The Mississippi River, the longest in North America, is really two rivers geophysically. The volume is less, the slope steeper, the velocity greater, and the channel straighter in its upper portion than in its lower portion. Below the mouth of the Ohio River, the Mississippi meanders through a continental depression that it has slowly filled with sediment over many millennia. Some limnologists and hydrologists consider the transitional middle portion of the Mississippi, where the waters of its two greatest tributaries, the Missouri and Ohio rivers, join it, to comprise a third river, in terms of its behavioral patterns and stream and floodplain ecologies.
The Mississippi River humans have known, with its two or three distinct sections, is a relatively recent formation. The lower Mississippi only settled into its current formation following the last ice age and the dissipation of water released by receding glaciers. Much of the current river delta is newer still, having taken shape over the last three to five hundred years.
Within the lower section of the Mississippi are two subsections, the meander zone and the delta. Below Cape Girardeau, Missouri, the river passes through Crowley’s Ridge and enters the wide and flat alluvial plain. Here the river meanders in great loops, often doubling back on itself, forming cut offs that, if abandoned by the river, forming lakes. Until modern times, most of the plain, approximately 35,000 square miles, comprised a vast and rich—rich in terms of biomass production—ecological wetland sustained by annual Mississippi River floods that brought not just water, but fertile sediment—topsoil—gathered from across much of the continent. People thrived in the Mississippi River meander zone. Some of the most sophisticated indigenous cultures of North America emerged here. Between Natchez, Mississippi, and Baton Rouge, Louisiana, at Old River Control, the Mississippi begins to fork into distributary channels, the largest of which is the Atchafalaya River. The Mississippi River delta begins here, formed of river sediment accrued upon the continental shelf. In the delta the land is wetter, the ground water table is shallower. Closer to the sea, the water becomes brackish and patterns of river sediment distribution are shaped by ocean tides and waves. The delta is frequently buffeted by hurricanes.
Over the last century and a half people have transformed the lower Mississippi River, principally through the construction of levees and drainage canals that have effectively disconnected the river from the floodplain. The intention has been to dry the land adjacent to the river, to make it useful for agriculture and urban development. However, an unintended effect of flood control and wetland drainage has been to interfere with the flood-pulse process that sustained the lower valley ecology, and with the process of sediment distribution that built the delta and much of the Louisiana coastline. The seriousness of the delta’s deterioration has become especially apparent since Hurricane Katrina, and has moved conservation groups to action. They are pushing politicians and engineers to reconsider their approach to Mississippi River management.
Juha Merilä and Ary A. Hoffmann
Changing climatic conditions have both direct and indirect influences on abiotic and biotic processes and represent a potent source of novel selection pressures for adaptive evolution. In addition, climate change can impact evolution by altering patterns of hybridization, changing population size, and altering patterns of gene flow in landscapes. Given that scientific evidence for rapid evolutionary adaptation to spatial variation in abiotic and biotic environmental conditions—analogous to that seen in changes brought by climate change—is ubiquitous, ongoing climate change is expected to have large and widespread evolutionary impacts on wild populations. However, phenotypic plasticity, migration, and various kinds of genetic and ecological constraints can preclude organisms from evolving much in response to climate change, and generalizations about the rate and magnitude of expected responses are difficult to make for a number of reasons.
First, the study of microevolutionary responses to climate change is a young field of investigation. While interest in evolutionary impacts of climate change goes back to early macroevolutionary (paleontological) studies focused on prehistoric climate changes, microevolutionary studies started only in the late 1980s. The discipline gained real momentum in the 2000s after the concept of climate change became of interest to the general public and funding organizations. As such, no general conclusions have yet emerged. Second, the complexity of biotic changes triggered by novel climatic conditions renders predictions about patterns and strength of natural selection difficult. Third, predictions are complicated also because the expression of genetic variability in traits of ecological importance varies with environmental conditions, affecting expected responses to climate-mediated selection.
There are now several examples where organisms have evolved in response to selection pressures associated with climate change, including changes in the timing of life history events and in the ability to tolerate abiotic and biotic stresses arising from climate change. However, there are also many examples where expected selection responses have not been detected. This may be partly explainable by methodological difficulties involved with detecting genetic changes, but also by various processes constraining evolution.
There are concerns that the rates of environmental changes are too fast to allow many, especially large and long-lived, organisms to maintain adaptedness. Theoretical studies suggest that maximal sustainable rates of evolutionary change are on the order of 0.1 haldanes (i.e., phenotypic standard deviations per generation) or less, whereas the rates expected under current climate change projections will often require faster adaptation. Hence, widespread maladaptation and extinctions are expected. These concerns are compounded by the expectation that the amount of genetic variation harbored by populations and available for selection will be reduced by habitat destruction and fragmentation caused by human activities, although in some cases this may be countered by hybridization. Rates of adaptation will also depend on patterns of gene flow and the steepness of climatic gradients. Theoretical studies also suggest that phenotypic plasticity (i.e., nongenetic phenotypic changes) can affect evolutionary genetic changes, but relevant empirical evidence is still scarce. While all of these factors point to a high level of uncertainty around evolutionary changes, it is nevertheless important to consider evolutionary resilience in enhancing the ability of organisms to adapt to climate change.
Mark V. Barrow
The prospect of extinction, the complete loss of a species or other group of organisms, has long provoked strong responses. Until the turn of the 18th century, deeply held and widely shared beliefs about the order of nature led to a firm rejection of the possibility that species could entirely vanish. During the 19th century, however, resistance to the idea of extinction gave way to widespread acceptance following the discovery of the fossil remains of numerous previously unknown forms and direct experience with contemporary human-driven decline and the destruction of several species. In an effort to stem continued loss, at the turn of the 19th century, naturalists, conservationists, and sportsmen developed arguments for preventing extinction, created wildlife conservation organizations, lobbied for early protective laws and treaties, pushed for the first government-sponsored parks and refuges, and experimented with captive breeding. In the first half of the 20th century, scientists began systematically gathering more data about the problem through global inventories of endangered species and the first life-history and ecological studies of those species.
The second half of the 20th and the beginning of the 21st centuries have been characterized both by accelerating threats to the world’s biota and greater attention to the problem of extinction. Powerful new laws, like the U.S. Endangered Species Act of 1973, have been enacted and numerous international agreements negotiated in an attempt to address the issue. Despite considerable effort, scientists remain fearful that the current rate of species loss is similar to that experienced during the five great mass extinction events identified in the fossil record, leading to declarations that the world is facing a biodiversity crisis. Responding to this crisis, often referred to as the sixth extinction, scientists have launched a new interdisciplinary, mission-oriented discipline, conservation biology, that seeks not just to understand but also to reverse biota loss. Scientists and conservationists have also developed controversial new approaches to the growing problem of extinction: rewilding, which involves establishing expansive core reserves that are connected with migratory corridors and that include populations of apex predators, and de-extinction, which uses genetic engineering techniques in a bid to resurrect lost species. Even with the development of new knowledge and new tools that seek to reverse large-scale species decline, a new and particularly imposing danger, climate change, looms on the horizon, threatening to undermine those efforts.
Fisheries science emerged in the mid-19th century, when scientists volunteered to conduct conservation-related investigations of commercially important aquatic species for the governments of North Atlantic nations. Scientists also promoted oyster culture and fish hatcheries to sustain the aquatic harvests. Fisheries science fully professionalized with specialized graduate training in the 1920s.
The earliest stage, involving inventory science, trawling surveys, and natural history studies continued to dominate into the 1930s within the European colonial diaspora. Meanwhile, scientists in Scandinavian countries, Britain, Germany, the United States, and Japan began developing quantitative fisheries science after 1900, incorporating hydrography, age-determination studies, and population dynamics. Norwegian biologist Johan Hjort’s 1914 finding, that the size of a large “year class” of juvenile fish is unrelated to the size of the spawning population, created the central foundation and conundrum of later fisheries science. By the 1920s, fisheries scientists in Europe and America were striving to develop a theory of fishing. They attempted to develop predictive models that incorporated statistical and quantitative analysis of past fishing success, as well as quantitative values reflecting a species’ population demographics, as a basis for predicting future catches and managing fisheries for sustainability. This research was supported by international scientific organizations such as the International Council for the Exploration of the Sea (ICES), the International Pacific Halibut Commission (IPHC), and the United Nations’ Food and Agriculture Organization (FAO).
Both nationally and internationally, political entanglement was an inevitable feature of fisheries science. Beyond substituting their science for fishers’ traditional and practical knowledge, many postwar fisheries scientists also brought progressive ideals into fisheries management, advocating fishing for a maximum sustainable yield. This in turn made it possible for governments, economists, and even scientists, to use this nebulous target to project preferred social, political, and economic outcomes, while altogether discarding any practical conservation measures to rein in globalized postwar industrialized fishing. These ideals were also exported to nascent postwar fisheries science programs in developing Pacific and Indian Ocean nations and in Eastern Europe and Turkey.
The vision of mid-century triumphalist science, that industrial fisheries could be scientifically managed like any other industrial enterprise, was thwarted by commercial fish stock collapses, beginning slowly in the 1950s and accelerating after 1970, including the massive northern cod crisis of the early 1990s. In the 1980s scientists, aided by more powerful computers, attempted multi-species models to understand the different impacts of a fishery on various species. Daniel Pauly led the way with multi-species models for tropical fisheries, where the need for such was most urgent, and pioneered the global database FishBase, using fishing data collected by the FAO and national bodies. In Canada the cod crisis inspired Ransom Myers to use large databases for fisheries analysis to show the role of overfishing in causing that crisis. After 1980 population ecologists also demonstrated the importance of life history data for understanding fish species’ responses to fishery-induced population and environmental change.
With fishing continuing to shrink many global commercial stocks, scientists have demonstrated how different measures can manage fisheries for species with different life-history profiles. Aside from the need for effective scientific monitoring, the biggest ongoing challenges remain having politicians, governments, fisheries industry members, and other stakeholders commit to scientifically recommended long-term conservation measures.