Changes In Population Size Structure Research Articles

Demographic recovery of corals at a wave-exposed reef following catastrophic disturbance

Rapid recovery of coral cover following acute disturbance has been documented on many reefs. Yet measuring coverage alone can mask shifts in community and demographic structure. Here, we quantify long-term changes in population size structure for three common genera (Acropora, Pocillopora, and Stylophora) at an eastern outer reef in Palau, Micronesia, following catastrophic loss of corals due to typhoon Bopha in 2012, based on size measurements from 3648 coral colonies. Mean colony size returned to pre-disturbance levels within 4 and 6 years for Stylophora and Pocillopora, respectively. By 2020, Pocillopora colony density far exceeded pre-disturbance levels, with rapid successful recruitment following typhoon Bopha. Despite recovery of Acropora colony density by 2020, populations remained dominated by smaller colonies. We demonstrate that full demographic recovery can occur more rapidly for pocilloporids (within 6 years) compared to Acropora which had not fully recovered by 8 years post-disturbance, possibly due to fewer annual recruitment events and larger maximum colony sizes. Our results highlight the value of demographic metrics as early indicators of recovery.

A discrete, stochastic model of colonial phytoplankton population size structure: Development and application to in situ imaging-in-flow cytometer observations of Dinobryon.

The scientific community lacks models for the dynamic changes in population size structure that occur in colonial phytoplankton. This is surprising, as size is a key trait affecting many aspects of phytoplankton ecology, and colonial forms are very common. We aim to fill this gap with a new discrete, stochastic model of dynamic changes in phytoplankton colonies' population size structure. We use the colonial phytoplankton Dinobryon as a proof-of-concept organism. The model includes four stochastic functions-division, stomatocyst production, colony breakage, and colony loss-to determine Dinobryon population size structure and populations counts. Although the functions presented here are tailored to Dinobryon, the model is readily adaptable to represent other colonial taxa. We demonstrate how fitting our model to in situ observations of colony population size structure can provide a powerful approach to explore colony size dynamics. Here, we have (1) collected high-frequency in situ observations of Dinobryon in Lac (Lake) Montjoie (Quebec, Canada) in 2013 with a moored Imaging FlowCytobot (IFCB) and (2) fit the model to those observations with a genetic algorithm solver that extracts parameter estimates for each of the four stochastic functions. As an example of the power of this model-data integration, we also highlight ecological insights into Dinobryon colony size and stomatocyst production. The Dinobryon population was enriched in larger, flagellate-rich colonies near bloom initiation and shifted to smaller and emptier colonies toward bloom decline.

Effects of stochastic growth on population dynamics and management quantities estimated from an integrated catch-at-length assessment model: Panopea globosa as case study

In size-based stock assessment models, the stochastic growth of individuals is expressed through a transition matrix representing the growth variability as probability of shift from one length class to another during a time period. This process is important because it describes the changes in population size structure brought about by the increase in length or weight of organisms over time. In this study, four stochastic growth matrices were developed within an integrated catch-at-length assessment model (ICLAM), and the changes in the population dynamics and quantities relevant to fishery management of Panopea globosa were analyzed. The growth increments were estimated through von Bertalanffy, Gompertz, Logistic, and Schnute models using a gamma probabilistic density function. A corrected Akaike information criterion was used to select the best performance among the four ICLAMs. The ICLAM associated with the von Bertalanffy matrix was the best for describing the catch-at-length data, providing conservative estimates on the condition of the stock; while the ICLAM for the Logistic matrix showed low performance, exhibiting the highest estimates for all the components of population dynamics, such as an increase of 75.3% in the recruitment and 58.5% in the total abundance with regard to the von Bertalanffy matrix. These differences emphasize the importance of determining a suitable stochastic growth matrix because it has serious implications in biasing stock assessments, resulting in inadequate management strategies.

Population regulation in a Neotropical seasonal wetland fish

Density-dependent regulation is a necessary process for the long-term persistence of populations. Nevertheless, ecologists still debate whether floodplain fish populations are subject to density-dependent dynamics or mostly regulated by the density-independent seasonal flooding. We surveyed Jewel tetra (Hyphessonbrycon eques) populations during 4 years in the floodplain lagoons of the Cuiaba River (Brazil) in order to determine if recruitment, growth, and survival are subject to density-dependent effects. We showed that population dynamics of this species are influenced by the seasonal drought, which affects the various life stages of the cohort in different ways. The flood had positive effects on the strength of recruitment, and indirectly affected the probability of survival of recruits during the following dry season as well as on the extinction probability of local populations. Additionally, by facilitating the dispersal of pre-larvae and juvenile recruits, it enables the re-establishment of locally extinct populations during the preceding dry season. Therefore, flooding is essential for the persistence of the species at the regional scale. On the other hand, the limiting conditions during the dry season cause high mortality rates among juveniles. More importantly, the mortality during the dry season is density-dependent and causes changes in population size structure and extinction of local populations, which will affect recruitment in the following year. Our results change the traditional belief that flood alone control population dynamics of fish dwelling in floodplains of larger rivers but a longer survey is required to better understand the effects of drought.

On the population dynamics, reproductive biology and growth of Succinea putris (Linnaeus, 1758) (Gastropoda: Pulmonata: Succineidae)

Selected life-cycle and population parameters of a common Euro-Siberian wetland snail – Succinea putris (L.) – were studied in the field and in the laboratory. In the field the snails reproduced at least from April till September, with April–May and August–September peaks; only one such peak (April) was observed in the laboratory, though the snails reproduced throughout the year. The changes in population size structure in the field and the laboratory results (life span 210–420 days, mean 309) indicated semelparity. Growth in the laboratory included two phases: slow (November–March) and fast (June–October); which phase came first during the life cycle depended on the date of hatching. The growth rate in the field corresponded closely with the fast-phase growth in the laboratory. The smallest reproducing individual was slightly over 2.8 whorls; snails of 3.0 whorls were regularly observed to produce eggs (maximum number of whorls in adults: 4.0). Sexual maturity was attained in ca. 160 days. The eggs (non-calcified, translucent, spherical, 1.7–2.0 mm in diameter) were laid in batches, 5–64 per batch; the eggs within a batch were glued together. The batch dimensions were 3.5–25.7×2.2–24.7 mm. The time to lay a batch was 20–35 minutes. Forty-eight snails produced 74 batches within 12 months. The duration of the egg phase was 11–28 days, which might indicate egg retention of varied duration; hatching was asynchronous, spanning 1–12 days within a batch. The hatchlings had shells of 1.1–1.2 whorls; hatching success was 95%. Neither uniparental reproduction, nor egg or juvenile cannibalism were observed. When compared to data in the literature, our results imply that Succinea putris displays substantial local variation in life-cycle traits. We also provide an overview table to discuss similarities and adaptive radiation in the European succineid species.

Climatic stress mediates the impacts of herbivory on plant population structure and components of individual fitness

SummaryPast studies have shown that the strength of top‐down herbivore control on plant physiological performance, abundance and distribution patterns can shift with abiotic stress, but it is still unclear whether herbivores generally exert stronger effects on plants in stressful or in nonstressful environments.One hypothesis suggests that herbivores' effects on plant biomass and fitness should be strongest in stressful areas, because stressed plants are less able to compensate for herbivore damage. Alternatively, herbivores may reduce plant biomass and fitness more substantially in nonstressful areas, either because plant growth rates in the absence of herbivory are higher and/or because herbivores are more abundant and diverse in nonstressful areas.We test these predictions of where herbivores should exert stronger effects by measuring individual performance, population size structure and densities of a common subshrub,Hibiscus meyeri, in a large‐scale herbivore exclosure experiment arrayed across an aridity gradient in East Africa.We find support for both predictions, with herbivores exerting stronger effects on individual‐level performance in arid (stressful) areas, but exerting stronger effects on population size structure and abundance in mesic (nonstressful) areas. We suggest that this discrepancy arises from higher potential growth rates in mesic areas, where alleviation of herbivory leads to substantially more growth and thus large changes in population size structure. Differences in herbivore abundance do not appear to contribute to our results.Synthesis. Our work suggests that understanding the multiple facets of plant response to herbivores (e.g. both individual performance and abundance) may be necessary to predict how plant species' abundance and distribution patterns will shift in response to changing climate and herbivore numbers.

Effects of water temperature and mixed layer depth on zooplankton body size

Ecological consequences of global warming include shifts of species ranges toward higher altitudes and latitudes as well as temporal shifts in phenology and life-cycle events. Evidence is accumulating that increasing temperature is also linked to reduced body size of ectotherms. While temperature can act directly on body size, it may also act indirectly by affecting the timing of life-cycle events and the resulting population age and size structure, especially in seasonal environments. Population structure may, in turn, be influenced by temperature-driven changes in resource availability. In a field mesocosm experiment, we investigated how water temperature and mixed surface layer depth (a temperature-dependent determinant of light availability to phytoplankton) affected population dynamics, population age and size structure, and individual size at stage (size at first reproduction) of Daphnia hyalina during and after a phytoplankton spring bloom. Mixed layer depth was inversely related to the magnitudes of the phytoplankton spring bloom and the subsequent Daphnia peak, but had no effect on the body size of Daphnia. Conversely, temperature had no effects on abundance peaks but strongly affected the timing of these events. This resulted in at times positive, at other times negative, transient effects of temperature on mean body size, caused by asynchronous changes in population size structure in cold versus warm treatments. In contrast to mean body size, individual size at stage consistently decreased with increasing temperature. We suggest that size at stage could be used as an unbiased response parameter to temperature that is unaffected by transient, demographically driven changes in population size structure.

Effects of Fire on Riparian Forests Along a Free-Flowing Dryland River

Riparian fire studies in the American Southwest have focused on flow-regulated rivers and typically show increase in introduced Tamarix, a species with high resprout capacity, and declines in Populus. Effects of fire, however, can vary with environmental setting. We examined riparian fire along the free-flowing Upper San Pedro River (Arizona) by making temporal comparisons supplemented by spatial contrasts between burned and unburned sites. Pre-fire, Populus fremontii and Salix gooddingii were dominant species, with Tamarix sparse in the understory. Species differed in mortality and resprout rates, producing post-fire vegetation change. Mortality was highest for Tamarix, intermediate for Salix, and lowest for Populus, and also varied among size classes. Resprout rate was low for Populus, high for Salix, and also high for the few surviving Tamarix. The net effect was changes in population size structure (relative shifts towards larger Populus but smaller individuals of other species) and forest composition (decreased abundance of Tamarix relative to Populus and Salix), and in spatial distribution of stems. Tamarix’s low ability to survive fire resulted from competitive suppression by Populus and Salix, with their dominance a product of the site’s flow regime. Results indicate that post-fire outcomes within the riparian Southwest are variable and context-dependent.

Simulation of alternative management strategies for red algae, luga roja, ( Gigartina skottsbergii Setchell and Gardner) in southern Chile

The objective of this study was to examine the response of a Gigartina skottsbergii population, a commercial red algal species from southern Chile, to different management strategies using a simulation model. The impact of present harvesting techniques, varying the recommended size at harvest and the rate of harvest were evaluated. Model development followed Grant et al. (Ecology and Natural Resource Management: Systems Analysis and Simulation. Wiley, New York, 1997) and the data base was obtained from available literature. The model simulates population dynamics of G. skottsbergii and in terms of both number of fronds and biomass per size class. Model predicts an annual pattern of variation for biomass determined by variations in seasonal factors. However, biomass has a 2 year cycle of alternate years with high biomass and low biomass. This production cycle can be attributed to changes in population size structure and density, and to the existence of discrete recruitment events. Results predicted from using different management strategies suggest that commercial harvest should be based on selection for larger fronds and restricted harvest period. Under this scenario harvest rate can be relaxed since no important changes in harvest biomass were predicted when increasing harvest rate from 50 to 90%, at least in the short term. However, it is recommendable to keep it low (50%) as a preventive mechanism, since results suggest that in the long term, a harvest rate of 50% could produce larger harvested biomass. The existence of a biennial cycle of biomass production suggests that populations should be harvested in rotation. Several aspects of the model must be improved in order to obtain better prediction of both specific values as well as dynamics. These include winter recruitment, frond breakage, longevity of larger sized fronds, and a more complete analysis of changes in the size structure of the population.

Assessing Population Responses to Multiple Anthropogenic Effects: A Case Study with Brook Trout

Population declines are often caused by multiple factors, including anthropogenic ones that can be mitigated or reversed to enhance population recovery. We used a size‐classified matrix population model to examine multiple anthropogenic effects on a population and determine which factors are most (or least) important to population dynamics. We modeled brook trout (Salvelinus fontinalis) in southern Appalachian mountain streams responding to multiple anthropogenic effects including the introduction of an exotic salmonid species (rainbow trout, Oncorhynchus mykiss), a decrease in pH (through acidic deposition), an increase in siltation (from roadbuilding and logging), and an increase in fishing pressure. Potential brook trout responses to rainbow trout include a decrease in survival rate of small fish, a change in density dependence in survival of small fish, and a decrease in growth rates of all sizes. When we included these responses in the population model, we found that population size tended to decrease with an increase in small‐fish growth rate (producing a population with fewer, but larger, fish). In addition, changes in patterns of density‐dependent survival also had a strong impact on both population size and size structure. Brook trout respond to decreases in pH with decreased growth rate in all size classes, decreased survival rates of small fish, and decreased egg‐to‐larva survival rates. This combination of effects, at magnitudes documented in laboratory experiments, had severe negative impacts on the modeled population. If siltation effects were severe, the extreme increase in egg‐to‐larva mortality could have strong negative effects on the population. However, even very strong increases in large fish mortality associated with sport harvesting were not likely to cause a local extinction. In all of these cases, the interaction of drastic changes in population size structure with randomly occurring floods or droughts may lead to even stronger negative impacts than those predicted from the deterministic model. Because these fish can reproduce at a small size, negative impacts on survival of the largest fish were not detrimental to the persistence of the population. Because survival of small juveniles is density dependent, even moderate decreases in survival in this stage had little effect on the ultimate population size. In general, a brook trout population will respond most negatively to factors that decrease survival of large juveniles and small adults, and growth rates of small juveniles

Population dynamics and morphometries of Gammarus pulex L.: evidence of seasonal food limitation in a freshwater detritivore

SUMMARY. 1. Seasonal changes in population size structure of Gammarus pulex L. in a Cotswold stream appeared to indicate a growth check in late summer.2. The relationships between dry weight and body length, and between the number of primary flagellar segments on the first antenna and either dry weight or body length provided further evidence of a reduction in growth in mid and late summer.3. Body fat content was minimal (4% of dry weight) at the end of summer, when large particulate organic detritus was scarce or of poor quality, and maximal in late winter (17.9% dry weight in females; 9.4% in males), after a period of high food availability. In a field experiment, the fat content of animals in summer was raised to levels typical of winter by providing high quality food.4. Field and experimental evidence together strongly infer that this population of G. pulex was subject to severe food limitation from early summer until leaf fall in autumn.

The population biology ofPlanaria torva (M�ller) (Turbellaria, Tricladida)

1. A quantitative study of the population biology ofPlanaria torva (Müller) living in a productive lake, based on monthly samples over a period of 20 months, is presented. 2. Samples of triclads, their cocoons and other organisms were taken from bricks placed at a shallow depth in the lake. They provided relative information on changes in population size structure and demonstrated the temporal breeding of this triclad. 3. Cocoon production byP. torva began in January and reached a peak in March-April. This was followed by a peak in recently-hatched young during May-June. Adults reached a maximal proportion of the total population in January and most had died by July. 4. Changes in size structure suggest that intra-specific competition occurred during June-July following the recruitment of young. This hypothesis is supported by other independent evidence. 5. Attempts to measure fecundity based on cocoons and young gave very different results attributed to the sampling technique.