The generalism–specialism debate: the role of generalists in the life and death of species

Specialisms on resources and for niches, leading to specialization, have been construed to be tantamount to speciation and vice versa, while the occurrence of true generalism in nature has also been questioned. We argue that generalism in resource use, biotope occupancy, and niche breadth not only exists, but also forms a crucial part in the evolution of specialists, representing a vital force in speciation and a more effective insurance against extinction. We model the part played by generalism and specialism in speciation and illustrate how a balance may be maintained between the number of specialists and generalists within taxa. The balance occurs as an ongoing cycle arising from turnover in the production of specialists and generalists, speciation, and species extinction. The nature of the balance depends on the type of resources exploited, biotopes, and niche space occupied. These vary between different regions and create taxonomic biases towards generalists or specialists. We envisage that the process may be sympatric/parapatric, although it is more likely initiated by allopatry driven by abiotic forces. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 725–737.     

Effective population size and evolutionary dynamics in outbred laboratory populations of Drosophila

Census population size, sex-ratio and female reproductive success were monitored in 10 laboratory populations of Drosophila melanogaster selected for different ages of reproduction. With this demographic information, we estimated eigenvalue, variance and probability of allele loss effective population sizes. We conclude that estimates of effective size based on gene-frequency change at a few loci are biased downwards. We analysed the relative roles of selection and genetic drift in maintaining genetic variation in laboratory populations of Drosophila. We suggest that rare, favourable genetic variants in our laboratory populations have a high chance of being lost if their fitness effect is weak, e.g. 1% or less. However, if the fitness effect of this variation is 10% or greater, these rare variants are likely to increase to high frequency. The demographic information developed in this study suggests that some of our laboratory populations harbour more genetic variation than expected. One explanation for this finding is that part of the genetic variation in these outbred laboratory Drosophila populations may be maintained by some form of balancing selection. We suggest that, unlike bacteria, medium-term adaptation of laboratory populations of fruit flies is not primarily driven by new mutations, but rather by changes in the frequency of preexisting alleles.     

Lake responses to reduced nutrient loading – an analysis of contemporary long-term data from 35 case studies

1. This synthesis examines 35 long‐term (5–35 years, mean: 16 years) lake re‐oligotrophication studies. It covers lakes ranging from shallow (mean depth <5 m and/or polymictic) to deep (mean depth up to 177 m), oligotrophic to hypertrophic (summer mean total phosphorus concentration from 7.5 to 3500 μg L^?1 before loading reduction), subtropical to temperate (latitude: 28–65°), and lowland to upland (altitude: 0–481 m). Shallow north‐temperate lakes were most abundant. 2. Reduction of external total phosphorus (TP) loading resulted in lower in‐lake TP concentration, lower chlorophyll a (chl a) concentration and higher Secchi depth in most lakes. Internal loading delayed the recovery, but in most lakes a new equilibrium for TP was reached after 10–15 years, which was only marginally influenced by the hydraulic retention time of the lakes. With decreasing TP concentration, the concentration of soluble reactive phosphorus (SRP) also declined substantially. 3. Decreases (if any) in total nitrogen (TN) loading were lower than for TP in most lakes. As a result, the TN : TP ratio in lake water increased in 80% of the lakes. In lakes where the TN loading was reduced, the annual mean in‐lake TN concentration responded rapidly. Concentrations largely followed predictions derived from an empirical model developed earlier for Danish lakes, which includes external TN loading, hydraulic retention time and mean depth as explanatory variables. 4. Phytoplankton clearly responded to reduced nutrient loading, mainly reflecting declining TP concentrations. Declines in phytoplankton biomass were accompanied by shifts in community structure. In deep lakes, chrysophytes and dinophytes assumed greater importance at the expense of cyanobacteria. Diatoms, cryptophytes and chrysophytes became more dominant in shallow lakes, while no significant change was seen for cyanobacteria. 5. The observed declines in phytoplankton biomass and chl a may have been further augmented by enhanced zooplankton grazing, as indicated by increases in the zooplankton : phytoplankton biomass ratio and declines in the chl a : TP ratio at a summer mean TP concentration of <100–150 μg L^?1. This effect was strongest in shallow lakes. This implies potentially higher rates of zooplankton grazing and may be ascribed to the observed large changes in fish community structure and biomass with decreasing TP contribution. In 82% of the lakes for which data on fish are available, fish biomass declined with TP. The percentage of piscivores increased in 80% of those lakes and often a shift occurred towards dominance by fish species characteristic of less eutrophic waters. 6. Data on macrophytes were available only for a small subsample of lakes. In several of those lakes, abundance, coverage, plant volume inhabited or depth distribution of submerged macrophytes increased during oligotrophication, but in others no changes were observed despite greater water clarity. 7. Recovery of lakes after nutrient loading reduction may be confounded by concomitant environmental changes such as global warming. However, effects of global change are likely to run counter to reductions in nutrient loading rather than reinforcing re‐oligotrophication.     

Primers and polymerase chain reaction conditions for DNA barcoding teleost fish based on the mitochondrial cytochrome b and nuclear rhodopsin genes

This report describes a set of 21 polymerase chain reaction primers and amplification conditions developed to barcode practically any teleost fish species according to their mitochondrial cytochrome b and nuclear rhodopsin gene sequences. The method was successfully tested in more than 200 marine fish species comprising the main Actinopterygii family groups. When used in phylogenetic analyses, its combination of two genes with different evolutionary rates serves to identify fish at the species level. We provide a flow diagram indicating our validated polymerase chain reaction amplification conditions for barcoding and species identification applications as well as population structure or haplotyping analyses, adaptable to high‐throughput analyses.     

Sources of protein in two semi‐arid zone mistletoe specialists: Insights from stable isotopes

Obtaining adequate levels of dietary protein is essential for the physiology of consumers. This presents potential problems for frugivorous birds because fruit is generally low in protein rendering it nutritionally inadequate and potentially explaining the rarity of exclusive frugivory in birds. We addressed this issue by determining the isotope composition (¹⁵N/¹⁴N) in the whole blood of two mistletoe consumers, that is, painted honeyeater (Grantiella picta, Meliphagidae) and mistletoebird (Dicaeum hirundinaceum, Dicaeidae) during the grey mistletoe (Amyema quandang, Loranthaceae) fruiting peak in a semi‐arid woodland, NSW, Australia. Grey mistletoe fruit pulp and arthropods were isotopically distinct (mean δ¹⁵N fruit 4.4‰vs. arthropods 7.1‰), thus readily discriminated using the stable isotope approach. Painted honeyeaters and mistletoebirds formed a single group based on their mean δ¹⁵N values and, on average, assimilated approximately half of their nitrogen from mistletoe fruit although individual variation was high. The importance of nitrogen derived from mistletoe fruit did not track its abundance in the environment, suggesting that at least during peak fruiting, this resource is not limiting at this site. Researchers should account for intraspecific variation and take a cautious approach when reconstructing diets using stable isotopes by incorporating individual‐based analyses rather than presenting mean values alone. This is the first study to use the isotope approach to investigate the dietary relationship of mistletoe frugivores and mistletoe fruit and has implications for our understanding of the nutritional ecology of frugivores and its functional relationship to ecosystem processes such as seed dispersal.     

A Remotely Operated Motorized Burrow Probe to Investigate Carnivore Neonates

Abstract: We describe a miniaturized infrared camera mounted on a remotely operated platform (burrow probe system) that we developed to investigate spotted hyena (Crocuta crocuta) cubs while still in their natal dens. In conjunction with the burrow probe system, we used a hook system to retrieve carcasses from within burrows. To test the utility of these systems, we compared results obtained using the burrow probe and hook to results obtained from intensive above-ground monitoring alone. With the probe, we documented births and litter sizes at significantly younger ages than with above-ground monitoring. The probe was instrumental in detecting cubs that died prior to emergence from the natal den. Retrieval of carcasses allowed collection of genetic samples and data on sex and cause of death. Thus, the burrow probe and hook systems can provide scientists and managers with empirical data on productivity and neonatal mortality of carnivores in the wild; data essential to accurately monitor and model population trends.     

ORAL ASSESSMENT OF HARDNESS BETWEEN ELASTIC AND PLASTIC PRODUCTS

This study examined how people compare the hardness of different materials. Simple models with either elastic or plastic behavior were prepared and controlled mechanically. Six elastomers were compared to 29 plastic dental waxes of different hardness. For each elastic sample, a psychophysical staircase method was used to determine its perceived hardness in comparison to the plastic samples. Single bites were performed and the forces were recorded by a small load-cell placed between the sample and the teeth. Nine subjects. free of dental pathology, participated in this study. Subjects were able to match samples of both materials with a specific stress ratio which depended on the hardness of the elastic samples. Results indicated that people do not use the same sensory cues and, in this case, no clear role of the bite force was established.     

Stage of Development Response and Recovery of Radish Plants from Episodic Exposure to Simulated Acidic Rain

Greenhouse-grown radish plants were exposed to repeated applicationsof simulated acidic rain at pH values from 2·6 to 5·0in order to determine whether growth and yield responses toacidic rain change with stage of development and whether plantshave the capacity to recover from injury during rain-free intervals.Solutions contained low concentrations of cations and anionscommon to rainfall of the eastern U.S. and sulphate to nitratemass ratios of 2/1. One-hour rain events were simulated by applicationof solutions through rain nozzles to plants on rotating turntables. Seedling were more susceptible to repeated applications of simulatedacidic rain than older plants as indicated by foliar injuryand reductions in the dry mass of shoots and hypocotyls. However,exposures at an intermediate stage when plants were growingmost rapidly caused the greatest reductions in dry mass of marketablesize hypocotyls. A rain-free interval, after a series of exposuresto simulated acidic rain, allowed plants to recover from injuryand compensate for growth reductions. Lengthening the durationof rain-free intervals between exposures to simulated acidicrain enhanced the capacity of plants to recover. These resultsindicate that the interaction of increased tolerana to acidityat certain growth stages and recovery from injury during rain-frecintervals with the episodic nature of rainfall should be consideredwhen determinations are made of reductions in yield from repeatedexposures to rainfall with a pH sufficient to cause foliar injury(below approximately pH 3·4). Key words: Acidic rain, Radish, Growth stage, Recovery