Estimating rates of local extinction and colonization in colonial species and an extension to the metapopulation and community levels

Coloniality has mainly been studied from an evolutionary perspective, but relatively few studies have developed methods for modelling colony dynamics. Changes in number of colonies over time provide a useful tool for predicting and evaluating the responses of colonial species to management and to environmental disturbance. Probabilistic Markov process models have been recently used to estimate colony site dynamics using presence–absence data when all colonies are detected in sampling efforts. Here, we define and develop two general approaches for the modelling and analysis of colony dynamics for sampling situations in which all colonies are, and are not, detected. For both approaches, we develop a general probabilistic model for the data and then constrain model parameters based on various hypotheses about colony dynamics. We use Akaike's Information Criterion (AIC) to assess the adequacy of the constrained models. The models are parameterised with conditional probabilities of local colony site extinction and colonization. Presence–absence data arising from Pollock's robust capture–recapture design provide the basis for obtaining unbiased estimates of extinction, colonization, and detection probabilities when not all colonies are detected. This second approach should be particularly useful in situations where detection probabilities are heterogeneous among colony sites. The general methodology is illustrated using presence–absence data on two species of herons. Estimates of the extinction and colonization rates showed interspecific differences and strong temporal and spatial variations. We were also able to test specific predictions about colony dynamics based on ideas about habitat change and metapopulation dynamics. We recommend estimators based on probabilistic modelling for future work on colony dynamics. We also believe that this methodological framework has wide application to problems in animal ecology concerning metapopulation and community dynamics.     

Nonlinearity in the growth of bacterial colonies: conditions and causes

The universally recognized kinetic model of colony growth, introduced by Pirt, predicts a linear increase of colony size. The linearity follows from the assumption that the colony expands through the growth of only such cells that are located immediately behind the moving colony front, in the so-called peripheral zone of constant width and density. In this work, Pirt's model was tested on two bacteria--Alcaligenes sp. and Pseudomonas fluorescens--having markedly distinct cultural properties and grown on agarized medium with pyruvate. The colony size dynamics was followed for different densities of the inoculum, ranging from a single cell to a microdroplet of bacterial suspension (10(5)-10(6) cells), and for different depths of the agar layer, determining the amount of available substrate. A linear growth mode was observed only with P. fluorescens and only in the case of growth from a microdroplet. When originating from a single cell, colonies of both organisms displayed nonlinear growth with a distinct peak of Kr (the rate of colony radius increase) occurring after 2-3 days of growth. The growth of P. fluorescens colonies showed virtually no dependence on the depth of the agarized medium, whereas the rate of colony size increase of Alcaligenes sp. turned out to be directly related to the medium layer thickness. The departure from linearity is consistently explained by a new kinetic chart stipulating a possible contribution to the colony growth not only of peripheral cells but also (much more distinct in Alcaligenes) of cells at the colony center. The colony growth dynamics is determined not only by the concentration of the limiting substrate but also by the amount of autoinhibitor, the synthesis of which is governed by age of cells. The distinctions of growth from a single cell and microdroplet could also originate as a result of dissociation into the R- and S-forms and competition between the corresponding subpopulations for oxygen and the common substrate.     

Nonlinearity in the Growth of Bacterial Colonies: Conditions and Causes

The universally recognized kinetic model of colony growth, introduced by Pirt, predicts a linear increase of colony size. The linearity follows from the assumption that the colony expands through the growth of only such cells that are located immediately behind the moving colony front, in the so-called peripheral zone of constant width and density. In this work, Pirt's model was tested on two bacteria—Alcaligenes sp. and Pseudomonas fluorescens—having markedly distinct cultural properties and grown on an agarized medium with pyruvate. The colony size dynamics was followed for different densities of the inoculum, ranging from a single cell to a microdroplet of bacterial suspension (10⁵–10⁶ cells), and for different depths of the agar layer, determining the amount of available substrate. A linear growth mode was observed only with P. fluorescens and only in the case of growth from a microdroplet. When originating from a single cell, colonies of both organisms displayed nonlinear growth with a distinct peak of K _r (the rate of colony radius increase) occurring after 2–3 days of growth. The growth of P. fluorescens colonies showed virtually no dependence on the depth of the agarized medium, whereas the rate of colony size increase of Alcaligenes sp. turned out to be directly related to the medium layer thickness. The departure from linearity is consistently explained by a new kinetic scheme stipulating a possible contribution to the colony growth not only of peripheral cells but also (much more distinct in Alcaligenes) of cells at the colony center. The colony growth dynamics is determined not only by the concentration of the limiting substrate but also by the amount of autoinhibitor, the synthesis of which is governed by the age of cells. The distinctions of growth from a single cell and microdroplet could also originate as a result of dissociation into the R- and S-forms and competition between the corresponding subpopulations for oxygen and the common substrate.     

Determining specific growth rates in different regions of Salmonella typhimurium colonies

Computer-aided image analysis (IA) techniques were used to determine specific growth rates (μ) in different regions of Salmonella typhimurium colonies grown on solid medium at 30°C. Captured transmission images were converted to optical density values, allowing calculation of μ. Data show that growth was exponential throughout the colony during the period studied (between 13 and 21 h after inoculation), but that μ varied from 0·69 to 0·37 h⁻¹ at the periphery and centre, respectively. Results demonstrate regional variation in μ values within colonies, and support conceptual models of colony development. The technique described illustrates that kinetic growth data can be derived within bacterial colonies, and this may lead towards an improved understanding of the growth dynamics of these structures.     

Nest specificity of the bacterial community in termite guts (Hodotermes mossambicus)

While recent results have provided strong evidence for the presence of a stable gut microbiota among several termite species, little is known about variations at the colony or individual level. Using a cultivation-independent approach, we investigated the structure of the bacterial community in the gut of termites from four different colonies of Hodotermes mossambicus. 16S rRNA-based terminal restriction fragment length polymorphism (T-RFLP) analysis of the bacterial gut microbiota revealed (1) a high consistency of the gut microbiota among nestmates and (2) subtle but distinct differences in community structure between individuals from different colonies. Since products of bacterial metabolism may contribute to a colony odor that can be used as discriminatory signal, the presence of a colony-specific bacterial community adds support to the hypothesis that the gut microbiota of termites is involved in nestmate recognition. Received 12 July 2005; revised 10 February and 15 March 2006; accepted 7 April 2006.     

Contrasting age-specific recruitment and survival at different spatial scales: a case study with the European storm petrel

Evolutionary studies on optimal decisions or conservation guidelines are often derived by generalising patterns from a single population, while inter‐population variability in life‐history traits is seldom considered. We investigated here how survival and recruitment probabilities changed with age at different geographical scales using the encounter histories of 5523 European storm petrels from three Mediterranean colonies, and also how our estimates of these parameters might be expected to affect population growth rates using population matrix models. We recorded similar patterns among colonies, but also important biological differences. Local survival, recruitment and breeding success increased with age at all colonies; the most distant of three colonies (Marettimo Is.) showed the largest differences. Strikingly, differences in recruitment were also found between two adjacent colonies (two caves from Benidorm Is.). Birds marked as adults from Marettimo and Benidorm colonies had a different survival, whereas we found no differences within Benidorm. Differences in survival were no longer apparent between the two islands at the end of the study following a reduction in predation by specialist gulls at Benidorm. Since birds marked as fledglings mostly recruited near the end of the study, their overall survival was high and in turn similar among colonies. Results from our population matrix models suggested that different age‐dependent patterns of demographic parameters can lead to similar population growth rates. Variability appeared to be greater for recruitment and the most sensitive parameter was adult survival. Thus conservation actions targeting this vulnerable species should focus on factors influencing adult survival. Differences in survival and recruitment among colonies could reflect the spatial heterogeneity in mortality due to predation and colony‐specific density dependent processes. Results highlight the importance of taking into account the potential spatio‐temporal heterogeneity among populations in vital rates, even in those traits that life‐history theory considers less important in driving population dynamics.     

The Use of Surrogate Data in Demographic Population Viability Analysis: A Case Study of California Sea Lions

Reliable data necessary to parameterize population models are seldom available for imperiled species. As an alternative, data from populations of the same species or from ecologically similar species have been used to construct models. In this study, we evaluated the use of demographic data collected at one California sea lion colony (Los Islotes) to predict the population dynamics of the same species from two other colonies (San Jorge and Granito) in the Gulf of California, Mexico, for which demographic data are lacking. To do so, we developed a stochastic demographic age-structured matrix model and conducted a population viability analysis for each colony. For the Los Islotes colony we used site-specific pup, juvenile, and adult survival probabilities, as well as birth rates for older females. For the other colonies, we used site-specific pup and juvenile survival probabilities, but used surrogate data from Los Islotes for adult survival probabilities and birth rates. We assessed these models by comparing simulated retrospective population trajectories to observed population trends based on count data. The projected population trajectories approximated the observed trends when surrogate data were used for one colony but failed to match for a second colony. Our results indicate that species-specific and even region-specific surrogate data may lead to erroneous conservation decisions. These results highlight the importance of using population-specific demographic data in assessing extinction risk. When vital rates are not available and immediate management actions must be taken, in particular for imperiled species, we recommend the use of surrogate data only when the populations appear to have similar population trends.     

The growth and form of bacterial colonies.

A simple method is described for measuring the profile of bacterial colonies. Profiles were determined for colonies of Bacillus cereus, Escherichia coli and Staphylococcus albus of different ages. In spite of differences in cell morphology, the colony profiles had a common basic structure consisting of steeply rising leading edge connected by a ridge to an interior region where height also rose, though less steeply, to a flat or domed centre. The colony mass increased exponentially through part of the growth phase. It is suggested that net colony growth consists of a combination of leading edge growth, which is unrestricted and approaches the maximum specific growth rate of the organism, and diffusion-limited growth in the colony interior. Common elements of profiles from each species may be a consequence of such differences in growth rate.     

Trade-offs limiting the evolution of coloniality: ecological displacement rates used to measure small costs

Multicellular organisms that benefit from division of labour are presumably descended from colonial species that initially derived benefits from larger colony size, before the evolution of specialization. Life in a colony can have costs as well as benefits, but these can be hard to measure. We measured physiological costs to life in a colony using a novel method based on population dynamics, comparing growth rates of unicells and kairomone-induced colonies of a green alga Desmodesmus subspicatus against a reference co-occurring species. Coloniality negatively affected growth during the initial log growth phase, while no adverse effect was detected under nutrient-limited competitive conditions. The results point to costs associated with traits involved in rapid growth rather than those associated with efficient growth under resource scarcity. Some benefits of coloniality (e.g. defence from herbivory) may be different from when this trait evolved, but our approach shows how costs would have depended on conditions.     

Why join a neighbour: fitness consequences of colony fusions in termites

The evolution of life is characterized by major evolutionary transitions during which independent units cooperated and formed a new level of selection. Relatedness is a common mechanism that reduces conflict in such cooperative associations. One of the latest transitions is the evolution of social insect colonies. As expected, they are composed of kin and mechanisms have evolved that prevent the intrusion of nonrelatives. Yet, there are exceptions an extreme case is the fusion of unrelated colonies. What are the advantages of fusions that have colonies with a high potential for conflict as a consequence? Here, we investigated fitness costs and benefits of colony fusions in a lower termite species, Cryptotermes secundus, in which more than 25% of all colonies in the field are fused. We found two benefits of colony fusion depending on colony size: very small colonies had an increased probability of survival when they fused, yet for most colony sizes mainly a few workers profit from colony fusions as their chance to become reproductives increased. This individual benefit was often costly for other colony members: colony growth was reduced and the current reproductives had an increased chance of dying when fusions were aggressive. Our study suggests that fusion of colonies often is the result of ‘selfish’ worker interests to become reproductives, and this might have been important for the termites' social evolution. Our results uniquely shows that selfish interests among related colony members can lead to the formation of groups with increased potential for conflict among less related members.     

Emergent polyethism as a consequence of increased colony size in insect societies

A threshold reinforcement model in insect societies is explored over a range of colony sizes and levels of task demand to examine their effects upon worker polyethism. We find that increasing colony size while keeping the demand proportional to the colony size causes an increase in the differentiation among individuals in their activity levels, thus explaining the occurrence of elitism (individuals that do a disproportionately large proportion of work) in insect societies. Similar results were obtained when the overall work demand is increased while keeping the colony size constant. Our model can reproduce a whole suite of distributions of the activity levels among colony members that have been found in empirical studies. When there are two tasks, we demonstrate that increasing demand and colony size generates highly specialized individuals, but without invoking any strict assumptions about spatial organization of work or any inherent abilities of individuals to tackle different tasks. Importantly, such specialization only occurs above a critical colony size such that smaller colonies contain a set of undifferentiated equally inactive individuals while larger colonies contain both active specialists and inactive generalists, as has been found in empirical studies and is predicted from other theoretical considerations.     

Daily and Seasonal Extranidal Behaviour Variations in the Invasive Yellow-Legged Hornet, <Emphasis Type="Italic">Vespa velutina</Emphasis> Lepeletier (Hymenoptera: Vespidae)

In eusocial insect species, the nest represents the fundamental element of the colony. Extranidal activities (foraging, nest maintenance, defence) are fundamental for the development and the survival of the colony. Therefore, they may represent interesting targets to disrupt to limit their expansion in case of pest species such as Vespa velutina, an alien predator of domestic honeybees in Europe. An accurate knowledge of the pattern of activity of this pest’s colonies is therefore required. Due to the highly defensive nature of this hornet, a video monitoring was realized on two colonies during their growth from August to November. Three major behaviours were monitored: nest maintenance, patrolling on the nest and foraging flights. Although of different size and monitored during different years, the two colonies exhibited similar patterns of daily and seasonal variation. This work is a first step in the study of this pest especially in view of control program.     

Colony‐age‐dependent variation in cuticular hydrocarbon profiles in subterranean termite colonies

Cuticular hydrocarbons (CHCs) have, in insects, important physiological and ecological functions, such as protection against desiccation and as semiochemicals in social taxa, including termites. CHCs are, in termites, known to vary qualitatively and/or quantitatively among species, populations, castes, or seasons. Changes to hydrocarbon profile composition have been linked to varying degrees of aggression between termite colonies, although the variability of results among studies suggests that additional factors might have been involved. One source of such variability may be colony age, as termite colony demographics significantly change over time, with different caste and instar compositions throughout the life of the colony. We here hypothesize that the intracolonial chemical profile heterogeneity would be high in incipient termite colonies but would homogenize over time as a colony ages and accumulates older workers in improved homeostatic conditions. We studied caste‐specific patterns of CHC profiles in Coptotermes gestroi colonies of four different age classes (6, 18, 30, and 42 months). The CHC profiles were variable among castes in the youngest colonies, but progressively converged toward a colony‐wide homogenized chemical profile. Young colonies had a less‐defined CHC identity, which implies a potentially high acceptance threshold for non‐nestmates conspecifics in young colonies. Our results also suggest that there was no selective pressure for an early‐defined colony CHC profile to evolve in termites, potentially allowing an incipient colony to merge nonagonistically with another conspecific incipient colony, with both colonies indirectly and passively avoiding mutual destruction as a result.     

光强对微囊藻群体形态的影响及其生理机制研究

为了探讨光照对微囊藻形态的影响,研究了6株不同种的群体微囊藻在不同光强下群体形态的变化及其响应机制。研究发现,随着光强的增加,6株群体微囊藻的群体尺寸变大。当光强为80200 mol/（m2s）时,群体微囊藻DH-M1和DC-M2的比生长速率显著增大,而另4株在高光强下比生长速率无显著性差异;对多糖含量分析发现,高光强对群体微囊藻TH-M2、DC-M1、FACHB1174和FACHB1027胞外及胶被多糖的分泌与释放有显著的促进效果,而DH-M1和DC-M2多糖含量增加不明显。对于不同的微囊藻株,高光强促进群体形态变化的作用机理不同:光饱和点低的微囊藻是通过分泌大量的胞外及胶被多糖使群体尺寸变大,而光饱和点高的微囊藻是通过生长来促进群体尺寸的增大。此外,对产毒藻株在不同光强下的毒素基因表达及胞内毒素测定发现,高光强组的群体微囊藻mcyB和mcyD表达量升高,且胞内微囊藻毒素含量增加显著,推测微囊藻毒素也可能是影响微囊藻群体形态及大小的作用因子之一。       

Multiple queens in founding colonies of the neotropical ant Pachycondyla striata Smith (Formicidae: Ponerinae)

In social insects, the typical mode of colony foundation occurs when a single queen is inseminated by a male and establishes a new colony, although we can find interspecific and intraspecific variations in queen number and queen-mating frequencies in a single colony. This study aimed to verify the queen number in Pachycondyla striata (Smith) colonies and to evaluate the level of aggressiveness among workers. We collected 14 colonies of P. striata. The behaviors of individuals from five multiple-queen colonies maintained in laboratory were studied by the method of scan sampling. In order to evaluate aggressiveness, dyadic encounters among heterocolonial and homocolonial workers were performed. The results showed that colonies of P. striata can have two or more mated queens (polygynous colonies) besides to monogynous ones (colony containing one queen). Because in polygynous colonies the number of workers was relatively low, such colonies could represent colonies in the foundation phase that characterize a pleometrosis state. In fact, ovarian development analysis from queens showed that the number of queens in the colonies seemed to be unstable. Despite a few cases of oophagy (egg cannibalism), social hierarchy among queens is unclear in comparison to other Pachycondyla species. In addition, aggressiveness increased with distance among nests. Nearby colonies (less than 1 m apart) showed a low level of aggressiveness, suggesting the presence of polydomy, that is, a unique colony can occupy multiple nests. Polygyny associated to polydomy in founding colonies may confer benefits on growth and dispersion of colonies in the studied environments.     

An integrated population model sheds light on the complex population dynamics of a unique colonial breeder

Climate models forecast increasing climatic variation and more extreme events, which could increase the variability in animal demographic rates. More variable demographic rates generally lead to lower population growth and can be detrimental to wild populations, especially if the particular demographic rates affected are those to which population growth is most sensitive. We investigated the population dynamics of a metapopulation of 25 colonies of a semi-arid bird species, the sociable weaver Philetairus socius, and how it was influenced by seasonal weather during 1993–2014. We constructed an integrated population model which estimated population sizes similar to observed population counts, and allowed us to estimate annual fecundity and recruitment. Variance in fecundity contributed most to variance in population growth, which showed no trend over time. No weather variables explained overall demographic variation at the population level. However, a separate analysis of the largest colony showed a clear decline with a high extinction probability (0.05 to 0.33) within 5 years after the study period. In this colony, juvenile survival was lower when summers were hot, and adult survival was lower when winters were cold. Rainfall was also negatively correlated with adult survival. These weather effects could be due to increased physiological demands of thermoregulation and rainfall-induced breeding activity. Our results suggest that the dynamics of the population on the whole are buffered against current weather variation, as individual colonies apparently react in different ways. However, if more and increasingly extreme weather events synchronize colony dynamics, they are likely to have negative effects.     

Effects of colony size on larval performance in a processionary moth

1. Some lepidopteran species have larvae that live gregariously, especially in early instars. Colony‐living species may benefit from improved protection from predators, thermoregulation, and feeding facilitation, for example. 2. While many studies have compared solitary and gregarious life styles, few data exist as to the relationship between size of the larval colony and larval performance in gregarious species. The present study was aimed at understanding the importance of colony size for growth and survival of the northern pine processionary moth (Thaumetopoea pinivora) larvae. 3. Field studies, comparing three different sizes of colonies of T. pinivora larvae, showed that individuals in larger colonies had a higher survival rate compared with those living in smaller colonies and also a faster growth rate. 4. The higher survival rate of large colonies was attributed to improved protection from predacious arthropods. 5. In early spring, the young larvae bask in the sun to increase their body temperature. In field experiments the thermal gain was higher in large colonies, and individuals in such colonies also grew faster. As growth rate was not affected by colony size when the ability to bask was experimentally removed in a laboratory experiment, the higher growth rate of the larger colonies was probably due to improved thermoregulation rather than feeding facilitation. 6. The size of larval colonies of gregarious insects depends on natural mortality events as well as on female oviposition strategy. Our results show that decreasing colony size can lead to a reduction in growth rate and survival. It is therefore important to understand whether or not small colonies will benefit equally from the gregarious behaviour.     

Multi-parameter flow cytometry and cell sorting reveal extensive physiological heterogeneity in <Emphasis Type="Italic">Bacillus cereus</Emphasis> batch cultures

Based on two staining protocols, DiOC₆(3)/propidium iodide (PI) and RedoxSensor Green (an indicator of bacterial reductase activity)/PI, multi-parameter flow cytometry and cell sorting has identified at least four distinguishable physiological states during batch cultures of Bacillus cereus. Furthermore, dependent on the position in the growth curve, single cells gave rise to varying numbers of colonies when sorted individually onto nutrient agar plates. These growing colonies derived from a single cell had widely different lag phases, inferred from differences in colony size. This further highlights the complex population dynamics of bacterial monocultures and further demonstrates that individual bacterial cells in a culture respond in markedly dissimilar ways to the environment, resulting in a physiologically heterogenous and dynamic population.     

Recruitment Strategies and Colony Size in Ants

Ants use a great variety of recruitment methods to forage for food or find new nests, including tandem running, group recruitment and scent trails. It has been known for some time that there is a loose correlation across many taxa between species-specific mature colony size and recruitment method. Very small colonies tend to use solitary foraging; small to medium sized colonies use tandem running or group recruitment whereas larger colonies use pheromone recruitment trails. Until now, explanations for this correlation have focused on the ants'' ecology, such as food resource distribution. However, many species have colonies with a single queen and workforces that grow over several orders of magnitude, and little is known about how a colony''s organization, including recruitment methods, may change during its growth. After all, recruitment involves interactions between ants, and hence the size of the colony itself may influence which recruitment method is used—even if the ants'' behavioural repertoire remains unchanged. Here we show using mathematical models that the observed correlation can also be explained by recognizing that failure rates in recruitment depend differently on colony size in various recruitment strategies. Our models focus on the build up of recruiter numbers inside colonies and are not based on optimality arguments, such as maximizing food yield. We predict that ant colonies of a certain size should use only one recruitment method (and always the same one) rather than a mix of two or more. These results highlight the importance of the organization of recruitment and how it is affected by colony size. Hence these results should also expand our understanding of ant ecology.