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A device to study the behavioral responses of zooplankton to food quality and quantity
No full textIn order to explore the behavioral mechanisms underlying aggregation of
foragers on local resource patches, it is necessary to manipulate the location, quality
and quantity of food patches. This requires careful control over the conditions in the
foraging arena, which may be a challenging task in the case of aquatic resourceconsumer
systems, like that of freshwater zooplankton feeding on suspended algal
cells. We present an experimental tool designed to aid behavioral ecologists in
exploring the consequences of resource characteristics for zooplankton aggregation
behavior and movement decisions under conditions where the boundaries and characteristics
(quantity and quality) of food patches can be standardized. The aggregation
behavior of Daphnia magna and D. galeata x hyalina was tested in relation to i)
the presence or absence of food or ii) food quality, where algae of high or low nutrient
(phosphorus) content were offered in distinct patches. Individuals of both Daphnia
species chose tubes containing food patches and D. galeata x hyalina also showed a
preference towards food patches of high nutrient content. We discuss how the
described equipment complements other behavioral approaches providing a useful tool to understand animal foraging decisions in environments with heterogeneous
resource distributions.
Single-cell versus population-level reproductive success of bacterial immigrants to pre-colonized leaf surfaces
No full textWe assessed how preemptive inoculation of plant leaves with bacteria affected the establishment of secondary colonizers. We quantified the latter in two ways: (i) at the population level, i.e. as counts of colony-forming units and (ii) at the level of single cells by tracking the reproductive success of individual bacteria. Both analyses showed that the ability of secondary immigrants to establish on the leaf was negatively correlated with the level of pre-population by primary colonizers. This effect was best described by an inverse dose–response curve with an apparent half-point inhibition efficacy of approximately 106 cells of primary colonizers per gram leaf. This efficacy was the same whether calculated from population- or average single-cell data. However, single-cell data revealed that even under conditions of heavy pre-population with primary colonizers, a small fraction of secondary immigrants still produced offspring, although the corresponding population measurement showed no increase in total population size. This observation has direct relevance for biocontrol strategies that are based on the principle of preemptive exclusion of foliar bacterial pathogens: even at seemingly saturating levels of primary inoculum, some secondary colonizers may still be able to reproduce and possibly reach a quorum to trigger behaviours that enhance survival or virulence.
Variation in eggshell traits between geographically distant populations of pied flycatchers Ficedula hypoleuca
No full textThe expression and impact of maternal effects may vary greatly between populations and environments. However, little is known about large-scale geographical patterns of variation in maternal deposition to eggs. In birds, as in other oviparous animals, the outermost maternal component of an egg is the shell, which protects the embryo, provides essential mineral resources and allows its interaction with the environment in the form of gas exchange. In this study, we explored variation of eggshell traits (mass, thickness, pore density and pigmentation) across 15 pied flycatcher populations at a large geographic scale. We found significant between-population variation in all eggshell traits, except in pore density, suggesting spatial variation in their adaptive benefits or in the females’ physiological limitations during egg laying. Between- population variation in shell structure was not due to geographic location (latitude and longitude) or habitat type. However, eggshells were thicker in populations that experienced higher ambient temperature during egg laying. This could be a result of maternal resource allocation to the shell being constrained under low temperatures or of an adaptation to reduce egg water loss under high temperatures. We also found that eggshell colour intensity was positively associated with biliverdin pigment concentration, shell thickness and pore density. To conclude, our findings reveal large- scale between-population variation of eggshell traits, although we found little environmental dependency in their expression. Our findings call for further studies that explore other environmental factors (e.g. calcium availability and pollution levels) and social factors like sexual selection intensity that may account for differences in shell structure between populations.
How to measure top-down vs. bottom-up effects: A new population metric and its calibration on Daphnia
No full textResearch on the role of top–down (predation) and bottom–up (food) effects in food webs has led to the understanding that the variability of these effects in space and time is a fundamental feature of natural systems. Consequently, our measurement tools must allow us to evaluate the effects from a dynamical perspective. A population-dynamics approach may be appropriate to the task. More specifically, because food and predators both affect birth rate, birth rate dynamics may be a key to understanding their impact on the population of interest. Based on the Edmondson–Paloheimo model for birth rate, we propose a new population metric to assess the relative strength of top–down vs bottom–up effects. The metric is the ratio of contributions of changes in proportion of adults and fecundity to change in birth rate. Proportion of adults reflects a top–down effect (predators are assumed to be size-selective), fecundity reflects a bottom–up effect, and birth rate appears as a common currency with which to compare the former and the latter. Using microcosm experiments and computer simulations on the cladoceran Daphnia, we calibrate the metric and show that, in both types of tests, the ratio of contributions is typically 0.5–0.7 under a strong bottom–up effect and 2.0–2.2 under a strong top–down effect. This provides experimental evidence that the ratio of contributions may allow one to distinguish a strong top–down effect from a strong bottom–up effect.
Sommige or sommigen? Factors conditioning the written –e or –n of independently used quantifiers
No full textThe effect of small doses of toxic cyanobacterial food on temperature response of Daphnia galeata: Is bigger better?
No full text1.Growing evidence shows that cyanobacterial blooms thrive in warmer temperatures, underlining the importance of understanding the relationship between cyanobacterial food, temperature and performance of the key freshwater herbivore Daphnia.
2.To evaluate potential effects of toxic cyanobacterial food and warmer temperatures, we first offered populations of Daphnia galeata a mixture of a low dosage of toxic non-colony-forming cyanobacteria [Microcystis aeruginosa (PCC7806 strain) and non-toxic, good-quality food (Scenedesmus obliquus) and S. obliquus as a sole food source]. After 2 weeks on the different diets, we exposed the daphnids to different temperatures (15 °C, 20 °C and 25 °C) for 2 weeks.
3.We expected that short-term exposure to toxins would reduce population growth, but populations would consist of larger individuals (‘bigger is better’). Upon long-term exposure, however, daphnids switched to higher temperature would be forced to mature at a smaller size and produce smaller, more vulnerable, offspring with subsequent reductions in population size (‘hotter is smaller’). Conversely, populations of daphnids switched to lower temperature should be able to cope better with prolonged exposure to toxins by producing more and larger offspring, less vulnerable to toxins (‘bigger is better’).
4.The amount of the toxin microcystin incorporated in the Daphnia tissue decreased in the first 7 days of exposure, suggesting triggering of detoxification mechanisms upon first exposure. In the remainder of the experiment, the amount of microcystin incorporated in Daphnia tissue increased to high levels, showing the differences between short-term exposure and long-term exposure to low-dosage toxins.
5.Contrary to our expectations, short-term exposure to low dosages of toxic Microcystis resulted in daphnids attaining a higher population density on the mixed diet. Furthermore, on mixed diets daphnids did not increase their body mass (‘bigger is not better’), but rather decreased strongly compared to daphnids growing on pure Scenedesmus diets. A potential explanation for this observation could be a positive hormetic response on short-term exposure to low-dosage toxin, leading to individuals living longer.
6.Prolonged exposure (>14 days) to low-dosage Microcystis resulted in a strong decrease in population densities, regardless of temperature. This decrease in population densities seemed to show a lagged response to the observed decreases in clutch size. On average, clutch size was much smaller on a low-dosage Microcystis diet than on a pure Scenedesmus diet. However, this difference in response between the two diets was smaller when daphnids were exposed to higher temperatures, as daphnids growing on pure Scenedesmus diet at higher temperatures had reduced clutch size.
7.Our observations indicate that the population-level responses to prolonged exposure to low-dosage cyanobacterial toxin mask potential responses to changes in temperatures. Warmer temperatures and toxins in diet seem to trigger similar responses in Daphnia populations: smaller individuals with smaller clutch sizes. The combined effect of toxic cyanobacteria and warmer temperatures on Daphnia populations might be additive rather than synergistic.
Hyperspectral reflectance of leaves and flowers of an outbreak species discriminates season and successional stage of vegetation
No full texthttp://www.sciencedirect.com/science/journal/03032434
