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We examined the population genetic structure of the amphipod Gammarus fasciatus (Crustacea: Amphipoda) over two years from a total of 11 sites within the Great Lakes - St. Lawrence River system using allozyme electrophoresis. In 1995, we examined differences within the St. Lawrence River (Cornwall to Quebec City), and in 1996, we focused on larger scale, whole-system differences from Lake Superior to Lake Champlain. We found very low levels of genetic differentiation among sites (Wright's FST< 0.03). This was not unexpected for an active disperser in a relatively contiguous system. However, we also found low levels of genetic variability within sites (Hobs= 0.07- 0.15), a result that was not anticipated. We surveyed the available literature on gammarid and other amphipods and found that limited genetic variability is characteristic of continuously distributed taxa, contrary to the expectations of current conservation theory. This is potentially of concern, especially in light of recently reported declines of G. fasciatus at several locations within the St. Lawrence River and Great Lakes. Given the importance of species such as G. fasciatus in aquatic food webs, further work is clearly needed on the consequences of population genetic structure to species' responses to novel environmental changes (e.g., exotic species introductions).

We tested the hypothesis that fish species diversity would be positively related to resource diversity in a series of northern and southeastern Ontario lakes. At a macrospatial scale (among lakes), the number of fish species per lake was significantly correlated with two indices of habitat heterogeneity: lake area and the shoreline development factor. At a microspatial scale (among habitats within lakes), detailed analyses revealed several significant correlations between fish species diversity and resource heterogeneity, although some effects of latitude were apparent. Fish species diversity in northern Ontario lakes was positively correlated with the diversity of invertebrate prey (benthos and zooplankton), but not with measures of physical habitat complexity. Species diversity in southern Ontario lakes was positively related to several measures of habitat heterogeneity. Reduction of redundancy in the habitat variables by principal components analysis, followed by multiple regression analyses, showed that fish species diversity in southern lakes was best predicted by substrate diversity and vertical vegetation complexity. Differences in the response of northern and southern fish communities to habitat structure likely relate to glacial history and to characteristics of the different habitats. Our results are consistent with earlier studies and suggest that species diversity – resource diversity relationships, previously reported for terrestrial communities, also apply in aquatic environments.

Growth in the first summer of life was compared in nine coexisting fish species with different life history strategies. Relative to hatching size, the various species grew 6- to 10-fold in length and up to 1000-fold in weight. Growth patterns were similar among species; length increments were linear early in the summer and reached a plateau by early September. Few species grew significantly in length or weight beyond September. There were also few appreciable differences in length–weight regressions among species, and in all cases growth was isometric. Spawning date had the greatest influence on the amount of growth attained, with early spawners achieving a greater mean length and weight than late spawners. Adult size did not influence the amount of growth; large-bodied species achieved proportionately less of their adult weight and length in the first summer than did small species. However, adult size did influence the duration of the spawning period. Small-bodied species had the most protracted spawning seasons and the greatest size range of young at the end of the summer. There was no apparent relationship between diet type and growth in young fish.

Two forms of brood amalgamation occur frequently in several species of North American waterfowl: (i) pre-hatch brood amalgamation, whereby a female lays her eggs in the nest of another female and the recipient thereafter provides all further care of the eggs and resulting offspring, and (ii) post-hatch brood amalgamation, whereby a female abandons or loses her young to another female after hatch, and the recipient subsequently tends the foster young. Some authors have viewed these behaviours as accidental or aberrant and of little evolutionary significance. More recently, a number of alternative hypotheses have been suggested. However, few of these hypotheses have been contrasted as viable alternatives and tested in the field, largely because an appropriate theoretical framework is lacking. We analyze the frequency of occurrence of brood amalgamation in North American anatids. We also review the hypotheses that have been proposed to explain these behaviours and erect a theoretical framework which applies to the evolution of both pre-hatch and post-hatch brood amalgamation, and which may apply to species other than those of the Anatidae. Finally, we show that the occurrence of brood amalgamation in North American waterfowl may be associated with low relative resource availability and K-type life-history traits.