Population cycles caused by selection by density dependent competitive interactions

Several animal species have cyclic population dynamics with phase-related cycles in life history traits such as body mass, reproductive rate, and pre-reproductive period. Although many mechanisms have been proposed there is no agreement on the cause of these cycles, and no population equation that deduces both the abundance and the life history cycles from basic ecological constraints has been formulated. Here I deduce a population dynamic equation from the selection pressure of density dependent competitive interactions in order to explain the cyclic dynamics in abundance and life history traits. The model can explain cycles by evolutionary changes in the genotype or by plastic responses in the phenotype. It treats the population dynamic growth rate as an initial condition, and its density independent fundament is Fisher’s (1930, The Genetical Theory of Natural Selection, Oxford: Clarendon) fundamental theorem of natural selection that predicts a hyper-geometrical increase in abundance. The predicted periods coincide with the cyclic dynamics of Lepidoptera, and the Calder hypothesis, which suggests that the period of population cycles is proportional to the 1/4 power of body mass, follows from first principles of the proposed density dependent ecology.     

The current flow of migrants and its contribution to butterfly faunas (Lepidoptera, Rhopalocera) on marine islands with young allochthonous biota

The concept that the mechanisms limiting immigration of new Lepidoptera species are analogous for islands with different ages and degrees of endemism of their biotas is discussed. Specific features of individual species play a key role in establishment of the fauna and population on small islands rather than competitive interactions between these species. It is most likely that the species stably coexisting on an island will display similar ecological characteristics and life cycles, which complies with the concepts of the neutral theory of biodiversity and biogeography.     

Functional properties of the isomorphic biphasic algal life cycle

Many species of marine algae have life cycles that involve multipleseparate, free-living phases that frequently differ in ploidylevels. These complex life cycles have received increasing scientificattention over the past few decades, due to their usefulnessfor both ecological and evolutionary studies. I present a synthesisof our current knowledge of the ecological functioning and evolutionaryimplications of the isomorphic, biphasic life cycles commonlyfound in many species of marine algae. There are both costsand benefits to life cycles with 2 morphologically similar butseparate, free-living phases that differ in ploidy levels (haploidsand diploids). Evolutionary theory predicts that the existenceof subtle yet important differences between the phases may bewhat allows these life cycles to be maintained. Different phasesof the same species can vary in abundance, in demographic parameterssuch as mortality and fecundity, in their physiology, and intheir resistance to herbivory. Some taxonomic groups withinthe red algae have received significant attention toward theseissues, while our knowledge of these properties for brown andgreen algae remains limited.     

Competition and coexistence with multiple life-history stages

Do complex life histories affect the conditions under which competitors can coexist? We investigated this using a two-species, two-stage Ricker model. With complex life cycles, the competition coefficients associated with each life-history stage suggest one of three competitive outcomes-coexistence, alternate stable states, or competitive exclusion-that depend on the relative magnitudes of intraspecific and interspecific competition. When the two stages suggest the same outcome, only that outcome can occur. When the stages suggest different outcomes, either one may prevail. It is also possible to have emergent outcomes, in which the outcome is not suggested by either stage. This can occur when the two stages suggest competitive exclusion by opposite species or when one stage suggests alternate stable states and the other suggests coexistence. Therefore, determining the mechanisms of coexistence in species with complex life histories may require consideration of competitive interactions within all life-history stages.     

Long-Term Competitive Dynamics of Two Cryptic Rotifer Species: Diapause and Fluctuating Conditions

Life-history traits may have an important role in promoting species coexistence. However, the complexity of certain life cycles makes it difficult to draw conclusions about the conditions for coexistence or exclusion based on the study of short-term competitive dynamics. Brachionus plicatilis and B. manjavacasare two cryptic rotifer species co-occurring in many lakes on the Iberian Peninsula. They have a complex life cycle in which cyclical parthenogenesis occurs with diapausing stages being the result of sexual reproduction. B. plicatilis and B. manjavacasare identical in morphology and size, their biotic niches are broadly overlapping, and they have similar competitive abilities. However, the species differ in life-history traits involving sexual reproduction and diapause, and respond differently to salinity and temperature. As in the case of certain other species that are extremely similar in morphology, a fluctuating environment are considered to be important for their coexistence. We studied the long-term competitive dynamics of B. plicatilis and B. manjavacas under different salinity regimes (constant and fluctuating). Moreover, we focused on the dynamics of the diapausing egg bank to explore how the outcome of the entire life cycle of these rotifers can work to mediate stable coexistence. We demonstrated that these species do not coexist under constant-salinity environment, as the outcome of competition is affected by the level of salinity—at low salinity, B. plicatilis excluded B. manjavacas, and the opposite outcome occurred at high salinity. Competitive dynamics under fluctuating salinity showed that the dominance of one species over the other also tended to fluctuate. The duration of co-occurrence of these species was favoured by salinity fluctuation and perhaps by the existence of a diapausing egg bank. Stable coexistence was not found in our system, which suggests that other factors or other salinity fluctuation patterns might act as stabilizing processes in the wild.     

Multi level ecological fitting: indirect life cycles are not a barrier to host switching and invasion

Many invasive species are able to escape from coevolved enemies and thus enjoy a competitive advantage over native species. However, during the invasion phase, non‐native species must overcome many ecological and/or physiological hurdles before they become established and spread in their new habitats. This may explain why most introduced species either fail to establish or remain as rare interstitials in their new ranges. Studies focusing on invasive species have been based on plants or animals where establishment requires the possession of preadapted traits from their native ranges that enables them to establish and spread in their new habitats. The possession of preadapted traits that facilitate the exploitation of novel resources or to colonize novel habitats is known as ‘ecological fitting’. Some species have evolved traits and life histories that reflect highly intimate associations with very specific types of habitats or niches. For these species, their phenological windows are narrow, and thus the ability to colonize non‐native habitats requires that a number of conditions need to be met in accordance with their more specialized life histories. Some of the strongest examples of more complex ecological fitting involve invasive parasites that require different animal hosts to complete their life cycles. For instance, the giant liver fluke, Fascioloides magna, is a major parasite of several species of ungulates in North America. The species exhibits a life cycle whereby newly hatched larvae must find suitable intermediate hosts (freshwater snails) and mature larvae, definitive hosts (ungulates). Intermediate and definitive host ranges of F. magna in its native range are low in number, yet this parasite has been successfully introduced into Europe where it has become a parasite of native European snails and deer. We discuss how the ability of these parasites to overcome multiple ecophysiological barriers represents an excellent example of ‘multiple‐level ecological fitting’.     

Changed plant and animal life cycles from 1952 to 2000 in the Mediterranean region

The available data on climate over the past century indicate that the earth is warming. Important biological effects, including changes of plant and animal life cycle events, have already been reported. However, evidence of such effects is still scarce and has been mostly limited to northern latitudes. Here we provide the first long‐term (1952–2000) evidence of altered life cycles for some of the most abundant Mediterranean plants and birds, and one butterfly species. Average annual temperatures in the study area (Cardedeu, NE Spain) have increased by 1.4 °C over the observation period while precipitation remained unchanged. A conservative linear treatment of the data shows that leaves unfold on average 16 days earlier, leaves fall on average 13 days later, and plants flower on average 6 days earlier than in 1952. Fruiting occurs on average 9 days earlier than in 1974. Butterflies appear 11 days earlier, but spring migratory birds arrive 15 days later than in 1952. The stronger changes both in temperature and in phenophases timing occurred in the last 25 years. There are no significant relationships among changes in phenophases and the average date for each phenophase and species. There are not either significant differences among species with different Raunkiaer life‐forms or different origin (native, exotic or agricultural). However, there is a wide range of phenological alterations among the different species, which may alter their competitive ability, and thus, their ecology and conservation, and the structure and functioning of ecosystems. Moreover, the lengthening of plant growing season in this and other northern hemisphere regions may contribute to a global increase in biospheric activity.     

Life cycles and challenges for fish species that breed in South African estuaries

The literature currently recognizes four guilds of estuarine resident fish species, namely solely estuarine, estuarine and marine, estuarine and freshwater, and estuarine migrant. In this review the life cycles of actual representatives from these four guilds are assessed to determine whether the current definitions, which have never been formally tested, are appropriate to fish species resident in South African estuaries. Detailed information and diagrammatic life cycles are provided for the selected species covered by this review. A potential new estuarine resident guild category is also identified, namely, those taxa that are primarily estuarine but also have subpopulations recorded in both adjacent marine and freshwater habitats. The full range of reproductive characteristics employed by estuary resident species is examined, ranging from live bearers, pouch and nest brooders, to a suite of oviparous taxa that attach their ova to estuarine rocks, shells, and submerged vegetation, all of which assists with larval retention within the estuarine environment. The small size and early reproductive maturity of most estuarine resident species is highlighted, with reduced vulnerability to predation in shallow, sheltered, often turbid estuary waters offering considerable protection during spawning events when compared to the open ocean. In addition, these small fish would not have to move considerable distances at any stage of their life cycle, since egg, larval, juvenile, and adult stages all occur in the same place. The existence of contingent subpopulations within many estuarine resident species is noted, physico-chemical stresses on these species are highlighted, and the eurytopic nature of these small fish taxa emphasized.     

Species composition, host association and niche differentiation in fleas of small mammals in the Ilmen-Volkhov lowland

Species composition, abundance, annual cycles, and host association of fleas parasitizing small mammals were investigated. The problem of niche differentiation in these insects is considered on the base of the comparative analysis of their annual cycles. The annual cycles of the fleas are revealed to be similar in the case of few number of flea species in parasite community. Thus, two species parasitizing Sorex araneus (Doratopsylla dasycnema and Palaeopsylla soricis), as well as three species associated with Apodemus uralensis (Megabothris turbidus, Ctenophthalmus agyrtes, and Ct. uncinatus) have equal phenology of parasitizing. The fleas community of Clethrionomys glareolus is characterized by a large species number and high diversity of the annual cycles. The differentiation by the seasons of parasitizing is observed most clearly in the dominant flea species, namely Amalaraeus penicilliger, Ct. uncinatus, and Peromyscopsylla bidentata. The periods of imaginal life are overlapped significanly in these species, but they are differed by the season of dominance. Ct. uncinatus predominates in spring and summer, while P. bidentata predominates in autumn, and A. penicilliger predominates in winter and early spring. It may be noted also, that niche partitioning was not observed in the fleas having wide range of hosts. The imaginal life of such fleas usually does not go beyond the warm season.     

The evolution of traits that determine ability in competitive contests

Summary We analyse mathematical models of the evolution of a trait that determines ability in contest competition. We assume that the value of the competitive trait affects two different components of fitness, one measuring the benefit of winning contests and the other measuring the cost of developing the competitive trait. Unlike previous analyses, we include the population dynamical consequences of larger competitive trait values. Exaggeration of the competitive trait reduces the mean probability of survival during the non-competitive stage of the life cycle. The resulting lower population density reduces competition and, therefore, reduces the advantages of greater competitive ability. Models without population dynamics often predict dimorphism in the competitive trait when resource possession is decided by interactions with many other individuals. If the competition involves a contest with a single other individual, models without population dynamics often predict cycles of increase and collapse in the trait or a continual increase, possibly resulting in extinction. When population dynamics are included, both of these results become less likely and a single stable trait value becomes more likely. Population dynamics also make it possible to have dimorphism when individuals have a single pairwise contest and alternative stable trait values when an individual has many contests. Increases in the value of the resource being contested may increase or decrease the evolutionarily stable size of the trait. Competition between very differently sized species will often decrease size in the larger species (character convergence).     

Optimal seasonal schedules and the relative dominance of heteromorphic and isomorphic life cycles in macroalgae

Marine macroalgae (seaweed) show diverse life cycles. Species with a heteromorphic life cycle have a large multicellular algal body in one generation but have a very small body in the second generation of the same year. In contrast, the diploid and haploid life forms of isomorphic species have similar morphology, and these species often have more than two generations in a year. Here, we first study the optimal life cycle schedule of marine macroalgae when daily mortality changes seasonally, and then we discuss the conditions for coexistence and relative dominance of different life cycles. According to the optimal life cycle schedule, heteromorphic species tend to have a generation with a large algal body when mortality is low, and a microscopic-sized generation when mortality is high. In contrast, isomorphic species tend to mature when body size reaches a threshold value that is the same for different generations. We then examine the coexistence of the two life cycles when growth rate decreases with biomass. The model predicts that (1) at high latitudes (i.e., in strongly seasonal environments), heteromorphic species are likely to dominate over isomorphic species, and (2) species with a heteromorphic life cycle should dominate in the supratidal and upper intertidal zones where macroalgae tend to suffer high mortality, and also in the subtidal zone, where mortality is low, whereas isomorphic species are likely to be more successful when mortality is intermediate. These predictions are consistent with the observed distribution patterns of the two life cycles in macroalgae.     

The evolution of growth rate, resource allocation and competitive ability in seeder and resprouter tree seedlings

Many woody plant species in fire disturbed communities survive disturbance events by resprouting. The resprouting life history is predicted to be costly to plants as resources are diverted into storage for post-fire regrowth rather than allocated to current growth, and resprouting species typically grow more slowly than seeder species (species that do not resprout after disturbance events). Differences in allocation to current growth are also predicted to make resprouter species poorer competitors compared to seeder species. We tested the predictions that the evolution of a resprouter life history is associated with slow growth, increased allocation to storage, and low competitive ability in woody plant seedlings. We grew eight phylogenetically independent pairs of seeder and resprouter species in competition and no competition treatments in a field experiment near Sydney, Australia. The presence of competitors reduced plant growth rates across taxa and fire response life histories. However, relative to seeder species, resprouter species were not slower growing, they did not allocate more resources to storage, and they did not have lower competitive abilities. We propose that differences in resource allocation to storage are not responsible for differences in growth rate and competitive ability. Rather, growth rate and competitive ability in seedlings are associated with key aspects of plant life history such as life-span and body size at maturity. These traits that are sometimes, but not always, related to fire response life histories.     

Life-history characteristics of eleotrid fishes of the western hemisphere, and perils of life in a vanishing environment

The term amphidromous was coined (Myers in Copeia 1949:89–97, 1949b) to describe diadromous life histories that include migrations, not associated with reproduction, that are between fresh and marine waters. This concept has facilitated evaluations of life cycles among a number of groups of fishes including some belonging to the family Eleotridae. Information was gathered on life history patterns of eleotrid fish species that have been recorded from both fresh and brackish or marine waters of the east and west coasts of North, Middle, and South America, including adjacent islands, seeking evidence of diadromy and especially of amphidromy. Convincing evidence was found of diadromy in the life cycles of four species from the east coast (with another two species possibly in this category), and of three species from the west coast of the Americas and adjacent islands. However, there was convincing evidence of amphidromy in only one species from the east coast and adjacent islands, with another three species or species populations possibly in this category, and in three species from the west coast and adjacent islands. It seems possible from the available information that there may be variation among populations in life cycles of some of these species. However, it remains for the use of modern techniques to allow more definitive determinations of life history patterns of these eleotrid species populations. Serious concern exists with respect to the conservation of all diadromous species because of the worldwide emphasis on river manipulations, especially of dam construction. Resident diadromous organisms are being impeded in their migrations, and thus imperiled in their potential for survival, depending on the nature of river alterations and the abilities of the organisms to cope with them.     

An algorithm for a decomposition of weighted digraphs: with applications to life cycle analysis in ecology

In the analysis of organism life cycles in ecology, comparisons of life cycles between species or between different types of life cycles within species are frequently conducted. In matrix population models, partitioning of the elasticity matrix is used to quantify the separate contributions of different life cycles to the population growth rate. Such partition is equivalent to a decomposition of the life cycle graph of the population. A graph theoretic spanning tree method to carry out the decomposition was formalized by Wardle [Ecology 79(7), 2539–2549 (1998)]. However there are difficulties in realizing a suitable decomposition for complex life histories using the spanning-tree method. One of the problems is the occurrence of life cycles that contain contradictory directions that defy biological interpretation. We propose an algorithmic approach for decomposing a directed, weighted graph. The graph is to be decomposed into two parts. The first part is a set of simple cycles that contain no contradictory directions and that consist of edges of equal weight. The second part of the decomposition is a subgraph in which no such simple cycles are obtainable. When applied to life cycle analysis in ecology, the proposed method will guarantee a complete decomposition of the life cycle graph into individual life cycles containing no contradictory directions. Although the research described in this article has been funded in part by the United States Environmental Protection Agency through STAR cooperative agreement R-82940201-0 to the University of Chicago, it has not been subjected to the Agency’s required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred.     

The parasitic dinoflagellates of marine crustaceans

Parasitic dinoflagellates have recently emerged as significant disease agents of commercially important crustaceans. For example, epizootics of Hematodinium have seriously affected certain crab and lobster fisheries. The parasitic dinoflagellates of crustaceans are, however, relatively unknown. Marine crustaceans are parasitized by two orders of dinoflagellates: the Blastodinida and the Syndinida. Crustaceans are also parasitized by the Paradinida and the Ellobiopsidae, taxa that have close historical ties and possible taxonomic affinities with the dinoflagellates. The taxonomy and life history patterns of the different parasitic species are largely dictated by their host-parasite relationships. For example, sporulation in the blastodinids occurs internally but is completed externally with the expulsion of spores via the anus of the host. The egg-parasitic chytriodinids sporulate externally after destroying their host egg. The tissue-dwelling syndinids have plasmodia that sporulate internally and generally kill their hosts upon the expulsion of the dinospores. Unfortunately, complete life cycles have not been elucidated for any of the parasitic forms, hence characteristics of the life cycles must be applied cautiously to the systematics of the taxa. For example, gamogony and the presence of resting cysts are only known from a few species; they probably occur in most species. Further work on the life cycles of the parasitic dinoflagellates of crustaceans should concentrate on establishing the life cycles of representative species from each order or family. Parasitic dinoflagellates infect copepods, amphipods, mysids, euphausiids, and decapods. Their pathogenicity varies with their invasiveness in the host. The gut-dwelling blastodinids are relatively benign, while the chytriodinids kill their host egg. Members of the pervasive Syndinida and Paradinida are overtly pathogenic and insidiously ramify throughout the hemal sinuses and organs of their hosts. Members of the Ellobiopsidae vary from the commensal Ellobiocystis to the overtly parasitic Thalassomyces. Host castration and feminization are common pathologic results of infection by these parasites. The severity of the castration is dependent upon the invasiveness of the parasitic species and the duration of the infection, while the degree of feminization is related to the stage at which the host acquires the infection. Most of the parasitic dinoflagellates occur in epizootics in their host populations. Recent epizootics of Hematodinium spp. have had severe effects on crustacean fisheries in Alaska, Virginia, and Scotland, and may potentially result in changes to the benthic communities of the hosts. The epizootics are often associated with host-parasite systems that occur in regions with unique hydrological features, such as fjords or poorly draining estuaries with shallow sills. These regions are ideal for the application of a “landscape” ecology approach that could lead to a better understanding of the epizootiology of parasitic dinoflagellates and other marine pathogens.     

Evolution of the maturation rate collapses competitive coexistence

Most theoretical studies on character displacement and the coexistence of competing species have focused attention on the evolution of competitive traits driven by inter-specific competition. We investigated the evolution of the maturation rate which is not directly related to competition and trades off with the birth rate and how it influences competitive outcomes. Evolution may result in the superior competitor becoming extinct if, initially, the inferior competitor has a lower, and the superior one a higher, maturation rate at the coexistence equilibrium. This counterintuitive result is explained by an explosive increase in the adult population of the inferior competitor as a result of the more rapid evolution of its maturation rate, which is caused by differences in the intensity and direction of selection on the maturation rates of the two species and in their adult densities, which are related to differences in their life histories. Thus, a life history trait trade-off with a competitive trait may cause a competitive ecological coexistence to collapse.     

Allelopathic adaptation can cause competitive coexistence

The maintenance of plant diversity is often explained by the ecological and evolutionary consequences of resource competition. Recently, the importance of allelopathy for competitive interactions has been recognized. In spite of such interest in allelopathy, we have few theories for understanding how the allelopathy influences the ecological and evolutionary dynamics of competing species. Here, I study the coevolutionary dynamics of two competing species with allelopathy in an interspecific competition system, and show that adaptive trait dynamics can cause cyclic coexistence. In addition, very fast adaptation such as phenotypic plasticity is likely to stabilize the population cycles. The results suggest that adaptive changes in allelopathy can lead to cyclic coexistence of plant species even when their ecological characters are very similar and interspecific competition is stronger than intraspecific competition, which should destroy competitive coexistence in the absence of adaptation.     

Evolutionarily stable age at first reproduction in a density-dependent model.

We develop a new model of life history evolution to investigate the evolution of age at first reproduction. Density dependence is taken into account. For a given "species", age of maturity, offspring survival, immature survival, adult survival, fecundity, immature age-classes entering in competition with adults and immature competitive ability are traits adjustable by natural selection, and constitute a particular strategy. On the contrary, the type of intraspecific competition (scramble or contest), strength of competition and inherent net reproductive rate Ro(inh) are fixed (specific) characteristics. As a consequence of fixing Ro(inh), the evolution of any trait will affect trade-offs between others. Evolutionarily stable strategies are determined numerically by using the mathematical concept of Lyapunov exponents. Altogether, we consider 960 different hypothetical "species" (i.e. different combinations of fixed traits). Corresponding ESSs are analyzed with respect to their age at first reproduction, adult survival and immature competitive ability components. They appear to be gathered in three groups. One is intuitive and characterized by a reduction of immature competitive ability and a correlation of age of maturity with adult survival; populations reach mainly equilibria. The two other groups respectively include "species" with low age of maturity but high adult survival, and "species" close to semelparity with delayed maturity; immature competitive ability may not be minimized, and populations possibly exhibit complex dynamics. In conclusion, the hypothesis that the evolution of a demographic parameter modifies trade-offs between others turns out to have important consequences. We argue that life history theory cannot ignore the source and mode-of-operation of density dependence and must regard potential short-term instability as essential.     

蔊菜杂草性的研究

本文通过对十字花科常见杂草芹菜[Rorippa in lica(L.)Hiern]在南京地区的自然群体的周期性观察,以了解其在不同生态环境下的生活周期。本文还通过一系列实验来观察其繁育系统、种子传播的动因、结实力、种子发芽率和植物抗人为干扰的耐受性。花蕾套袋试验证明蔊菜主要是自交可亲和的,即使存在异型杂交也是微不足道的。种子传播效应试验揭示水和风都是种子传播的自然力。该种具有范围很广的耐受性,踏践和刈割试验证明在严重的人为于扰下仍能完成其生活周期。蔊菜尽管本质上属于多年生草本,能产生大量的种子,在很大程度上靠种子繁殖。种子萌芽试验证明它的种子萌发参差不齐。总之,蔊菜具有典型杂草的许多特性,而这些特性给它以适合在多种自然的和人为干扰的环境中正常地生长的能力。     

STUDIES ON MARINE PLANKTON DIATOMS. II. RESTING SPORE MORPHOLOGY1

A morphological study of resting spores in five marine planktonic diatom species using electron microscopy indicates that Bacteriastrum delicatulum Cleve and Leptocylindrus danicus Cleve spores bear little resemblance, to their vegetative cells. Detonula confervacea (Cleve) Gran and Stephanopyxis turris (Grev. & Arn.) Ralfs spores have several features in common with their vegetative cells, and Rhizosolenia setigera Brightwell lies between the two extremes. The function of resting spores in relation to diatom life cycles is briefly discussed. Spore formation may be a primitive characteristic in the life cycle and may no longer have significant survival value for the species.