In its simplest definition, a trait is a surrogate of organismal performance, and this meaning of the term has been used by evolutionists for a long time. Over the last three decades, developments in community and ecosystem ecology have forced the concept of trait beyond these original boundaries, and trait-based approaches are now widely used in studies ranging from the level of organisms to that of ecosystems. Despite some attempts to fix the terminology, especially in plant ecology, there is currently a high degree of confusion in the use, not only of the term "trait" itself, but also in the underlying concepts it refers to. We therefore give an unambiguous definition of plant trait, with a particular emphasis on functional trait. A hierarchical perspective is proposed, extending the "performance paradigm" to plant ecology. "Functional traits" are defined as morpho-physiophenological traits which impact fitness indirectly via their effects on growth, reproduction and survival, the three components of individual performance. We finally present an integrative framework explaining how changes in trait values due to environmental variations are translated into organismal performance, and how these changes may influence processes at higher organizational levels. We argue that this can be achieved by developing "integration functions" which can be grouped into functional response (community level) and effect (ecosystem level) algorithms.

/pdf/let-the-concept-of-trait-be-functional-2ppbgwsn3s.pdf

Let the concept of trait be functional

It is widely agreed that fecundity selection and sexual selection are the major evolutionary forces that select for larger body size in most organisms. The general, equilibrium view is that selection for large body size is eventually counterbalanced by opposing selective forces. While the evidence for selection favoring larger body size is overwhelming, counterbalancing selection favoring small body size is often masked by the good condition of the larger organism and is therefore less obvious. The suggested costs of large size are: (1) viability costs in juveniles due to long development and/or fast growth; (2) viability costs in adults and juveniles due to predation, parasitism, or starvation because of reduced agility, increased detectability, higher energy requirements, heat stress, and/or intrinsic costs of reproduction; (3) decreased mating success of large males due to reduced agility and/or high energy requirements; and (4) decreased reproductive success of large females and males due to late reproduction. A review of the literature indicates a substantial lack of empirical evidence for these various mechanisms and highlights the need for experimental studies that specifically address the fitness costs of being large at the ecological, physiological, and genetic levels. Specifically, theoretical investigations and comprehensive case studies of particular model species are needed to elucidate whether sporadic selection in time and space is sufficient to counterbalance perpetual and strong selection for large body size.

/pdf/the-evolution-of-body-size-what-keeps-organisms-small-1yb7tgm94a.pdf

The evolution of body size: what keeps organisms small?

We describe the impact of recent life-history plasticity theory on insect studies, particularly on the interface between genetics and plasticity. We focus on the three-dimensional relationship between three key life-history traits: adult size (or mass), development time and growth rate, and the connections to life cycle regulation, host plant choice, and sexual selection in seasonal environments. The review covers fitness consequences of variation in size, development time and growth rate, and effects of sex, photoperiod, temperature, diet, and perceived mortality risk on these traits. We give special attention to evidence for adaptive plasticity in growth rates because of the important effects of such plasticity on the expected relationships between development time and adult size and, hence, on the use of life-history, fitness, and optimality approaches in ecology, as well as in genetics.

Plasticity in Life-History Traits

Question: Associations of body size and of body temperature with fitness have complex relationships for ectotherms, but three general patterns are known. Bigger is better: Larger body size is frequently associated with greater fitness within populations. Hotter is smaller: Smaller adult body sizes typically result from development at higher temperatures. Hotter is better: Greater maximal performance at the optimal temperature is frequently associated with higher optimal temperatures. How do we – or even can we – reconcile these three apparently conflicting empirical patterns about temperature, size, and fitness of ectotherms? Methods: We summarize available evidence supporting or contradicting these three rules. We present a conceptual framework that describes how developmental and adult body temperatures affect causal connections among size, performance, and key components of fitness. Findings: There is strong empirical support for Bigger is better and Hotter is smaller (≥ 79% of studies/estimates), primarily for terrestrial insects, reptiles, and annual plants. Evidence regarding Hotter is better is still limited (and primarily from terrestrial insects), but most available information supports the rule. Analyses of counterexamples are particularly instructive. The rules operate at different levels. Bigger is better describes phenotypic variation within populations. Hotter is smaller describes phenotypic plasticity of a genotype. Hotter is better describes evolved variation in reaction norms among genotypes or between species. Conclusions: We unify these three rules into a path diagram that describes how temperature impacts critical rate processes throughout the life cycle. Adult body size and development time are key traits that are not only consequences of temperature-dependent processes, but also are causes of variation in fitness. An unresolved issue involves how to determine the appropriate fitness metric for a particular ecological context (population and environment). For example, the intrinsic rate of population increase (r) is strongly influenced by generation time (and development time), whereas net reproductive rate (R0) is strongly influenced by fecundity (and size). Because the relative strengths of different paths contributing to fitness change differ for these fitness metrics, the choice of metric can affect whether Hotter is better is ‘better’ than Bigger is better.

https://faculty.washington.edu/hueyrb/pdfs/KingsolverHuey08.pdf

Size, temperature, and fitness: Three rules

The extent to which behavioural choices reflect fine-tuned evolutionary adaptation remains an open debate. For herbivorous insects, the preference-performance hypothesis (PPH) states that female insects will evolve to oviposit on hosts on which their offspring fare best. In this study, we use meta-analysis to assess the balance of evidence for and against the PPH, and to evaluate the role of individual factors proposed to influence host selection by female insects. We do so in an explicitly bitrophic context (herbivores versus plants). Overall, our analyses offer clear support for the PPH: Offspring survive better on preferred plant types, and females lay more eggs on plant types conducive to offspring performance. We also found evidence for an effect of diet breadth on host choice: female preference for 'good quality plants' was stronger in oligophagous insects than in polyphagous insects. Nonetheless, despite the large numbers of preference-performance studies conducted to date, sample sizes in our meta-analysis are low due to the inconsistent format used by authors to present their results. To improve the situation, we invite authors to contribute to the data base emerging from this work, with the aim of reaching a strengthened synthesis of the subject field.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1461-0248.2009.01433.x

A meta-analysis of preference-performance relationships in phytophagous insects.

The relationship between intra-specific variation in female body size and potential fecundity was investigated using the published literature on 57 oviparous species of Coleoptera, Diptera, Ephemeroptera, Heteroptera, Homoptera, Hymenoptera, Lepidoptera, and Trichoptera, and 11 species of larviparous Aphidina and Diptera. Female body sizes were converted to dry body weight. Variation in body weight and fecundity was expressed as percentage deviation from the median values. The increase in fecundity with body weight was similar in most taxa, with only a few important exceptions. The common regression for oviparous and larviparous species predicts a 0.95% increase in median fecundity for each 1% increase in dry body weight. The number of ovarioles (in 10 species of Coleoptera, Diptera, Hymenoptera and Orthoptera) also increased with body weight. The general relationship predicted a 0.81% increase in ovariole number for each 1% increase in dry body weight. The slope of ovariole number versus weight relationship was greater in species with many ovarioles than in species with few. The common slope of the fecundity/size relationship is close to 1 and this indicates that female size is a principal constraint on insect potential fecundity.

https://www.jstor.org/stable/pdfplus/3544943.pdf

Alois. Honek

Papers

Intraspecific variation in body size and fecundity in insects: a general relationship