Evolution Restricts the Coexistence of Specialists and Generalists - The Role of Trade-Off Structure

Environmental variability and adaptive foraging behavior have been shown to favor coexistence of specialists and generalists on an ecological time scale. This leaves unaddressed the question whether such coexistence can also be expected on an evolutionary time scale. In this article we study the attainability, through gradual evolution, of specialist-generalist coexistence, as well as the evolutionary stability of such communities when allowing for immigration. Our analysis shows that the potential for specialist-generalist coexistence is much more restricted than originally thought, and strongly depends on the trade-off structure assumed. We establish that ecological coexistence is less likely for species facing a trade-off between per capita reproduction in different habitats than when the trade-off acts on carrying capacities alone. We also demonstrate that coexistence is evolutionarily stable whenever it is ecologically stable, but that in most cases such coexistence cannot be reached through gradual evolution. We conclude that an evolutionarily stable community of specialists and generalists may only be created through immigration from elsewhere or through mutations of large effect. Our results highlight that trade-offs in fitness-determining traits can have counterintuitive effects on the evolution of specialization

Farmers perception of coconut mite damage and crop diversification alternatives in the coastal belt of Tanzania

Published online: 30 May 2012This article analysed farmers' perceptions of the effects of coconut mite in their livelihood and assessed crop diversification as a copping strategy for reduced coconut production. A socio-economic model of farmers' decisions on intercropping as an indicator for overall crop diversity was developed. The study was conducted between November 2009 and March 2010 in five districts in Tanzania, which were selected on the basis of the coconut's economic importance, using structured questionnaires which were administered to 200 household heads. Respondents were categorized in three groups – resource-poor farmers (43% of sample), medium-level farmers (50%) and well-off farmers (7%) according to six criteria. More than 80% of farmers were aware of the negative effects of the coconut mite. The result further indicated that the damaged nuts cause a loss of more than 30% of the cash income from coconut. Intercropping coconut with cassava, maize, cashew nut, sorghum and pineapples were the alternatives used by farmers to cope with declining coconut production caused by coconut mite and lethal yellowing disease. Land ownership and size, income from crops, non-farm income and family size were the main factors that influenced the farmer's decision to diversify crops. Although farmers diversify their cropping systems in order to be self-reliant, there is still a need to promote policies and programmes that will address coconut production constraints such pests and diseases such as rehabilitation of old plantations

Influence of prey density, species and developmental stages on the predatory behaviour of Amblyseius longispinosus (Acari: Phytoseiidae)

The influence of prey density, species and developmental stages on the predatory behaviour of Amblyseius longispinosus (Evans) was studied. A 24 h exposure revealed that gravid females were more voracious compared to young females. The trends in the number of eggs and larvae consumed by each young and gravid female predator were about the same, showing an increase with density of the red and the yellow strains of T. urticae levelling off at a prey density of 40 per predator. The highest mean number of eggs consumed in 24 h was 16.7 for the young female and 33.3 for the gravid female, and a mean high of 17 larvae in 24 h for the young female and 27.8 for the gravid female. With adult prey, however, the predators reached satiation point at a lower density of five to ten adult prey per female. In general, the response curves were adequately described by the Holling's Type II model. Under continuous exposure for five days, a significant reduction in consumption was observed with the gravid female from the second day onwards, to a level similar to the number of eggs and larvae consumed by a young female predator

Verified and potential pathogens of predatory mites (Acari: Phytoseiidae)

Several species of phytoseiid mites (Acari: Phytoseiidae), including species of the genera Amblyseius, Galendromus, Metaseiulus, Neoseiulus, Phytoseiulus and Typhlodromus, are currently reared for biological control of various crop pests and/or as model organisms for the study of predator¿prey interactions. Pathogen-free phytoseiid mites are important to obtain high efficacy in biological pest control and to get reliable data in mite research, as pathogens may affect the performance of their host or alter their reproduction and behaviour. Potential and verified pathogens have been reported for phytoseiid mites during the past 25 years. The present review provides an overview, including potential pathogens with unknown host effects (17 reports), endosymbiotic Wolbachia (seven reports), other bacteria (including Cardinium and Spiroplasma) (four reports), cases of unidentified diseases (three reports) and cases of verified pathogens (six reports). From the latter group four reports refer to Microsporidia, one to a fungus and one to a bacterium. Only five entities have been studied in detail, including Wolbachia infecting seven predatory mite species, other endosymbiotic bacteria infecting Metaseiulus (Galendromus, Typhlodromus) occidentalis (Nesbitt), the bacterium Acaricomes phytoseiuli infecting Phytoseiulus persimilis Athias-Henriot, the microsporidium Microsporidium phytoseiuli infecting P. persimilis and the microsporidium Oligosproridium occidentalis infecting M. occidentalis. In four cases (Wolbachia, A. phytoseiuli, M. phytoseiuli and O. occidentalis) an infection may be connected with fitness costs of the host. Moreover, infection is not always readily visible as no obvious gross symptoms are present. Monitoring of these entities on a routine and continuous basis should therefore get more attention, especially in commercial mass-production. Special attention should be paid to field-collected mites before introduction into the laboratory or mass rearing, and to mites that are exchanged among rearing facilities. However, at present general pathogen monitoring is not yet practical as effects of many entities are unknown. More research effort is needed concerning verified and potential pathogens of commercially reared arthropods and those used as model organisms in research

Effects of kinship or familiarity? Small thrips larvae experience lower predation risk only in groups of mixed-size siblings

In many species of insects, larvae are distributed in an aggregated fashion. As they may differ in size and size matters to predation risk, small larvae may be less likely to fall prey to predators when near large and therefore better-defended larvae. We hypothesize that the small larvae may profit even more when these large larvae are siblings. We tested this hypothesis on kinship-dependent survival in groups of larvae of the Western flower thrips (Frankliniella occidentalis) exposed to a predatory mite (Iphiseius degenerans). Our experiments showed that small larvae in sibling groups survive significantly better than in non-sibling groups, but only when such groups consisted of a mixture of small and large larvae. To test whether the survival effect we found is due to familiarity of thrips larvae growing up together (i.e. on one leaf), we also measured survival in sibling groups of larvae grown up on different leaves and in non-sibling groups of larvae grown up on the same leaf. These experiments showed an increased survival of small thrips larvae only in groups of sibling larvae from the same leaf. Non-sibling larvae did not show an increased survival when they come from the same leaf. Our results indicated that the increased survival in sibling groups was only partly due to the familiarity effect we tested. Growing up together did not return the same survival effect for non-siblings as it did for siblings. We conclude that growing up together is a necessary but not sufficient condition for discrimination in thrips larvae