most available, lizard activity was least constrained. S. merriami timed the initiation, peak, and cessation of overall and microclimate-specific activity to coincide with the overall and microclimate-specific avail- ability of favorable operative temperatures.
TL;DR: Thermal environments in a large, topographically diverse serpentine soil- based grassland were quantified and ranked using a computer model of clear sky insolation and shading on different slopes to determine the effects of microclimate on the rates of development of each of the life stages of the butterfly Euphydryas editha bayensis.
Abstract: Thermal environments in a large, topographically diverse serpentine soil- based grassland were quantified and ranked using a computer model of clear sky insolation and shading on different slopes to determine the effects of microclimate on the rates of development of each of the life stages of the butterfly Euphydryas editha bayensis. Larvae developed to pupation earlier on warm slopes than on progressively cooler slopes. Avail- ability of sunlight can be limiting for larvae, which bask in direct sun to raise body temperature. Larvae can disperse > 10 m/d, allowing them to transfer between microcli- mates. Pupae on warmer slopes also developed faster than those on cooler slopes. Microclimate also affects the phenology of host plants of larvae and nectar sources of adults. Larval and pupal development and host-plant phenology determine the phase relationship between adult butterfly flight and host-plant senescence, which in turn deter- mines mortality rates of prediapause larvae. Adult females that eclosed early in the season could have their offspring survive on almost all slopes; survivorship of offspring from adults that eclosed in the middle of the flight season was restricted to cooler slopes in the habitat. Some butterflies eclosed too late for their offspring to survive on any slope. Topographic diversity on several scales is a prime indicator of habitat quality for this butterfly. Areas of high local topographic diversity on a scale of tens of metres appear particularly important for long-term population persistence under variable climatic condi- tions.
TL;DR: Variation in the long- term response of communities reflects complex interactions between species life history, disturbance intensity, and chance, suggesting that both deterministic and stochastic factors must be considered in evaluating community stability and response to disturbance.
Abstract: Vegetation changes were studied for 21 yr in two clearcut logged and slash- burned Pseudotsuga forests in the western Cascade Range of Oregon. Detrended corre- spondence analysis (DCA) was used to examine the successional relationships among six understory communities exposed to a gradient of disturbance intensity. Euclidean distances between pre- and postdisturbance samples in ordination space were used to compare community resistance to disturbance and long-term recovery, or resilience. Ordination through time for plant communities revealed a common pattern of rapid floristic change away from predisturbance composition, followed by gradual, unidirectional return. Early, but transient, convergence of successional pathways was common among mesic- and dry- site communities, reflecting the broad distribution of colonizers and the floristic similarity of predisturbance understories. Distinct sequences were observed on moist sites, reflecting more unique residual and colonizing floras. Ordinations also revealed increasing compo- sitional change with disturbance intensity. Successional sequences were dominated by residual species on relatively undisturbed sites and by alternate suites of invading species on moderately disturbed and burned sites. Variation in the response gradient between watersheds reflected the modifying influence of local environment, stand history, and chance in succession. Resistance and resilience varied little among plant communities but were generally lowest for the depauperate Coptis community and greatest for the compositionally and structurally diverse Polystichum and Rhododendron-Gaultheria types. Both measures were strongly influenced by disturbance intensity. The stability of Pseudotsuga understories derives from the moderate tolerance of initial understory dominants to burning and in their ability to subsequently perennate from subterranean structures. Variation in the long- term response of communities reflects complex interactions between species life history, disturbance intensity, and chance, suggesting that both deterministic and stochastic factors must be considered in evaluating community stability and response to disturbance.
TL;DR: In this article, the authors used a statistic called the "time to independence" to make an ecologically meaningful estimate of the amount of time required for an individual to traverse its home range.
Abstract: The area occupied or traversed by an animal is a function of the time period considered, but few empirical estimates of the temporal component of home range use are available. We used a statistic called the "time to independence" to make an ecologically meaningful estimate of the amount of time required for an individual to traverse its home range. Data from 23 species of terrestrial mammals indicated the existence of a size- dependent time scale governing the rate of home range use. Foraging mode influenced the rate of home range use; central place foragers traversed their home ranges approximately five times as rapidly as comparably sized noncentral place foragers. Numerous physiological measures of time are related to body mass raised to the ?4 power. Our results suggest that the time scale governing the rate of space use by mammals is related similarly to body mass. This relationship permits a more critical examination of factors thought to influence home range size, including habitat productivity and social organization.
TL;DR: In this paper, the authors measured the projected leaf area index (LAI) of coniferous forest stands in western Montana using an integrating radiometer, and found that projected LAI was highly correlated with tree transmittance.
Abstract: Canopy transmittance was measured at 1200 and 1400 local solar time using an integrating radiometer on seven coniferous forest stands in western Montana, ranging in projected leaf area index (LAI) from 1.7—5.3 m2/m2. Transmittance of each 1—ha stand was measured at 96,000 points, yet measurement required <1 h because the instrument instantaneously integrates 80 radiometer measurements at once. The Beer—Lambert Law was inverted to estimate LAI using measured transmittance and an extinction coefficient of 0.52. LAI estimated by transmittance was highly correlated with LAI measured by sapwood—based allometric equations at both the 1200 (R2 = 0.97) and 1400 (R2 = 0.94) measurement times. The results suggest that the technique has a wide applicability given the range of LAIs, stand densities (450—4140 trees/ha) and illumination angles (32°—57°) under which it was tested. See full-text article at JSTOR
TL;DR: Elevated water levels, which occur during late fall when seeds are released, influence the distribution of bald cypress and water tupelo seeds and their avail- ability for recruitment, and short-term, deeper floods may also be important for seed transport among bottomland hardwood communities that are spatially separated or differ in species composition.
Abstract: The importance of hydrochory, or seed dispersal by water, to the regeneration of Taxodium distichum (bald cypress) and Nyssa aquatica (water tupelo) was examined in a forested floodplain of the Savannah River in South Carolina. Seedfall and dispersal by water were quantified for 2 yr using floating seed traps. Water depth, surface velocity, and flow direction were monitored over the same period. Seedfall for bald cypress and water tupelo occurred primarily from early fall throughout the winter, when water levels were rising in the swamp. Extended buoyancy periods for newly released seeds and fruits (bald cypress: 42 + 37 d; water tupelo: 85 ? 36 d) prolonged dispersal for both species. Flowing water transported experimentally released seeds long distances, but in a uniform direction, and concentrated them nonrandomly against logs, trees, knees, and other emergent substrates. An examination of the soil seed banks in five microsite types supported the results of the seed transport study. Lowest woody seed densities occurred in the open areas, and highest seed densities occurred in sediments adjacent to emergent substrates such as logs. Additionally, elevated water levels of 1-2 m caused by short-term, high-discharge floods scoured seeds of Nyssa sylvatica var. biflora, Quercus spp., Liquidambar styraciflua, Pinus taeda, and other species from adjacent bot- tomland hardwood communities and transported them into the bald cypress-water tupelo forest. We conclude that elevated water levels, which occur during late fall when seeds are released, influence the distribution of bald cypress and water tupelo seeds and their avail- ability for recruitment. Short-term, deeper floods may also be important for seed transport among bottomland hardwood communities that are spatially separated or differ in species composition.