Abstract
The identification of trade-offs is necessary for understanding the evolution and maintenance of diversity. Here we employ the supply-demand (SD) body size optimization model to predict a trade-off between asymptotic body size and growth rate. We use the SD model to quantitatively predict the slope of the relationship between asymptotic body size and growth rate under high and low food regimes and then test the predictions against observations for Daphnia ambigua. Close quantitative agreement between observed and predicted slopes at both food levels lends support to the model and confirms that a ‘rate-size’ trade-off structures life history variation in this population. In contrast to classic life history expectations, growth and reproduction were positively correlated after controlling for the rate-size trade-off. We included 12 Daphnia clones in our study, but clone identity explained only some of the variation in life history traits. We also tested the hypothesis that growth rate would be positively related to intergenic spacer length (i.e. the growth rate hypothesis) across clones, but we found that clones with intermediate intergenic spacer lengths had larger asymptotic sizes and slower growth rates. Our results strongly support a resource-based optimization of body size following the SD model. Furthermore, because some resource allocation decisions necessarily precede others, understanding interdependent life history traits may require a more nested approach.
Highlights
The study of trade-offs underlies much of evolutionary ecology [1,2,3,4]
Standing variation in life history traits may have a genetic underpinning, so we examined whether variation in massspecific demand for resources may be linked to genotypic variability in the ribosomal intergenic spacer (IGS) region
We identified a rate-size trade-off in a population of D. ambigua under experimentally controlled conditions. Such a tradeoff is predicted by the supply-demand (SD) model based on the principle that the optimal body size is that which matches total bodily demand for resources with resource supply (Fig. 1)
Summary
Trade-offs arise under constraints because not all competing ends can be maximized simultaneously, and investment of resources in one trait or activity comes at the expense of investment in another. Traits that are related by a trade-off typically are negatively correlated. Such negative relationships do not, automatically confirm the existence of a trade-off (see discussion in [7]). A negative correlation between growth and reproduction in oaks was caused not by a trade-off in biomass production but by climatic factors influencing both variables simultaneously [8]. To establish the existence of a trade-off, it is necessary to identify the constraint and to understand how organisms may have to ‘choose’ between competing ends that each influence fitness
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