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SUMMARY(1) Competitive ability can be compared between species in two ways: effect of different neighbour species on performance of a single target species or response of different target species to a single neighbour species. In a 5-week glasshouse experiment, an additive design was used for all combinations of seven species as both target and neighbour species to determine if there were consistent hierarchies in competitive effect and/or response, what traits of individuals determined position in these hierarchies, and whether or not effect and response competitive ability were related during the early stages of competition.(2) Five weeks after sowing, significant non-linear regressions of target biomass on neighbour density were found for 59% of the forty-nine species combinations and significant linear regressions on neighbour biomass were found for 51% of the species combinations. The slopes of these regressions represent per-plant and per-gram competition coefficients, respectively.(3) Neighbour species differed strongly in competitive effect per plant. Differences in effect per gram, response per plant, and response per gram were much weaker. Nevertheless, consistent competitive hierarchies were found for both effect and response on both a per-plant and per-gram basis.(4) Different traits determined position in the effect and response hierarchies. Neighbour species with larger seed mass and larger maximum potential mass had stronger per-plant competitive effects, whilst neighbour species with higher maximum relative growth rates had stronger per-gram competitive effects. The reverse of this latter pattern was seen for competitive response: target species with lower maximum relative growth rates were better response competitors. Mean effect and response competitive ability of the seven species were uncorrelated with each other.(5) These differences in traits associated with strong effect and strong response competitive ability emphasize the importance of distinguishing between them in experimental studies, at least during the early stages of competition.
In this study, I examine the effects of natural and experimentally induced variation in life cycle timing on offspring fitness in Arphia sulphurea and Chortophaga viridifasciata, to understand the selective pressures shaping phenology in these two species of nymph-overwintering grasshoppers. Because these species lack embryonic diapause, hatching varies over a two month range under natural conditions. I used a cold treatment to delay hatching of some egg pods and extend the natural range of hatching dates. Due to the shorter time for growth and poorer growing conditions late in the fall, late-hatching nymphs of both species grew to a smaller size before winter and suffered higher overwinter mortality, compared to early nymphs. In addition, late nymphs that did survive the winter became reproductive later in the following year's breeding season. Size- dependent mortality of offspring during the winter is a strong selective pressure favoring early reproduction in these species. Female adult life history traits appear responsive to the seasonal declines in offspring fitness, in that late-maturing females began reproducing sooner after adult maturation and reproduced at a more rapid rate, even at the expense of having shorter adult longevity and producing fewer total egg pods. Experimental manipulations were crucial in understanding the fitness consequences of intrapopulation variation in the timing of specific life-cycle events for these species.
Offspring size has been reported to vary seasonally in a diverse group oforganisms: for example, flowering plants, isopods, cladocerans, insects, fish, amphibians,
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