Recent theoretical studies have shown that spatial redistribution of surface water may explain the occurrence of patterns of alternating vegetated and degraded patches in semiarid grasslands. These results implied, however, that spatial redistribution processes cannot explain the collapse of production on coarser scales observed in these systems. We present a spatially explicit vegetation model to investigate possible mechanisms explaining irreversible vegetation collapse on coarse spatial scales. The model results indicate that the dynamics of vegetation on coarse scales are determined by the interaction of two spatial feedback processes. Loss of plant cover in a certain area results in increased availability of water in remaining vegetated patches through run-on of surface water, promoting within-patch plant production. Hence, spatial redistribution of surface water creates negative feedback between reduced plant cover and increased plant growth in remaining vegetation. Reduced plant cover, however, results in focusing of herbivore grazing in the remaining vegetation. Hence, redistribution of herbivores creates positive feedback between reduced plant cover and increased losses due to grazing in remaining vegetated patches, leading to collapse of the entire vegetation. This may explain irreversible vegetation shifts in semiarid grasslands on coarse spatial scales.

Light is an essential resource for phytoplankton and fluctuates on a wide range of timescales. To understand how light fluctuations affect phytoplankton community structure and diversity, we have studied a set of simple models using a combination of analytical and numerical techniques. Light fluctuations can affect community structure when species exhibit the gleaner-opportunist trade-off between competitive ability and ability to reach carrying capacity quickly. Fast fluctuations can switch the competitive dominant from a gleaner to an opportunist; slow fluctuations can cause this switch and also lead to stable coexistence. Coexistence is easiest between species that are highly differentiated along the gleaner-opportunist trade-off. Our results remain qualitatively unchanged when more realistic light fluctuations such as daily and seasonal changes in irradiance and the presence of a spatial gradient in light are considered. Seasonal change in day length may be one of the factors driving the seasonal succession of phytoplankton, from opportunist species dominant under shorter day lengths (spring and autumn) to gleaner species dominant under longer day length (summer). These results illustrate how resource fluctuations can have an important role in structuring ecological communities.

Terrestrial isopod species recorded from Canada and the continental United States are listed with the original literature citation for each province and state record. The major habitat preference of each species is given. Marine littoral and cave dwelling species are generally endemics and the majority of isopods from other habitat types are assumed immigrants. Distribution and habitat data are analysed for implications regarding the origins of various elements of the North American fauna and conservation concerns. RÉSUMÉ La liste des espèces d'isopodes terrestres du Canada et des Etats-Unis continentaux est fournie, ainsi que la citation originale dans la littérature pour chaque signalement de province et d'état. La préférence dominante pour l'habitat est indiquée pour chaque espèce. Les espèces marines littorales et souterraines sont généralement endémiques et la majorité des isopodes des autres types d'habitat sont considérés comme immigrants. Les données sur la répartition et l'habitat sont analysées pour déterminer les implications en ce qui concerne les origines des divers éléments de la faune d'Amérique du Nord et leur prise en compte en vue de leur conservation.

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Addresses the effects of the current EPA Method 20 correction factors on predicted NOx levels vs. actual observed NOx emissions based upon data from some 800 engine field test data sets. The report demonstrates that correction procedure to the 85 degrees F / 75 grains of water per pound of dry air standard introduces considerable over-prediction and presents a case for the elimination of the procedure or at a minimum a modification to permit regional correction factors. The data sets are displayed in Appendix of some 200 p.