Density-dependent prey behaviours and mutable predator foraging modes induce Allee effects and over-prediction of prey mortality rates

1. Predator–prey models are often used to represent consumptive interactions between species but, typically, are derived using simple experimental systems with little plasticity in prey or predator behaviours. However, many prey and predators exhibit a broad suite of behaviours. Here, we experimentally tested the effect of density-dependent prey and predator behaviours on per capita relative mortality rates using Florida bass (Micropterus floridanus) consuming juvenile Bluegill (Lepomis macrochirus).
2. Experimental ponds were stocked with a factorial design of low, medium, and high prey and predator densities. Prey mortality, prey–predator behaviours, and predator stomach contents were recorded over or after 7 days. We assumed the mortality dynamics followed foraging arena theory. This pathologically flexible predator–prey model separates prey into invulnerable and vulnerable pools where predators can consume prey in the latter. As this approach can represent classic Lotka–Volterra and ratio-dependent dynamics, we fit a foraging arena predator–prey model to the number of surviving prey.
3. We found that prey exhibited density-dependent prey behaviours, hiding at low densities, shoaling at medium densities, and using a provided refuge at high densities. Predators exhibited ratio-dependent behaviours, using an ambush foraging mode when one predator was present, hiding in the shadows at low prey–high predator densities, and shoaling at medium and high prey–high predator densities. The foraging arena model predicted the mortality rates well until the high prey–high predator treatment where group vigilance prey behaviours occurred and predators probably interfered with one another resulting in the model predicting higher mortality than observed.
4. This is concerning given the ubiquity of predator–prey models in ecology and natural resource management. Furthermore, as Allee effects engender instability in population regulation, it could lead to inaccurate predictions of conservation status, population rebuilding or harvest rates.

     

Altered Immunity of Laboratory Mice in the Natural Environment Is Associated with Fungal Colonization

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A low‐cost mesocosm for the study of behaviour and reproductive potential in Afrotropical mosquito (Diptera: Culicidae) vectors of malaria

A large‐scale mesocosm was constructed and tested for its effectiveness for use in experiments on behaviour, reproduction and adult survivorship in the Afrotropical malaria vector Anopheles gambiae s.s. Giles (Diptera: Culicidae) in temperate climates. The large space (82.69 m³) allowed for semi‐natural experiments that increased demand on a mosquito's energetic reserves in an environment of widely distributed resources. A one‐piece prefabricated enclosure, made with white netting and vinyl, prevented the ingress of predators and the egress of mosquitoes. Daylight and white materials prompted the mosquitoes to seclude themselves in restricted daytime resting sites and allowed the easy collection of dead bodies so that daily mortality could be assessed accurately using a method that accounts for the loss of a proportion of bodies. Here, daily, age‐dependent mortality rates of males and females were estimated using Bayesian Markov chain Monte Carlo simulation. In overnight experiments, mosquitoes successfully located plants and took sugar meals. A 3‐week survival trial with a single cohort demonstrated successful mating, blood feeding, oviposition and long life. The relatively low cost of the mesocosm and the performance of the mosquitoes in it make it a viable option for any behavioural or ecological study of tropical mosquitoes in which space and seasonal cold are constraining factors.     

Contribution of biological processes to self-purification of water with respect to petroleum hydrocarbon associated with No. 0 diesel in Changjiang Estuary and Jiaozhou Bay, China

Mesocosms in 25-m³ ethylene vinyl acetate or 4-m³ polyethylene bags were performed at two sites in China: Changjiang Estuary in spring–summer 1998, and Jiaozhou Bay in autumn 1999 and spring–summer 2000. The experiments were designed to show the contributions of physical, physicochemical and biological processes to self-purification of water with respect to the water-soluble fraction (WSF) of petroleum hydrocarbon associated with No.0 diesel (PHAD). Over the experimental periods, the WSF-PHAD concentration in water declined significantly. A kinetic model for WAS-PHAD distribution in multiphase environments – in this case water (W), suspended sediment (SS), phytoplankton (PPT), zooplankton (ZPT) and atmosphere (ATM) – was developed to estimate the contributions of the various processes. In the model, the key processes responsible for self-purification include (1) physical transport by volatilization from W to ATM; (2) biological transformation through biodegradation; (3) biological transfer involving bioconcentration by PPT and ZPT; and (4) physicochemical transfer through sorption by SS. Model parameters were determined from the mesocosm experiments and parallel laboratory experiments, or taken from the literature. It indicated that volatilization plays an important role in short-term self-purification especially regarding alkanes with relatively low molecular weight, accounting for 82.9±2.5%. After the volatilization stops, bioconcentration by PPT contributes greatly to long-term self-purification especially regarding alkanes of higher molecular weight and polycyclic aromatic hydrocarbons (PHAs), accounting for 0.7 – 17.8% with an average of 7.8±7.3%. The bioconcentration is dominated by biomass of PPT as well as the WSF-PHAD concentration. However, the biodegradation process makes the greatest contribution to both the short- and long-term self-purification, affecting alkanes of both lower and higher molecular weight as well as PHAs. The results suggest that it is necessary to take into account the biological processes of biodegradation and bioconcentration in estimating the self-purification of water with respect to oil pollutants.     

Can chaoborid larvae control their zooplanktonic prey populations in a neotropical lake?

Although it is well known that predatory Chaoborus Lichtenstein 1800 larvae have a strong negative impact on zooplankton populations in many Holarctic lakes, this top‐down control is still not entirely clear in the Neotropics. In Lake Carioca, a Brazilian monomictic lake, chaoborids became one of the main zooplankton predators after the depletion of planktivorous fish by non‐native piscivorous fish. The aim of this research was to test the hypothesis that Chaoborus spp. larvae control the density of their prey populations. Enclosures simulated conditions with higher (Pr+) and lower (Pr?) predatory Chaoborus spp. larva densities over 30 days in two periods (stratification and mixing). Our results indicated that the dominant copepod species was affected only in the stratification period when its population was smaller. In this circumstance, predated individuals were probably not replaced in the same proportion by reproductive output of the smaller population. Rotifers were not sensitive to changes in chaoborids densities in both periods. The lack of a strong top‐down effect recorded by our mesocosm experiments emphasises the importance of continuing to investigate the ecological roles played by this invertebrate predator in aquatic food webs and if its ability to regulate the zooplankton in most Holarctic lakes can (or cannot) be widely generalised to Neotropical communities. Such a result also suggests that alternative research approaches, besides in‐situ experiments, may be necessary to better clarify this topic. Specifically for cladocerans, parallel paleolimnological analyses in Lake Carioca have supported the idea of a possible negative impact of Chaoborus larvae on bosminids after the introduction of non‐native piscivorous fishes. Abstract in Portuguese is available with online material.     

Littoral macrofauna (secondary) responses to experimental nutrient addition to rocky shore mesocosms and a coastal lagoon

Macrofauna secondary responses to controlled eutrophication within two MARICULT/MAST-III projects, EULIT and COMWEB, are compared. EULIT utilises a nutrient gradient established in eight hard bottom mesocosms, whereas the data from COMWEB originate from a whole-ecosystem study – the case of experimental nutrient addition to Hopavågen lagoon. In both systems, nutrient addition started in May 1998, after initial studies of the background macrofauna communities, thus allowing application of Before-After-Control-Impact-Pairs techniques (BACIP). The main objectives have been to investigate the macrofauna responses to eutrophication in littoral rocky shore ecosystems and to evaluate if similar responses could occur in mesocosm and `field' systems, despite their inherent differences. Apart from a distinct increase in numbers of Littorina littorea L. and some indications of increased abundance of the genus Jaera, no significant changes, caused by 2.5 years of nutrient addition, could be detected in the mesocosm fauna. It is interesting, however, that these two possibly stimulated animal groups have two things in common: (1) both belong to the rather few mobile taxa, which are not flushed out of the system through the mesocosm outlets, (2) both feed on microalgae and green algae in the upper littoral zone, i.e., algal groups that have shown the clearest response to the nutrient addition. In Hopavågen, 1.5 years of nutrient addition only caused modest plant and animal responses in the rocky shore ecosystem, although there was a markedly increased settlement of Mytilus edulis L. at one lagoon site. This increase was probably due to elevated levels of digestible particles (increased phytoplankton production) caused by the nutrient addition. There were also some indications of increased production of filamentous algae at this same lagoon site. All in all, both studies demonstrate only minor responses to increased nutrient levels within the littoral community (both plants and animals). One explanation to this may be that longer time-scales are needed in order for clear-cut changes to occur. Several observations also indicate that wave exposure (significant in both systems) may largely modulate the impact of increased nutrient load on the structure of littoral communities. Another explanation is the high degree of stability among littoral macroalgae-dominated communities with internal biological regulation factors (like grazing) possibly being able to counteract effects.     

Warming and depth interact to affect carbon dioxide concentration in aquatic mesocosms

1. Climate change may significantly influence lake carbon dynamics and consequently the exchange of CO₂ with the atmosphere. Warming will accelerate multiple processes that either absorb or release CO₂, making predicting the net effect of warming on CO₂ exchange with the atmosphere difficult. Here we experimentally test how the CO₂ flux of deep and shallow systems responds to warming. To do this, we conducted a greenhouse experiment using mesocosms of two depths, experiencing either ambient or warmed temperatures. 2. Deeper mesocosms were found to have a lower average CO₂ concentration than shallower mesocosms under ambient temperature conditions. In addition, warming interacts with mesocosm depth to affect the average CO₂ concentration; there was no effect of warming on the average CO₂ concentration of deep mesocosms, but shallow mesocosms had significantly lower average CO₂ concentrations when warmed. 3. The difference in CO₂ concentration resulting from the depth manipulation was due to varying loss rates of particulate carbon to the sediments. There was a strong negative correlation between CO₂ and sedimentation rates in the deep mesocosms suggesting that high particulate carbon loss to the sediments lowered the CO₂ concentration in the water column. There was no correlation between CO₂ and sedimentation rates observed for shallow mesocosms suggesting enhanced carbon regeneration from the sediments was maintaining higher CO₂ concentrations in the water column. 4. Relationships between CO₂ and algal concentrations indicate that the reduction in CO₂ concentrations resulting from warming is due to increased per capita algal turnover rates depleting CO₂ in the water column. Our results suggest that the carbon dynamics and CO₂ flux of shallow systems will be affected more by climate warming than deep systems and the net effect of warming is to increase CO₂ uptake.