Population structure of spotted salamanders (Ambystoma maculatum) in a fragmented landscape

Understanding the impacts of landscape-level processes on the population biology of amphibians is critical, especially for species inhabiting anthropogenically modified landscapes. Many pond-breeding amphibians are presumed to exist as metapopulations, but few studies demonstrate the extent and consequences of this metapopulation structure. Gene flow measures may facilitate the construction of more realistic models of population structure than direct measures of migration. This is especially true for species that are cryptic, such as many amphibians. We used eight polymorphic microsatellite loci to determine the genetic population structure of spotted salamanders ( Ambystoma maculatum ) breeding at 17 ponds in northeastern Ohio, a landscape fragmented by roads, agriculture, urban areas and the Cuyahoga River. Using a variety of analyses (Bayesian clustering, F -statistics, AMOVA) we generated a model of salamander population genetic structure. Our data revealed patterns of genetic connectivity that were not predicted by geographical distances between ponds (no isolation by distance). We also tested for a relationship between population structure and several indices of landscape resistance, but found no effect of potential barriers to dispersal on genetic connectivity. Strong overall connectivity among ponds, despite the hostile habitat matrix, may be facilitated by a network of riparian corridors associated with the Cuyahoga River; however, high gene flow in this system may indicate a general ability to disperse and colonize beyond particular corridors.     

Interaction of an insecticide with larval density in pond-breeding salamanders (Ambystoma)

1. Amphibian populations residing in or near agricultural areas are often susceptible to pesticide contamination. Recent evidence suggests that the effects of pesticides on amphibians often exceed those estimated in laboratory toxicity tests because other environmental factors (e.g. predators, resource abundance) can influence pesticide toxicity. 2. To examine the effects of an insecticide (carbaryl) on two species of Ambystoma salamanders experiencing the natural stress of competition, we manipulated chemical concentration (control, 3.5 and 7.0 mg L^?1) and larval density (low and high). We determined the effect of treatments on snout‐vent length (SVL), growth rate, lipid reserves, time to metamorphosis, per cent survival and per cent metamorphosis. 3. Carbaryl negatively affected all response variables of Ambystoma maculatum significantly, and significantly reduced survival and metamorphosis of A. opacum. Increased density significantly influenced SVL, lipid reserves, growth rate and metamorphosis of A. maculatum. 4. The effects of carbaryl and increased density on per cent metamorphosis were nearly additive, but were generally less than additive on other variables. 5. The negative effects of chemical contamination on salamanders were likely because of pesticide‐induced reductions of food resources, as zooplankton abundance decreased by as much as 97% following carbaryl application. 6. Our study demonstrates the importance of the interactive effects that chemical contamination and natural environmental factors have on salamanders.     

Landscape genetics of alpine Sierra Nevada salamanders reveal extreme population subdivision in space and time

Quantifying the influence of the landscape on the genetic structure of natural populations remains an important empirical challenge, particularly for poorly studied, ecologically cryptic species. We conducted an extensive microsatellite analysis to examine the population genetics of the southern long‐toed salamander (Ambystoma macrodactylum sigillatum) in a naturally complex landscape. Using spatially explicit modelling, we investigated the influence of the Sierra Nevada topography on potential dispersal corridors between sampled populations. Our results indicate very high‐genetic divergence among populations, high within‐deme relatedness, and little evidence of recent migration or population admixture. We also discovered unexpectedly high between‐year genetic differentiation (F_ST) for breeding sites, suggesting that breeding groups vary over localized space and time. While environmental factors associated with high‐elevation montane habitats apparently play an important role in shaping population differentiation, additional, species‐specific biological processes must also be operating to account for observed deviations from temporal, among‐year panmixia. Our study emphasizes the population‐level insights that can be gained from high‐density sampling in space and time, and the highly substructured population biology that may characterize amphibians in extreme montane habitats.     

Body size of virtual rivals affects ejaculate size in sticklebacks

Sperm competition occurs when sperm of two or more males competeto fertilize a given set of eggs. Theories on sperm competitionexpect males under high risk of sperm competition to increaseejaculate size. Here we confirm this prediction experimentallyin the three-spined stickleback (Gasterosteus aculeatus). Inthis species, sneaking (i.e., stealing of fertilizations byneighboring males) can lead to sperm competition. Sneaking malesinvade foreign nests, and the owners vigorously try to preventthis intrusion. In such fights, male body size is assumed tobe an important predictor of success. Consequently, the riskof sperm competition may depend on the size of a potential competitor.We experimentally confronted males before spawning with eithera large or a small computer-animated rival. We show that malesejaculated significantly more sperm after the presentation ofthe larger virtual rival than after the small stimulus. In addition,the time between the initiation of courting and the spawningwas shorter in the large virtual male treatment. The resultssuggest that stickleback males tailor ejaculate size relativeto the risk of sperm competition perceived by the size of apotential competitor.     

Zooplankton community size structure and taxonomic composition affects size-selective grazing in natural communities

The body size of an individual zooplankton is well related to its grazing rate and to the range of particle sizes it can ingest, and since cladocerans and copepods feed differently, they follow different relationships. Based on these general patterns in individual organisms, we tested whether the size structure and taxonomic composition of more complex natural zooplankton communities are related to their in situ grazing rate and to the range of algal sizes they graze. We compared community grazing rates on individual algal taxa in two communities dominated by small cladocerans, three communities dominated by large cladocerans and three copepod-dominated communities. Small algae were usually grazed most intensively, but grazing rates were poorly related to algal size alone. The range in size of grazed algae increased with increasing mean zooplankton body size, but differed systematically with their taxonomic composition. Communities dominated by Ceriodaphnia or Holopedium grazed a narrower size range of algae [maximum greatest axial length dimension (GALD)=16–36 μm)] than communities with large biomasses of Bosmina or Daphnia (maximum GALD=28–78 μm). Copepod-dominated communities followed the same general relationship as cladocerans. Daphnia-dominated communities grazed the broadest range of algal sizes, and their total grazing rates were up to 2.4 times their grazing rates on small (<35 μm) “highly edible” algae, a difference of similar magnitude to those found in successful trophic cascade biomanipulations. Received: 31 March 1998 / Accepted: 19 October 1998     

Individual responses to population size structure: the role of size variation in controlling expression of a trophic polyphenism

We investigated how size structure affects development of alternative larval phenotypes in Arizona tiger salamanders, Ambystoma tigrinum nebulosum, by testing the hypothesis that population size structure per se is a significant component of an individual's environment. Larvae of this subspecies exhibit one of two feeding phenotypes; typical larvae eat zooplankton and macroinvertebrates and cannibalistic larvae feed primarily on conspecifics. Previous laboratory experiments showed that larval density positively affected expression of the cannibalistic phenotype. In this study we tested the hypothesis that size variation among larvae also serves as a cue triggering development of the cannibalistic phenotype. We report laboratory experiments and field observations showing that both an individual larva's position in a size distribution and the amount of size vaiation among larvae serve as cues stimulating development of cannibalistic larvae. Larval density and population size structure provide a larva with an indication of the abundance and vulnerability of potential conspecific prey. Size variation among larvae, in turn, appears to be influenced by larval density. Thus, a complex relationship exists between larval density, population size structure, and the frequency of cannibals within a habitat.     

Seasonal variations in phytoplankton size structure in Lake Cromwell (Laurentian Shield), Quebec

Seasonal variation in phytoplankton size classes was studied over the period of a year in a small, mesotrophic, brown-water lake. Throughout the sampling period, species from 2–16 μm (GALD) dominated the phytoplankten community. Small phytoplankters (2–8 μm) were predominant in winter and early spring populations while algae of 8–16 μm became important in summer (phytoplankton < 2 μm was not counted). Principal coordinate analysis resulted in a cyclic pattern of dates in the reduced space, winter and early spring samples being well separated from the other dates. In multiple linear regressions between environmental parameters and size classes, temperature was always the most important variable.     

The effect of patch size and competitor number on aggression among foraging house sparrows

We examined the effect of patch size and competitor number onaggression among house sparrows, Passer domesticus, foragingat patches of seven different sizes in a doubling series (0.014,0.029, 0.058, 0.116, 0.230, 0.462, and 0.922 m²). Contrary toour expectations, the birds did not defend an entire patch,even when it was small as 0.014 m². The frequency of aggressionamong the birds decreased gradually with increasing patch size,in contrast to the step decline predicted by resource defensetheory. Moreover, the birds fought more frequently and moreintensely as competitor density increased. Both results areconsistent with the predictions of a modified hawk-dove modelfor shared patches. Females were more aggressive and fed ata higher rate than did males. The proportion of females increasedas patch size decreased, and aggression became more frequentand intense. Even when patches are shared, patch size has animportant effect on the frequency and intensity of foragingcompetition and the size and composition of foraging groups.     

Group size affects the metabolic rate of a tropical beetle

ABSTRACT. . Dormancy is metabolically very costly for insects and other poikilotherms of the tropics because energy reserves are rapidly utilized at high temperature. Yet, the beetle Stenotarsus rotundus Arrow spends up to 10 months each year in a tropical lowland forest as a non-feeding adult, dependent upon its fat reserve as an energy source. While in this dormant state, the beetles aggregate in huge numbers and the size of the group greatly influences the metabolic rate. As group size increases, the metabolic rate decreases, and the minimum rate observed in the field (22 μ l O₂ g^-1 h^-1) is estimated to occur when group size reaches about 300 individuals. A decrease in metabolic rate can also be achieved by elevating relative humidity, thus suggesting that one function of the aggregation is to increase humidity within the group and thereby decrease metabolic rate.     

Development of size structure in tiger salamanders: the role of intraspecific interference

Cannibalism affects patterns of density-dependent mortality and may regulate population size. In many cases, rates of cannibalism depend on size structure, the frequency distribution of body sizes in the population, because cannibals can often only capture and consume smaller individuals. Size differences within single-age groups can be caused by a variety of factors. In this research we tested the hypothesis that size variation among larval tiger salamanders is due, in part, to interference interactions among individuals of different sizes. We found that size variation was greater when we raised larvae in groups rather than in isolation. This increase in size variation was due more to a relative deceleration of growth among smaller individuals rather than acceleration among larger individuals. We also found that smaller larvae had lower feeding rates than larger larvae when in groups, but not when isolated. Including spatial structure to limit physical interactions did not affect the size specificity of feeding rate, although it reduced feeding rates overall. We argue that these results are consistent with the hypothesis that larger larvae interfere, probably indirectly, with the feeding behavior of small individuals and this contributes to increases in size variation over time. We hypothesize that this indirect interference is caused by a behavioral response of smaller larvae to the risk of predation (cannibalism) by larger individuals. Received: 18 May 1998 / Accepted: 29 April 1999     

Transmission mode affects the population genetic structure of Daphnia parasites

Parasite life cycle variation can shape parasite evolution, by predisposing them towards different population genetic structures. We compared the population genetic structure of two co-occurring parasite species of Daphnia, to collect evidence for their expected transmission modes. The ichthyosporean Caullerya mesnili has a direct life cycle, whereas the microsporidian Berwaldia schaefernai is hypothesized to require passage through a secondary host. The parasites were collected from three geographically isolated Daphnia populations. The nucleotide variation in the internal transcribed spacer (ITS) region was assessed at the within-individual, within-population and among-population levels, using amova. We detected significant partitioning at all levels, except for a lack of among-population variation in Berwaldia. This was confirmed by neighbour-joining and principal component analyses; Caullerya populations were distinct from each other, while there was much overlap among parasite isolates representing different populations of Berwaldia. This all implies a higher amount of gene flow for Berwaldia, consistent with the hypothesized transmission mode.     

Growth depensation and aggression in laboratory reared coho salmon: the effect of food distribution and ration size

Groups of recently emerged coho salmon fry Oncorhynchus kisutch were reared for 3 months on food that appeared either asynchronously at a single location (localized) or synchronously and spatially dispersed (dispersed). Groups were further subdivided into those receiving low (1%) or high (3% body weight per day) rations, with five replicate groups for each treatment combination. At low ration there was greater growth depensation, i.e. growth variation, in groups receiving localized as compared to dispersed food. At high ration there was no difference. There was no effect of food distribution upon mean fish weight, but groups receiving high rations had greater mean fish weights than groups receiving low rations. There was no overall difference in the frequency of chasing between any of the treatment combinations. However, in localized food groups, dominants defended positions close to where food entered the tank, giving them greater access than subordinates. In dispersed food groups, while dominants also defended particular areas, this did not result in greater access to food. These results demonstrate that although feeding methodology may not directly influence the frequency of aggressive interactions, feeding methods which facilitate food monopolization by dominants can accelerate the growth of these individuals at the expense of subordinates. In aquacultural applications where greater size is desirable, or otherwise selected for, this may result in the unintentional selection for increased aggressiveness.     

Review of the population structure and ecology of S. mentella in the Irminger sea and adjacent waters

Significant controversies exist over the three types of Sebastes mentella found in the Irminger Sea and adjacent waters. Preliminary genetic studies have given evidence for the existence of three distinct groups, characterized using several molecular genetic markers. The biological or ecological significance of these between-group differences has, however, not been evaluated. In the present paper, we review the life cycle of S. mentella in this area, based on published data. Spawning of S. mentella in the Irminger Sea takes place in a single area above Reykjanes Ridge. The larvae drift towards East Greenland from where they are carried to West Greenland (NAFO Subareas 0+1). Later they return to East Greenland where the main nursery area has been identified. There is evidence for a migration of juvenile fish from the nursery area into the open Irminger Sea, where many different cohorts spawn. We also compare these ecological data with the genetic results and we conclude that the observed genetic differences can be derived from possible genetic drift, selection or mostly from temporal variation (age dependency). which has also been reported for other species with as high longevity as S. mentella. We conclude that S. mentella in the Irminger Sea and adjacent waters comprises one single population.     

The effective number of a population that varies cyclically in size. II. Overlapping generations

We consider haploid and dioecious age-structured populations that vary over time in cycles of length k. Results are obtained for both autosomal and sex-linked loci if the population is dioecious. It is assumed that k is small in comparison with numbers of haploid individuals (or of numbers of males and females) in any generation of a cycle. The inbreeding effective population size N(e) is then approximately given by the expression [T summation operator (k-1)(j=0)1/[N(e)(j)T(j)]](-1), where N(e)(j) and T(j) are, respectively, the effective population size and generation interval that would hold if the population was at all times generated in the same way as at time j. The constant T, which is the effective overall generation interval, is defined to be k times the harmonic mean of the quantities T(j). Our expressions for T and N(e), in terms of N(e)(j) and T(j), are general, but the N(e)(j)s are derived under the assumption that offspring are produced according to Poisson distributions.     

Population size,habitat fragmentation,and the nature of adaptive variation in a stream fish

Whether and how habitat fragmentation and population size jointly affect adaptive genetic variation and adaptive population differentiation are largely unexplored. Owing to pronounced genetic drift, small, fragmented populations are thought to exhibit reduced adaptive genetic variation relative to large populations. Yet fragmentation is known to increase variability within and among habitats as population size decreases. Such variability might instead favour the maintenance of adaptive polymorphisms and/or generate more variability in adaptive differentiation at smaller population size. We investigated these alternative hypotheses by analysing coding-gene, single-nucleotide polymorphisms associated with different biological functions in fragmented brook trout populations of variable sizes. Putative adaptive differentiation was greater between small and large populations or among small populations than among large populations. These trends were stronger for genetic population size measures than demographic ones and were present despite pronounced drift in small populations. Our results suggest that fragmentation affects natural selection and that the changes elicited in the adaptive genetic composition and differentiation of fragmented populations vary with population size. By generating more variable evolutionary responses, the alteration of selective pressures during habitat fragmentation may affect future population persistence independently of, and perhaps long before, the effects of demographic and genetic stochasticity are manifest.     

Intra-specific variations of egg size,clutch size and larval survival relatedto maternal size in amphidromous <Emphasis Type="BoldItalic">Rhinogobius</Emphasis> goby

Synopsis Newly hatched amphidromous Rhinogobius sp. CB (cross band type) larvae drift downstream to the sea to grow and develop before returning upstream as juveniles. Since larger and older individuals usually inhabit the upper reaches of rivers, larvae from larger females are more likely to suffer higher risks of starvation or predation during their longer migration to the sea. We examined the relationship between reproductive parameters (egg volume and clutch size) and maternal length. We collected adult Rhinogobius sp. CB along the course of the Aizu River (Wakayama Prefecture, Japan) and spawned them under laboratory conditions. We measured egg volumes and clutch sizes, as well as larval starvation tolerance. Both egg volume and clutch size increased with standard length or age of the maternal fish, while egg density in these clusters did not correlate with standard length. Gonad-somatic index (GSI) also tended to increase with maternal standard length. There was a significant positive correlation between egg size and the 72-h survival rate of unfed hatchlings. Intra-specific variation of egg volumes and clutch sizes in this species seems to be an adaptation for enhancing offspring survivorship during migration to the sea. Some females spawned a second time. Second spawned egg sizes were smaller than first spawned egg sizes, although there was no difference in clutch size between the two. Egg size variation between subsequent spawns may be an adjustment to the changes in seasonal environmental conditions.     

The relationship between body size and population size in bromeliad tank faunas

Relationships between body size and abundance in collections of animals from the tanks of 73 bromeliads belonging to five species were analysed. Unlike data in previously published studies on this relationship, these collections of species are not taxonomically restricted and represent complete communities over the macroscopic range of organisms. There is no overall tendency for there to be a positive or negative relationship between population abundance and body size of morphospecies. We can find no evidence that body size-abundance patterns are triangular in complete communities. However, there is weak evidence that the relationship in the aquatic subsets of those communities may have some underlying triangularity, with medium-sized species having the largest populations.     

Regulation of body size during larval development in the German cockroach, Blattella germanica

It may be advantageous for insects to attain a certain standard size when they become adult. Recent studies have demonstrated that in some species of insects, a threshold size must be reached in the last instar in order that the adult will attain the standard size. It has been revealed also, however, that there may be another type of regulation of body size during larval development in the German cockroach. Head width and other characteristics were measured in every larval instar up to the adult. Body size is distributed clearly into a small group and a large group in the 5th instar, and even in the 4th instar. This suggests that a larva has already determined the number of following moults by the late 3rd instar. Adult size is almost the same in 5- and 6-instar types. Among individuals which require the same number of instars to reach the adult stage, the coefficient of variation in body size increases until the 3rd instar then decreases to the adult stage. Smaller individuals in a particular instar after the 3rd tend to grow more than larger ones during that instar. These results suggest that a larva regulates its body size after the 3rd instar in order to attain the ‘norm’ for adult size.     

The anatomy of the interspecific abundance–range size relationship for the British avifauna: I. Spatial patterns

Data from the British Trust for Ornithology Common Birds Census and two atlases of breeding birds were used to examine the form of the interspecific abundance–range size relationship for the British avifauna. The relationship is positive for both farmland and woodland habitats and over two different periods, with some evidence of curvilinearity, using either proportion of occupied sites or numbers of occupied 10 × 10 km squares as measures of range size, and mean density at occupied sites as a measure of abundance. A log-linear plot gives the highest correlation. The relationship is stronger if based on maximum local densities than if based on average densities, but there is no relationship using minimum local densities. Relationships based on abundances at individual sites are uniformly positive for all sites, although the relationships for many sites also show evidence of curvilinearity, especially when range size is measured as the proportion of occupied sites. Species show significant concordance in their rank abundances across sites. We discuss some implications of these results.     

A test of alternative hypotheses for kin recognition in cannibalistic tiger salamanders

The function of kin recognition is controversial. We investigatedtheadaptive significance of kin discrimination in cannibalistictiger salamanderlarvae, Ambystoma tigrinum. Previous laboratoryexperiments show thatcannibals preferentially consume lessrelated individuals. We hypothesizedthat this example of kinrecognition (1) is a laboratory artifact, (2) is aby-productof sibship-specific variation in escape responses, becausecannibalsfrom families with rapid responses may be more likely to cannibalizeslowlyescaping non-kin, (3) is an epiphenomenon of species recognition,(4)functions in disease avoidance, because kin may be moreinfectious thannon-kin, or (5) is favored by kin selection.We evaluated these fivehypotheses by using laboratory and fieldexperiments to test specificpredictions made by each hypothesis.We rejected hypotheses 1-4 above because(1) kin recognitionwas expressed in the wild, (2) escape responses did notreliablypredict whether a cannibal would ingest kin or non-kin, (3)kinrecognition was not most pronounced in populations wheretiger salamandersco-occur with other species of salamanders,and (4) non-kin prey were morelikely than kin to transmit pathogensto cannibals. However, we establishedthat the necessary conditionfor kin selection, Hamilton's rule, was met.Thus, our resultsimplicate kin selection as the overriding reason thatcannibalistictiger salamanders discriminate kin.