Geology topics
P.D. Steinberg
Publications and source records attributed to P.D. Steinberg.
Predation, herbivory and kelp evolution
We propose that the kelps (Laminariales) radiated in the North Pacific following the onset of late Cenozoic polar cooling. The evidence is that (1) extant kelps occur exclusively in cold-water habitats; (2) all but one of 27 kelp genera occur in the North Pacific, 19 of these exclusively; and (3) limpets and herbivorous marine mammals obligately associated with kelps or other stipitate brown algae appeared late in the Cenozoic, even though more generalized forms of both groups are much older. We propose, further, that sea otters and perhaps other groups of benthic-feeding predatory mammals, whose late Cenozoic distributions all were limited to the North Pacific, created an environment for the evolution of kelps in which the intensity of herbivory was unusually low. We hypothesize that this interaction created predictable differences among habitats in the intensity of herbivory on several spatial scales, with resulting trade-offs between anti-herbivore defenses and plant competitive abilities in their respective floras. Sea otters incur time and energy costs for diving, resulting in depth-related reductions to foraging efficiency and thus increased sizes and densities of herbivorous sea urchins. Thus, the deep-water flora is well defended, but competitively subordinate, compared with the shallow-water flora. Similarly, we argue that during the same period of earth history, predation had less of a limiting influence on herbivorous invertebrates in the temperate southwestern Pacific. We hypothesize that (1) consequent biogeographical differences in the intensity of herbivory may have selected the phenolic-rich brown algal flora in temperate Australia/New Zealand; and (2) tightly coevolved plant/herbivore interactions may explain why Australian and New Zealand herbivores are undeterred by phenolics and why other classes of secondary compounds in the Australian/New Zealand flora significantly deter herbivores.
Complex trophic interactions in kelp forest ecosystems
The distributions and abundances of species and populations change almost continuously. Understanding the processes responsible is perhaps ecology’s most fundamental challenge. Kelp-forest ecosystems in southwest Alaska have undergone several phase shifts between alga- and herbivore-dominated states in recent decades. Overhunting and recovery of sea otters caused the earlier shifts. Studies focusing on these changes demonstrate the importance of top-down forcing processes, a variety of indirect food-web interactions associated with the otter-urchin-kelp trophic cascade, and the role of food-chain length in the coevolution of defense and resistance in plants and their herbivores. This system unexpectedly shifted back to an herbivore-dominated state during the 1990s, because of a sea-otter population collapse that apparently was driven by increased predation by killer whales. Reasons for this change remain uncertain but seem to be linked to the whole-sale collapse of marine mammals in the North Pacific Ocean and southern Bering Sea. We hypothesize that killer whales sequentially "fished down" pinniped and sea-otter populations after their earlier prey, the great whales, were decimated by commercial whaling. The dynamics of kelp forests in southwest Alaska thus appears to have been influenced by an ecological chain reaction that encompassed numerous species and large scales of space and time.
A new assay for quantifying brown algal phlorotannins and comparisons to previous methods
Quantitative measurement of phlorotannins (polyphenolics) in brown algae (Phaeophyta) by colorimetric assays can be confounded because: (1) most such assays also react to nonphlorotannin substances (interferences) and (2) the appropriate reference compound for such assays is not always clear, although phloroglucinol is typically used. We developed a new assay in which 2,4-dimethoxybenzaldehyde (DMBA) reacts specifically with 1,3-and 1,3,5-substituted phenols (e.g., phlorotannins) to form a colored product. This new assay, as well as eliminating the problem of measuring interferences, is inexpensive, rapid, and can be used with small sample volumes. We recommend it for all assays of phlorotannins from one or a set of closely related species where the structural types of phlorotannins present are likely to be similar among samples. It is also appropriate for broader surveys of phlorotannin levels across many species, but in this case a reference must be chosen with care. We also compared the DMBA assay to existing assays, including the Folin-Denis [both before and after the samples were mixed with polyvinylpolypyrrolidone (PVPP)] and the Prussian blue assays. PVPP was not 100% efficient (and often much less) at removing phlorotannins from solution, and its effectiveness varied among different phlorotannins. Thus, in contrast to previous studies, measuring phenolic levels in extracts before and after treatment with PVPP will not necessarily result in an interference-free measure of phlorotannins. Based on an analysis of reactive substances in red and green algae (which do not contain phlorotannins) in the Folin-Denis and Prussian blue assays, we estimate that the average level of interferences (nonphlorotannins) in brown algae measured in these two assays is on the order of 0.5% by dry weight.
Evolutionary consequences of food chain length in kelp forest communities
Explore the source record for details and available documents.