Search

The Mara River basin is a trans-boundary basin of international importance. It forms the headwaters of the Nile River and serves as the primary dry season water source for an estimated 1.1 million rural people and the largest remaining overland migration of 1.4 million wildebeest in the Serengeti-Mara Ecosystem. Changes throughout the basin are impacting the quantity and quality of the Mara River, yet the historical context of environmental conditions in the basin is not well known. We collected sediment cores throughout the wetland at the mouth of the Mara River, and we used isotopic dating methods and a suite of analyses to examine historical patterns of sediment quantity and source, mercury contamination, and carbon and nutrient loading. Our results show that ecological conditions in the Mara River basin were fairly stable over paleoecological time scales (2000–1000 years before present), but there has been a period of rapid change in the basin over the last 250 years, particularly since the 1960s. A shift in the source and quantity of sediments in the river began in the late 1700s and became much more pronounced in the 1950s and 1960s, coincident with increasing mercury concentrations. The quantity of sediment from the Upper Mara increased, particularly since 1960, but the proportion of total sediment from this region decreased as the Talek and Middle Mara portions of the basin began producing more sediment. The decadal oscillation in sediment accumulation was congruent with known periods of extreme precipitation events. Carbon and nitrogen loading also increased since the 1960s, and the shift in the isotopic ratio of nitrogen provides evidence for increased anthropogenic loading. Altogether, these data likely reflect patterns of change also experienced in other basins throughout East Africa.

The value of biodiversity is widely appreciated, but we are only beginning to understand the interplay of processes that generate biodiversity and their consequences for coevolutionary interactions. Whereas predator–prey coevolution is most often analyzed in the context of evolutionary arms races, much less has been written about how predators are affected by, and respond to, evolutionary diversification in their prey. We hypothesize here that adaptive radiation of prey may lead to diversification and potentially speciation in predators, a process that we call an upwards adaptive radiation cascade. In this paper we lay out the conceptual basis for upwards adaptive radiation cascades, explore evidence for such cascades, and finally advocate for intensified research.

Benthic communities in Long Island Sound (LIS) have experienced over 150 years of commercial shellfishing and excess nutrient loading (eutrophication) which causes hypoxia. We established an ecological baseline using a combination of live, dead, archaeological, and fossil material to investigate the impacts of these stressors on the molluscan community. We expected that ecological change would increase with eutrophication-hypoxia west towards New York City. Instead we found that taxonomic similarity, rank-order abundance, and drilling frequency are more strongly controlled by commercial fishing pressure than by decreasing dissolved oxygen. Commercial fisherman collecting quahog clams (Mercenaria mercenaria), physically disrupt surface-dwelling organisms and also kill large numbers of predatory gastropods, including the channeled whelk, Busycotypus canaliculatus, and the drilling moonsnails Neverita duplicata and Euspira heros, to protect hard clam stocks. As a result, areas dredged by commercial fishermen yield fewer shells with drill-holes and fewer surface-dwelling organisms than unfished sites. In spite of recent reductions in lobster fishing, crushing predation by crabs and lobsters on clams has been suppressed below baseline levels throughout LIS, even in the well oxygenated east. The absence of a clear relationship between eutrophication-hypoxia and ecological change questions the effectiveness of nitrogen reduction alone as a restoration strategy. LIS fossils revealed a relatively ancient loss of those mollusks associated with seagrass and oyster habitats (e.g., oysters, Crassostrea virginica; jingle shells, Anomia simplex; scallops, Argopecten irradiens; and the gastropod Bittiolum alternatum) that predates the accumulation of dead shells and underscores the need for older material to reveal the shifting baseline. The interactive nature of multiple stressors means that past overfishing may have dampened the response of communities in LIS to eutrophication or inhibited their capactiy to recover. The unexpected role of hypoxic areas protected from commercial fishing as refuges highlights the utility of no-take marine preserves in eutrophied estuaries worldwide.

Stable isotope methods are powerful, frequently used tools which allow diet and trophic position reconstruction of organisms and the tracking of energy sources through ecosystems. The majority of ecosystems have multiple food sources which have distinct carbon and nitrogen isotopic signatures despite occupying a single trophic level. This difference in the starting isotopic composition of primary producers sets up an isotopic baseline that needs to be accounted for when calculating diet or trophic position using stable isotopic methods. This is particularly important when comparing animals from different regions or different times. Failure to do so can cause erroneous estimations of diet or trophic level, especially for organisms with mixed diets. The isotopic baseline is known to vary seasonally and in concert with a host of physical and chemical variables such as mean annual rainfall, soil maturity, and soil pH in terrestrial settings and lake size, depth, and distance from shore in aquatic settings. In the fossil record, the presence of shallowing upward suites of rock, or parasequences, will have a considerable impact on the isotopic baseline as basin size, depth and distance from shore change simultaneously with stratigraphic depth. For this reason, each stratigraphic level is likely to need an independent estimation of baseline even within a single outcrop. Very little is known about the scope of millennial or decadal variation in isotopic baseline. Without multi-year data on the nature of isotopic baseline variation, the impacts of time averaging on our ability to resolve trophic relationships in the fossil record will remain unclear. The use of a time averaged baseline will increase the amount of error surrounding diet and trophic position reconstructions. Where signal to noise ratios are low, due to low end member disparity (e.g., aquatic systems), or where the observed isotopic shift is small (≤ 1‰) the error introduced by time averaging may severely inhibit the scope of one's interpretations and limit the types of questions one can reliably answer. In situations with strong signal strength, resulting from high amounts of end member disparity (e.g., terrestrial settings), this additional error maybe surmountable. Baseline variation that is adequately characterized can be dealt with by applying multiple end-member mixing models.

Forging stronger linkages between geomorphology and ecosystem ecology depends, in part, upon developing common conceptualizations of an ecosystem. Because most ecosystem processes are scale dependent, the choice of boundaries is of profound importance to the conceptualization of an ecosystem and to the scope and validity of questions being asked within that ecosystem. Indeed, any conceptualization of an ecosystem requires constraining the spatial and temporal scales of analysis. Thus, it is of particular importance to match the ecosystem boundaries to the question being asked or to the processes being studied and, to facilitate better communication among disciplines, to be explicit in the definitions adopted for an ecosystem. Defining an ecosystem can be problematic when the processes of interest operate at potentially different scales, and little research exists comparing scales of geomorphic processes with those of ecological processes. Here we will discuss the importance of scale in geomorphic and ecological research, and compare and contrast disciplinary biases and inclinations. To highlight the problem of conflicting spatial scales, we will draw on recent attempts to link the structure of food webs to measures of ecosystem size. In particular, problems arise where little or no strong association exists among community membership, resource supply, and physical boundaries. Similar problems arise when trying to link geomorphologic and ecological processes that can operate at different, but variable, temporal scales.

Food-chain length is a central characteristic of ecological communities that has attracted considerable attention for over 75 years because it strongly affects community structure, ecosystem processes and contaminant concentrations. Conventional wisdom holds that either resource availability or dynamical stability limit food-chain length; however, new studies and new techniques challenge the conventional wisdom and broaden the discourse on food-chain length. Recent results suggest that resource availability limits food-chain length only in systems with very low resource availability, and call into question the theoretical basis for dynamical stability as a determinant of food-chain length. Evidence currently points towards a complex and contingent framework of interacting constraints that includes the history of community organization, resource availability, the type of predator–prey interactions, disturbance and ecosystem size. Within this framework, the debate has shifted from a search for singular explanations to a search for when and where different constraints operate to determine food-chain length.