Showing papers in "Annual Review of Ecology, Evolution, and Systematics in 1998"

Roads and their major ecological effects

Abstract: A huge road network with vehicles ramifies across the land, representing a surprising frontier of ecology. Species-rich roadsides are conduits for few species. Roadkills are a premier mortality source, yet except for local spots, rates rarely limit population size. Road avoidance, especially due to traffic noise, has a greater ecological impact. The still-more-important barrier effect subdivides populations, with demographic and probably genetic consequences. Road networks crossing landscapes cause local hydrologic and erosion effects, whereas stream networks and distant valleys receive major peak-flow and sediment impacts. Chemical effects mainly occur near roads. Road networks interrupt horizontal ecological flows, alter landscape spatial pattern, and therefore inhibit important interior species. Thus, road density and network structure are informative landscape ecology assays. Australia has huge road-reserve networks of native vegetation, whereas the Dutch have tunnels and overpasses perforating road barriers to enhance ecological flows. Based on road-effect zones, an estimated 15‐20% of the United States is ecologically impacted by roads.

Pathways, mechanisms, and rates of polyploid formation in flowering plants

Abstract: Polyploidy is widely acknowledged as a major mechanism of adaptation and speciation in plants. The stages in polyploid evolution include frequent fertility bottlenecks and infrequent events such as gametic nonreduction and interspecific hybridization, yet little is known about how these and other factors influence overall rates of polyploid formation. Here we review the literature regarding polyploid origins, and quantify parameter values for each of the steps involved in the principal pathways. In contrast to the common claim that triploids are sterile, our results indicate that the triploid bridge pathway can contribute significantly to autopolyploid formation regardless of the mating system, and to allopolyploid formation in outcrossing taxa. We estimate that the total rate of autotetraploid formation is of the same order as the genic mutation rate (10 i5 ), and that a high frequency of interspecific hybridization (0.2% for selfing taxa, 2.7% for outcrossing taxa) is required for the rate of tetraploid formation via allopolyploidy to equal that by autopolyploidy. We conclude that the rate of autopolyploid formation may often be higher than the rate of allopolyploid formation. Further progress toward understanding polyploid origins requires studies in natural populations that quantify: (a) the frequency of unreduced gametes, (b) the effectiveness of triploid bridge pathways, and (c) the rates of interspecific hybridization.

ENDANGERED MUTUALISMS: The Conservation of Plant-Pollinator Interactions

Abstract: ▪ Abstract The pollination of flowering plants by animals represents a critical ecosystem service of great value to humanity, both monetary and otherwise. However, the need for active conservation of pollination interactions is only now being appreciated. Pollination systems are under increasing threat from anthropogenic sources, including fragmentation of habitat, changes in land use, modern agricultural practices, use of chemicals such as pesticides and herbicides, and invasions of non-native plants and animals. Honeybees, which themselves are non-native pollinators on most continents, and which may harm native bees and other pollinators, are nonetheless critically important for crop pollination. Recent declines in honeybee numbers in the United States and Europe bring home the importance of healthy pollination systems, and the need to further develop native bees and other animals as crop pollinators. The “pollination crisis” that is evident in declines of honeybees and native bees, and in damage to web...

The chemical cycle and bioaccumulation of mercury

Abstract: Because it is very toxic and accumulates in organisms, particularly in fish, mercury is an important pollutant and one of the most studied. Nonetheless we still have an incomplete understanding of the factors that control the bioconcentration of mercury. Elemental mercury is efficiently transported as a gas around the globe, and even remote areas show evidence of mercury pollution originating from industrial sources such as power plants. Besides elemental mercury, the major forms of mercury in water are ionic mercury (which is bound to chloride, sulfide, or organic acids) and organic mercury, particularly methylmercury. Methylmercury rather than inorganic mercury is bioconcentrated because it is better retained by organisms at various levels in the food chain. The key factor determining the concentration of mercury in the biota is the methylmercury concentration in water, which is controlled by the relative efficiency of the methylation and demethylation processes. Anoxic waters and sediments are an important source of methylmercury, apparently as the result of the methylating activity of sulfatereducing bacteria. In surface waters, methylmercury may originate from anoxic

Bacterial growth efficiency in natural aquatic systems

Abstract: Heterotrophic bacteria perform two major functions in the transformation of organic matter: They produce new bacterial biomass (bacterial secondary production [BP]), and they respire organic C to inorganic C (bacterial respiration [BR]). For planktonic bacteria, a great deal has been learned about BP and its regulation during the past several decades but far less has been learned about BR. Our lack of knowledge about BR limits our ability to understand the role of bacteria in the carbon cycle of aquatic ecosystems. Bacterial growth efficiency (BGE) is the amount of new bacterial biomass produced per unit of organic C substrate assimilated and is a way to relate BP and BR: BGED (BP)=(BPC BR). Estimates of BGE for natural planktonic bacteria range from <0.05 to as high as 0.6, but little is known about what might regulate this enormous range. In this paper we review the physiological and ecological bases of the regulation of BGE. Further, we assemble the literature of the past 30 years for which both BP and BR were measured in natural planktonic ecosystems and explore the relationship between BGE and BP. Although the relationship is variable, BGE varies systematically with BP and the trophic richness of the ecosystem. In the most dilute, oligotrophic systems, BGE is as low as 0.01; in the most eutrophic systems, it plateaus near 0.5. Planktonic bacteria appear to maximize carbon utilization rather than BGE. A consequence of this strategy is that maintenance energy costs (and therefore maintenance respiration) seems to be highest in oligotrophic systems.

The functional significance of the hyporheic zone in streams and rivers

Abstract: ▪ Abstract The hyporheic zone is an active ecotone between the surface stream and groundwater. Exchanges of water, nutrients, and organic matter occur in response to variations in discharge and bed topography and porosity. Upwelling subsurface water supplies stream organisms with nutrients while downwelling stream water provides dissolved oxygen and organic matter to microbes and invertebrates in the hyporheic zone. Dynamic gradients exist at all scales and vary temporally. At the microscale, gradients in redox potential control chemical and microbially mediated nutrient transformations occurring on particle surfaces. At the stream-reach scale, hydrological exchange and water residence time are reflected in gradients in hyporheic faunal composition, uptake of dissolved organic carbon, and nitrification. The hyporheic corridor concept describes gradients at the catchment scale, extending to alluvial aquifers kilometers from the main channel. Across all scales, the functional significance of the hyporheic z...

FUNGAL ENDOPHYTES: A Continuum of Interactions with Host Plants

Abstract: ▪ Abstract Endophytic fungi living asymptomatically within plant tissues have been found in virtually all plant species. Endophytes are considered plant mutualists: They receive nutrition and protection from the host plant while the host plant may benefit from enhanced competitive abilities and increased resistance to herbivores, pathogens, and various abiotic stresses. Limited evidence also indicates that endophytes may influence population dynamics, plant community diversity, and ecosystem function. However, most of the empirical evidence for this mutualism and its ecological consequences has been based on a few agronomic grass endophytes. More recent studies suggest that endophyte-host plant interactions are variable and range from antagonistic to mutualistic. A more comprehensive view of the ecology and evolution of endophytes and host plants is needed. This article discusses how life history traits—such as fungal reproduction and pattern of infections and genotypic variation and ecological factors—in...

Carbon and carbonate metabolism in coastal aquatic ecosystems

Abstract: The coastal zone is where land, ocean, and atmosphere interact. It exhibits a wide diversity of geomorphological types and ecosystems, each one displaying great variability in terms of physical and biogeochemical forcings. Despite its relatively modest surface area, the coastal zone plays a considerable role in the biogeochemical cycles because it receives massive inputs of terrestrial organic matter and nutrients, is among the most geochemically and biologically active areas of the biosphere, and exchanges large amounts of matter and energy with the open ocean. Coastal ecosystems have therefore attracted much attention recently and are the focus of several current national and international research programs (e.g. LOICZ, ELOISE). The primary production, respiration, calcification, carbon burial and exchange with adjacent systems, including the atmosphere, are reviewed for the major coastal ecosystems (estuaries, macrophyte communities, mangroves, coral reefs, and the remaining continental shelf ). All ecosystems

Evolution of helping behavior in cooperatively breeding birds

Abstract: ▪ Abstract It has recently been argued that the paradox of helping behavior in birds has been solved (73). This optimism may be premature. I argue that there is no obvious dichotomy between cooperative societies based on natal philopatry and the formation of extended families, and those formed via recruitment of unrelated individuals into coalitions. Tests of the effect of helping behavior suggest that kinship may have been overemphasized for male helpers but underestimated for females. The first studies applying molecular techniques to resolve genealogy in these societies suggest that reproductive sharing occurs commonly across all types of social organization. Incest avoidance may be an important constraint on sharing in families, but molecular techniques have thus far been inappropriate to assess its importance. The interests of males and female helpers may be quite different because females often have less opportunity to inherit a territory vacancy on the death of the breeder, less opportunity to cour...

The role of introduced species in the degradation of island ecosystems: A case history of guam

Abstract: The accidental introduction of the brown treesnake (Boiga irregularis) on Guam around 1950 induced a cascade of extirpations that may be unprecedented among historical extinction events in taxonomic scope and severity. Birds, bats, and reptiles were affected, and by 1990 most forested areas on Guam retained only three native vertebrates, all of which were small lizards. Of the hypotheses to account for the severity of this extinction event, we find some support for the importance of lack of coevolution between introduced predator and prey, availability of alternate prey, extraordinary predatory capabilities of the snake, and vulnerabilities of the Guam ecosystem. In addition, there were important interactions among these factors, especially the presence of introduced prey (possessing coevolutionary experience) that were thus able to maintain their populations and provide alternate prey to the introduced predator while it was driving the native prey species to extinction. This complex of vulnerabilities is common on oceanic islands.

EARLY EVOLUTION OF LAND PLANTS: Phylogeny, Physiology, and Ecology of the Primary Terrestrial Radiation

Abstract: ▪ Abstract The Siluro-Devonian primary radiation of land biotas is the terrestrial equivalent of the much-debated Cambrian “explosion” of marine faunas. Both show the hallmarks of novelty radiations (phenotypic diversity increases much more rapidly than species diversity across an ecologically undersaturated and thus low-competition landscape), and both ended with the formation of evolutionary and ecological frameworks analogous to those of modern ecosystems. Profound improvements in understanding early land plant evolution reflect recent liberations from several research constraints: Cladistic techniques plus DNA sequence data from extant relatives have prompted revolutionary reinterpretations of land plant phylogeny, and thus of systematics and character-state acquisition patterns. Biomechanical and physiological experimental techniques developed for extant plants have been extrapolated to fossil species, with interpretations both aided and complicated by the recent knowledge that global landmass positi...

FLORAL SYMMETRY AND ITS ROLE IN PLANT-POLLINATOR SYSTEMS: Terminology, Distribution, and Hypotheses

Abstract: Floral symmetry has figured prominently in the study of both pollination biology and animal behavior. However, a confusion of terminology and the diffuse nature of the literature has limited our understanding of the role that this basic characteristic of flower form has played in plant-pollinator interactions. Here, we first contribute a classification scheme for floral symmetry that we hope will resolve some of the confusion resulting from the inconsistent application of terms. Next, we present a short review of the distribution of floral forms in angiosperm families. Finally, we provide a list of hypotheses and, when available, supporting evidence for the causes of the evolution of floral symmetry.

Sex determination, sex ratios and genetic conflict

Abstract: Genetic mechanisms of sex determination are unexpectedly diverse and change rapidly during evolution. We review the role of genetic conflict as the driving force behind this diversity and turnover. Genetic conflict occurs when different components of a genetic system are subject to selection in opposite directions. Conflict may occur between genomes (including paternal-maternal and parentalzygotic conflicts) or within genomes (between cytoplasmic and nuclear genes or sex chromosomes and autosomes). The sex-determining system consists of parental sex-ratio genes, parental-effect sex determiners, and zygotic sex determiners, which are subject to different selection pressures because of differences in their modes of inheritance and expression. Genetic conflict theory is used to explain the evolution of several sex-determining mechanisms, including sex chromosome drive, cytoplasmic sex-ratio distortion, and cytoplasmic male sterility in plants. Although still limited, there is growing evidence that genetic conflict could be important in the evolution of sex-determining mechanisms.

Molecular trans-species polymorphism

Abstract: ▪ Abstract Trans-species polymorphism (TSP) is the occurrence of similar alleles in related species. Excluding instances in which the similarity arose by convergent evolution, TSP is generated by the passage of alleles from ancestral to descendant species. Closely related, recently diverged species, such as those of the Lake Victoria cichlid flock, may share neutral alleles, but long-lasting TSPs occur only in genetic systems evolving under balancing selection. Two such systems have been studied extensively, the major histocompatibility complex (Mhc) of jawed vertebrates and the self-incompatibility (SI) system of flowering plants. Allelic lineages that diverged many millions of years ago and passed through numerous speciation events have been described in both systems. The lineages may differ at up to 50% of their coding sites, both synonymous and nonsynonymous. The differences arise by the process of incorporation of mutations, which is different from the process of fixation. TSP, on the one hand, compl...

VERTEBRATE HERBIVORES IN MARINE AND TERRESTRIAL ENVIRONMENTS: A Nutritional Ecology Perspective

Abstract: ▪ Abstract The study of digestive physiology provides a framework for analyzing food resources, feeding patterns, and evolutionary trends in vertebrate herbivores. Most of the research in this field, nutritional ecology, has been focused on terrestrial herbivores, especially mammals. By integrating physiological, demographic, and evolutionary approaches, the study of terrestrial herbivores has generated several important hypotheses, notably on factors determining body mass. Marine vertebrate herbivores are abundant and locally diverse, but with the exception of reptiles and mammals, we lack information on digestive physiology and processing of plant foods, the key element in terrestrial studies. This review provides a foundation for a nutritional ecology of marine vertebrate herbivores, especially teleost fish, by summarizing the available information on their digestive physiology and identifying research priorities in the field.

Principles of phylogeography as illustrated by freshwater and terrestrial turtles in the southeastern united states

Abstract: ▪ Abstract Geographic patterns in mtDNA variation are compiled for 22 species of freshwater and terrestrial turtles in the southeastern United States, and the results are employed to evaluate phylogeographic hypotheses and principles of genealogical concordance derived previously from similar analyses of other vertebrates in the region. The comparative molecular findings are interpreted in the context of intraspecific systematics for these taxa, the historical geology of the area, traditional nonmolecular zoogeographic information, and conservation significance. A considerable degree of phylogeographic concordance is registered with respect to (a) the configuration of intraspecific mtDNA subdivisions across turtle species, (b) the principal molecular partitions and traditional morphology-based taxonomic boundaries, (c) genetic patterns in turtles versus those described previously for freshwater fishes and terrestrial vertebrates in the region, and (d) intraspecific molecular subdivisions versus the bounda...

Phylogeny of vascular plants

Abstract: ▪ Abstract Morphological and molecular analyses resolve many aspects of vascular plant phylogeny, though others remain uncertain. Vascular plants are nested within bryophytes; lycopsids and zosterophylls are one branch of crown-group vascular plants, and euphyllophytes (Psilophyton, sphenopsids, ferns, seed plants) are the other. In Filicales, Osmundaceae are basal; water ferns and Polypodiaceae sensu lato are both monophyletic. Seed plants are nested within progymnosperms, and coniferophytes are nested within platyspermic seed ferns. Morphology indicates that angiosperms and Gnetales are related, but detailed scenarios depend on uncertain relationships of fossils; molecular data are inconsistent but indicate that both groups are monophyletic. Amborella, Nymphaeales, Austrobaileya, and Illiciales appear basal in angiosperms. Groups with tricolpate pollen form a clade (eudicots), with ranunculids and lower hamamelids basal. Most eudicots belong to the rosid and asterid lines, with higher hamamelids in the ...

Possible largest-scale trends in organismal evolution : Eight live hypotheses

Abstract: Historically, a great many features of organisms have been said to show a trend over the history of life, and many rationales for such trends have been proposed. Here I review eight candidates, eight “live hypotheses” that are inspiring research on largest-scale trends today: entropy, energy intensiveness, evolutionary versatility, developmental depth, structural depth, adaptedness, size, and complexity. For each, the review covers the principal arguments that have been advanced for why a trend is expected, as well as some of the empirical approaches that have been adopted. Also discussed are three conceptual matters arising in connection with trend studies: 1. Alternative bases for classifying trends: pattern versus dynamics; 2. alternative modes in which largest-scale trends have been studied: “exploratory” versus “skeptical”; and 3. evolutionary progress.

The scientific basis of forestry

Abstract: A sufficiently wise and flexible silvicultural art can be developed on the ground only by practitioners who understand the forest as a biological entity. F. S. Baker (10) …the existing level of knowledge about forests is inadequate to develop sound forest management policies. National Research Council (114) ▪ Abstract Over the past two decades forestry in the United States has diverged into two approaches with quite different objectives and scientific priorities. The management focus of most industrial lands is on increasing productivity of wood fiber via plantations and various cultural tools, especially genetic selection, fertilization, and control of noncrop vegetation. Federal forest management has shifted from a similar focus to greater emphasis on protecting diversity and water. Issues of long-term sustainability are important regardless of ownership. Science has played and continues to play a fundamental role in all aspects. Selection for fast-growing genotypes has increased yields on the...

The ecological evolution of reefs

Abstract: ▪ Abstract Many groups of extinct and extant organisms have aggregated to form reefs for over 3.5 billion yr (Ga). Most of these communities, however, grew under ecological and environmental controls profoundly different from those that govern modern coral reefs. Not only has the global distribution of reefs varied considerably through geological time—determined largely by sea level, and latitudinal temperature/saturation gradients—but more importantly the trophic demands of reef-building organisms have changed, as has the degree of biological disturbance faced by sessile biota in shallow marine environments. Reefs differentiated into open surface and cryptic communities as soon as open frameworks developed in the Proterozoic, some 1.9 million yr ago (mya) and diverse and complex ecosystems were established by the early Cambrian (∼520 mya). Calcified heterotrophs were conspicuous in reefs during the Paleozoic and early Mesozoic, but considerable rigidity was imparted to these often otherwise fragile commu...