Some population consequences of variation in preference among individual predators

A number of foraging studies have demonstrated that populations of predators rarely consist of individuals with identical preferences for particular types of prey. Variation among predators can lead to frequency-dependent changes in population preference, because those predators mat prefer the rarer type of prey generally have the greatest influence on population preference. In this study we develop a series of theoretical models which demonstrate how anti-apostatic selection (i.e. selection against the rare form) can arise out of (a) bimodal and (b) normal variation in preference among individuals of the same species. We show that population level anti-apostatic selection can occur even when individual predators show pro-apostatic selection (i.e. selection against the common form). Furthermore, patterns of population prey selection that arise out of variation in preference can potentially be pro-apostatic over one range of relative densities and anti-apostatic over another range of relative densities. Finally, we examine a case study involving predation by female waterboatmen Notonecta glauca and show that the variation in preference in this species is large enough to generate higher anti-apostatic selection than would be expected from the diet selected by the average individual.     

An East–West distribution of divergent mitochondrial haplotypes in British populations of the land snail, Cepaea nemoralis (Pulmonata)

Some continental European populations of the land snail Cepaea nemoralis have mitochondrial haplotypes that differ by up to 20% at the 16S rRNA locus. I mapped the distribution of different lineages in populations from 36 different sites in Britain and Ireland. In 93% of individuals, one of two mitochondrial lineages was found, A or. N , which differ from each other by about 6% using a 16S rRNA fragment {approximately 300 base pairs). The distribution of these two types is very striking–one is confined to Wales, West and central England, and Scotland, while the other is found mainly in East and central England. The two types meet in a transition zone. The most likely explanation for the distribution is that it reflects two routes of colonization after the last ice age. Cepaea dispersal is leptokurtic, and only limited gene flow occurs between established populations, so that the original pattern could have been retained since the post-glacial colonization. However, many environmental gradients are orientated East-West, so alternative selective explanations are possible. A distinct mitochondrial lineage, as well as fossil evidence, suggests that Ireland was colonized separately from Britain. The implications of these distributions for the origins of the puzzling geographical patterns of shell types known as 'area effects' is discussed.     

Recruitment variation in a stream galaxiid fish: multiple influences on fry dynamics in a heterogeneous environment

1. The relative importance of density‐dependent and density‐independent processes in explaining fluctuations in natural populations has been widely debated. In particular, the importance of larval supply and whether it may control the type of regulatory processes a population experiences has proved contentious. 2. Using surveys and field experiments conducted in streams in Canterbury, New Zealand, we investigated how variation in the survival of non‐migratory Galaxias vulgaris fry was affected by density‐dependent and density‐independent processes and how this variation influenced recruitment dynamics. 3. Fry populations with high settlement densities experienced a 70–80% reduction in population size from density‐related mortality during the first fourteen days after peak settlement but thereafter the influence of density‐dependent processes on fry was weak. The impact of environmental conditions on fry populations was dependent on fry size and the magnitude of the perturbation, such that flooding effects on fry survival were most severe when fry were small. 4. In streams not affected by flooding, the size and density of introduced trout (Salmo trutta and Oncorhynchus mykiss) were the most significant factors determining the abundance of eventual recruits. A field experiment manipulating brown trout access to fry populations revealed that trout as small as 110 mm may be capable of greatly reducing and possibly preventing galaxiid recruitment. 5. Overall, the results indicated density‐dependent population regulation was only possible at sites with high native fish densities because trout were likely to be suppressing the number of potential recruits at sites with low native fish numbers. Whilst density‐dependent processes had a strong effect on fry survival following the period of peak fry abundance, density‐independent processes associated with flow and predatory trout influences on fry survival largely determined recruitment variability among galaxiid populations. Focusing conservation efforts on improving habitat to increase fry retention and reducing the impacts of trout on galaxiids would ensure more native fish populations reached their potential abundance.     

Shell shape and habitat use in the North‐west Pacific land snail Mandarina polita from Hahajima,Ogasawara: current adaptation or ghost of species past?

The endemic land snail genus Mandarina of the Ogasawara Islands provides an excellent model system to investigate adaptive radiation. Previously, it has been shown that coexisting species of the islands segregate by microhabitat, so that they are either predominantly found on the ground in relatively wet and sheltered sites, dry and exposed sites, or else are arboreal. Moreover, shell morphology correlates with microhabitat, so that species in wet and sheltered sites tend to have high-spired shells with a high aperture, and those in dry and exposed sites tend to have relatively low-spired shells with a wide aperture. We have now found that on Hahajima, Mandarina polita have variable shell morphology, and there is a correlation between morphology and the depth of leaf litter, as well as the presence/absence of other terrestrial species. Specifically, when high-spired terrestrial Mandarina ponderosa is present, M. polita tend to be low-spired and have a large aperture, indicative of character displacement. When M. ponderosa is absent, the shell shape of M. polita is much more variable, the overall spire is higher, individuals are found in deeper litter, and there is a strong correlation between litter depth and spire height. We argue that these patterns are due to local adaptation, but it remains possible that they are an artefact due to the 'ghost of species past'.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 149–159.     

New Axonemal Dynein Heavy Chains from Tetrahymena thermophila

Two dyneins can be extracted from Tetrahymena ciliary axonemes. The 22S dynein contains three heavy chains (HC), sediments at 22S in a sucrose gradient, and makes up the outer arms. The 14S dynein contains two to six HCs, sediments at 14S, and is thought to contribute to formation of the inner arms. We have identified two large proteins that are extracted from Tetrahymena axonemes with high salt and that sediment together at approximately 18S. The two large proteins cleave when subjected to UV light in the presence of ATP and vanadate, suggesting both proteins are dynein HC. Antibodies against one of the 18S HCs do not recognize 22S dynein HCs. Antibodies to 22S dynein HC do not bind appreciably to 18S dynein photocleavage fragments. Taken together, these results indicate that the large proteins that sediment at 18S are axonemal dynein heavy chains.     

Location of demographic sources affects the distributions of a vulnerable native fish in invaded river networks

1. Invasive predators negatively affect native prey to varying degrees across landscapes, and spatial configuration of invader‐free refugia may affect prey distributions across invaded river networks. In New Zealand, introduced trout (Salmo trutta and Oncorhynchus mykiss) create source‐sink dynamics in native Galaxias vulgaris populations, and their co‐occurrence with trout may be enhanced by immigration from trout‐free reaches. 2. We investigated how network configuration of trout‐free demographic sources affected the distribution of G. vulgaris across trout‐invaded riverscapes. Using quantitative biomass surveys and spatially extensive presence–absence surveys, the interaction between habitat variability and location relative to sources in limiting distributions of G. vulgaris in trout‐invaded reaches was assessed. 3. Galaxias vulgaris biomass at invaded sink sites decreased with increasing network distance to the nearest trout barrier. The maximum distance to barriers at which G. vulgaris occurred in the riverscape was limited, so that galaxiids were excluded from small and stable streams far from sources. 4. Large predatory trout (i.e. >150 mm fork length) occurred in high densities at stable sites all year round and were seasonally excluded from sites disturbed by flooding. Large streams probably provide increased refugia for galaxiids to avoid predation from trout, but narrowness and habitat stability may act synergistically to extirpate G. vulgaris from sites that are too far from sources to receive regular immigrants. 5. The interaction between immigration and habitat configuration in mediating effects of trout on G. vulgaris distributions indicates the spatial dependency of predator–prey interactions in river networks. 6. These results indicate creating new invader‐free source habitat should enhance co‐occurrence in nearby invader‐occupied reaches. Moreover, adding source habitat in sink streams far from existing sources, and ensuring barriers prevent future invasion, will also allow native fish dispersal between sources and sinks and will maximise the conservation gains from management across invaded riverscapes.