Diseases and pathogens are receiving increasing recognition as sources of mortality in animal populations. Immune system strength is clearly important in fending off pathogen attack. Physical barriers to pathogen entry are also important. Various individual behaviors are efficacious in reducing contact with diseases and pests. This paper focuses on a fourth mode of defense: social barriers to transmission. Various social behaviors have pathogen transmission consequences. Selective pressures on these social behaviors may therefore exist. Effects on pathogen transmission of mating strategies, social avoidance, group size, group isolation, and other behaviors are explored. It is concluded that many of these behaviors may have been affected by selection pressures to reduce transmission of pathogens.

BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access titles in the biological, ecological, and environmental sciences published by nonprofit societies, associations, museums, institutions, and presses.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Wiley-Blackwell andNordic Society Oikos are collaborating with JSTOR to digitize, preserve and extend access to Oikos. . 1990. On the relationship between r/K selection and environmental carrying capacity: a new habitat templet for plant life history strategies. -Oikos 58: 239-250.We propose a revision of the habitat templet approach for modelling the relationship between r/K selection and selection related to the level of resource impoverishment within vegetation. The latter is referred to in our model as "I (Impoverishment)-selection" and is represented as a continuum on one axis of the templet by the mean annual environmental carrying capacity of the habitat. This is perpendicular to the second axis representing the traditional r/K selection continuum and defined by the mean annual distance below environmental carrying capacity that the vegetation is maintained at, usually as a consequence of different levels of disturbance. The model thus recognizes three broad categories of selection but their relationship is defined by the interaction of two independent two-way selection continua rather than by a three-axes' selection continuum as in both the triangular CU-S-R model and the Southwood-Greenslade habitat templet. Each of the three categories of selection (r, K and I) reaches its maximum intensity along a separate side of the quadrangular templet. Thus, two of these are at maximum intensity simultaneously in each of two corners: r-and I-selection in one corner and K-and I-selection in the other. This leads to four rather than three 'extreme' types of strategy (one in each of the four corners) which are contrasted on our version of the temple in terms of predicted differences in the relative allocation of photosynthate to photosynthesizing, reproductive, structural and defensive plant parts. Unlike the triangular C-S-R model and the Southwood-Greenslade habitat templet, the model proposed here does not predict a general decrease in competition intensity with increasing resource impoverishment. This leads to predictions for secondary successional trajectories which differ from those derived from the triangular model. Based on this revised habitat templet, the "species-pool" hypothesis is proposed as an alternative to the "hump-back" model of species density variation across habitat fertility gradients.

Analysis of energy partitioning between defensive investments and growth in woody plants indicates that increasing a tree's life-span should require increased energy investment in protective measures such as thick bark and defensive chemicals. Increased investment in such defenses, however, logically must slow down the growth rate, thereby raising the mortality rate for juveniles in competition for height growth. Early reproduction should also reduce the growth rate. It is hypothesized that rapid growth can substitute for these defenses, but the consequence is rapid decline upon reaching maturity. These predictions are tested with data compiled from the literature for 159 species of North American trees. Data analysis supports predictions. Longevity of angiosperms, but not of gymnosperms was correlated with increased investment in defenses as measured by volumetric heat content of the wood. Wood density was not as good a measure. Longevity of gymnosperms was predicted by resistance to wood decay. For both taxa there was a negative correlation between growth rate and longevity, supporting the hypothesis of growth trade-offs. Age of sexual maturity was closely predicted by longevity in angiosperms. There was no such relationship for conifers as a whole, though there was for pines. The lack of relationship for all conifers might be explained by (i) variation in reproductive opportunities for young trees of different species, or (ii) variation in growth rates of young trees in certain adverse habitats occupied by conifers.

Abstract. Identifying the biological determinants of range limits of trees is an unsolved problem of critical importance for predicting the effects of climate change on forests. Data showing that many boreal trees can grow in temperate climates indicate that southern range limits do not necessarily result from excessive temperature per se. A growth tradeoff could exist between freezing tolerance and height growth rate if adaptations to tolerate cold climates interfered with growth. Analysis of height growth rate versus freezing tolerance for twenty‐two North American trees provided evidence for such a tradeoff. Provenance trials of numerous tree species also showed that a tradeoff exists within species, indicating a genetic basis for these traits. The result of this tradeoff is that at their southern range margins most species do not suffer from too much heat but rather face competitors with a faster growth rate. The implication for future climate change is that forests will not suffer catastrophic dieback due to increased temperatures but will rather be replaced gradually by faster growing types, perhaps over hundreds of years.