Jon Loman

CAN VERTEBRATE PREDATORS REGULATE THEIR PREY

Whether vertebrate predators can regulate their prey or not has long been a controversial question. At the one extreme it has been claimed that predators have no impact on prey numbers but consume only a doomed surplus (Errington 1946), and at the other that predators strongly interact with their prey causing either stable equilibria or cycles (Tanner 1975; Keith et al. 1977). However,.there are no field studies showing a regulatory effect of predation among vertebrates. Here we report on such a study.

Increased fitness from multiple matings, and genetic heterogeneity: a model of a possible mechanism

Females from various taxa may mate with several males before the conception of a brood. Examples include insects (Cobbs 1977), snakes (Stille et al. 1986), birds (Gladstone 1979, Gowaty and Karlin 1984), and mammals (Bertram 1975, Hanken and Sherman 1981). Several explanations have been suggested to account for the evolutionary background for this behaviour in particular groups of animals (e.g. langurs, Hrdy 1974; lions, Bertram 1975; blue birds, Gowaty and Karlin 1984). However, most of the proposed explanations are based on female-male social interactions after mating (eg. male paternal care, male infanticide). Here we present a model that is applicable to species where male contribution to the female is restricted to his transfer of

Temperature, genetic and hydroperiod effects on metamorphosis of brown frogs Rana arvalis and R. temporaria in the field

Time for metamorphosis and metamorph size of moor frogs Rana arvalis and common frogs R. temporaria were measured in 22 ponds for 18 years. Environmental data in these ponds were also measured. Metamorphosis of the frogs took place from the beginning of June to the beginning of August. When both species were found in one pond, the common frogs metamorphosed up to 20 days earlier than the moor frogs. Most variation in time for metamorphosis, among ponds and years, is explained by temperature effects, but a causal relationship was not established. Within a pond, metamorphosis was later in cold summers than in warm summers. Size at metamorphosis was affected by tadpole density; at high densities metamorphs were smaller. Size at metamorphosis was not related to time for metamorphosis. There was an effect of pond drying; if ponds were about to dry up, metamorphosis was accelerated by about 2.4 days. Tadpoles from ponds with a late metamorphosis in the field had tadpoles that metamorphosed early in a common garden experiment, suggesting counter gradient selection.Time for metamorphosis and metamorph size of moor frogs Rana arvalis and common frogs R. temporaria were measured in 22 ponds for 18 years. Environmental data in these ponds were also measured. Metamorphosis of the frogs took place from the beginning of June to the beginning of August. When both species were found in one pond, the common frogs metamorphosed up to 20 days earlier than the moor frogs. Most variation in time for metamorphosis, among ponds and years, is explained by temperature effects, but a causal relationship was not established. Within a pond, metamorphosis was later in cold summers than in warm summers. Size at metamorphosis was affected by tadpole density; at high densities metamorphs were smaller. Size at metamorphosis was not related to time for metamorphosis. There was an effect of pond drying; if ponds were about to dry up, metamorphosis was accelerated by about 2.4 days. Tadpoles from ponds with a late metamorphosis in the field had tadpoles that metamorphosed early in a common garden experiment, suggesting counter gradient selection.Time for metamorphosis and metamorph size of moor frogs Rana arvalis and common frogs R. temporaria were measured in 22 ponds for 18 years. Environmental data in these ponds were also measured. Metamorphosis of the frogs took place from the beginning of June to the beginning of August. When both species were found in one pond, the common frogs metamorphosed up to 20 days earlier than the moor frogs. Most variation in time for metamorphosis, among ponds and years, is explained by temperature effects, but a causal relationship was not established. Within a pond, metamorphosis was later in cold summers than in warm summers. Size at metamorphosis was affected by tadpole density; at high densities metamorphs were smaller. Size at metamorphosis was not related to time for metamorphosis. There was an effect of pond drying; if ponds were about to dry up, metamorphosis was accelerated by about 2.4 days. Tadpoles from ponds with a late metamorphosis in the field had tadpoles that metamorphosed early in a common garden experiment, suggesting counter gradient selection.A correction to this article is found in Journal of Zoology (2003)260(2)p.217-217 (Less)

Early metamorphosis in common frog Rana temporaria tadpoles at risk of drying: an experimental demonstration

Does the development rate of common frog tadpoles accelerate if their habitat dries? To study this, the water level in experimental tanks was reduced shortly before time of metamorphosis. Water level remained high in control tanks. The experiment was performed at two different tadpole densities and replicated four times, with tadpoles from different source ponds. The experimental treatment, simulating a drying pond, resulted in earlier metamorphosis while no significant difference in size at metamorphosis was found. Resources per capita decreased as a result of the decreased water level so the increase in development rate was not an effect of feeding conditions. Temperatures in the tanks were such that it is unlikely that the increased development rate was due to temperature effects. I interpret the advancement of metamorphosis as an adaptive response to the threat of drying. This response has been documented for several other anuran species. All those breed in temporary water bodies, supporting the hypothesis that the trait is an evolved adaptation for breeding in such waters.

Density regulation in tadpoles of Rana temporaria: A full pond field experiment

Tadpole density dependence has repeatedly been studied in laboratories and mesocosms, but rarely in natural ponds. The present study manipulated density in three ponds; each pond was divided into one low density section and one high density section. The experiment was run for eight years, switching density treatments within ponds among years. This experimental design cannot definitively separate the experimental density effects from certain lag effects. This problem is discussed. Nonetheless, in all ponds tadpole survival was affected by the density manipulation. The reduced survival of larvae at high density was most pronounced in the pond with the overall highest density. In this pond, the density-dependent mortality almost offset the original, experimental, density difference between sections. Effects on tadpole and metamorph size and development rate were found in those cases in which density-dependent mortality was weak and treatment effects on density persisted to the later stages of tadpole development. However, even if significant, these were generally of small magnitude compared to the variation among ponds and years. I suggest that different modes and degrees of regulation among populations of one species of frog are to be expected, not only due to variation in pond quality but also due to variation in the surrounding terrestrial habitat. This may determine the balance between tadpole and adult population regulation.

Growth or reproduction? Resource allocation by female frogs Rana temporaria

The decision how to allocate marginal resources to reproduction and growth can have important effects on associated life-history parameters as well as on population dynamics. In addition to showing variation among individuals in a population, such allocation rules may be either condition-dependent or fixed in different individuals. While many studies on anuran amphibians have focused on egg numbers and egg sizes in females of different sizes, virtually no data exist on the relative allocation of marginal resources to growth versus reproduction. In the laboratory, we therefore offered female common frogs (Rana temporaria) low versus high food rations for a full reproductive cycle, and monitored their growth and later reproductive investment (egg number and egg size the following breeding season). Feeding rates had an effect both on female growth and on egg number and size. There was no trade-off found between the two forms of investment. A flexible allocation rule could not be supported as there was no significant effect of feeding rate on the relative allocation of resources to growth versus reproduction.

Conservation Biology in Agricultural Habitat Islands

Modern agriculture in temperate areas has a twofold impact on natural populations of plants and animals: it causes fragmentation and decreases the quality of habitats. Economy forces modern farmers to establish large fields that are easily worked. Ditches, habitat islands (piles of stone or shallow soils covered with vegetation) and irregularities in the fields are considered as obstacles and are therefore often removed. From a conservation point of view this development is often detrimental, because populations are fragmented, reducing migration between them.

Reproductive tactics of large and small male toads Bufo bufo

During the period from 5 to 27 April 1982, 385 male and 72 female toads were individually marked. Males that arrived early at a breeding pond were larger than those arriving late (r = -0.31, p 0.1). We suggest that large and small males employed different breeding tactics. Large males arrived early at the pond where they waited for females to arrive. They relied upon their size to secure a female. Small males often clasped a female on land, where pair formation might be a more random process and their small size is of no great disadvantage. By remaining clasped with the females small males could obtain matings especially when females, as was the case in the studied pond, only remained in the breeding pond for a short period of time.

The Effects of Age and Size on Reproductive Timing in Female Chamaeleo chamaeleon

Reproductive timing in relation to the size and age of individual female common Chame- leons (Chamaeleo chamaeleon) was studied in a population in southern Spain. Thirty-one radio-tagged fe- males were observed during two summers, 1993 and 1995. The mating season (courting, pair formation, and copulation) lasted from late July to mid-September. Egg-laying was recorded from 25 September to 28 October, 34-40 d after copulation. Timing of reproduction varied between individual females, old and large females timing the reproductive events (i.e, mating and egg-laying) earlier than young and small ones. Specifically, timing was affected by size when age was controlled for but not the reverse. Four alternative explanations for delayed breeding of younger and smaller female chameleons are discussed: (1) late breeding could be a consequence of physiological constrains in the onset of maturation of younger females; (2) late breeding allows young females to secure more resources for reproduction thus achieving larger clutches; (3) young females avoid competition for attractive males from large females, and (4) if young females are less preferred, they may not be courted until males no longer are courting larger females. This happens when the latter become pregnant.

Growth and Development of Larval Rana temporaria: Local Variation and Countergradient Selection

Abstract I raised tadpoles of the Common Frog (Rana temporaria) from populations in eight source ponds in southern Sweden, in a common garden experiment at two densities. Tadpoles from different populations differed in development rate; those from source ponds with high tadpole densities developed faster than those from less crowded ponds. Thus, differences among ponds in tadpole performance, which were documented in previous field studies, must have a genetic or maternal component. This result of source pond crowding likely resulted from microevolution and is an example of countergradient selection. In contrast, I found no significant effect of source pond hydroperiod; tadpoles from temporary ponds grew and developed at a rate similar to those from permanent ponds. Tadpoles of R. temporaria can respond plastically to pond drying by increasing development rate. I suggest adaptive plasticity in development rate decreased selection by pond drying in natural ponds.