Resource competition among saline-lake diatoms under varying N/P ratio, salinity and anion composition

1. A series of resource competition experiments were performed with four diatom taxa isolated from saline lakes in the Northern Great Plains of North America. Three variables were tested in a factorial design: N/P ratio (6 : 1, 12 : 1, 24 : 1, 48 : 1), salinity (5, 8, 11‰), and anion composition (sulphate versus bicarbonate-dominated medium).
2. Coexistence of Chaetoceros elmorei , Cyclotella quillensis , and Cymbella pusilla occurred at the two lowest N/P ratios. These three taxa have similar N requirements and thus competitive exclusion by the taxon with the lowest N requirement may have required more time.
3. Anomoeoneis costata , which had the highest nitrogen requirements and slowest growth rate of these four taxa, declined in all cases. Cyclotella biovolume concentration consistently declined at the two highest N/P ratios, indicating a higher P requirement, which is consistent with its distribution pattern.
4. In general, we found opposite trends with respect to all variables for Chaetoceros , a taxon that mainly inhabits sulphate systems, versus Cymbella , a bicarbonate-associated taxon. Our results suggest that Cymbella may be found mainly in bicarbonate systems because of a higher C requirement, whereas Chaetoceros may be less abundant in these systems, in part, because of competitive interactions with bicarbonate-associated taxa.     

Size-asymmetric competition and size-asymmetric growth in a spatially explicit zone-of-influence model of plant competition

Size-asymmetric competition among plants is usually defined as resource pre-emption by larger individuals, but it is usually observed and measured as a disproportionate size advantage in the growth of larger individuals in crowded populations (“size-asymmetric growth”). We investigated the relationship between size-asymmetric competition and size-asymmetric growth in a spatially explicit, individual-based plant competition model based on overlapping zones of influence (ZOI). The ZOI of each plant is modeled as a circle, growing in two dimensions. The size asymmetry of competition is reflected in the rules for dividing up the overlapping areas. We grew simulated populations with different degrees of size-asymmetric competition and at different densities and analyzed the size dependency of individual growth by fitting coupled growth functions to individuals. The relationship between size and growth within the populations was summarized with a parameter that measures the size asymmetry of growth. Complete competitive symmetry (equal division of contested resources) at the local level results in a very slight size asymmetry in growth. This slight size asymmetry of growth did not increase with increasing density. Increased density resulted in increased growth asymmetry when resource competition at the local level was size asymmetric to any degree. Size-asymmetric growth can be strong evidence that competitive mechanisms are at least partially size asymmetric, but the degree of size-asymmetric growth is influenced by the intensity as well as the mode of competition. Intuitive concepts of size-asymmetric competition among individuals in spatial and nonspatial contexts are very different.     

Climate change vulnerability of forest biodiversity: climate and competition tracking of demographic rates

Forest responses to climate change will depend on demographic impacts in the context of competition. Current models used to predict species responses, termed climate envelope models (CEMs), are controversial, because (i) calibration and prediction are based on correlations in space (CIS) between species abundance and climate, rather than responses to climate change over time (COT), and (ii) they omit competition. To determine the relative importance of COT, CIS, and competition for light, we applied a longitudinal analysis of 27 000 individual trees over 6–18 years subjected to experimental and natural variation in risk factors. Sensitivities and climate and resource tracking identify which species are vulnerable to these risk factors and in what ways. Results show that responses to COT differ from those predicted based on CIS. The most important impact is the effect of spring temperature on fecundity, rather than any input variable on growth or survival. Of secondary importance is growing season moisture. Species in the genera Pinus, Ulmus, Magnolia, and Fagus are particularly vulnerable to climate variation. However, the effect of competition on growth and mortality risk exceeds the effects of climate variation in space or time for most species. Because sensitivities to COT and competition are larger than CIS, current models miss the most important effects. By directly comparing sensitivity to climate in time and space, together with competition, the approach identifies which species are sensitive to climate change and why, including the heretofore overlooked impact on fecundity.     

Evolution of extraordinary female-biased sex ratios: the optimal schedule of sex ratio in local mate competition

Female-biased sex ratio in local mate competition has been well studied both theoretically and experimentally. However, some experimental data show more female-biased sex ratios than the theoretical predictions by Hamilton [1967. Science 156, 477-488] and its descendants. Here we consider the following two effects: (1) lethal male-male combat and (2) time-dependent control (or schedule) of sex ratio. The former is denoted by a male mortality being an increasing function of the number of males. The optimal schedule is analytically obtained as an evolutionarily stable strategy (ESS) by using Pontrjagin's maximum principle. As a result, an ESS is a schedule where only males are produced first, then the proportion of females are gradually increased, and finally only females are produced. Total sex ratio (sex ratio averaged over the whole reproduction period) is more female-biased than the Hamilton's result if and only if the two effects work together. The bias is stronger when lethal male combat is severer or a reproduction period is longer. When male-male combat is very severe, the sex ratio can be extraordinary female-biased (less than 5%). The model assumptions and the results generally agree with experimental data on Melittobia wasps in which extraordinary female-biased sex ratio is observed. Our study might provide a new basis for the evolution of female-biased sex ratios in local mate competition.     

Species coexistence under resource competition with intraspecific and interspecific direct competition in a chemostat

Competition theory has developed separately for direct competition and for exploitative competition. However, the combined effects of the two types of competition on species coexistence remain unclear. To examine how intraspecific and interspecific direct competition contributes to the coexistence of species competing for a single resource, we constructed a chemostat-type resource competition model. With general functions for intraspecific and interspecific direct competition, we derived necessary and sufficient conditions (except for a critical case that rarely occurs in a biological sense) that determine the number of stably coexisting species. From these conditions, we found that the number of coexisting species is determined just by the invasibility of each species into subcommunities with a smaller number of species. In addition, using a combination of rigorous mathematical theory and a simple graphical method, we can demonstrate how the stronger intraspecific direct competition facilitates species invasion, leading to a larger number of coexisting species.     

Parental care, cost of reproduction and reproductive skew: a general costly young model

Understanding the mechanisms by which animals resolve conflicts of interest is the key to understanding the basis of cooperation in social species. Conflict over reproductive portioning is the critical type of conflict among cooperative breeders. The costly young model represents an important, but underappreciated, idea about how an individual's intrinsic condition and cost of reproduction should affect the resolution of conflict over the distribution of reproduction within a cooperatively breeding group. However, dominant control in various forms and fixed parental care (offspring fitness dependent solely on total brood size) are assumed in previous versions of costly young models. Here, we develop a general costly young model by relaxing the restrictive assumptions of existing models. Our results show that (1) when the complete-control assumption is relaxed, the costly young model behaves very differently from the original model, and (2) when the fixed parental care assumption is relaxed, the costly young-costly care model displays similar predictions to the tug-of-war model, although the underlying mechanisms causing these similar patterns are different. These results, we believe, help simplify the seemingly divergent predictions of different reproductive skew models and highlight the importance of studying the group members' intrinsic conditions, costs of producing young, and costs of parental care for understanding breeding conflict resolution in cooperatively breeding animals.     

Optimal resource allocation to seeds and vegetative propagules under density-dependent regulation in Syneilesis palmata (Compositae)

Syneilesis palmata reproduces by both seeds and vegetative propagules (short rhizomes). The latter result in the production of new plants that are larger in size and hence have a higher survival probability and a higher growth rate than seeds. A previous study predicted that the optimal reproductive strategy, in terms of maximizing population growth rate (a fitness measure under no density regulations), was pure vegetative reproduction. However, high resource investment to vegetative propagules can cause local crowding resulting in reduced demographic performances of the plants, because the vegetative propagules of Syneilesis are produced close to one another. We examined, in this situation, the impact of allocating a certain proportion of reproductive resource to seeds with relatively greater capacity for dispersal. We simulated dynamics of hypothetical Syneilesis populations with various reproductive resource allocation balances (from pure seed to pure vegetative reproduction), using a density-dependent matrix model. In the model, it was assumed that plants from vegetative propagules experienced density-dependent reduction in their survival probabilities, but this was not the case for plants originating from seeds. Each allocation strategy was evaluated based on an equilibrium population density, a fitness measure under density-dependent regulations. The optimal reproductive strategy predicted was pure vegetative reproduction. Unrealistic conditions were required for seed reproduction to be favoured, such as the production of seeds one hundred times the normal number per unit resource investment. However, the conditions were fairly relaxed compared with those required in the model where no density effects were incorporated. This indicates that escape from local crowding is likely to be one of the roles of seed production in Syneilesis.     

Vegetative reproduction capacities of floodplain willows--cutting response to competition and biomass loss

While several studies on regeneration in Salicaceae have focused on seedling recruitment, little is known about factors controlling their vegetative reproduction. In two greenhouse experiments, we studied the response of floodplain willows (Salix fragilis, S. viminalis, S. triandra) to competition with Poa trivialis, and to shoot and root removal when planted as vegetative cuttings. In the first experiment, growth performance variables were analysed in relation to full competition, shoot competition, root competition and control, taking into account two different water levels. After 9 weeks, shoots were removed and the resprouting capacity of the bare cuttings was recorded. In the second experiment, the cutting performance of the three floodplain and an additional two fen willow species (S. cinerea, S. aurita) was compared when grown in three different soil compositions and with two different water levels. After 9 weeks, shoot and root biomass was removed and the bare cuttings were replanted to test their ability to resprout. Cutting performance and secondary resprouting were negatively affected by full and shoot competition while root competition had no or weak effects. The floodplain species performed better than the fen species in all soil types and water levels. Secondary resprouting capacity was also higher in the floodplain species, which showed an additional strong positive response to the previous waterlogging treatment. The results contribute to understanding of the vegetative regeneration ecology of floodplain willows, and suggest that the use of vegetative plantings in restoration plantings could be an effective strategy for recovering floodplain forests.     

Long adult life, low reproduction and competition in two sub-Antarctic carabid beetles

ABSTRACT. 1. On the biologically impoverished sub-Antarctic Ile de la Possession, Iles Crozet, Amblystogenium pacificum Putzeys and A.minimum Luff (Carabidae), both opportunistic carnivores, coexist in fellfield habitat above 120 m altitude, where they hunt prey in inter-stone spaces.
2. Beetles show marked claw wear showing that many individuals survive to breed in their third or later year of adult life, and in some years, particularly in A.minimum , populations consisted largely of very old beetles. This coupled with evidence from ripe egg numbers, of rarity of tenerals and of larvae, shows that adult recruitment rates were usually very low.
3. In fellfield the coexisting species pair have non-overlapping size distributions with A.pacificum the larger. In moorland A.pacificum adults occur in isolation and are smaller and overlap the size ranges of both species in fellfield. These facts are interpreted as character displacement or release, reflecting present-day interspecific competition, probably for food. Supporting evidence was obtained in terms of apparent size-differential survival of beetles of different ages.     

Immune investment and sperm competition in a beetle

Individuals that invest more in immunity may not be able to invest as much in other life history traits. The overall effects on fitness depend on the balance of investment in life history traits and unnecessary investment in immunity may lower fitness. Adult mealworm beetles (Tenebrio molitor L.) modulate their investment according to the perceived risk of infection as larvae; the amount of investment can be assessed by body coloration. This prophylactic investment in immunity can be used to assess the costs of investment when no immune challenge is present. Whether investment in immunity is traded off against sperm competitive ability, another important fitness trait in insects, was investigated. Males that had invested more in immunity (dark males) competed against males that had invested less (light males) for fertilization of offspring. Dark males did lose sperm precedence over time, whereas light males did not. However, this decrease in sperm offensive ability may not result in decreased fitness for darker males under normal female mating frequencies; the decrease in offspring did not occur for 1 week, but females that have constant access to males mate once a day, which would negate any long‐term effects of male mating order. Thus, prophylactic investment in immunity does not produce immediate reductions in a male's ability to gain fertilizations. The costs to immune investment may be born by other fitness traits in T. molitor.     

Resource competition and suppression of plants colonizing early successional old fields

Early colonizing annual plants are rapidly suppressed in secondary succession on fertile midwestern old fields, while later colonizing perennials persist. Differences in competitive ability for above- and belowground resources may be partly responsible for differences in species persistence during succession, as both light and nutrient availability may change rapidly. We found that, although both above- and belowground competition suppress growth of colonizing plants, belowground competition was the dominant factor in the suppression of the annual Ambrosia artemisiifolia in 2nd-year-old fields near the W.K. Kellogg Biological Station in southwestern Michigan. Despite an ability to persist in later successional fields, seedling transplants of the perennial Achillea millefolium were also suppressed by above- and belowground competition, with belowground competition having the strongest effect. As in many old fields, nitrogen availability is the primary factor limiting plant productivity. There was no clear difference between the species in ability to compete for ¹⁵N from an enriched patch, although there was an indication of greater precision of foraging by Achillea. Life history differences between these species and consequent differences in the phenology of root growth relative to other old-field plants are likely to play a large role in the persistence of Achillea in successional fields where Ambrosia is suppressed. Received: 8 January 1998 / Accepted: 16 September 1998     

Capacity-Building for Natural Resource Management: Lessons from the Health Sector

This paper examines capacity-building measures used by the health sector relevant to natural resource management (NRM) using the delivery of two programs in Australia through its recently formalized 56 community-based regional NRM Boards as a reference point. The delivery of NRM outcomes through devolved governance arrangements has been hampered by inadequate capacity in other countries (e.g., New Zealand, Canada). The inherent complexities of NRM, coupled with professional “comfort zones,” often limit actors from looking to other sectors confronting similar issues. This paper summarizes findings from ∼70 sources in health reviewing nine major capacity-building measures. The outcomes of research evaluating the relative benefits and disbenefits of applying these measures are discussed. The paper identifies several opportunities to trial some of these measures in NRM.

Resource Optimization and Symbiotic Nitrogen Fixation

In temperate forests, symbiotic nitrogen (N) fixation is restricted to the early phases of succession despite the persistence of N limitation on production late in succession. This paradox has yet to be explained adequately. We hypothesized that the restriction of N fixation to early stages of succession results from the optimization of resource allocation in the vegetation. Because of this optimization, N fixation should be restricted to periods when fixation is less costly than N uptake. Our analysis differs from others in the way we calculate the cost of N uptake; we assess the cost of N uptake as the amount of carbon (C) that could be assimilated if the resources necessary to acquire one gram of N from the soil were allocated instead to photosynthesis. We then simulate N fixation as an asymptotic function of the difference in cost between N uptake and N fixation and proportional to the abundance of host tissues for the N-fixing symbionts. The factors that contribute to conditions that favor N fixation are (a) elevated-carbon dioxide (CO₂) concentrations, (b) an open canopy, (c) low available N in the soil, and (d) a soil volume already well exploited by roots. Our results indicate that changes in the relative cost of uptake vs fixation can explain most of the pattern in fixation through both primary and secondary succession, but that competitive interactions with nonfixing species play a role in the final exclusion of fixation in later stages of succession. Received 26 September 2000; accepted 31 January 2001.     

Root proliferation and seed yield in response to spatial heterogeneity of below-ground competition

Here, we tested the predictions of a 'tragedy of the commons' model of below-ground plant competition in annual plants that experience spatial heterogeneity in their competitive environment. Under interplant competition, the model predicts that a plant should over-proliferate roots relative to what would maximize the collective yield of the plants. We predict that a plant will tailor its root proliferation to local patch conditions, restraining root production when alone and over-proliferating in the presence of other plants. A series of experiments were conducted using pairs of pea (Pisum sativum) plants occupying two or three pots in which the presence or absence of interplant root competition was varied while nutrient availability per plant was held constant. In two-pot experiments, competing plants produced more root mass and less pod mass per individual than plants grown in isolation. In three-pot experiments, peas modulated this response to conditions at the scale of individual pots. Root proliferation in the shared pot was higher compared with the exclusively occupied pot. Plants appear to display sophisticated nutrient foraging with outcomes that permit insights into interplant competition.