Torbjörn Fagerström

Limiting dissimilarity in plants: randomness prevents exclusion of species with similar competitive abilities

We present a model in which each of two competing plant species is characterized by three parameters: the coefficient of competitiveness in the seedling stage, the expected life time of the adult plant, and the annual seed production of the adult. It is found that when seed production varies stochastically in time, species too similar with respect to these parameters can not outcompete each other, a special case of which is that ecologically identical species coexist stably. This contrasts with classical niche theory which predicts that there is a limit to similarity of species competing for the same resources.

The meristem-meristem cycle as a basis for defining fitness in clonal plants

In clonal plants the genetic individual may be exceedingly long-lived, and the traditional way of defining Darwinian fitness − i.e. on the basis of the zygote-zygote cycle − may therefore be impracticable. Yet, an obvious alternative, viz. to define fitness on the basis of the meristem-meristem cycle, has frequently been rejected. I suggest here that this rejection is unwarranted, and I present a quantitative measure of fitness in clonal plants that I consider to be of general applicability. Meristems are considered to have three developmental options : (i) to propagate vegetatively; (ii) to propagate sexually; (iii) to remain dormant. Each option leads to the perpetuation of the genome of a mother meristem from the time t 1 , to daughter meristems at the time t 2 ; in the case of option (ii) a gene runs a risk of getting lost through meiosis (...)

Genetic variation and clonal diversity in four clonal sedges (Carex) along the Arctic coast of Eurasia

We studied the structure of genetic variation (at both ramet‐ and genet‐level) and clonal diversity within and among populations in the four closely related arctic clonal sedges Carex bigelowii, C. ensifolia, C. lugens and C. stans by use of allozyme markers. Compared to other sedges and arctic plants, the studied taxa all had high levels of genetic variation, both within populations and taxa. These taxa contained most of the total gene diversity (HT) within populations and a small part of the diversity among populations (GST ranged 0.05–0.43). Carex bigelowii had genetic variation (HS = 0.173, mean for populations) at a comparable level to other outbreeding arctic plants and to other widespread, rhizomatous and mainly outbreeding Carex species. In contrast, C. ensifolia (HS = 0.335), C. lugens (HS = 0.339) and C. stans (HS = 0.294) had within‐population variations that were higher than in most other studied Carex species and for arctic plants in general. Genetic variation was not related to any tested environmental variable, but it was lower in areas deglaciated only 10 000 years bp compared to areas deglaciated 60 000 years bp or not glaciated at all during the Weichselian. All the populations were multiclonal, except for two populations of C. stans that were monoclonal. In contrast to genetic variation, clonal diversity decreased with latitude and did not differ between areas with different times of deglaciation. In accordance with previous studies, C. bigelowii and C. lugens were found to be outbreeding, while C. ensifolia and C. stans had mixed mating systems.

Theory for Coexistence of Species Differing in Regeneration Properties

In many organisms the life cycle is characterized by a regular shift between two distinctly different lifeforms, one spatially fixed phase and one dispersal phase. Interspecific differentiation with regard to the dispersal phase may then enable a species to survive in a community of otherwise superior competitors. This paper analyses how much differentiation is required with respect to (1) temporal average, (2) temporal variance, (3) phenology of diaspore production, and (4) longevity of the fixed phase, in order for two species to co-exist when one of them always outcompetes the other in the fixed phase. It is shown that differentiation solely in (1), (2) or (3) can suffice, whereas differentiation in (4) cannot. The existence of temporal fluctuations in the environment permits the adoption of qualitatively new strategies, not available in a constant environment. In particular, increasing the magnitude of the environmental fluctuations allows competing species to become more similar. Also, it is shown that differences in flowering or fruiting periods between coexisting plant species need not be the result of competition for pollinators or seed dispersal agents as has been suggested. The formalism is compared with that used by previous workers; it is concluded that much of existing competition theory, based on the assumption of global interaction, is less well suited for the understanding of mechanisms behind maintained species richness in many kinds of communities.

Evolution of mitotic cell-lineages in multicellular organisms

Adaptive evolution in multicellular organisms is generally assumed to occur through natural selection acting differentially among the phenotypes programmed by sexually-generated zygotic genotypes. Under this view, only genetic changes in the gamete-zygote-germline-gamete cycle are considered relevant to the evolutionary process. Yet asexuality - production of progeny through proliferation of mitotic cell-lineages - is found in over one half of all eukaryotic phyla, and is likely to contribute to adaptive changes, as suggested by recent evidence from both animals and plants. Adaptive changes in mitotic lineages can be reconciled with contemporary evolutionary thought by fully abandoning the weismannian concept of individuality.

Does seed dormancy benefit the mother plant by reducing sib competition

SummarySeed dormancy has been considered, almost without exception, as a bet-hedging strategy in a temporally varying environment. However, in this paper we show that seed dormancy can improve the reproductive success of the mother plant when competition between sibling seedlings and adult plants is intense even if the environment is temporally invariable. We allow a cohort of sibling seeds to germinate simultaneously in the same patch and assume a density dependent survival and fecundity of seedlings. In the model, the mother plant is assumed to control the germination behaviour of the seeds, e.g. by enclosing the seeds in coats of different hardiness. When sib competition is intense, a postponed germination of her seeds can increase the reproductive success of the mother plant up to four times, measured in terms of the number of grandchildren. Consequently, our results suggest that postponed germination may function as a mechanism that alters local interactions in viscous plant populations with limited dispersal.

Competition, defense and games between plants

SummaryCoexistence of defended and undefended plants may be maintained by herbivory. In the present paper this phenomenon is analyzed by means of evolutionary game theory. The plants in the model play either a defensive or a non-defensive strategy and they interact indirectly: when a plant is grazed its competitive ability decreases, because of this a neighboring plant makes a profit. The solution to the game leads to three qualitatively different cases depending on whether the profit is equal for the two strategies, defended and undefended, or if the profit is higher for one type than for the other. When the results are applied to intea-specific interactions, the model predicts that polymorphic populations should be expected only under certain specific conditions. When the results are applied to inter-specific interactions, the model predicts either stable coexistence, i.e., increased diversity, or a paradoxical situation without increased diversity.

Model for accumulation of methyl mercury in northern pike Esox lucius

A compartmental model of mercury residue accumulation in individual fishes is presented. The model assumes a single residue pool that acquires methyl mercury from respired water and with the diet. Uptake and clearance of residue are treated as functions of metabolic rate, which is itself size-dependent. The parameters for the model are estimated from a release-recapture experiment and from literature data, and the solutions produced by the model when using these estimates are compared with real-world data where the latter set of data is independent of the set that was used for parameter estimation. The correspondence between the calculatedconcentration of methylmercuryin lateral muscle tissue and empirically observed values is acceptable though the calculated values in the lower weight range are somewhat high. The possibility that the model may apply to other substances is discussed.

Population dynamics in sessile organisms: some general results from three seemingly different theory-lineages

Sessile organisms may interact with their close neighbours by competition for resources, by allelopathy or by symbiosis, but interaction with distant individuals is solely through the dispersal of propagules. Accordingly, competitive ability of juvenile stages, as well as demographic parameters such as fecundity, longevity and dispersibility assume great significance for the population dynamics of sessile species. Three parallel research traditions, or theory-lineages (summarized in Fig. 1, and to be presented below) have addressed the question how such parameters influence succession or coexistence of sessile species. Some confusion prevails concerning the generality of results obtained, because these different theory-lineages emerge from different historical roots, focus on different questions, use different terminologies, and employ different kinds of mathematics. Not unexpectedly, therefore, several recent papers have produced theoretical predictions that were perceived as new and exciting within their own theory-lineage, whereas the results in fact had much in common with those obtained within other lineages. Conversely, some authors (including the senior author of this paper) have claimed that their models were similar to those of other authors, when in fact some fundamental assumptions were different. Our purpose with this short review is to bring out certain general results that emerge from the superficially different models employed within the three lineages. Our intent is not to provide a comprehensive review of the literature, so the reader is referred to references in the relevant papers mentioned below for a full coverage of background papers. We hope the review may help to fuse the separate theory-lineages into a compound lineage from here on, and new problems may be addressed on the foundation of the general results now established. Three theory-lineages

Adaptations and resistance to anoxia in Cloeon dipterum (Ephemeroptera) and Nemoura cinerea (Plecoptera)

The survival of larvae of Cleon dipterum L. (Ephemeroptera) and Nemoura cinerea (Retzius) (Plecoptera) was studied after exposure to anoxia for 40 h at temperatures between 0?C and 11.5?C. If the larvae were acclimated to 2?C for 6 wk C. dipterum survived temperatures close to 0?C better than N. cinerea. If they were acclimated to 10?C, most C. dipterum died at all experimental temperatures. N. cinerea died to 60 % at 5?C; at higher or lower temperatures the mortality increased. A 6-d stay in 2?C of C. dipterum previously acclimated to 10?C was not long enough to induce resistance to anoxia. When exposed to anoxia N. cinerea became unconscious within 10 min, whereas larvae of C. dipterum did so first within 60 min. It is concluded that a long term acclimation to low temperatures induces in C. dipterum but not in N. cinerea a high degree of resistance to anoxia at temperatures close to 09C. This resistance is of great importance for C. dipterum since in cold temperated regions this species usually overwinters in anoxic conditions. The different rate at which the species lose consciousness in anoxia may be important for their survival in natural habitats.