RESUMO

What factors limit ecosystem evolution? Like human economies, ecosystems are arenas where agents compete for locally limiting resources. Like economies, but unlike genes, ecosystems are not units of selection. In both economies and ecosystems, productivity, diversity of occupations or species and intensity of competition presuppose interdependence among many different agents. In both, competitive dominants need abundant, varied resources, and many agents' products or services, to support the activity and responsiveness needed to maintain dominance. Comparing different-sized land masses suggests that productivity is lower on islands whose area is too small to maintain some of the interdependences that maintain diversity, productivity and competitiveness in mainland ecosystems. Islands lacking the rare, metabolically active dominants that make competition so intense in mainland ecosystems are more easily invaded by introduced exotics. Studies of islets in reservoirs identify mechanisms generating these phenomena. These phenomena suggest how continued fragmentation will affect future 'natural' ecosystems.

Assuntos

RESUMO

To resolve a panselectionist paradox, the population geneticist Kimura invented a neutral theory, where each gene is equally likely to enter the next generation whatever its allelic type. To learn what could be explained without invoking Darwinian adaptive divergence, Hubbell devised a similar neutral theory for forest ecology, assuming each tree is equally likely to reproduce whatever its species. In both theories, some predictions worked; neither theory proved universally true. Simple assumptions allow neutral theorists to treat many subjects still immune to more realistic theory. Ecologists exploit far fewer of these possibilities than population geneticists, focussing instead on species abundance distributions, where their predictions work best, but most closely match non-neutral predictions. Neutral theory cannot explain adaptive divergence or ecosystem function, which ecologists must understand. By addressing new topics and predicting changes in time, however, ecological neutral theory can provide probing null hypotheses and stimulate more realistic theory.

Assuntos

RESUMO

Adam Smith proposed that individuals can share sufficient interest in their society's welfare to profit by cooperating for its benefit and by jointly suppressing behavior hurtful to it. Similarly, a genome's genes share a common interest in 'honest' meiosis, which ensures that alleles can spread only by benefitting their carriers. Honeybee workers express a common interest in raising their queen's, rather than their half-sisters', eggs by eating eggs laid by half sisters. Can analogous principles explain the evolution of harmony at other levels of biological organization, such as ecosystems or organismic development?

RESUMO

Despite unprecedented research attention in recent years, the tropics remain an unexplored frontier. To achieve a better understanding of tropical ecosystems in the face of rapid and irrevocable destruction, it is essential to develop and improve field facilities for long-term comparative research worldwide. This article describes the work of one such facility - the Smithsonian Tropical Research Institute in Panama - as model for future investigations.

RESUMO

MacArthur and Levins suggested that species persist by specializing as much as variation in their environments allows, thus avoiding competitive displacement. Accordingly, more species should coexist in stabler environments. Empirical analyses of trade-offs suggest that, indeed, 'the jack of all trades is master of none'. Diversity represents a balance between speciation and extinction. Theory and experiment suggest that competitive overlap hastens a population's extinction. Currently, ecological specialization and environmental stability seem nearly unmeasurable. Nevertheless, new theoretical analyses, and empirical studies of extinction among small populations, may help us to understand how specialization and environmental variation affect a population's susceptibility to extinction.

RESUMO

In the northeastern Pacific, intertidal zones of the most wave-beaten shores receive more energy from breaking waves than from the sun. Despite severe mortality from winter storms, communities at some wave-beaten sites produce an extraordinary quantity of dry matter per unit area of shore per year. At wave-beaten sites of Tatoosh Island, WA, sea palms, Postelsia palmaeformis, can produce > 10 kg of dry matter, or 1.5 x 10(8) J, per m(2) in a good year. Extraordinarily productive organisms such as Postelsia are restricted to wave-beaten sites. Intertidal organisms cannot transform wave energy into chemical energy, as photosynthetic plants transform solar energy, nor can intertidal organisms "harness" wave energy. Nonetheless, wave energy enhances the productivity of intertidal organisms. On exposed shores, waves increase the capacity of resident algae to acquire nutrients and use sunlight, augment the competitive ability of productive organisms, and protect intertidal residents by knocking away their enemies or preventing them from feeding.

RESUMO

In a system of N populations of n reproductive individuals apiece, in which each population has constant variance v(2) and lasts L generations, group selection on a quantitative character has a reasonable chance of overriding selection within populations if (and only if) the populations never exchange migrants, each population is founded by colonists from a single parent population, and the number of populations exceeds the effective number of reproductive individuals per population. If each population derives from a single parent population, then the exchange of a single successful migrant per population per L generations can triple the strength of group selection required to overcome a given selection within populations. If populations exchange no migrants, then the derivation of one in every N populations from two equally represented parents (while the others all derive from a single parent) doubles the strength of group selection required to prevail. Group selection is accordingly likely to be effective only in certain categories of parasites.