Differences In Population Growth Rates Research Articles

Licensed control does not reduce local Cormorant Phalacrocorax carbo population size in winter

Cormorants Phalacrocorax carbo have increased on European freshwaters, creating conflicts with fishing interests. As a result, control measures have been implemented in several countries, although their effect on the English population has yet to be determined. Wetland Bird Survey data was used to derive population growth rates (PGR) of non-coastal Cormorant populations in England. PGR was analysed in relation to control intensity at different scales (5- to 30-km radius) from 2001 to 2009 in order to determine (1) the extent to which control intensity (proportion of the local population shot per winter) was associated with site-level population change, and (2) whether potential effects of control intensity were evident on Special Protection Areas (SPAs). There were no clear differences in PGR when comparing sites which had experienced control versus sites where control had never been carried out. The few significant relationships between control intensity and Cormorant PGR detected were mostly positive, i.e. population growth was associated with higher control intensity. Control intensity was not related to Cormorant numbers in SPAs. Positive associations with control may arise because control is reactive, or because non-lethal effects cause greater dispersal of Cormorants. These results provide no evidence that Cormorant removal at local scales is having an effect on longer term (i.e. year-to-year) population size at a site level. They also suggest that control measures have not affected national population trends, although a better understanding of site use and movements of individual Cormorants needs to be developed at smaller scales (including those due to disturbance caused by control measures) to more fully understand processes at larger scales. Further research is also needed into the extent to which lethal and non-lethal effects of control on Cormorants are having the desired impact on predation rates of fish, and so help resolve the conflict between Cormorants and fisheries.

Demography and Growth: A Unified Treatment of Overlapping Generations

We construct a unified overlapping-generations (OLG) framework of equilibrium growth that includes the Blanchard “perpetual youth” model, the Samuelson model, and the infinitely-lived representative agent growth model as limit specifications of a “realistic,” two-parameter survivorship function. We analyze how exogenous changes in demographic conditions affect the equilibrium growth and savings rates by computing equilibrium rates under different specifications of the survivorship function. Differences in population growth rates, life-expectancies, retirement durations, and the degree of concavity of the survivorship function are found to have significant impacts on equilibrium growth rates. The observed effects are consistent with some cross-country correlations between demographic conditions and growth rates. We also identify a potential “Malthusian growth trap” in economies where life expectancy is short, fertility rates are high, and households work most of their lives - conditions often found in less developed economies.

Adaptive significance of temperature-induced egg size plasticity in a planktonic rotifer, Brachionus calyciflorus

Larger egg size and body size at lower temperature are widely observed in zooplankton and other groups. Theoretically, considering the increased cost of producing larger eggs, females would produce larger and fewer eggs only if the newborn from such eggs perform better under certain environmental constraints, such as low temperature. In temperate lakes, low water temperature is a fairly reliable cue of future declining dietary conditions for herbivorous zooplankters. Under these conditions, females that produce more resistant offspring are likely to be selected. Nevertheless, it is not efficient for them to do this at high temperature, when the dietary conditions are usually more abundant. In this study, the adaptive significance of temperature-induced plasticity in a planktonic rotifer, Brachionus calyciflorus, was investigated. We found that: first, the egg size and body size were negatively related to temperature; second, neonates from larger eggs induced by low a temperature of 138C were more resistant to starvation at three different experimental temperatures; third, life table experiments showed that offspring from larger eggs had a higher population growth rate and shorter juvenile period than those from smaller ones at low temperature, and vice versa, which suggests that this may be attributed to some form of anticipatory maternal effect. These differences in population growth rates were mainly due to higher fertility during the first 1 or 2 days after maturation. All of these findings support the view that temperature-induced egg size plasticity has adaptive significance in rotifers.

Increased population growth rate in invasive polyploid Centaurea stoebe in a common garden

Biological invasions are inherently demographic processes, but trait differences between native and introduced genotypes are rarely linked to population growth rates. Native European Centaurea stoebe occurs as two cytotypes with different life histories (monocarpic diploids, polycarpic tetraploids); however, only tetraploids have been found in its introduced range in North America. In a common garden experiment using artificial populations, we compared the demographic performance of the three geo-cytotypes in the presence and absence of a specialist herbivore using periodic matrix models. We found no difference in population growth rate between the two European cytotypes and no significant effects of herbivory in all geo-cytotypes. However, there was a pronounced increase in population growth rate for North American compared with European tetraploids due to increased seed production and juvenile establishment. These results suggest that genetic drift or rapid evolution, rather than pre-adaptation through polyploidy may explain the invasion success of tetraploids.

Variation in life history and demography of the American black bear

AbstractVariation in life history and demography across a species' range informs researchers about regional adaptations and affects whether managers can borrow information from other populations in decision‐making. The American black bear (Ursus americanus) is a long‐lived game species whose continued persistence depends on management of harvest and removal of habituated bears that come into conflict with humans. Understanding the demography of black bears guides efforts at management and conservation, yet detailed knowledge of many populations is typically lacking. I performed a hierarchical Bayesian meta‐analysis of black bear demographic studies across the geographic range of the species to explore how vital rates vary across the range, what information they give us about population growth, and whether managers can justify borrowing information from other studies to inform management decisions. Cub, yearling, and adult survival and fecundity varied between eastern and western North America, whereas subadult survival did not show geographic structuring. Adult survival and fecundity appeared to trade off, with higher survival in the western portions of bears' range and higher fecundity in the east. Although adult survival had the highest elasticity, differences in reproduction drove differences in population growth rate. Mean population growth rate was higher in the east (0.99; 95% credible interval [CrI]: 0.96, 1.03) than the west (0.97; 95%CrI: 0.93, 1.01). Despite declining trends in the west, 34% of the distribution of population growth rate was >1, compared to 55% in the east. Further work needs to be done to address the cause of the apparent trade‐off between adult survival and fecundity and explore how the estimated growth rates are likely to affect population status of black bears. Because population growth rates are close to 1 and small deviations could impact whether a population is considered increasing or decreasing, managers need to employ caution in borrowing vital rates from other populations. © 2011 The Wildlife Society.

Demographic Variability and Population Viability of an Endemic Variety of the Siberian Primrose

We used matrix population models to analyse spatial and temporal variation in the demography of the Siberian primrose (Primula nutans ssp. finmarchica var. jokelae), an early successional species of seashore meadows in the Gulf of Bothnia. We compared the dynamics of populations in habitats that were partially overgrown by woody shrubs with those in open ones. The major threat to the populations was the invasion of the habitat by woody plants, as population growth was very sensitive to a change in the transition from a sterile rosette to a flowering one and this transition occurred less frequently in overgrown meadows. This transition also had the greatest contribution to the differences in population growth rates between habitats. Although the invasion of shrubs negatively influenced population growth rate, most populations are still expected to persist and only two small populations out of eleven examined populations were expected to disappear within 50 years. However, with proper habitat management the problem of coarse-scale interactions, such as asymmetric competition for light with shrubs, can be substantially alleviated.

Demographic processes of upward range contraction in a long-lived Mediterranean high mountain plant

We analyzed demographic data of a long-lived high mountain Mediterranean plant, Silene ciliata Poirret, over a 4-yr period. Selected populations were located at contrasting altitudes at the southernmost margin of the species (Sierra de Guadarrama, central Spain), representing a local altitudinal range at the rear edge of its overall distribution. Previous studies have suggested that differences in the reproduction and performance of individuals at upper and lower populations may have implications for population dynamics. We used matrix analysis to assess their demographic behaviour. Life Table Response Experiments were used to identify the life history stages most relevant to observed differences in population growth rates between populations. Transition matrices revealed great spatio-temporal variability in demographic traits. Seedling recruitment was very low each year in all populations. Maximum longevity of S. ciliata individuals in the lower peripheral population was much lower compared to the central population, probably due to higher adult mortality. Population growth rate (λ) showed a declining trend at the lowest altitude and a relatively stable trend at the central population. Long-term simulations also indicated a great risk of quasi-extinction at the lowest population. Our results suggest that rear edge populations of S. ciliata at Sierra de Guadarrama are suffering demographic processes that may be leading to the latitudinal displacement of the species' range.

Size‐dependent flowering and costs of reproduction affect population dynamics in a tuberous perennial woodland orchid

Summary 1. In woodland herbs, the probability of flowering and costs associated with reproduction may strongly depend on environmental context (shade vs. light habitats) and on plant size. This may be particularly true for tuberous orchids that inhabit woodlands, as the amount of incoming radiation and total leaf area strongly determine photosynthetic capacity and hence the amount of carbohydrates that can be relocated to below‐ground storage organs that form next year’s rosette and flowering stalk. 2. To fully comprehend the impact of size‐dependent reproduction on population dynamics under varying light conditions, life cycle models should therefore include plant size in a continuous manner. In this study, annual changes in plant size and demographic behaviour of the tuberous perennial orchid Orchis purpurea were monitored during seven consecutive years (2003–2009) in open and shaded woodland. Integral projection models (IPMs) and life table response experiments (LTRE) were used to investigate the extent to which variation in plant size affected the overall population dynamics of this species and to decompose differences in population growth rates between populations of open and shaded woodland into contributions from growth, survival and reproduction. 3. Both plants in shaded and light environments needed to be a certain size to initiate flowering, but this threshold size was almost three times as large in shaded environments as in light environments. Plants in open woodlands flowered more frequently over the years, showed less size regression after flowering and produced significantly more fruits than plants in shaded environments, resulting in significantly larger population growth rates. 4. Our life cycle models revealed that costs of reproduction, measured at the population‐level, were small in the light environment, and more than buffered by the increase in survival of flowering plants compared to non‐flowering plants. In the shade environment, however, the costs of reproduction were significant and made the difference between a stable (current) and a growing (without reproduction costs) population. 5. Synthesis. Light penetration to the soil is a key variable determining population dynamics of woodland orchids. Our analyses show that differences in vital rates related to size‐dependent reproduction (flowering) and growth are essential drivers of changes in orchid population dynamics in different light environments. The combination of IPMs and LTREs thus proves to be very useful when studying the fitness consequences of size‐ and context‐dependent phenomena like flowering strategies and costs of reproduction.

Population-level effects of spinosad and Bacillus thuringiensis israelensis in Daphnia pulex and Daphnia magna: comparison of laboratory and field microcosm exposure conditions

Because exposure to toxicants not only results in mortality but also in multiple sublethal effects, the use of life-table data appears particularly suitable to assess global effects on exposed populations. The present study uses a life table response approach to assess population-level effects of two insecticides used against mosquito larvae, spinosad (8 μg/l) and Bacillus thuringiensis var. israelensis (Bti, 0.5 μl/l), on two non target species, Daphnia pulex and Daphnia magna (Crustacea: Cladocera), under laboratory versus field microcosms conditions. Population growth rates were inferred from life table data and Leslie matrices under a model with resource limitation (ceiling). These were further used to estimate population risks of extinction under each tested condition, using stochastic simulations. In laboratory conditions, analyses performed for each species confirmed the significant negative effect of spinosad on survival, mean time at death, and fecundity as compared to controls and Bti-treated groups; for both species, population growth rate λ was lower under exposure to spinosad. In field microcosms, 2 days after larvicide application, differences in population growth rates were observed between spinosad exposure conditions, and control and Bti exposure conditions. Simulations performed on spinosad-exposed organisms led to population extinction (minimum abundance = 0, extinction risk = 1), and this was extremely rapid (time to quasi-extinction = 4.1 one-week long steps, i.e. one month). Finally, D. magna was shown to be more sensitive than D. pulex to spinosad in the laboratory, and the effects were also detectable through field population demographic simulations.

A quantitative assessment of the decline in the U.S. current account

Low frequency changes in the U.S. current account can be understood in terms of the influence of differences in productivity growth rates across time and across countries using standard growth theory. In particular, the secular decline is primarily driven by the increase in the U.S. TFP growth rate relative to its trading partners. Differences in population growth rates or fiscal policy have no significant effects on the low frequency changes in the U.S. current account.

Comparative demographic analysis in contrasting environments of Magnolia dealbata: an endangered species from Mexico

AbstractThe study of population regulation and demography in natural habitats is critical for the conservation of rare and endangered species. We address the impact of cattle exclusion on the demographics of Magnolia dealbata (Magnoliaceae), an endangered species according to IUCN and Federal Mexican laws. Sixteen permanent plots were established, eight of which were enclosed to livestock, in the mountain cloud forest in Coyopolan, Mexico, which has the largest population of M. deadlbata. Censuses of the plots were undertaken annually during three annual cycles to record seed number, recruitment, mortality, and growth (defined as length and diameter at breast height). The effects of two treatments (with and without livestock exclusion) on the dynamics of M. dealbata were investigated using transition matrix models and life table response experiment (LTRE) analysis. Contrary to expectations, there was no significant effect of cattle exclusion on population growth rates (λ). Furthermore, the λ, estimated from the mean transition matrix for both treatments was greater than one. The transitions with the highest elasticity values were similar between the two treatments, while reproductive stage contributed more to differences in population growth rate and were less variable than the non‐reproductive stage. LTRE analysis showed that treatment differences had little effect on λ. Livestock exclusion appears to lead more to differences in the arrangement of the values of the transition matrix than to the rate of population growth for M. dealbata.

INVASION DYNAMICS OF THE VARNISH CLAM (NUTTALLIA OBSCURATA): A MATRIX DEMOGRAPHIC MODELING APPROACH

Marine invaders have become a significant threat to native biodiversity and ecosystem function. In this study, the invasion of the varnish clam (Nuttallia obscurata) in British Columbia, Canada, is investigated using a matrix modeling approach to identify the life history characteristics most crucial for population growth and to investigate population differences. Mark-recapture analyses and field collections from 2003 to 2004 were used to determine individual growth, survival rates, and fecundity for two sites. A multi-state matrix model was used to determine population growth rates and to conduct sensitivity and elasticity analyses. A life table response experiment was also used to determine what life history stage contributed most to observed differences in population growth rates. Population survey data were used in conjunction with the matrix model to determine plausible recruitment levels and to investigate recruitment scenarios. Both populations are currently declining but are likely sustainable because of the pulsed nature of large recruitment events. Survival of larger clams (>40 mm) is the most important for population growth based on elasticity and sensitivity analyses. Adult survival also had the largest influence on observed differences between site-specific population growth rates. The two populations studied differed in recruitment dynamics; one experiencing annual recruitment with higher post-settlement mortality and the other, episodic recruitment and lower post-settlement mortality. The most influential factor for the successful invasion of the varnish clam appears to be survival of the larger size classes. Therefore, any process that decreases adult survival (e.g., predation, commercial harvest) will have the greatest impact on population growth.

Genetic quality of individuals impacts population dynamics

AbstractAmple evidence exists that an increase in the inbreeding level of a population reduces the value of fitness components such as fecundity and survival. It does not follow, however, that these decreases in the components of fitness impact population dynamics in a way that increases extinction risk, because virtually all species produce far more offspring than can actually survive. We analyzed the effects of the genetic quality (mean fitness) of individuals on the population growth rate of seven natural populations in each of two species of wolf spider in the genus Rabidosa, statistically controlling for environmental factors. We show that populations of different sizes, and different inbreeding levels, differ in population dynamics for both species. Differences in population growth rates are especially pronounced during stressful environmental conditions (low food availability) and the stressful environment affects smaller populations (<500 individuals) disproportionately. Thus, even in an invertebrate with an extremely high potential growth rate and strong density‐dependent mortality rates, genetic factors contribute directly to population dynamics and, therefore, to extinction risk. This is only the second study to demonstrate an impact of the genetic quality of individual genotypes on population dynamics in a wild population and the first to document strong inbreeding–environment interactions for fitness among populations. Endangered species typically exist at sizes of a few hundred individuals and human activities degrade habitats making them innately more stressful (e.g. global climate change). Therefore, the interaction between genetic factors and environmental stress has important implications for efforts aimed at conserving the Earth's biodiversity.

Effects of harvester behavior on population growth rate affects sustainability of ginseng trade

When long-term, intense levels of harvest nearly extirpated Chinese ginseng ( Panax ginseng C. Meyer) in the early 18th century, commercial harvest of American ginseng ( P. quinquefolius L.) began and large quantities of the roots were exported to East Asia. Annual export figures have fallen over the past 200 years, but demand for wild American ginseng has not abated. Persistent harvest of long-lived, slow-maturing species can have negative impacts on population growth rates, yet those closest to wild resources are often in a position to be the best stewards of that resource. This study explored the consequences of different harvester behaviors on the population dynamics of American ginseng. Drawing on known behaviors, we developed three harvester ‘types’ and ran demographic simulations on wild ginseng populations, partitioning the sources of differences in population growth rates using a life table response experiment (LTRE). The simulations showed that ignoring size class limits and harvest season onset dates dramatically affected population growth rates. Existing laws in many states are not adequate to protect wild ginseng populations. A stewardship-oriented harvester, who delays harvest onset by two weeks, self-limits harvest intensity and plants ginseng seeds at the time of harvest can reverse declining population growth rates.

Habitat Change and Demography of Primula veris: Identification of Management Targets

Although the effects of deterministic factors on population viability often are more important than stochasticity, few researchers have dealt with the effect of deterministic habitat changes on plant population demography. We assessed population viability for the perennial herb Primula veris L. and identified targets for management based on demographic data from five different habitat types representing different degrees of canopy closure. We conducted replicate studies at the border of the distribution area and in more central parts. Demographic patterns were similar between the two regions. Most study populations had a positive population growth, and only populations in late phases of forest succession showed consistently negative trends. The populations of open habitats had high seedling recruitment, and the populations of early and middle forest succession had high seed production. The importance of survival for population growth rate increased with increasing habitat closure, whereas the importance of growth and reproduction decreased. Results of the elasticity analysis suggested that the best method to manage decreasing late-successional populations is to increase survival of the largest individuals. The life-table response experiment (LTRE) analysis, however showed that survival of the largest individuals contributed little to differences in population growth rates of different habitats, whereas seed production and growth of small individuals were more important. Moreover direct perturbation of the performance of the largest stages showed that late-successional populations would not attain positive population growth even if the largest stages had no mortality at all. We conclude that restoration of recruitment is the only possibility for positive population growth in late-successional populations of P. veris, although the elasticities of recruitment transitions are low. Our results also suggest that retrospective demographic methods such as LTRE constitute an important and necessary complement to prospective methods such as elasticities in identifying management targets.

A demographic analysis of vole population responses to fragmentation and destruction of habitat

AbstractFragmentation and destruction of natural habitats is currently considered to be the major threat to wildlife populations. We here perform a comprehensive analysis of the demographic effects of habitat fragmentation and destruction on 14 populations of the root vole. The experiment was divided into two consecutive periods. During the first period, we contrasted populations with the same initial size and structure in continuous and fragmented habitat. During the second period, we fragmented the continuous habitat into the same configuration as the permanently fragmented habitat so that the effect of habitat destruction could be evaluated. We estimated survival and fecundity parameters and combined them into population projection matrices to evaluate their relative impact on population growth. In the first period of the experiment there was no difference in population growth rate between fragmented and continuous populations, although litter size was significantly higher in the continuous populations. In the second period, we found higher population growth rates in populations that had experienced habitat destruction. By applying the transition matrix model to empirical estimates of demographic parameters, we demonstrate that the difference in population growth rate in the second period of the experiment was the result of a nonsignificant difference in adult survival. Movements out of the habitat patches were significantly lower in populations that had experienced habitat destruction. We conclude that predator‐caused mortality of animals moving out of the habitat patches was the main determinant of demographic variation in this system.

The effect of changes in light availability caused by mixing on the growth of Anabaena circinalis (Nostocales, Cyanobacteria) and Aulacoseira sp. (Centrales, Bacillariophyceae)

Experiments were undertaken to study the effects of vertical distribution controlled by mixing on the population growth of Australian isolates of the toxic cyanobacterium Anabaena circinalis and the diatom Aulacoseira sp. Transparent tubular microcosms were identically illuminated in all experiments, with a surface maximum irradiance of 225 μmol photons m−2 s−1 attenuating to a minimum of 18 μmol photons m−2 s−1 at the base of the microcosm at I m depth. In separate experiments, microcosms containing A. circinalis or Aulacoseira sp. were mixed at 10 min (MIXED treatment) or 48 h (CALM treatment) intervals to simulate well-mixed and calm environments, respectively. The vertical distribution of cells was predicted using a model that utilized growth-irradiance relationships and sinking and flotation rates measured during the experiments. For A. circinalis, there was no significant difference in population growth rate between MIXED and CALM treatments (MIXED, μ = 0.47 d−1; CALM, μ=0.45 d−1). The predicted vertical distribution of A. circinalis was skewed toward the surface in the CALM treatment, indicating that an increased portion of the population received greater irradiance. However, both predicted and actual population growth were no greater in the CALM treatment, owing to the low flotation rate of the cells. Any light-harvesting advantage gained through buoyancy was insufficient to produce a significant difference in growth. Aulacoseira sp. had significantly different population growth rates in different treatments (MIXED, μ = 0.39 d−1; CALM, μ=0.28 d−1). Predicted differences in the vertical distribution of Aulacoseira sp. indicate that a large proportion of the cellular population received reduced light as a consequence of sinking in the CALM treatment. These results indicate that population growth in Aulacoseira sp. is highly sensitive to reduced light caused by sinking from the well-lit surface waters. Our microcosm experiments suggest that, in the natural environment, reduced growth of Aulacoseira sp. in calm conditions in thermally stratified and/or low flow conditions would put it at a competitive disadvantage relative to A. circinalis.

Population Characteristics of Sympatric Mule and White-Tailed Deer on Rocky Mountain Arsenal, Colorado

We studied population characteristics of sympatric mule deer (Odocoileus hemionus) and white-tailed deer (O. virginianus) on Rocky Mountain Arsenal, Colorado, USA, 1990-1993. Both species' populations increased during the study. Mule deer increased nearly 3 times faster than white-tailed deer. We did not detect any differences in adult survival, overwinter fawn survival, or natality between these species. Differences in population growth rates over 4 years were attributed to an initial 4:1 numerical advantage for mule deer and a 2:1 advantage in early survival of mule deer fawns.

GENETIC INFLUENCES ON EXPERIMENTAL POPULATION DYNAMICS OF THE LEAST KILLIFISH

When natural populations differ in density or in the dynamic fluctuations of population size, some of those differences may result from their different ecological conditions, and some may originate from genetically based differences in life history expression. Natural populations of the live-bearing poeciliid fish, Heterandria formosa, vary considerably in their population dynamics, with densities that differ between populations by as much as sevenfold. This system offers an excellent opportunity to explore the potential role of genetically based differences in life history expression in creating different dynamic patterns in a common environment. We created five different genetic stocks of H. formosa by carrying out a series of crosses using fish from two North Florida populations (the Wacissa River and Trout Pond) and used them to initiate replicate experimental populations in artificial ponds. The five stocks consisted of two “controls,” which were pure Wacissa River and Trout Pond stocks, and three types of hybrid stocks. The hybrid stocks differed in a regular way in the proportion of genes from one population or the other. The crossing scheme was designed so that each hybrid stock would have the same proportion of heterozygous (or “heterodemic”) loci but would differ in the proportion and/or identity of homozygous (or “homodemic”) loci from the Wacissa River and Trout Pond populations. These populations were chosen because a previous study had found that population densities in the Wacissa River greatly exceeded those of Trout Pond and exhibited a higher range of population fluctuation during the breeding season. We addressed four questions in this experiment: (1) Are there genetically based differences in life history traits of fish from the two populations? (2) If so, do differences in life history expression produce differences in population dynamics in a common environment? (3) Which traits have the greatest influence on population dynamics? (4) How do changes in density affect the phenotypes of individual traits that govern the rates of birth and death in a population? We followed experimental populations of the five genetic stocks from their initiation at low density through 4–6 generations of population growth and decline. The mean offspring size differed among stocks by as much as 50%. At low densities, offspring size exhibited a trade-off with brood size: Trout Pond alleles were associated with more, smaller offspring. At higher densities, offspring sizes were similar among stocks, and the trade-off with offspring number was not evident. Stocks differed in realized population growth rate by as much as 70%; the rank order differences among stocks with respect to population growth rate appeared to match the genetic relatedness among stocks based on the expected percentage of Trout Pond alleles. Differences in population growth rate appeared to be due to differences in brood size among stocks at low density. Stocks did not differ in the equilibrium population size, which indicated the absence of a trade-off between population growth rate and carrying capacity in this environment. Adult survival and recruitment of juveniles into the adult population both declined linearly with increasing density; and stocks did not generally differ in those rates after the effects of density had been taken into account. The stocks differed in their response to the depressant effects of density on life history trait expression. The offspring size of the pure Wacissa River stock was much more sensitive to density than was the offspring size of the pure Trout Pond stock. However, the brood sizes of the Wacissa River stock were reduced much less than those of the Trout Pond stock when exposed to the same high density. These results suggest that life history distinctions among populations, both in the mean values and plasticity of traits, play a role in creating different dynamics. We discuss the ways in which phenotypic plasticity in reproductive traits potentially acts as a mechanism to stabilize population dynamics in this species.

Ecological factors affecting gene flow in the Brachionus plicatilis complex (Rotifera).

We investigated how adaptation to salinity and temperature acts as reproductive barriers in three sympatric species from the Brachionus plicatilis species complex. These species co-occur in a salt marsh in Spain, and a previous electrophoretic study of variation revealed no hybrids between them. A factorial experiment was designed to test for differences in population growth rates and patterns of bisexual reproduction. The design combined representative strains from each species in different salinity and temperature conditions, representing the range over which these rotifers are found in their natural environment. We found differences in the growth response of the three species to both factors and in the pattern of bisexual reproduction. These differences help to explain patterns of succession observed in the field. We conclude that these ecological factors, together with mate recognition systems, account for the absence of gene flow in these sympatric species.