The consequences of among-individual phenotypic variation for the performance and ecological success of populations and species has attracted growing interest in recent years. Earlier reviews of this field typically address the consequences for population processes of one specific source of variation (plasticity or polymorphism), or consider one specific aspect of population performance, such as rate of speciation. Here we take a broader approach and study earlier reviews in order to summarize and compare predictions regarding several population-level consequences of phenotypic variation stemming from genetic polymorphism, developmental plasticity or randomized phenotype switching. Unravelling cause-dependent consequences of variation may increase our ability to understand the ecological dynamics of natural populations and communities, develop more informed management plans for protection of biodiversity, suggest possible routes to increased productivity and yield in natural and managed biological systems, and resolve inconsistencies in patterns and results seen in studies of different model systems. We find an overall agreement regarding the effects of higher levels of phenotypic variation generated by different sources, but also some differences between fine-grained and coarse-grained environments, modular and unitary organisms, mobile and sessile organisms, and between flexible and fixed traits. We propose ways to test the predictions and identify issues where current knowledge is limited and future lines of investigation promise to provide important novel insights.

Biological diversity is threatened by exploitation, fragmentation of natural habitats, pollution, climate change, and anthropogenic spread of species. The question of how among‐individual variation influences the performance of populations and species is a poorly explored but currently growing field of research. Here, we review 31 experimental and 14 comparative studies and first investigate whether there is empirical support for the propositions that higher levels of among‐individual phenotypic and genetic variation promote the ecological and evolutionary success of populations and species in the face of environmental change. Next, we examine whether and how the effect of diversity depends on environmental conditions. Finally, we explore whether the relationship linking population fitness to diversity is typically linear, asymptotic, or whether the benefits peak at intermediate diversity. The reviewed studies provide strong, almost invariable, evidence that more variable populations are less vulnerable to environmental changes, show decreased fluctuations in population size, have superior establishment success, larger distribution ranges, and are less extinction prone, compared with less variable populations or species. Given the overwhelming evidence that variation promotes population performance, it is important to identify conditions when increased variation does not have the theoretically expected effect, a question of considerable importance in biodiversity management, where there are many other practical constraints. We find that experimental outcomes generally support the notion that genetic and phenotypic variation is of greater importance under more stressful than under benign conditions. Finally, population performance increased linearly with increasing diversity in the majority (10 of 12) of manipulation studies that included four or more diversity levels; only two experiments detected curvilinear relationships.

Evolutionary theory predicts an interactive process whereby spatiotemporal environmental heterogeneity will maintain genetic variation, while genetic and phenotypic diversity will buffer populations against stress and allow for fast adaptive evolution in rapidly changing environments. Here, we study color polymorphism patterns in pygmy grasshoppers (Tetrix subulata) and show that the frequency of the melanistic (black) color variant was higher in areas that had been ravaged by fires the previous year than in nonburned habitats, that, in burned areas, the frequency of melanistic grasshoppers dropped from ca. 50% one year after a fire to 30% after four years, and that the variation in frequencies of melanistic individuals among and within populations An improved knowledge of how environmental change affects natural populations of plants and animals is crucial for our understanding of evolution of biological diversity as well as for the development of successful plans for protection and management of biodiversity (Hanski 1998;Lande 1998;Bell 2010). The influence of natural selection on biodiversity in unstable and heterogeneous environments has been debated since the middle of the 20th century. Theory posits that overall, spatially divergent selection in combination with gene flow generally provides broad conditions for maintenance of genetic polymorphism and that fluctuating selection in temporally changing environments may maintain genetic variation under restricted conditions, but the consequences are scale dependent. The effect of spatial variation depends on if the environment is fine or coarse grained relative to the dispersal capacity of the organism, and whether temporally changing environments will promote or erode genetic variation depends on the frequency of change relative to the life span of the organism and on whether generations are overlapping or discrete (Haldane and

Environmental changes currently pose severe threats to biodiversity, and reintroductions and translocations are increasingly used to protect declining populations and species from extinction. Theory predicts that establishment success should be higher for more variable groups of dissimilar individuals. To test this 'diversity promotes establishment' hypothesis, we introduced colour polymorphic pygmy grasshoppers (Tetrix subulata) to different sites in the wild. The number of descendants found at the release sites the subsequent year increased with increasing number of colour morphs in the founder group, and variation in founder groups also positively affected colour morph diversity in the established populations. Since colour morphs differ in morphology, physiology, behaviour, reproductive life history and types of niche used, these findings demonstrate that variation among individuals in functionally important traits promotes establishment success under natural conditions, and further indicate that founder diversity may contribute to evolutionary rescue and increased population persistence.

Gene flow is often regarded a random process that homogenizes differences between populations and constrains local adaptation. However, the matching habitat choice hypothesis posits that individuals actively choose those microhabitats that best match their specific phenotype to maximize fitness. Dispersal (and possibly gene flow) may thus be directed. Many studies report associations between habitats and phenotypes, but they may reflect selection, plasticity or adaptation rather than matching choice. Here, we test two predictions from the matching habitat choice hypothesis by manipulating the dorsal colour of Tetrix subulata, a pygmy grasshopper. (1) Is microhabitat choice flexible such that differently manipulated phenotypes distribute themselves differently in a microclimatic and solar radiation mosaic? (2) If they do, are their fitness prospects higher in the more preferred microhabitat? We find that individuals painted white or black do distribute themselves differently, with black individuals residing in habitats with less radiation, on average, than white individuals, demonstrating that microhabitat choices are plastic. Furthermore, white females had more hatchlings than black ones in the increased radiation treatment, and this was mainly due to increased mortality of black females under increased radiation. These findings provide rare experimental evidence in line with predictions from the matching habitat choice hypothesis.

That colour polymorphism may protect prey populations from predation is an old but rarely tested hypothesis. We examine whether colour polymorphic populations of prey exposed to avian predators in an ecologically valid visual context were exposed to increased extinction risk compared with monomorphic populations. We made 2976 artificial pastry prey, resembling Lepidoptera larvae, in four different colours and presented them in 124 monomorphic and 124 tetramorphic populations on tree trunks and branches such that they would be exposed to predation by free-living birds, and monitored their 'survival'. Among monomorphic populations, there was a significant effect of prey coloration on survival, confirming that coloration influenced susceptibility to visually oriented predators. Survival of polymorphic populations was inferior to that of monomorphic green populations, but did not differ significantly from monomorphic brown, yellow or red populations. Differences in survival within polymorphic populations paralleled those seen among monomorphic populations; the red morph most frequently went extinct first and the green morph most frequently survived the longest. Our findings do not support the traditional protective polymorphism hypothesis and are in conflict with those of earlier studies. As a possible explanation to our findings, we offer a competing 'giveaway cue' hypothesis: that polymorphic populations may include one morph that attracts the attention of predators and that polymorphic populations therefore may suffer increased predation compared with some monomorphic populations.

Abstract. Large founder groups and habitat match have been shown to increase the establishment success of reintroduced populations. Theory posits that the diversity of founder groups should also be important, but this has rarely been investigated. Here, experimental introductions of color-polymorphic Tetrix subulata pygmy grasshoppers into outdoor enclosures were used to test whether higher phenotypic diversity promotes establishment success. We show that the number of individuals present one year after introduction increases with color morph diversity in founder groups. Variance in establishment success did not decrease with increasing founder diversity, arguing against an important contribution of sampling effects or evolutionary rescue. Color morphs in T. subulata covary with a suite of other functionally important traits and utilize different resources. The higher establishment success in more diverse founder groups may therefore result, in part, from niche complementarity. Variation in establishment among groups was not associated with differences among source populations in reproductive capacities.

The matching habitat choice hypothesis posits that individuals actively choose those microhabitats that best match their specific phenotype to maximize fitness. Despite the profound implications, matching habitat choice has not been unequivocally demonstrated. We conducted two experiments to examine the impact of pigmentation pattern in the color polymorphic pygmy grasshopper Tetrix subulata on habitat choice in a laboratory thermal mosaic arena. We found no behavioral differences in the thermal mosaic among pygmy grasshoppers belonging to either pale, intermediate or dark natural color morphs. However, after manipulating the grasshoppers’ phenotype, the utilization through time of warmer and colder parts of the arena was different for black-painted and white-painted individuals. White-painted individuals used warmer parts of the arena, at least during the initial stage of the experiment. We conclude that microhabitat choice represents a form of behavioural plasticity. Thus, even if the choice itself is flexible and not genetically determined, it can still lead to spatial genetic structure in the population because the phenotypes themselves may be genetically mediated.