Timing of avian reproduction in unpredictable environments

Organisms living in periodically varying environments adjust their life history events to the changes in food availability. When these changes are not entirely predictable animals face a trade-off between maintaining physiological preparedness (which can be costly) and being unprepared (which decreases the chances of successful reproduction). To investigate this problem, we developed an optimal annual routine model of gonad regulation in birds. Most birds regress their reproductive organs during non-breeding periods, but to start breeding again they need to have functional gonads. Maintaining the gonads in this state is costly, but because it takes time to achieve this state, if gonads are not functional the bird may miss a possible breeding opportunity. We explore the resolution of this trade-off in environments where favorable periods can occur at any time of the year and variability in the length of good and bad periods can be altered. Consistent with empirical studies of reproductive behavior in unpredictable environments, we find that birds maintain the gonads partially activated during unfavorable conditions in many cases. However, gonad regulation may differ strikingly depending on the consistency of the good and bad periods. Furthermore, seasonal changes in food availability lead to the entrainment of reproduction and the segregation of the breeding and non-breeding season, even if the magnitude of seasonality is small compared to the degree of environmental fluctuations. These results indicate that several aspects of the environment need to be taken into account to understand reproductive behavior in unpredictable environments. Given that the trade-off between the costs and benefits of maintaining physiological preparedness is not limited to birds, our results have implications for understanding behavioral flexibility in other organisms as well.     

Population differentiation of temperate amphibians in unpredictable environments

Amphibians are a globally distributed and diverse lineage, but much of our current understanding of their population genetic structure comes from studies in mesic temperate habitats. We characterize the population genetic structure of two sympatric explosive breeding amphibians in the southwestern deserts of the United States: the Great Plains toad ( Anaxyrus cognatus ) and Couch's spadefoot toad ( Scaphiopus couchii ). For both species, we find limited genetic differentiation even between populations in adjacent valleys separated by dispersal barriers such as mountainous habitats. To understand how population genetic patterns in these two arid-adapted species compare to taxa in more mesic environments, we computed a standardized measure of population differentiation for A. cognatus , S. couchii , and for pond-breeding amphibians that inhabit mesic temperate environments. Our results indicate that the arid-adapted species have lower population genetic structure at fine and moderate scales than most other amphibian species we surveyed. We hypothesize that stochasticity in the availability of appropriate breeding sites as well as landscape homogeneity may result in increased population connectivity in desert-adapted frogs. Future work examining fine-scale population structure in amphibians from a diversity of habitats will test the generality of our findings. Intraspecific comparisons among localities with varied seasonality and habitats will be particularly useful for investigating the interaction between species-typical population dynamics and environmental characteristics as determinants of population connectivity in pond-breeding amphibians.     

Variable timing of reproduction in unpredictable environments: adaption of flood plain plants

We study the evolutionarily stable reproductive timing of annual plants that face unpredictable environmental disturbances. Plants living in a riverbed often experience a disturbance before they reproduce, suffering major fitness loss. Plants reproducing prior to the flood season are free from the risk of lost reproduction, but a small flowering plant can produce only a few numbers of seeds. If the date of disturbance is unpredictable, a mixed strategy of reproductive timing may evolve in which individuals of the same genotype have different reproductive dates. We calculate the evolutionarily stable phenotype distribution analytically. Depending on parameters, the ESS distribution is either (1) a timid strategy-the plant reproduces when small, prior to the major disturbance season; (2) a bold strategy-the plant reproduces only when it is fully grown; (3) a mixture of early and late reproduction; or (4) dates of reproduction spread over a wide interval. We also examine the effects of developmental and environmental noises that make realized flowering dates deviate from that programmed by the genotype, which follows the ESS distribution. In the presence of noise, the ESS distribution of programmed timing of reproduction is discrete.     

Understorey environments influence functional diversity in tank‐bromeliad ecosystems

1. A substantial fraction of the freshwater available in neotropical forests is impounded within the rosettes of bromeliads that form aquatic islands in a terrestrial matrix. The ecosystem functioning of bromeliads is known to be influenced by the composition of the contained community but it is not clear whether bromeliad food webs remain functionally similar against a background of variation in the understorey environment. 2. We considered a broad range of environmental conditions, including incident light and incoming litter, and quantified the distribution of a very wide range of freshwater organisms (from viruses to macroinvertebrates) to determine the factors that influence the functional structure of bromeliad food webs in samples taken from 171 tank‐bromeliads. 3. We observed a gradient of detritus‐based to algal‐based food webs from the understorey to the overstorey. Algae, rotifers and collector and predatory invertebrates dominated bromeliad food webs in exposed areas, whereas filter‐feeding insects had their highest densities in shaded forest areas. Viruses, bacteria and fungi showed no clear density patterns. Detritus decomposition is mainly due to microbial activity in understorey bromeliads where filter feeders are the main consumers of microbial and particulate organic matter (POM). Algal biomass may exceed bacterial biomass in sun‐exposed bromeliads where amounts of detritus were lower but functional diversity was highest. 4. Our results provide evidence that tank‐bromeliads, which grow in a broad range of ecological conditions, promote aquatic food web diversity in neotropical forests. Moreover, although bromeliad ecosystems have been categorised as detritus‐based systems in the literature, we show that algal production can support a non‐detrital food web in these systems.     

Leaving home ain't easy: non-local seed dispersal is only evolutionarily stable in highly unpredictable environments

It is widely understood that in the presence of asynchronous environmental variation, seeds disperse to escape disturbances, avoid crowding or colonize newly favourable habitat before a superior competitor can arrive. If seeds are dispersing for any of these reasons, it seems intuitive that they should travel far enough to reach conditions uncorrelated with their natal environment: why ‘escape in space’ only to land somewhere more or less like where they started? However, in this paper, I present a series of mathematical experiments which show that the evolutionarily stable mean dispersal distance remains well short of the spatial correlation length of the environmental variation, regardless of disturbance severity, coevolution with a superior competitor or the presence of a small fraction of seeds which travel well beyond the mean distance. Non-local dispersal arises only as part of a polymorphism that evolves when favourable conditions are fleeting. To the degree that non-local dispersal is a response to environmental variation, it appears to be a response to environmental unpredictability.     

Regime shift and robustness of organism-created environments: A model for microbial ecosystems

Organism-environment interactions are different from organism-resource interactions in two respects: (1) resources can only be consumed by organisms whereas environmental conditions can be increased or decreased depending on the species; (2) high resource conditions generally stimulate the growth of organisms, whereas extreme environmental conditions are not necessarily favored because each species usually has an optimum range for growth. To investigate the properties of an organism-environment feedback system, we analyze a model for microbial ecosystems in which a single microorganism species can modify the environmental pH. We demonstrate that the equilibrium level of the environmental pH can be partially regulated at a relatively constant value even if the pH in the influx to the ecosystem changes over a wide range. For species that acidify the medium, the equilibrium pH is somewhat lower than the pH optimal for the species. The pH-stabilizing effect of microorganisms is stronger if their growth is self-limited by the environmental pH. When the influx becomes sufficiently alkaline, the population of the organism suddenly disappears and the environmental pH changes abruptly. The system shows bi-stability and hysteresis and therefore differs from a standard resource competition model composed of a single species that consumes resources.     

Biological equilibrium in ecosystems 2. Parameters of ecosystems

In this paper the Theory of Biological Equilibrium (TBE), as the theoretical basis of the Canonical Hypothesis, is used to compute a set of theoretical parameters of natural ecosystems. These parameters are the individuals: species (I/S); the commonness: rarity (C/U) ration as well as the expected density (d _e) and total area (TA) of ecosystems. Results of sampling with nested quadrates are checked against theoretical values. Deviations from expected values of 5 to 10% occur in samples with Natural Biological Equilibrium (NBE); in disturbed and transitional sites a compensation mechanism is found which results in simulated NBE.     

Biological equilibrium in ecosystems 3. Classification of ecosystems

The Theory of Biological Equilibrium (TBE) is applied to the classification of ecosystems. It is shown that one of the basic factors, the habitat-type, governs the distribution of species through the subfactors climate and substrate in accordance with the Theory of Tolerance. Groups of species can be distinguished which show a very similar distribution, and combinations of species in these groups form the kengroups of classification units, or associations. Because the habitat-type of associations is often fragmented, the classification procedures can be applied to explaint the natural extinction of species, as well as the probability of species extinction through man's actions upon biocenoses.     

Limiting similarity in subterranean ecosystems: a case of niche differentiation in Elmidae (Coleoptera) from epigean and hypogean environments

Hydrobiologia - Environmental pressures in caves can act on species, selecting functional attributes that allow their best performance in such habitats. Caves are simplified habitats, which makes...     

Site fidelity in predictable and unpredictable habitats

Summary Site fidelity, the tendency to return to a previously occupied location, has been observed in numerous species belonging to at least three phyla. In this paper I develop a general model using dynamic programming to investigate conditions under which fidelity to a previously occupied territory will be advantageous. The results predict that site fidelity should be inversely related to heterogeneity in territory quality and the animal's lifespan and positively related to the cost of changing territories, age and probability of mortality in the habitat. The predictability of reproductive outcome (defined as the probability that next period's outcome will be the same as this period's outcome) also affects site fidelity. In predictable habitats, changing territories may be favoured after a bad previous outcome. In contrast, settlement should be independent of the previous outcome in unpredictable habitats. Individuals should also be site-faithful in unpredictable habitats, as long as the mean territory quality is equal among available territories. I also investigate the success of two potential decision rules (always stay and win-stay: lose-switch) relative to the optimal settlement strategy. The results show that these rules may perform as well as the optimal strategy under certain conditions. The always stay strategy does well in unpredictable habitats, when the mean quality within a territory is equal among territories. In contrast, the win-stay: lose-switch strategy performs best in predictable habitats.     

Arterial thrombosis--insidious, unpredictable and deadly

The formation of blood clots--thrombosis--at sites of atherosclerotic plaque rupture is a major clinical problem despite ongoing improvements in antithrombotic therapy. Progress in identifying the pathogenic mechanisms regulating arterial thrombosis has led to the development of newer therapeutics, and there is general anticipation that these treatments will have greater efficacy and improved safety. However, major advances in this field require the identification of specific risk factors for arterial thrombosis in affected individuals and a rethink of the 'one size fits all' approach to antithrombotic therapy.     

Cyanobacteria in mangrove ecosystems

Mangroves are subject to the effects of tides and fluctuations in environmental conditions, which may reach extreme conditions. These ecosystems are severely threatened by human activities despite their ecological importance. Although mangroves are characterized by a highly specialized but low plant diversity in comparison to most other tropical ecosystems, they support a diverse microbial community. Adapted microorganisms in soil, water, and on plant surfaces perform fundamental roles in nutrient cycling, especially nitrogen and phosphorus. Cyanobacteria contribute to carbon and nitrogen fixation and their cells act as phosphorus storages in ecosystems with extreme or oligotrophic environmental conditions such as those found in mangroves. As the high plant productivity in mangroves is only possible due to interactions with microorganisms, cyanobacteria may contribute to these ecosystems by providing fixed nitrogen, carbon, and herbivory-defense molecules, xenobiotic biosorption and bioremediation, and secreting plant growth-promoting substances. In addition to water, cyanobacterial colonies have been detected on sediments, rocks, decaying wood, underground and aerial roots, trunks, and leaves. Some mangrove cyanobacteria were also found in association to algae or seagrasses. Few studies on mangrove cyanobacteria are available, but together they have reported a substantial number of species in these ecosystems. However, the cyanobacterial diversity in this biome has been traditionally underestimated. Though mangrove communities generally host cyanobacterial taxa commonly found in marine environments, unique microhabitats found in mangroves potentially harbor several undescribed cyanobacterial taxa. The relevance of cyanobacteria for mangrove conservation is highlighted in their use for the recovery of degraded mangroves as biostimulants or in bioremediation.     

The organization of a honeyeater community in an unpredictable environment

The honeyeater community of an open-layered forest was studied in the New England National Park over a 2.5 year period. Various aspects of honeyeater behaviour (aggression, social and foraging) were examined during transects and by use of time-budgets. Time-budgets were used to estimate the daily energy requirements for each species. Since nectar availability in the study area was variable, the energy produced per day by the nectar sources was occasionally insufficient to meet the daily energy demands of the honeyeater community. No strong spatial partitioning of the nectar resources by the birds was found. Use of the resources appeared to be based primarily on dominance interactions (interspecific aggression), with the larger species tending to dominate the smaller ones. Smaller species survive in the area because of their behavioural attributes and greater foraging efficiency which enables them to use the sparse supply of nectar on poor days. Temporal gradients in nectar richness may be just as important as spatial gradients in permitting the coexistence of species which use the same resource in the same habitat. Most of the results from this study fail to support the predictions made elsewhere concerning the organization of communities in unpredictable environments.