Showing papers in "Ecology in 1996"

Generalization in pollination systems, and why it matters

Abstract: One view of pollination systems is that they tend toward specialization. This view is implicit in many discussions of angiosperm evolution and plant-pollinator coevolution and in the long-standing concept of pollination syndromes. But actual pollination systems often are more generalized and dynamic than these traditions might suggest. To illustrate the range of specialization and generalization in pollinators' use of plants and vice versa, we draw on studies of two floras in the United States, and of members of several plant families and solitary bee genera. We also summarize a recent study of one local flora which suggests that, although the colors of flowers are aggregated in phenotype space, there is no strong association with pollinator types as pollination syndromes would predict. That moderate to substantial generalization often occurs is not surprising on theoretical

An Evaluation of the Accuracy of Kernel Density Estimators for Home Range Analysis

Abstract: Kernel density estimators are becoming more widely used, particularly as home range estimators. Despite extensive interest in their theoretical properties, little empirical research has been done to investigate their performance as home range estimators. We used computer simulations to compare the area and shape of kernel density estimates to the true area and shape of multimodal two—dimensional distributions. The fixed kernel gave area estimates with very little bias when least squares cross validation was used to select the smoothing parameter. The cross—validated fixed kernel also gave surface estimates with the lowest error. The adaptive kernel overestimated the area of the distribution and had higher error associated with its surface estimate. See full-text article at JSTOR

The Varying Success of Invaders

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Dispersal data and the spread of invading organisms.

Abstract: Models that describe the spread of invading organisms often assume that the dispersal distances of propagules are normally distributed. In contrast, measured dispersal curves are typically leptokurtic, not normal. In this paper, we consider a class of models, integrodifference equations, that directly incorporate detailed dispersal data as well as population growth dynamics. We provide explicit formulas for the speed of invasion for compensatory growth and for different choices of the propagule redistribution kernel and apply these formulas to the spread of D. pseudoobscura. We observe that: (1) the speed of invasion of a spreading population is extremely sensitive to the precise shape of the redistribution kernel and, in particular, to the tail of the distribution; (2) fat-tailed kernels can generate accelerating invasions rather than constant-speed travelling waves; (3) normal redistribution kernels (and by inference, many reaction-diffusion models) may grossly underestimate rates of spread of invading populations in comparison with models that incorporate more realistic leptokurtic distributions; and (4) the relative superiority of different redistribution kernels depends, in general, on the precise magnitude of the net reproductive rate. The addition of an Allee effect to an integrodifference equation may decrease the overall rate of spread. An Allee effect may also introduce a critical range; the population must surpass this spatial thresh-old in order to invade successfully. Fat-tailed kernels and Allee effects provide alternative explanations for the accelerating rates of spread observed for many invasions.

An Experimental Study of Plant Community Invasibility

Abstract: A long—term field experiment in limestone grassland near Buxton (North Derbyshire, United Kingdom) was designed to identify plant attributes and vegetation characteristics conducive to successful invasion. Plots containing crossed, continuous gradients of fertilizer addition and disturbance intensity were subjected to a single—seed inoculum comprising a wide range of plant functional types and 54 species not originally present at the site. Several disturbance treatments were applied; these included the creation of gaps of contrasting size and the mowing of the vegetation to different heights and at different times of the year. This paper analyzes the factors controlling the initial phase of the resulting invasions within the plots subject to gap creation. The susceptibility of the indigenous community to invasion was strongly related to the availability of bare ground created, but greatest success occurred where disturbance coincided with eutrophication. Disturbance damage to the indigenous dominants (particularly Festuca ovina) was an important determinant of seedling establishment by the sown invaders. Large seed size was identified as an important characteristic allowing certain species to establish relatively evenly across the productivity—disturbance matrix; smaller—seeded species were more dependent on disturbance for establishment. Successful and unsuccessful invaders were also distinguished to some extent by differences in germination requirements and present geographical distribution.

Nutrients in Senesced Leaves: Keys to the Search for Potential Resorption and Resorption Proficiency

Abstract: Analyses of nitrogen and phosphorus in the senesced leaves of 89 species of deciduous and evergreen woody perennials were used (1) to discover the limits of ultimate potential resorption (maximal withdrawal of nutrients from senescing leaves), (2) to determine a means by which resorption can be categorized as complete or incomplete, (3) to develop the concept of resorption proficiency (measured as the levels to which nutrients have been reduced in senesced leaves), (4) to compare resorption in evergreen vs. deciduous species, (5) to assess the impact of phylogeny on resorption, (6) to compare resorption in actinorhizal vs. non—nitrogen—fixing species, and (7) to consider the efficacy of using multiple measures of resorption to answer questions regarding the function and evolution of this process, rather than relying solely on analyses of resorption efficiency (percentage reduction of nutrients between green and senesced leaves). Concentrations of 0.3% nitrogen and 0.01% phosphorus in senesced leaves represent ultimate potential resorption of these nutrients in woody perennials. Resorption proficiency and potential resorption were quantitatively defined in two models that describe both resorption that is maximal and biochemically complete, and that which is not. Resorption is highly proficient in plants that have reduced nitrogen and phosphorus in their senescing leaves to concentrations below 0.7% and 0.05%, respectively. An important feature of knowing the levels to which nutrients can be reduced in senescing leaves is that these values offer an objective gauge by which to measure the success of resorption as a nutrient conservation mechanism. Evergreens were significantly more proficient at resorbing phosphorus than were deciduous species (0.045% vs. 0.067% P in senesced leaves, respectively) and plants capable of symbiotic nitrogen fixation were significantly less proficient at resorbing nitrogen than were nonfixers (1.6% vs. 0.9% N in senesced leaves, respectively). Resorption proficiency appeared to parallel some phylogenic trends, yet the influence of phylogeny was not so significant as to overwhelm the effects of recent selection. The ability of plants to reduce nitrogen in senescing leaves was significantly correlated with their ability to reduce phosphorus. Measurement and analysis of resorption proficiency, when coupled with concurrent consideration of potential resorption and resorption efficiency, should facilitate and expedite the ongoing attempt to resolve complex questions regarding the environmental constraints that influence resorption, and the selection pressures that have directed the evolution of this process.

Biological Invasions and Cryptogenic Species

Abstract: Ecologists and biogeographers routinely categorize species as "native" with a confidence that for a surprising number of taxa belies the quality of the supporting data. In classical views of the origins of community assemblages, species are either native (that is, indigenous or endemic taxa, including prehistorical invasions) or exotic (that is, historical invasions, including both natural range expansions and human-mediated introductions). The default in this view is to classify species without any obvious record of introduction as "native." Given, however, the historical prevalence of the global movement of plants and animals by humans, combined with a lack of data that would either support or dismiss such movements for many individual species, it is clear that in communities that have been under the sustained influence of human endeavor there are many species that cannot reliably be assigned to either category. The members of this latter group are called "cryptogenic species," a term I introduced in 1982 in abstract form and that has been occasionally but informally used in the literature since that time (e.g., Kinzie 1984). I here define a cryptogenic species (crypt-, Greek, kryptos, secret; -genic, New Latin, genic, origin) as a species that is not demonstrably native or introduced. The word "cryptogenic" (= cryptogenetic) also appears rarely in older medical literature, and was originally, curiously enough, "applied to those cases in which the port of entry of the germs is no longer visible (for example, a hypodermic puncture), or cannot be positively determined" (Oxford English Dictionary 1972). In this note I make two points: (1) cryptogenic species may be remarkably common, and (2) the existence of cryptogenic species has important consequences for understanding biological invasions.

Managing Tropical Rice Pests Through Conservation of Generalist Natural Enemies and Alternative Prey

Abstract: The cultivation of tropical Asian rice, which may have originated 9000 yr ago, represents an agricultural ecosystem of unrivaled ecological complexity. We undertook a study of the community ecology of irrigated tropical rice fields on Java, Indonesia, as a supporting study for the Indonesian National Integrated Pest Management Programme, whose purpose is to train farmers to be better agronomists and to employ the principles of integrated pest management (IPM). Two of our study objectives, reported on here, were (1) to explore whether there exist general and consistent patterns of arthropod community dynamics related to natural or intrinsic levels of biological control, and (2) to understand how the exist ing levels of biological control are affected by insecticide use, as well as by large -scale habitat factors relating to differing patterns for vegetational landscapes, planting times, and the length of dry fallow periods. We performed a series of observat ional studies and two experimental studies. Abundant and well-distributed populations of generalist predators can be found in most early -season tropical rice fields. We took samples from plants and water surface using a vacuum -suction device, and from the subsurface using a dip net. Our results show that high populations of generalist predators are likely to be supported, in the early season, by feeding on abundant populations of detritus feeding and plankton -feeding insects, whose populations consistently peak and decline in the first third of the season. We hypothesize that since this abundance of alternative prey gives the predator populations a "head start" on later -developing pest populations, this process should strongly suppress pest populations and g enerally lend stability to rice ecosystems by decoupling predator populations from a strict dependence on herbivore populations. We experimentall y tested our hypothesis of trophic linkages among organic matter, detritivores and plankton feeders, and generalist predators and showed that by increasing organic matter in test plots we could boost populations of detritivores and plankton -feeders, and in turn significantly boost the abundance of generalist predators. These results hold for populations found on t he plant, on the water surface, and below the water surface. We also demonstrated the link between early season natural enemy populations and later-season pest populations by experimentally reducing early -season predator populations with insecticide applications, causing pest populations to resurge later in the season. Overall, these results demonstrate the existence of a mechanism in tropical irrigated rice systems that supports high levels of natural biological control. This mechanism depends on season l ong successional processes and interactions among a wide array of species, many of which have hitherto been ignored as important elements in a rice ecosystem. Our results support a management strategy that promotes the conservation of existing natural biol ogical control through a major reduction in insecticide use, and the corresponding increase in habitat heterogeneity.

How Risky is Biological Control

Abstract: The potential harmful effects of non—indigenous species introduced for biological control remain an important unanswered question, which we addressed by undertaking a literature review. There are few documented instances of damage to non—target organisms or the environment from non—indigenous species released for biological pest control, relative to the number of such releases. However, this fact is not evidence that biological control is safe, because monitoring of non—target species is minimal, particularly in sites and habitats far from the point of release. In fact, the discovery of such impacts usually rests on a remarkable concatenation of events. In addition to trophic and competitive interactions between an individual introduced species and a native one, many effects of introduced species on ecosystems are possible, as are numerous types of indirect interactions. Predicting such impacts is no mean feat, and the difficulty is exacerbated by the fact that introduced species can disperse and evolve. Current regulation of introduced biological—control agents, particularly of entomophages, is insufficient. At the very least, strong consideration should be given to the likely impact of both the pest and its natural enemy on natural ecosystems and their species, and not only on potential costs to agriculture, silvi—culture, and species of immediate commercial value.

Using Landscape Indices to Predict Habitat Connectivity

Abstract: The ecological consequences of habitat fragmentation include the direct effects of habitat loss and the indirect effects of reduced inter-patch dispersal. In particular, habitat patches that survive the process of fragmentation become increasingly isolated from one another, and this can cause species' declines in excess of predictions based strictly on reductions in habitat area. To quantify the hindrance of dispersal caused by habitat frag- mentation, landscape ecologists have invented the notion of habitat connectivity. Indices of landscape pattern are frequently used to estimate habitat connectivity, but whether they actually do so remains undocumented. If indices of habitat pattern do indeed estimate habitat connectivity, then these indices should correlate well with predictions of dispersal success. To test this possibility, I looked for correlations between nine common indices of habitat pattern and the results of a simulated dispersal process conducted using GIS data on the distribution of old-growth forest through the Pacific Northwest. The nine indices of habitat pattern that I examined were only weakly correlated with the results from the dispersal modeling, but I identified a new pattern index, termed patch cohesion, for which the fit was much better. Moreover, I found patch cohesion to be insensitive to the details and artifacts of the dispersal model. The methodology described here will be useful to inves- tigators using indices of landscape pattern to quantify habitat fragmentation.

Population Size, Pollinator Limitation, and Seed Set in the Self‐ Incompatible Herb Lythrum Salicaria

Abstract: In reward-producing animal-pollinated plants, small populations are likely to be less attractive to pollinators than large populations. The likelihood of pollinator limitation of seed production should therefore increase with decreasing population size. I documented the number of flowering plants and plant fecundity in 18 populations of the self-incompatible, tristylous herb Lythrum salicaria within an archipelago in northern Swe- den in two consecutive years. To test the hypothesis that a positive correlation between population size and seed set is due to a higher degree of pollinator limitation in small than in large populations, I performed supplemental hand-pollinations in eight (1993) and 13 (1994) of the study populations. To test the hypothesis that common mating types are more likely than rare types to experience inadequate pollination, I compared the natural level of seed production and the effect of supplemental pollination in different style morphs in the five populations in which -10 plants per morph were included in the experiment. There was no significant correlation between population size and plant size in terms of number of floral shoots or number of flower-producing leaf nodes per shoot. However, there was a positive relationship between population size and seed production per flower and between population size and total seed number per plant. In contrast, there was no significant correlation between population size and seed production of flowers that had received sup- plemental pollination. In both years, the difference in mean seed production per flower between hand-pollinated flowers and controls decreased with increasing population size. In two of five populations, the effect of supplemental pollination differed significantly among morphs. Seed production was more likely to be pollinator limited in long-styled than in short-styled plants, but this difference could not be attributed to a preponderance of the long-styled morph in the studied populations. Results of the supplemental hand- pollinations indicate that the positive correlation between population size and seed pro- duction is a function of insufficient pollen transfer in small populations. Additional de- mographic studies are needed to determine to what extent the reduced level of seed pro- duction in small populations limits the growth of young populations, and to what extent it may threaten the local persistence of L. salicaria.

Altered Behavior of Parasitized Killifish Increases Susceptibility to Predation by Bird Final Hosts

Abstract: Parasites that are transmitted from prey to predator are often associated with altered prey behavior. Although many concur that behavior modification is a parasite strategy that facilitates transmission by making parasitized prey easier for predators to capture, there is little evidence from field experiments. We observed that conspicuous behaviors exhibited by killifish (Fundulus parvipinnis) were associated with parasitism by larval trematodes. A field experiment indicated that parasitized fish were substantially more susceptible to predation by final host birds. These results support the behavior-modification hypothesis and emphasize the importance of parasites for predator-prey interactions.

The Effect of Landscape Heterogeneity on the Probability of Patch Colonization

Abstract: The effect of landscape heterogeneity on the dispersal of organisms between habitat islands is poorly understood. Preferred pathways for dispersal (i.e., corridors), as well as dispersal barriers, are difficult to identify when the landscape matrix is composed of a complex mixture of land cover types. We developed an individual-based dispersal model to measure immigration and emigration rates between habitat islands within hetero- geneous landscapes. Dispersing individuals of a model organism were simulated as self- avoiding random walkers (SAW) traversing a digital land cover map, with each habitat type assigned a priori a probability that the SAW would enter that habitat type. Each individual began the dispersal process on a random site at the edge of a deciduous forest patch and was allowed to move until it reached a different deciduous forest patch. Visu- alization of the movement patterns across the landscape was achieved by tabulating the frequency of visitation Qf successful dispersers to each grid cell on the map. The model was used to estimate the probabilities of disperser transfer between patches by varying the a priori probabilities of movement into each habitat type in order to: (1) estimate the effect of changing landscape heterogeneity on the transfer probabilities, and (2) visualize dispersal corridors and barriers as perceived by model organisms operating by specific movement rules and at specific scales. The results show that 89% of the variability in dispersal success can be accounted for by differences in the size and isolation of forest patches, with closer and larger patches having significantly greater exchange of dispersing organisms. However, changes in the heterogeneity of the landscape matrix could significantly enhance or decrease emigration success from an individual patch, depending on the landscape. Changes in emigration success from an individual patch resulting from changes in matrix heterogeneity were not predictable, and transfer rates between patches were not symmetrical due to differences in the proximity of neighboring patches, and differences in the funneling at- tributes of certain landscape patterns. Visualizations showed that corridors are often diffuse and difficult to identify from structural features of the landscape. A wide range of organisms with differing movement capabilities can be simulated using the approach presented to increase our understanding of how landscape structure affects organism dispersal.

Facilitation between Higher Plant Species in a Semiarid Environment

Abstract: A major role of facilitation between higher plant species, particularly in stressful environments, has recently been reported in several papers that suggest that ben- eficial interactions could be more important in the dynamics of plant communities than has been recognized to date. In a semiarid region in southeastern Spain we determined the effect of a leguminous shrub, Retama sphaerocarpa, on its environment, testing the hy- pothesis that facilitation by the shrub of one species of its understory, Marrubium vulgare, is reciprocal and that the shrub benefits from sheltering herbs beneath its canopy. Marrubium plants under Retama had greater specific leaf area, leaf mass, shoot mass, leaf area, more flowers, a higher nitrogen (N) concentration in leaf tissue, and more N per plant than isolated plants, suggesting a facilitative effect of Retama on Marrubium including increased availability of resources. Biomass of 1-yr-old cladodes, total biomass, total ni- trogen content of 3-yr-old branches, and shoot water potential at midday were higher for Retama shrubs with Marrubium beneath them than in shrubs without Marrubium. Retama strongly improved its own environment, facilitated the growth of Marrubium and other species underneath its canopy, and at the same time obtained benefits from sheltering herbs underneath. The interaction between these two species was indirect, as- sociated with differences in soil properties and with improved nutrient availability under shrubs compared with plants growing on their own. We propose that the mutual benefit of the association between Retama and Marrubium is best termed a facultative mutualism. Each partner benefits from greater availability of resources in the "island of fertility" that results from their association. If beneficial plant- plant interactions as described here are widespread, positive relationships may have a major role in determining the pattern and structure of plant communities.

Analysis of Frequency Count Data Using the Negative Binomial Distribution

Abstract: The statistical distributions of the counts of organisms are generally skewed, and hence not normally distributed, nor are variances constant across treatments. We present a likelihood—ratio testing framework based on the negative binomial distribution that tests for the goodness of fit of this distribution to the observed counts, and then tests for differences in the mean and/or aggregation of the counts among treatments. Inferences about differences in means among treatments as well as the dispersion of the counts are possible. Simulations demonstrated that the statistical power of ANOVA is about the same as the likelihood—ratio testing procedure for testing equality of means, but our proposed testing procedure also provides information on dispersion. Type I error rates of Poisson regression exceeded the expected 5%, even when corrected for overdispersion. Count data on Orange—crowned Warblers (Vermivora celata) are used to demonstrate the procedure.

Population dynamics : new approaches and synthesis

Abstract: Introduction. N. Cappuccino, Novel Approaches to the Study of Population Dynamics. Observation and Comparative Approaches: P. Turchin, Population Regulation: Old Arguments and a New Synthesis. A.F. Hunter, Ecology, Life History and Phylogeny of Outbreak and Nonoutbreak Species. N. Cappuccino, H. Damman, and J.-F. Dubuc, Spatial Behavior and Temporal Dynamics of Outbreak and Nonoutbreak Species. M.J. Auerbach, E.F. Connor, and S. Mopper, Minor Miners and Major Miners: Population Dynamics of Leaf-Mining Insects. Mechanisms and Processes of Population Dynamics: R.F. Denno and M.A. Peterson, Density-Dependent Dispersal and its Consequences for Population Dynamics. S. Harrison and N. Cappuccino, Using Density-Manipulation Experiments to Study Population Regulation. I. Hanski and M. Kuussaari, Butterfly Metapopulation Dynamics. S.J. Walde, Internal Dynamics and Metapopulations: Experimental Tests with Predator-Prey Systems. J. Roland and P.D. Taylor, Herbivore-Natural Enemy Interactions in Fragmented and Continuous Forests. G. Dwyer, Simple Models and Complex Interactions. J.H. Myers and L. Rothman, Field Experiments to Study Regulation of Fluctuating Populations. M.C. Rossiter, Impact of Life History Evolution on Population Dynamics: Predicting the Presence of Maternal Effects. Case Studies: C. Solbreck, Long-Term Population Dynamics of a Seed-Feeding Insect in a Landscape Perspective. T. Ohgushi, Adaptive Behavior Produces Stability in Herbivorous Lady Beetle Populations. P.W. Price, T.P. Craig, and H. Roininen, Working Toward Theory on Galling Sawfly Population Dynamics. J.D. Reeve, M.P. Ayres, and P.L. Lorio, Host Suitability, Predation, and Bark Beetle Population Dynamics. G.E. Belovsky and A. Joern, The Dominance of Different Regulating Factors for Rangeland Grasshoppers. Conclusion: P.W. Price and M.D. Hunter, Novelty and Synthesis in the Development of Population Dynamics. Subject Index.

Are shade tolerance, survival, and growth linked? Low light and nitrogen effects on hardwood seedlings

Abstract: Variation in shade tolerance is a primary mechanism driving succession in northern deciduous forests. However, little is known about interspecific differences in the traits responsible for shade tolerance. Is shade tolerance due to the ability to grow or survive in deep shade, or both? How do plant morphology and photosynthesis relate to growth in shade? Is low light the sole critical stress determining differences in "shade tolerance" or do belowground resources interact with low light to affect growth and survival? In this study we address these questions for seedlings of Betula papyrifera Marsh., Betula al- leghaniensis Britton, Ostrya virginiana (Mill.) K. Koch, Acer saccharum Marsh., and Quer- cus rubra L. grown for 2 yr in outdoor shade houses in a complete factorial of low light (2 and 8% open sky) and nitrogen (forest soil and forest soil plus 200 kg N.ha-'.yr-'). For these seedlings we examined effects of light and nitrogen on the interrelationships among survival, growth, and shade tolerance and explored the physiological bases of shade tol- erance by examining the relationship of plant morphology and photosynthesis to growth. Nitrogen amendments did not have a significant effect on any plant trait at either light level. In 8% light, growth and survival were highest for shade-intolerant Betula papyrifera and mid-tolerant Betula alleghaniensis, lower for shade-tolerant Ostrya and Acer, and lowest for disturbance-adapted Quercus. In 2% light, species rankings reversed as Ostrya and Acer had higher growth and survival than the other species. Second-year survival was strongly related to 1st-yr growth (P < 0.001), whereas relationships with 1st-yr plant mass and 1st- yr absolute growth rates were weak. Therefore, survival of shade-tolerant species at 2% light was related to their maintenance of positive growth, whereas intolerant species had growth near zero and high rates of mortality. In both 2 and 8% light photosynthetic rates on mass (but not area) bases and the proportion of the plant in leaves (leaf area ratio and leaf mass ratio) were positively related to growth. Greater rates of growth and survival for shade-tolerant species in very low light, and for intolerant species in higher light, suggest that there is a species-based trade-off between maximizing growth in high light and min- imizing the light compensation point for growth. This trade-off may be an important mech- anism driving forest community dynamics in northern hardwood forests.

Untangling Multiple Factors in Spatial Distributions: Lilies, Gophers, and Rocks

Abstract: Despite broad consensus on the power of experiments, correlational studies are still important in ecology, and may become more so as spatial studies proliferate. Conventional correlation analysis, however, (1) fundamentally conflicts with the basic ecological concept of limiting factors, and (2) ignores spatial structure in data, which can produce spuriously high correlations. Especially for field data, bivariate scattergrams often show factor—ceiling" distributions wherein data points are widely scattered beneath an upper limit, due to the action of other factors. Although most ecological information in such a graph resides in the upper limit, standard correlation/regression does not characterize such limits. If other factors have been measured, path analysis may be useful, but otherwise, direct description of ecological ceilings is desirable. Objective methods for doing so are barely known to ecologists; we review recent proposals for statistical testing and data display. For correcting correlations for spatial patchiness of the variables, another new technique has been proposed by Clifford, Richardson, and Hemon: by reducing the effective sample size to account for the autocorrelation it allows significance tests. We discuss these issues with reference to counts of glacier lily (Erythronium grandiflorum) seedlings, vegetative plants, and flowering plants in a square grid of 256 contiguous $2 \times 2$ m quadrats in subalpine meadow in western Colorado. We also measured soil moisture, pocket gopher activity, and soil rockiness. All six variables showed significant patchiness (spatial autocorrelation) at similar scales. The abundance of flowering plants was positively correlated with rockiness and negatively correlated with moisture and gopher activity. Although limited seed dispersal suggests that seedlings should be spatially associated with flowering plants, no such correlation existed: indeed, examination of the bivariate scatterplot suggests a negative association, in the particular and restricted sense that seedlings are abundant only in quadrats where flowering is low. We hypothesize that seed germination is higher in less rocky areas of deeper, moister soil than in the rocky areas where most seeds land, but that seedlings seldom reach maturity unless they are in a rocky refuge from predation. Results from path analysis are consistent with this hypothesis. Such an ecological situation should weaken natural selection on characters enhancing seed dispersal.

Physiological and Growth Responses of Arctic Plants to a Field Experiment Simulating Climatic Change

Abstract: Field manipulations of light, temperature, nutrients, and length of growing season in directions simulating global environmental change altered biomass of the four most abundant vascular plant species in tussock tundra of northern Alaska. These species are Betula nana, Ledum palustre, Vaccinium vitis-idaea, and Eriophorum vaginatum. Biomass response reflected changes in both growth and mortality, with growth being stimulated by treatments that enhanced biomass,a and mortality being enhanced by all treatments (except in Vaccinium). Those species with highest leaf and stem turnover (the graminoid and deciduous shrub) initially showed large positive responses to nutrient addition. By contrast, slow-turnover evergeen species showed little initial change in production in response to our manipulations, and their long-term biomass responses were in the opposite direction to those of the responsive species. Short-term measurement of leaf expansion, photosynthesis, and phosphate uptake showed little correlation with net production of biomass change in response to manipulations because of compensatory mechanisms at levels of growth and allocation. Changes in nutrient distribution among species accounted for many of the long-term changes in biomass and productivity. Processes that are readily integrated at annual time steps (e.g., shoot growth, shoot mortality, allocation) were more useful than instantaneous physiological measurements in predicting decadalmore » vegetation changes because (1) compensating responses among physiological processes buffer plant responses at progressively longer time scales, (2) species interactions in the community buffer ecosystem processes such as productivity and nutrient cycling from changes in growth of individual species, and (3) different time lags between physiological, demographic, and ecosystem processes complicate modeling of long-term responses from short-term mechanism. 76 refs., 11 figs., 5 tabs.« less

Fossil Carbon/Nitrogen Ratios as a Measure of Peat Decomposition

Abstract: Net accumulation rates of carbon in a 9000 calendar year record of Sphagnum fuscum peat in western boreal Canada range from 13.6 to 34.9 g.m-2 yr-1. The depth vs. age curve is slightly convex due to generally declining net productivity at the site. Fossil carbon/nitrogen ratios of bulk Sphagnum fuscum peat and its components are used to calculate the rate and total amount of decay in the deeper anoxic peat deposits. The pro- portional rate of decay in the catotelm of the peatland declines linearly over time. Carbon loss from the catotelm is estimated at 50% after 1700 calendar years and 65% after 7500 calendar years. Carbon has been added to the catotelm at an average rate of 28.0 g.m-2*yr-1 over the last 1174 years, whereas, at present, the cumulative loss of carbon over the entire catotelm is 19.4 g.m-2*yr-1. The peatland continues to represent a sink for carbon.

Fire Severity and Vegetation Response in the Boreal Swedish Forest

Abstract: We studied plant survival and colonization over an experimental gradient, from fire lightly scorching the soil to fire consuming most of the organic soil layer, at two forest sites in northern Sweden. The gradient was achieved by adding different amounts of fuel to small plots that were burned in 1988 and 1989. Temperature was recorded at four soil strata during burning. We analyzed survival of seeds and rhizomes in the soil immediately after fire, and followed vegetation cover and seedling establishment until 1993. During fire, there was a steep decline in maximum temperature with increasing depth below the char, irrespective of the depth of burn in the mor layer, indicating that burn depth can be used as a general indicator of heat impact below ground. Lethal temperature was not recorded deeper than 20-30 mm under the burn boundary. Plant survival was determined both by depth of burn and by depth distribution of regenerative structures in the soil. Three rhizomatous species, the dwarf shrubs Vaccinium myrtillus and Vaccinium vitis-idaea and the grass Deschampsia fiexuosa, were dominant in the prefire vegetation. For all three species, the bulk of the soil bud bank was located within the mor layer, but was more superficial for D. flexuosa. Initial mortality in the bud bank was progressively higher with increasing depth of burn, and this determined the regrowth over the following years. After fires that consumed only the moss layer, cover of the Vaccinium species returned to prefire levels within 2-4 yr, and D. fiexuosa showed a dramatic increase in cover as well as in fruiting. Fires that burned slightly deeper nearly eliminated D. flexuosa, and the deepest burning fires also eliminated Vaccinium s-pp. In contrast to regrowth from rhizomes, col- onization from seed was better after relatively deep-burning fire, both for species with a soil seed bank and for species dispersing seed onto the burnt soil. However, after fires consuming most of the organic soil layer, seed bank species were also badly affected, whereas dispersers showed progressively better establishment with increasing depth of burn. Differences between treatments were still great after 5 yr, indicating that variation in depth of burn will have a long-lasting impact on the vegetation. These results from ex- perimentally burned plots were corroborated by an analysis of depth distribution of viable plant rhizomes and seeds, and the initial colonization at a site newly burned in a wildfire. The precise response patterns of boreal vegetation to variation in burn depth will depend on characteristics of the species present. However, we assume that these results have a high degree of generality, since, in podzolized soils, most rhizomatous species are predominantly located in the mor layer, since the dormant seed bank typically is concentrated at the interface of mor and mineral soil, and also since a thick organic soil layer is a poor seedbed for incoming seeds. The results indicate that in boreal forest, depth of burn is a more important variable than fire front intensity for the understory vegetation, in contrast to the situation in ecosystems with little accumulation of organic material on the mineral soil.

Disease in Metapopulation Models: Implications for Conservation

Abstract: Several conservation measures are intended to enhance the movement of individuals among populations. These include the establishment of wildlife corridors, cap- tive breeding and release programs, and translocation of individuals among populations. Many metapopulation models show that increasing movement among populations reduces the chance of metapopulation extinction. However, epidemiological models indicate that increased contact among populations enhances the spread of disease and can trigger epi- demics. I have synthesized elements of mathematical epidemiology with metapopulation models. An analytic model showed that highly contagious diseases of moderate severity spread widely, increasing the probability of metapopulation extinction. I also used a sim- ulation model to examine four spatial arrangements of populations: island, necklace, loop, and spider. When infected individuals were allowed to move freely among populations, all of the arrangements exhibited qualitative behavior similar to that exhibited by the analytic models. The most dangerous diseases were those for which infected populations grew large enough to produce dispersers that infected other populations, but which also reduced the geometric rate of increase for infected populations to near unity. Under those conditions, random demographic and environmental events caused metapopulation extinction. Major differences among the spatial arrangements emerged when a quarantine population was established. A centralized quarantine in the spider and necklace arrangement yielded the most dramatic reductions in metapopulation extinction probability. A single quarantine patch was of little value in an island arrangement. These results have several implications for managing metapopulations. Most notably, some spatial arrangements of populations are more amenable to disease control than others, and establishing a quarantine population can increase the probability of detecting new diseases and reduce the impact of diseases that do appear.

Seed Banks in Desert Annuals: Implications for Persistence and Coexistence in Variable Environments

Abstract: It is widely believed that desert annual plants maintain between-year seed banks, yet few field studies actually have measured the proportion of the viable seed bank that remains dormant through a season. Dormancy and germination fractions were quantified for a guild of winter annuals on a creosote flat in the Sonoran Desert for three years. Predictions from two types of theoretical models applicable to temporally variable envi- ronments were examined: (1) the evolution of life history traits promoting persistence in the face of temporal variation and (2) the role of temporal variation in mediating species coexistence. The density of ungerminated seeds was estimated by collecting soil samples after germination, but prior to new seed set. Seedlings were followed in nearby plots to estimate the density of germinated seedlings and their reproductive success. Long-term data collected from permanent plots over a 10-yr period were used to calculate temporal variation in reproductive success for each species. Species with higher temporal variation in reproductive success had lower germination fractions and smaller seeds, consistent with the theory that seed dormancy and large seed size are partially substitutable bet-hedging strategies. The data also suggested that this system possesses traits that are necessary for temporal variation to promote coexistence. First, between-year seed banks, necessary to buffer populations in unfavorable years, were documented for 17 species. Second, there was a strong tendency for year-to-year variation in germination fractions to vary among species. Finally, plants germinated more in years of higher reproductive success. We discuss

Assessing habitat selection when availability changes

Abstract: We present a method of comparing data on habitat use and availability that allows availability to differ among observations. This rnethod is applicable when habitats change over time and when animals are unable to move throughout a predeter- mined study area between observations. We used maximum-likelihood techniques to de- rive an index that estimates the probability that each habitat type would be used if all were equally available. We also demonstrate how these indices can be used to compare relative use of available habitats, assign them ranks, and assess statistical differences between pairs of indices. The set of these indices for all habitats can be compared between groups of animals that represent different seasons, sex or age classes, or experimental treatments. This method allows quantitative comparisons among types and is not affected by arbitrary decisions about which habitats to include in the study. We provide an example by comparing the availability of four categories of sea ice concentration to their use by adult female polar bears (Ursus maritimus), whose movements were monitored by satellite radio tracking in the Bering and Chukchi Seas during 1990. Use of ice categories by bears was nonrandom, and the pattern of use differed between spring and late summer seasons.

Ontogenetic Niche Shifts in Largemouth Bass: Variability and Consequences for First‐Year Growth

Abstract: The ontogeny of many size-structured organisms is marked by a series of niche shifts that divides a population into ecologically distinct stages. Consequently, overall population dynamics are affected both by processes operating within individual stages and by factors that influence transitions between stages. In largemouth bass (Micropterus sal- moides), there is a diet shift from littoral invertebrates to fish that typically occurs sometime in their 1st yr. I examined factors that influenced this shift by tracking diet and growth trajectories of young-of-year (YOY) bass cohorts in several Michigan lakes over 2 yr. Cohorts differed in consumption rates of invertebrate prey, which led to an early divergence in bass growth among lakes. This variation in early growth had dramatic consequences for the timing of the shift to piscivory. In lakes with rapid growth in the invertebrate feeding stage, bass were able to gain a size advantage over YOY bluegill (Lepomis macrochirus), their primary fish prey, which facilitated a shift to piscivory as soon as these bluegill became available as food. In contrast, slow growth during the invertebrate feeding stage reduced the size advantage of bass over bluegill and delayed or even prevented the shift to piscivory in the 1 st yr. Niche shifts in bass were also sensitive to environmental conditions early in the growing season. In 1992, low temperatures caused a reduction in growth rates that prevented the establishment of a size advantage over bluegill in all but one lake. Bass grew much faster after switching to piscivory; thus the shift from invertebrates to fish strongly influenced sizes attained by YOY bass by autumn. Because survivorship of YOY bass is strongly size dependent (particularly through the first winter), the timing of the shift to piscivory may be important in determining survival and recruitment rates. Therefore, processes that operate on bass in the invertebrate feeding stage can influence the shift to piscivory, and this can potentially have important effects on the dynamics of the population as a whole.

Permanence and the assembly of ecological communities

Abstract: A numerical technique for assembly of ecological communities of Lotka- Volterra form is described. The technique is based upon a global criterion for coexistence of species known as permanence. This provides a relatively fast and accurate method to determine the sequence of communities that develops when species are drawn sequentially and in an arbitrary order from a regional pool of species. Steps in the assembly sequence that cannot be resolved by this method are determined by numerical integration. The results are as follows. (1) At each step in an assembly sequence, a species that succeeds in invading when rare persists in the resulting community even if one or more of the resident species becomes extinct. (2) Assembly sequences are terminated with a community that is unin- vadable by any of the remaining species from the pool. The number of these endpoints is small, even when the species pool is large. (3) In some cases, the final community cannot be reassembled from the species left in it; other species, which are absent at the end, are needed for the endpoint to be reached. (4) Invasion resistance builds up in three stages during an assembly sequence. Over much of the sequence, invasion resistance shows little if any increase; during this period, species composition continues to change until the se- quence happens to land on an endpoint. (5) Communities assembled from large species pools are more resistant to invasion than those assembled from small species pools.