Variation In Reproductive Rate Research Articles

Range-wide sources of variation in reproductive rates of northern spotted owls.

We conducted a range-wide investigation of the dynamics of site-level reproductive rate of northern spotted owls using survey data from 11 study areas across the subspecies geographic range collected during 1993-2018. Our analytical approach accounted for imperfect detection of owl pairs and misclassification of successful reproduction (i.e., at least one young fledged) and contributed further insights into northern spotted owl population ecology and dynamics. Both nondetection and state misclassification were important, especially because factors affecting these sources of error also affected focal ecological parameters. Annual probabilities of site occupancy were greatest at sites with successful reproduction in the previous year and lowest for sites not occupied by a pair in the previous year. Site-specific occupancy transition probabilities declined over time and were negatively affected by barred owl presence. Overall, the site-specific probability of successful reproduction showed substantial year-to-year fluctuations and was similar for occupied sites that did or did not experience successful reproduction the previous year. Site-specific probabilities for successful reproduction were very small for sites that were unoccupied the previous year. Barred owl presence negatively affected the probability of successful reproduction by northern spotted owls in Washington and California, as predicted, but the effect in Oregon was mixed. The proportions of sites occupied by northern spotted owl pairs showed steep, near-monotonic declines over the study period, with all study areas showing the lowest observed levels of occupancy to date. If trends continue it is likely that northern spotted owls will become extirpated throughout large portions of their range in the coming decades.

Long‐term trends of reproductive success of black grouse Lyrurus tetrix in the southern Swiss Alps in relation to changes in climate and habitat

Breeding success of an Alpine black grouse Lyrurus tetrix population in southern Switzerland was monitored from 1981 to 2020. This long‐term dataset allows exploring relationships of reproductive rates with climate and habitat, which have shown marked changes during this period. Over the 40 years, the average elevation of black grouse breeding sites increased by around 100 m in central/southern Ticino but showed only a slight increase in northern Ticino, where black grouse occur at higher elevations. Average reproductive rates in northern Ticino remained constant throughout the study period but declined in central/southern Ticino. Relationships between reproductive success and weather as well as habitat variables were analysed with a multiple regression model. Temperature during the early chick‐rearing phase and the time of egg‐laying was positively correlated with reproductive rate. Correlations between reproductive rates and precipitation were less clear, and only small proportions of the variance in reproductive rates could be explained by precipitation. Brush forest explained the greatest amount of variation in reproductive rate (6.2%). Forest, alpine agricultural areas and unproductive vegetation all showed a positive relationship with reproductive rate, but the proportion of the variance explained was small. Year (5.1%) and its interaction with region (2.3%) explained considerable amounts of the variance. While in northern Ticino reproductive success did not show a negative trend when correcting for weather and habitat changes, there remained a negative trend over the years in central/southern Ticino. Despite the positive correlations of reproductive rate with temperature, increasing temperatures do not appear to have improved reproductive success, likely as a result of habitat changes that forced black grouse towards higher elevations. Changes in reproductive success were limited to the southern region, indicating deteriorating conditions at the edge of the distribution range.

Dolphin population specialized in foraging with artisanal fishers requires zero‐bycatch management to persist

Abstract The small population paradigm assumes that populations with low numbers of individuals intrinsically have a high probability of extinction. The small population of Lahille’s bottlenose dolphins Tursiops truncatus gephyreus that specializes in foraging with artisanal fishers in Laguna, southern Brazil, faces human pressures including bycatch in fishing gear. The viability of this population was modelled over 30 and 100 years under different levels of bycatch, including the current scenario of two bycatches every year, two scenarios with higher incidence of bycatches and three management scenarios. The sensitivity of predicted growth rates to fixed‐proportion and observed‐variation changes in life history parameters was explored. The current scenario predicted a declining population (r = −0.014; λ = 0.986) with a high probability of extinction in the long term (PE = 0.71). A small increase in bycatches would result in a marked increase in the probability of extinction. Management scenarios seem promising, but only the zero‐bycatch management scenario would make the difference between a declining and an increasing population. As expected for slow‐growing species, population growth rate was most sensitive to proportional changes in adult female and juvenile survival. However, considering observed variation in vital rates, population dynamics were most influenced by variation in reproductive rates. To determine the highest priority for management action, another simulation was made of how additional threat scenarios of recognized human activities (i.e. bycatch influencing adult survival and increased underwater noise or pollution influencing calf survival) would affect population dynamics. Population growth rate was very sensitive to changes in adult bycatch (especially females), as expected, and only subtly sensitive to a reduction in calf survival. The current level of bycatch is unsustainable. Bycatch needs to be eliminated to maximize the probability of long‐term persistence of this dolphin population. However, this population’s persistence could be threatened by natural variation in reproductive rates.

The Goldilocks effect: female geladas in mid-sized groups have higher fitness.

The cost–benefit ratio of group living is thought to vary with group size: individuals in ‘optimally sized’ groups should have higher fitness than individuals in groups that are either too large or too small. However, the relationship between group size and individual fitness has been difficult to establish for long-lived species where the number of groups studied is typically quite low. Here, we present evidence for optimal group size that maximizes female fitness in a population of geladas (Theropithecus gelada). Drawing on 14 years of demographic data, we found that females in small groups experienced the highest death rates, while females in mid-sized groups exhibited the highest reproductive performance. This group size effect on female reproductive performance was largely explained by variation in infant mortality (and, in particular, by infanticide from immigrant males) but not by variation in reproductive rates. Taken together, females in mid-sized groups are projected to attain optimal fitness due to conspecific infanticide and, potentially, predation. Our findings provide insight into how and why group size shapes fitness in long-lived species.

Reproductive constraints in frugivorous phyllostomid bats: seasonal and elevational variation in reproductive rates in the Brazilian Atlantic Forest

Reproductive rates in bats are higher when there is a greater availability of food, and when temperatures and rainfall are higher. Along elevational gradients, variation in these factors occurs over a small area, making them ideal natural laboratories for studies that aim to describe reproductive patterns and to identify possible vertical migrations for reproduction. We characterized the seasonal and elevational variation in reproductive rates of female and male Artibeus spp., Carollia perspicillata, and Sturnira lilium in the Brazilian Atlantic Forest. We sampled bats with mist nets at 13 sites in southeastern Brazil, between 23° and 24° of latitude, at elevations between 60 and 1,800 m a.s.l. The reproductive patterns of Artibeus spp. and S. lilium were seasonal, with higher reproductive rates in the period with higher rainfall, temperatures, and, consequently, greater availability of food. Moreover, elevation influenced reproductive activity in all study species. Reproductive rates for females of Artibeus spp. were higher at intermediate elevations (between 501 and 1,000 m a.s.l.), and for females of S. lilium at high elevations (above 1,000 m a.s.l.) with a less evident peak at intermediate elevations. Similarly, the proportions of reproductively active male Artibeus spp. and S. lilium were greater at intermediate and high elevations, respectively. In the dry season, reproductive rates of all females increased with local abundance of all females. Differences in reproductive rates along elevational gradients could influence population dynamics, and therefore entire elevational ranges should be protected to ensure conservation of the best reproductive habitats for a variety of bat species. Public policies are needed to ensure effective conservation of biodiversity, biological processes, and the ecosystem services provided by local fauna from the lower lands to the mountain peaks.

Adult survival and reproductive rate are linked to habitat preference in territorial, year‐round resident Song Sparrows Melospiza melodia

Individual animal fitness can be strongly influenced by the ability to recognize habitat features which may be beneficial. Many studies focus on the effects of habitat on annual reproductive rate, even though adult survival is typically a greater influence on fitness and population growth in vertebrate species with intermediate to long lifespans. Understanding the effects of preferred habitat on individuals over the annual cycle is therefore necessary to predict its influences on individual fitness. This is particularly true in species that are resident and territorial year‐round in the temperate zone, which may face potential trade‐offs between habitat that maximizes reproduction and that which maximizes non‐breeding season (‘over‐winter’) survival. We used a 37‐year study of Song Sparrows Melospiza melodia residing territorially year‐round on a small island to examine what habitat features influenced adult over‐winter survival, how site‐specific variation in adult survival vs. annual reproductive rate influenced long‐term habitat preference, and if preferred sites on average conferred higher individual fitness. Habitat features such as area of shrub cover and exposure to intertidal coastline predicted adult over‐winter survival independent of individual age or sex, population size, or winter weather. Long‐term habitat preference (measured as occupation rate) was better predicted by site‐specific annual reproductive rate than by expected over‐winter survival, but preferred sites maximized fitness on average over the entire annual cycle,. Although adult over‐winter survival had a greater influence on population growth (λ) than did reproductive rate, the influence of reproductive rate on λ increased in preferred sites because site‐specific variation in reproductive rate was higher than variation in expected over‐winter survival. Because preferred habitats tended to have higher mean site‐specific reproductive and adult survival rates, territorial birds in this population do not appear to experience seasonal trade‐offs in preferred habitat but are predicted to incur substantial fitness costs of settling in less‐preferred sites.

Blood may buy goodwill: no evidence for a positive relationship between legal culling and poaching in Wisconsin.

Chapron & Treves [1] tested the hypothesis that allowing some lethal legal control reduces the level of poaching. Using an analysis of data from the wolf populations in Michigan and Wisconsin, they discounted this hypothesis, concluding: … ‘allowing wolf ( Canis lupus ) culling was substantially more likely to increase poaching than reduce it’ [1, p. 1]. However, the text and analysis have in my opinion severe shortcomings, including: (i) biased reporting of previously published results, (ii) the use of a statistical model to evaluate density dependence in wolf area use that does not have theoretical or empirical support, and (iii) a failure to evaluate how between-year variations in reproductive rates affect their conclusions. When variation in reproductive rates is taken into account in their analysis, the conclusion is the opposite—allowing wolf culling is more likely to decrease poaching than increase it. Chapron & Treves [1] cite, but seem to give a biased report of the findings from several published articles. They [1] claim that their analysis is the first to evaluate the relationship between legal culling and poaching in wolves. However, poaching of radio-tagged wolves has previously been shown to decrease in association with legal state culling in Wisconsin [2]. In the discussion they state: ‘As with prior studies on Wisconsin's wolf population [3], we did not detect any negative density dependence’ [1, p. 5]. However, the article they refer to does indeed find recruitment to be density-dependent. In addition, the occurrence of density dependence in wolf area use in Wisconsin is supported by a decrease in average wolf territory size as the population size increases [4]. Overall, it seems to me that results from the same study system that do not corroborate their own …

The importance of pollinators and autonomous self-fertilisation in the early stages of plant invasions: Banksia and Hakea (Proteaceae) as case studies.

Reproduction is a crucial stage in the naturalisation of introduced plant species. Here, using breeding system experiments and observations of floral visitors, we investigate whether a lack of pollinators or an inability to autonomously self-fertilise limits naturalisation in five Australian Banksia species and the co-familial Hakea salicifolia in South Africa. Banksia species were heavily utilised by native insects and nectar-feeding birds. Although Banksia produced fruit when pollinators were excluded, pollinators significantly increased seed set in four of the five species. H.salicifolia flowers were visited by 11 insect species; honeybees (Apis mellifera) were the main visitors. Flowers in naturalised H.salicifolia populations received almost four times the number of visits as flowers in non-naturalised populations; the latter showed both pollen limitation (PLI 0.40) and partial self-incompatibility. This should not prevent invasion, since H.salicifolia produces fruits via autonomous selfing in the absence of pollinators. The results suggest a limited role of breeding systems in mediating naturalisation of introduced Proteaceae species. Other factors, such as features of the recipient environments, appear to be more important. Spatial variation in rates of reproduction might, however, explain variation in the extent and rate of naturalisation of different populations.

Dental eruption in East African wild chimpanzees

Knowledge of chimpanzee development has played an essential role in our understanding of the evolution of human ontogeny. However, recent studies of wild ape dentitions have cast doubt on the use of developmental standards derived from captive individuals. Others have called into question the use of deceased wild individuals to infer normative development. We conducted a high resolution photographic study of living known-age subadults in the Kanyawara community (Kibale National Park, Uganda) to generate a comprehensive three year record of dental eruption (including tooth emergence ages). These non-invasive data allow comparisons of captive and wild chimpanzees, establish accurate developmental standards for relatively healthy wild individuals, and facilitate direct assessments of primate-wide associations between dental development and life history. Emergence ages in the Kanyawara chimpanzees are very similar to living Gombe chimpanzees, and are broadly comparable to deceased Taï Forest chimpanzees. Early-emerging teeth such as the deciduous dentition and first molar (M1) appear during a time of maternal dependence, and are almost indistinguishable from captive chimpanzee emergence ages, while later forming teeth in the Kanyawara population emerge in the latter half of captive age ranges or beyond. Five juveniles whose lower M1s emerged by or before 3.3 years of age continued to nurse for a year or more beyond M1 emergence, and their mothers showed considerable variation in reproductive rates. The third molars of two adolescent females emerged several months to several years prior to the birth of their first offspring. Given that broad primate-wide relationships between molar emergence and life history do not necessarily hold within this population of chimpanzees, particularly for variables that are reported to be coincident with molar emergence, we suggest that further study is required in order to predict life history variables in hominins or hominoids.

Prey Use and Selection in Relation to Reproduction by Peregrine Falcons Breeding along the Yukon River, Canada

Some raptors are considered to be generalist predators and switch among different species of prey according to their availability in the environment. In contrast, other raptor species are specialists with narrow dietary niche widths, selecting only one or a few types of prey; reproductive success in these species is often closely related to variability in abundance of preferred prey species. Determining whether raptors are generalist or specialist predators is important for understanding the degree to which fluctuations in their population sizes are influenced by individual responses to changes in prey availability. We assessed diet components based on prey remains collected at nest ledges to determine whether Peregrine Falcons breeding along the Yukon River in northern Canada were specialist feeders or, conversely, whether they took prey according to its relative abundance. We also examined whether the use of certain types of prey had consequences for reproductive success. Compositional analyses showed that peregrines exhibited nonrandom prey selection, with shorebirds, jays and thrushes, and waterbirds being used more than expected based on their abundance, whereas sparrows and other small passerines were used significantly less than expected. Similarly, medium-sized prey was selected by peregrines, but small (<50 g) and large (>1000 g) prey were used significantly less than expected. Pairs occupying territories in which the diet included large numbers of waterbirds, and few sparrows and small passerines, fledged more young. Although prey >1000 g were among the least selected by pairs in our study population, pairs that used these larger prey species fledged more offspring. These patterns may relate directly to the contribution of female parents to provisioning. Females that are able to capture large prey items may deliver larger amounts of biomass to their nests and, as a consequence, may have high reproductive output. We conclude that Peregrine Falcons in the Yukon are selective predators and that some of the variation in reproductive rates in this population may be attributed to variation in the types of prey available to and utilized by individual pairs.

Spatial and Temporal Variation in Reproductive Rates of the Red-Shouldered Hawk in Suburban and Rural Ohio

Abstract. We measured the reproductive rate of Red-shouldered Hawks (Buteo lineatus) nesting in suburban southwestern Ohio and in a rural forested region in Hocking Hills, southeastern Ohio, from 1997 to 2005. In the suburban region, the reproductive rate varied greatly from nest to nest, less so from year to year, indicating that some nest areas consistently produced more young than others. The most productive 25% of nest areas produced 44% of all the nestlings in the study, whereas the least productive 25% of nest areas produced only 7% of all nestlings. In the rural area, the reproductive rate varied significantly from year to year, less so from nest area to nest area. Overall reproductive rates in the two study areas differed only in 2000. We suggest that differences among the nest areas in reproductive rate likely indicate differences in habitat quality of individual nest areas, whereas differences among years likely reflect regional factors that influence nesting birds, such as weather.

Spatial heterogeneity of mortality and temporal fluctuation in fertility promote coexistence but not vice versa: A random-community approach

Both spatial heterogeneity and temporal fluctuation of the environment are important mechanisms promoting species coexistence, but they work in different manners. We consider many pairs of species with randomly generated survivorship and fertility in the lottery model, and examine how the variability in demographic processes affects the outcome of competition. The results are: [1] Coexistence is easier if habitat difference in mortality is greater, or if year-to-year variation in reproductive rate is larger. But neither habitat-difference in fertility nor temporal variation in mortality promotes coexistence. [2] Mean fertility does not affect the outcome if CV remains constant. In contrast, enhanced mean mortality decreases the fraction of coexisting pairs if the environment fluctuates temporally. [3] We also investigate the effect of limited dispersal of propagules between habitats. Compared with the complete mixing case, the fraction of coexisting pairs is clearly enhanced if the spatial heterogeneity is the major source of environmental variation, but shows slight increase if the temporal fluctuation is dominant. We conclude that spatial heterogeneity is likely to work more effectively in promoting species coexistence than temporal fluctuation, especially when the species suffer relatively high mortality, and disperse their propagules in a limited spatial scale.

Demographics in an alpine reindeer herd: effects of density and winter weather

We examined how population density, winter weather, snow conditions, and 2 large‐scale climatic indices (North Atlantic Oscillation, NAO, and Arctic Oscillation, AO) influenced demography (reproduction and mortality) in an alpine herd of semi‐domesticated reindeer Rangifer tarandus between 1959 and 2000 in Finnish Lapland. The herd lived on heavily grazed lichen pastures, with winter densities between 0.8 and 3.9 individuals km−2. Icing conditions occurred every 7th yr, on an average, and decreased reproductive rate (calves/females) by 49%. In general linear models icing remarkably increased the fit of snow models to reproductive rate. Incorporation of an interaction term between icing and the snow depth index provided better fit than a model without interaction. Delayed snowmelt decreased reproductive rate. For the day of snowmelt, however, the model without interaction was better than the interaction model. These 3 models provided the best fit to the data and accounted for 51–54% of the variation in reproductive rate. Winter mortality was related to density and large‐scale climatic indices, but not to local winter weather except a slight increase in mortality during an icing winter. The best model for winter mortality, including reindeer density and NAO, accounted for 26% of variation in mortality. Three factors may be involved explaining weak density dependence or the lack of such dependence; climate change scenarios that predict higher winter temperature, more frequent thawing‐freezing periods, and deeper snow would be expected to decrease reproductive rate and increase winter mortality of reindeer and thus to reduce profitability of reindeer husbandry. In contrast, early springs would be advantageous for reindeer in the short term.

Forest edge creates small‐scale variation in reproductive rate of sika deer

AbstractHabitat edges are considered to have an important role in determining the abundance of deer in forest landscapes, but to our knowledge there are few lines of evidence indicating that forest edge enhances the vital rate of deer. We examined pregnancy of female sika deer in Boso peninsula, central Japan, and explored how forest edges, food availability in forests, and local population density influence the pregnancy rate of sika deer. Local deer density was estimated by the number of fecal pellets, and food availability in forests was estimated by combining GIS data of vegetation distribution and the relationship between vegetation biomass and local deer density. Forest edge length was also determined by GIS data. Model selection was performed with multiple logistic regression analyses using the AIC to find the best model for accounting for the observed variation in pregnancy rates of the deer. Multiple logistic regression analysis showed that the length of forest edge had a positive effect on the pregnancy rate of females, whereas food availability in forests and local deer density had little effect. This forest edge effect was detected in a 100–200‐m radius from deer captured locations, indicating that deer pregnancy is primarily determined by habitat quality within a 10‐ha area. This result was confirmed by tracking females with GPS telemetry, which found that the core areas of the home range were less than 12 ha. The positive effect of edges and the lack of density dependence could be a result of high plant productivity in open environments that produces forages not depleted by high deer densities. Our results support the view that land management is the cause of the current problem of deer overabundance.

Spatiotemporal variation in reproductive parameters of yellow-bellied marmots

Spatiotemporal variation in reproductive rates is a common phenomenon in many wildlife populations, but the population dynamic consequences of spatial and temporal variability in different components of reproduction remain poorly understood. We used 43 years (1962-2004) of data from 17 locations and a capture-mark-recapture (CMR) modeling framework to investigate the spatiotemporal variation in reproductive parameters of yellow-bellied marmots (Marmota flaviventris), and its influence on the realized population growth rate. Specifically, we estimated and modeled breeding probabilities of two-year-old females (earliest age of first reproduction), >2-year-old females that have not reproduced before (subadults), and >2-year-old females that have reproduced before (adults), as well as the litter sizes of two-year old and >2-year-old females. Most reproductive parameters exhibited spatial and/or temporal variation. However, reproductive parameters differed with respect to their relative influence on the realized population growth rate (lambda). Litter size had a stronger influence than did breeding probabilities on both spatial and temporal variations in lambda. Our analysis indicated that lambda was proportionately more sensitive to survival than recruitment. However, the annual fluctuation in litter size, abetted by the breeding probabilities, accounted for most of the temporal variation in lambda.

Adapting to the unpredictable: reproductive biology of vertebrates in the Australian wet–dry tropics

In the wet-dry tropics of northern Australia, temperatures are high and stable year-round but monsoonal rainfall is highly seasonal and variable both annually and spatially. Many features of reproduction in vertebrates of this region may be adaptations to dealing with this unpredictable variation in precipitation, notably by (i) using direct proximate (rainfall-affected) cues to synchronize the timing and extent of breeding with rainfall events, (ii) placing the eggs or offspring in conditions where they will be buffered from rainfall extremes, and (iii) evolving developmental plasticity, such that the timing and trajectory of embryonic differentiation flexibly respond to local conditions. For example, organisms as diverse as snakes (Liasis fuscus, Acrochordus arafurae), crocodiles (Crocodylus porosus), birds (Anseranas semipalmata) and wallabies (Macropus agilis) show extreme annual variation in reproductive rates, linked to stochastic variation in wet season rainfall. The seasonal timing of initiation and cessation of breeding in snakes (Tropidonophis mairii) and rats (Rattus colletti) also varies among years, depending upon precipitation. An alternative adaptive route is to buffer the effects of rainfall variability on offspring by parental care (including viviparity) or by judicious selection of nest sites in oviparous taxa without parental care. A third type of adaptive response involves flexible embryonic responses (including embryonic diapause, facultative hatching and temperature-dependent sex determination) to incubation conditions, as seen in squamates, crocodilians and turtles. Such flexibility fine-tunes developmental rates and trajectories to conditions--especially, rainfall patterns--that are not predictable at the time of oviposition.

Diet reconstruction and historic population dynamics in a threatened seabird

Summary For the overwhelming majority of species, we lack long‐term information on the dynamics of populations. As a consequence, we face considerable uncertainty about how to discriminate among competing hypotheses of population decline and design conservation plans. The marbled murrelet Brachyramphus marmoratus is a small seabird that nests in coastal old‐growth forest but feeds year‐round in near‐shore waters of the north‐eastern Pacific. Although a decline in nesting habitat is the primary reason why marbled murrelets are listed as threatened in Canada, nest predation and food availability may also influence population abundance. To examine the hypothesis that murrelet populations are influenced by variation in diet quality, we analysed stable‐carbon and ‐nitrogen isotopes in feathers of museum specimens collected in the Georgia Basin, British Columbia. Between 1889 and 1996, we found a decline in stable isotopic signatures that was approximately equal to a 62% drop in trophic feeding level. We also found that the estimated proportion of fish in murrelet diet was related closely to murrelet abundance over the past 40 years, as estimated from volunteer surveys. Using these isotopic data, we modelled population size as a function of variation in reproductive rate due to changes in diet quality and found that our model matched closely the 40‐year field estimates. We then applied our 107‐year isotopic record to the model to back‐cast estimates of population growth rate to 1889. Our results suggest that, up to the 1950s, murrelet populations in the Georgia Basin were capable of growing and were probably limited by factors other than diet quality. After this period, however, our results imply that murrelets were often, but not solely, limited by diet quality. Synthesis and applications. Protecting nesting habitat may not be sufficient to rebuild populations of this highly secretive and threatened seabird and recovery might also require the restoration of marine habitat quality, as well as a better understanding of how ocean climate affects prey abundance and reproductive rate. Combined with contemporary demographic data, stable isotope analysis of historic samples provides a unique opportunity to reconstruct population histories for species where we lack long‐term information.

Factors influencing length of maternal care in brown bears ( Ursus arctos ) and its effect on offspring

Length of maternal care, i.e. the interval between successfully raised litters, is the most important factor explaining the variation in reproductive rate among brown-bear (Ursus arctos) populations. In this paper, we examine the variation in length of maternal care in radio-marked brown bears and its effect on their offspring in northern Sweden. Young stayed with their mothers for 1.4–1.5 or 2.4–2.5 (in one case 3.5) years and were weaned with body masses varying from 17 to 69 kg. The probability of yearling litters staying with their mother for a 2nd year increased with decreasing yearling body mass, and was higher for litters with two offspring than for litters with one or three to four offspring. Staying with their mothers for a 2nd year had a positive effect on mass gain in yearlings and this effect was more pronounced in litters with two than three to four offspring. Body mass of 2-year-olds was not related to age of weaning, suggesting that keeping offspring for an additional year mainly compensated for low yearling body mass. If large offspring body mass positively affects later offspring survival and reproduction, mothers may be able to optimize the length of maternal care according to the litter size and the size of their yearlings.

Genetic variation in sexual and clonal lineages of a freshwater snail

Sexual reproduction within natural populations of most plants and animals continues to remain an enigma in evolutionary biology. That the enigma persists is not for lack of testable hypotheses but rather because of the lack of suitable study systems in which sexual and asexual females coexist. Here we review our studies on one such organism, the freshwater snail Potamopyrgus antipodarum (Gray). We also present new data that bear on hypotheses for the maintenance of sex and its relationship to clonal diversity. We have found that sexual populations of the snail are composed of diploid females and males, while clonal populations are composed of a high diversity of triploid apomictic females. Sexual and asexual individuals coexist in stable frequencies in many ‘mixed’ populations; genetic data indicate that clones from these mixed populations originated from the local population of sexual individuals without interspecific hybridization. Field data show that clonal and sexual snails have completely overlapping life histories, but individual clonal genotypes are less variable than individuals from the sympatric sexual population. Field data also show segregation of clones among depth-specific habitat zones within a lake, but clonal diversity remains high even within habitats. A new laboratory experiment revealed extensive clonal variation in reproductive rate, a result which suggests that clonal diversity would be low in nature without some form of frequency-dependent selection. New results from a long-term field study of a natural, asexual population reveal that clonal diversity remained nearly constant over a 10-year period. Nonetheless, clonal turnover occurs, and it occurs in a manner that is consistent with parasite-mediated, frequency-dependent selection. Reciprocal cross-infection experiments have further shown that parasites are more infective to sympatric host snails than to allopatric snails, and that they are also more infective to common clones than rare clones within asexual host populations. Hence we suggest that sexual reproduction in these snails may be maintained, at least in part, by locally adapted parasites. Parasite-mediated selection possibly also contributes to the maintenance of local clonal diversity within habitats, while clonal selection may be responsible for the distribution of clones among habitats. © 2003 The Linnean Society of London. Biological Journal of the Linnean Society 2003, 79, 165–181.

Extending the quasi-neutral concept

In caricature, the equilibrium paradigm of community ecology states that plant communities are stable entities consisting of competing species - and that such species coexist because each has its "niche". This paradigm, in its extreme, has been dead for some time. Nevertheless, it has yet to be replaced by a credible "non" - equilibrium paradigm. The quasi-neutral concept of plant communities, proposed by Kristjan Zobel, Folia Geobot. 36: 3-8, 2001, possesses some of the key ingredients of a nonequilibrium theory of diversity. It recognizes that there are inescapable relationships between diversity at different scales, that similarity can influence the rate of competitive exclusion, that successional change is typically associated with changes in life form, and that rarefaction (i.e. "sampling artifacts") can have strong effects on fine-scale diversity. However, the current formulation of the quasi-neutral concept is incomplete in that it relies on an unrealistic definition of community, it assumes that random sampling means that species richness at one scale will be linearly related to richness at finer scales, it ignores the possibility of fine-scale processes producing broader-scale patterns, and it avoids the subject of fine-scale environmental heterogeneity. But the most serious limitation of the quasi-neutral concept is that similarity of species alone is not sufficient to allow indefinite coexistence. I present the results of a simple simulation to demonstrate that: (1) identical species will eventually be lost from communities due to stochastic "drift", (2) slight variations in reproductive rates accelerate this loss, but (3) adding a miniscule "cost of commonness" to the model allows the indefinite coexistence of species. I conclude that the quasi-neutral model cannot work without some kind of trade-off.