No abstract

There is substantial environmental variance at small spatial scales (1 m or less) in both natural and disturbed environments. We have investigated the spatial structure of physical variables at larger scales (up to 10 m). We analysed surveys of edaphic properties of Wisconsin forest soils, of the water chemistry of lakes in Ontario and Labrador, and of temperature and precipitation in northeastern North America. We found no clear indication that the variance among sites approaches some maximal value as the distance between them increases. We suggest instead that the variance of the physical environment tends to increase continually with distance. The slope of the log-log regression of variance on distance provides a means of comparing the heterogeneity of different environments with respect to a given factor, or of comparing different factors within a given environment. This slope provides a useful measure of environmental structure that can be related to the biodiversity or plasticity of native organisms.

We studied predation by a hdal migrant, Calliopius laewusculus (Amphlpoda Gammaridea) on the eggs of beach-spawnlng capelln Mallotus vlllosus Handling tlme d~d not constran predation Attack rate m e d~a t e d by search bme and gut fullness and assessed here through funchonal response tnals and gut f~n g / c l e a r a n c e rate experiments determined the magnitude of predation on capelin eggs The relationsh~p between egg density and egg consumption was slgmoid Compensatory mortality occurred over an egg density range of ca 1 to 3 eggs cm-3 At these low egg densties compensatory mortality should only have a small stabilizing Influence on capelin egg surv~val At hlgher egg densities, attack rate declined with egg density Sat~ation occurred at ca 11 eggs amphipod-' At surplus egg density gut f l l h g rate expenments showed that satiation is possible wlthln 2 h and functional response tnals indicated that average egg densihes found in sltu (40 egg cm-') are sufficiently high to allow sahatlon withn thls time penod Gut clearance requued 36 h T~d a l perlodiclty should not constrain amphlpod predation on capehn eggs We suggest that predation by C laev~usculus on the intert~dal eggs of capelln can have an important influence on egg abundance We calculated that C laeviusculus could consume roughly 39 % of capelin egg production We speculate that predation by gammands and other invertebrates on subtidal fish eggs should be higher owing to longer e g g development tlmes and therefore exposure to predation which occur at lower incubation temperatures in deeper water Predation by small invertebrates on demersal f~s h eggs may therefore be an Important regulator of population size in many manne fish specles

In the coastal regions of Newfoundland, Canada, the most active growth period for Calliopius laeviusculus overlaps the intertidal development of capelin eggs. At one capelin spawning beach located at Bryant's Cove (Newfoundland), we calculated that 37.6 mg dwt (dry weight) of food per cni2 of intertidal substrate were required to support C. laeviusculus population growth from June 17 to August 6, 1988. The total seasonal deposition of capelm eggs (236 mg dwt cm-*) was more than sufficient to meet C. laeviusculus energetic requirements over this period. The cumulative production of all other potential intertidal prey for C. laeviusculus in the community over the summer was estimated at 0.3 mg dwt cm-' and insufficient to contribute significantly to amphipod growth. We estimated that consumption of capelin eggs accounted for ca 50 % of the total production of C. laeviusculus in 1988. In any given year and if there is little interannual variation in amphipod population size, the impact of C. laeviusculus predation on capelin eggs will vary with capelin egg deposition. In 1988, we found that C. laeviusculus consumed approximately 16 % of the total capelin egg deposition. Interannually, predation by C. laeviusculus could potentially account for ca 5 to 30 % of the total capelin egg deposition.

The amphipod Calliopius laeviusculus actively preys on the intertidal eggs of capelin (Mallotus villosus). From samples collected in 1988, we found that the patterns of changes in C. laeviusculus biomass closely matched changes in capelin egg biomass throughout the capelin egg development period. Calliopius laeviusculus and capelin egg biomass exhibited pronounced and overlapping peaks in mid June. Endogenous reproductive patterns influenced by exogenous seasonal stimuli in both C. laeviusculus and capelin could facilitate yearly biomass coupling between these two species. Calliopius laeviusculus exhibited a semiannual life cycle, overwintering adults releasing young in April, May and July. Cohorts released in April and May reached sexual maturity within the summer, reproduced and produced a second maximum in amphipod biomass at the beginning of August. This second abundance peak appeared to be a numerical response to predation on capelin eggs by the adult C. laeviusculus present in mid-summer. Given this response and the reported quiescence of C. laeviusculus during the winter, we speculate that amphipod biomass in spring may be strongly linked to feeding conditions in the previous year. The implications of this coupling for capelin egg survival are considered.