Recruitment and the local dynamics of open marine populations

The majority of marine populations are demographically open; their replenishment is largely or exclusively dependent on a supply of juveniles from the plankton. In spite of much recent research, no consensus has yet been reached regarding the importance of recruitment relative to other demographic processes in determining local population densities. We argue 1. that demographic theory suggests that, except under restrictive and unlikely conditions, recruitment must influence local population density to some extent. Therefore, 2. the question as to whether the size of a particular population is limited by recruitment is misguided. Finally, 3. the effect of recruitment on population size can be difficult to detect but is nonetheless real. A major weakness of most existing studies is a lack of attention to the survival of recruits over appropriate scales of time and space. Acknowledgment of the multifactorial determination of population density should guide the design of future experimental studies of the demography of open populations

Pancreatic diabetes manifests when beta cell area declines by approximately 65% in humans.

AIMS/HYPOTHESIS: Diabetes frequently develops in patients with pancreatic disorders. We aimed to determine the lower threshold of beta cell area for diabetes manifestation as well as the impact of insulin sensitivity on glucose homoeostasis in patients with pancreatic diabetes. METHODS: Eighty-two patients undergoing pancreatic surgery underwent pre-operative oral glucose challenge. Fractional pancreatic beta cell area was determined, and indices of insulin sensitivity and beta cell function were calculated. RESULTS: HbA(1c) and glucose levels were similar in patients with high and intermediate beta cell area, but were significantly higher in those with the lowest beta cell area (p\u2009<\u20090.0001). Insulin secretion was reduced only in patients with the lowest beta cell area (p\u2009<\u20090.001). The relative beta cell deficits at the onset of diabetes and impaired glucose tolerance were 64% and 21%, respectively, based on 2 h glucose levels. Deteriorating insulin sensitivity was associated with a small increase in the incidence of diabetes. CONCLUSIONS/INTERPRETATION: In conclusion, pancreatic diabetes probably develops after a reduction in beta cell area of ~65%. Post-challenge glucose excursions are much more closely related to pancreatic beta cell area than to fasting glycaemia, thereby underlining the usefulness of the OGTT in patients with pancreatic disorders

BOTTOM-UP AND TOP-DOWN INTERACTIONS IN COASTAL INTERFACE SYSTEMS

The land–sea margin encompasses a variety of hard and soft-bottom habitats where organisms are exposed to a dynamic range of aquatic and atmospheric conditions dependent on a rhythm set by the tides. In this chapter, we focus on rocky intertidal and salt marsh ecosystems, which have been extensively studied on many continents. Both rocky shore and salt marsh communities exhibit strong and consistent patterns of intertidal zonation over relatively compressed spatial scales, making them excellent systems for understanding the context-dependency of species interactions. Hard-bottomed rocky intertidal communities are dominated by marine macroalgae and sessile marine invertebrates extending their reach to the furthest edge of the influence of sea spray, while soft-bottomed salt marsh communities are anchored by terrestrial plants with adaptations or tolerance to inundation by salty and brackish waters. Rocky shore communities may be battered by the full force of large ocean waves or gently lapped with seawater on more protected shorelines. In contrast, salt marshes are restricted to quiet waters where sediment accretion by plants is the main mechanism for habitat creation. Both communities may experience very large tidal excursions or only minimal ones, depending on the local dynamics of the tides, with corresponding consequences for the spatial extent of these communities across the shoreline. The steep environmental gradients and distinctive biological zonation patterns that characterize both rocky shore and salt marsh ecosystems (Fig. 7.1) have provided ecologists with accessible and highly tractable ecosystems for investigating the role of bottom-up and top-down factors along environmental gradients