Patch choice from a distance and use of habitat information during foraging by the parasitoid Ibalia leucospoides

1. In environments in which resources are distributed heterogeneously, patch choice and the length of time spent on a patch by foragers are subject to strong selective pressures. This is particularly true for parasitoids because their host foraging success translates directly into individual fitness. 2. The aim of this study was to test whether: (i) females of the parasitoid Ibalia leucospoides (Hymenoptera: Ibaliidae) can discriminate among patches according to host numbers; (ii) the surrounding context affects the initial choice of patch, as well as time spent on patch; and (iii) the perceived quality of a given patch is affected by the quality of the surrounding patches. 3. Each female was randomly exposed to one of three different three‐patch environments which differed in host number per patch, mean environment host number and host distribution among patches. For each treatment level, the first patch chosen and the time allocated to each patch visited by the female were recorded. 4. Females of I. leucospoides were able to discriminate different levels of host numbers among patches from a distance. The patch bearing the highest number of hosts was, predominantly, the first choice. Patch host number in association with mean habitat profitability influenced the length of time spent on the first patch visited. By contrast, variance in habitat profitability did not influence time allocation decisions. Contrary to the study prediction, there were no significant habitat‐dependent time allocation differences among patches holding the same number of hosts. 5. The results indicate that, for I. leucospoides, patch exploitation decisions are partially influenced by information obtained from the habitat as a whole, a behaviour that may prove to indicate adaptive ability in highly patchy environments, as well as suggesting the presence of good cognitive abilities in this parasitoid species.     

Annual wood production in a tropical rain forest in NE Costa Rica linked to climatic variation but not to increasing CO2

Increased atmospheric [CO₂] could theoretically lead to increased forest productivity (‘CO₂ fertilization’). This mechanism was hypothesized as a possible explanation for biomass increases reported from tropical forests in the last 30+ years. We used unique long‐term records of annually measured stands (eighteen 0.5 ha plots, 10 years) and focal tree species (six species, 24 years) to assess the effects of rainfall, temperature, and atmospheric [CO₂] on annual wood production in a neotropical rain forest. Our study area was a meso‐scale section (600 ha) of old‐growth Tropical Wet Forest in NE Costa Rica. Using the repeated remeasurements we directly assessed the relative effects of interannual climatic variation and increasing atmospheric [CO₂] on wood production. A remarkably simple two‐factor model explained 91% of the interannual variance in stand‐level tree growth; the statistically independent factors were total dry season rainfall (positive effect, r²=0.85) and night‐time temperature (negative effect, r²=0.42). Stand‐level tree mortality increased significantly with night‐time temperature. After accounting for dry season rainfall and night‐time temperature, there was no effect of annual [CO₂] on tree growth in either the stand or focal species data. Tree growth in this Tropical Wet Forest was surprisingly sensitive to the current range of dry season conditions and to variations in mean annual night‐time temperature of 1–2°. Our results suggest that wood production in the lowland rainforests of NE Costa Rica (and by extension in other tropical regions) may be severely reduced in future climates that are only slightly drier and/or warmer.     

The changing ecosystem of a shallow, brackish lake, Hickling Broad, Norfolk, U.K. I. Trophic relationships with special reference to the role of Neomysis integer

• 1 Hickling Broad, a shallow, brackish lake in England, changed from dominance by submerged aquatic plants to dominance by phytoplankton in the 1970s. These changes were ascribed to the effects of guanotrophicatioh by black-headed gulls, and increased salinity which together resulted in fish kills caused by a toxic alga, Prymnesium parvum. A mysid, Neomysis integer, was believed to be important in switching the system to plankton dominance through its presumed selective feeding on Cladocera and increasing population size as fish predation decreased.
• 2 Studies on laboratory predation rates of the mysid on Cladocera and on the population dynamics, predation rates and diet of the mysid and the populations of the major zooplankter, Eurytemora affinis in the Broad in the 1980s, have shown that the former explanations were incomplete. The mysid could have markedly reduced the cladoceran community, for it has potentially high predation rates, but Cladocera were probably lost through salinization. Neomysis feeds efficiently on Eurytentora affinis, but the latter reproduces rapidly and its populations are unlikely to be controlled by mysid predation. Nauplii and small copepodites are selectively taken. Alternative and probably major food sources for the mysid are periphyton and benthic algae and detritus.
• 3 Roach and bream readily take the mysid but the present fish stock in the Broad is very low and can now exert minimal pressure on the mysid population. An invertebrate predator on the mysid, Palaemometes varians, is also unlikely to have major effects. The former model of the operation of the Broad's ecosystem needs reconsideration in view of the findings.

     

The allocation of foragers in red wood ants

Abstract. 1. We studied how colonies of the red wood ant, Formica polyctena , adjust the numbers of foragers allocated to different foraging trails. In a series of field experiments, foragers were marked and transferred from one nest to another, related nest, where they joined the foraging force. Transferred workers acted as a reserve of uncommitted, available foragers.
2. Previous work shows that each individual forager habitually uses one trail. We found that for an uncommitted forager, the influence of recruitment initially is stronger than that of directional fidelity. Transferred workers were likely to use trails leading to new food sources. When transferred to a new nest, foragers were not likely to use a trail in the same direction as their original trail in the donor nest.
3. After a week, transferred foragers tended to develop route fidelity. Even after bait was no longer present, they continued to use the trail that had formerly led to a bait source.
4. We examined how colonies adjust numbers on a trail by experimentally depleting some trails. Colonies usually did not compensate for depletion: foragers were not recruited to depleted trails.
5. In general, the dynamics of foraging in this species facilitate a consistent foraging effort rather than rapid adjustments of forager allocation.     

Characterization of 20 microsatellite loci in the long‐tailed tit Aegithalos caudatus (Aegithalidae,AVES)

We characterized 20 microsatellite loci in the long‐tailed tit, Aegithalos caudatus. Polymorphic loci were identified by testing 114 loci that had been originally isolated in other avian species. The loci were characterized in 23–163 unrelated long‐tailed tits from a British population and displayed between two and 42 alleles, with observed heterozygosity ranging between 0.09 and 0.97.