A genetic paradox1,2 exists in invasion biology: how do introduced populations, whose genetic variation has probably been depleted by population bottlenecks, persist and adapt to new conditions? Lessons from conservation genetics show that reduced genetic variation due to genetic drift and founder effects limits the ability of a population to adapt, and small population size increases the risk of extinction1,3,4. Nonetheless, many introduced species experiencing these same conditions during initial introductions persist, expand their ranges, evolve rapidly and become invasive. To address this issue, we studied the brown anole, a worldwide invasive lizard. Genetic analyses indicate that at least eight introductions have occurred in Florida from across this lizard's native range, blending genetic variation from different geographic source populations and producing populations that contain substantially more, not less, genetic variation than native populations. Moreover, recently introduced brown anole populations around the world originate from Florida, and some have maintained these elevated levels of genetic variation. Here we show that one key to invasion success may be the occurrence of multiple introductions that transform among-population variation in native ranges to within-population variation in introduced areas. Furthermore, these genetically variable populations may be particularly potent sources for introductions elsewhere. The growing problem of invasive species introductions brings considerable economic and biological costs5,6. If these costs are to be mitigated, a greater understanding of the causes, progression and consequences of biological invasions is needed7.

/pdf/genetic-variation-increases-during-biological-invasion-by-a-37rl4o29f0.pdf

Genetic variation increases during biological invasion by a Cuban lizard

Identification of general properties of evolutionary radiations has been hindered by the lack of a general statistical and phylogenetic approach applicable across diverse taxa. We present a comparative analytical framework for examining phylogenetic patterns of diversification and morphological disparity with data from four iguanian-lizard taxa that exhibit substantially different patterns of evolution. Taxa whose diversification occurred disproportionately early in their evolutionary history partition more of their morphological disparity among, rather than within, subclades. This inverse relationship between timing of diversification and morphological disparity within subclades may be a general feature that transcends the historically contingent properties of different evolutionary radiations.

http://science.sciencemag.org/content/sci/301/5635/961.full.pdf

Tempo and Mode of Evolutionary Radiation in Iguanian Lizards

Egg size is a widely-studied trait and yet the causes and consequences of variation in this trait remain poorly understood. Egg size varies greatly within many avian species, with the largest egg in a population generally being at least 50% bigger, and sometimes twice as large, as the smallest. Generally, approximately 70% of the variation in egg mass is due to variation between rather than within clutches, although there are some cases of extreme intra-clutch egg-size variation. Despite the large amount of variation in egg size between females, this trait is highly consistent within individuals between breeding attempts; the repeatability of egg size is generally above 0.6 and tends to be higher than that of clutch size or laying date. Heritability estimates also tend to be much higher for egg size (> 0.5) than for clutch size or laying date (< 0.5). As expected, given the high repeatability and heritability of egg size, supplemental food had no statistically significant effect on this trait in 18 out of 28 (64%) studies. Where dietary supplements do increase egg size, the effect is never more than 13% of the control values and is generally much less. Similarly, ambient temperature during egg formation generally explains less than 15% of the variation in egg size. In short, egg size appears to be a characteristic of individual females, and yet the traits of a female that determine egg size are not clear. Although egg size often increases with female age (17 out of 37 studies), the change in egg size is generally less than 10%. Female mass and size rarely explain more than 20% of the variation in egg size within species. A female's egg size is not consistently related to other aspects of reproductive performance such as clutch size, laying date, or the pair's ability to rear young. Physiological characteristics of the female (e.g. endogenous protein stores, oviduct mass, rate of protein uptake by ovarian follicles) show more promise as potential determinants of egg size. With regards to the consequences of egg-size variation for offspring fitness, egg size is often correlated with offspring mass and size within the first week after hatching, but the evidence for more long-lasting effects on chick growth and survival is equivocal. In other oviparous vertebrates, the magnitude of egg-size variation within populations is often as great or greater than that observed within avian populations. Although there are much fewer estimates of the repeatability of egg size in other taxa, the available evidence suggests that egg size may be more flexible within individuals. Furthermore, in non-avian species (particularly fish and turtles), it is more common for female mass or size to explain a substantial proportion of the variation in egg size. Further research into the physiological basis of egg-size variation is needed to shed light on both the proximate and ultimate causes of intraspecific variation in this trait in birds.

/pdf/avian-egg-size-variation-within-species-and-inflexibility-8qzh45viyq.pdf

Avian egg size: variation within species and inflexibility within individuals

Aim My goals here are to (1) assess the generality of the island rule – the graded trend from gigantism in small species to dwarfism in larger species – for mammals and other terrestrial vertebrates on islands and island-like ecosystems; (2) explore some related patterns of body size variation in insular vertebrates, in particular variation in body size as a function of island area and isolation; (3) offer causal explanations for these patterns; and (4) identify promising areas for future studies on body size evolution in insular vertebrates.

Location  Oceanic and near-shore archipelagos, and island-like ecosystems world-wide.

Methods  Body size measurements of insular vertebrates (non-volant mammals, bats, birds, snakes and turtles) were obtained from the literature, and then regression analyses were conducted to test whether body size of insular populations varies as a function of body size of the species on the mainland (the island rule) and with characteristics of the islands (i.e. island isolation and area).

Results  The island rule appears to be a general phenomenon both with mammalian orders (and to some degree within families and particular subfamilies) as well as across the species groups studied, including non-volant mammals, bats, passerine birds, snakes and turtles. In addition, body size of numerous species in these classes of vertebrates varies significantly with island isolation and island area.

Main conclusions  The patterns observed here – the island rule and the tendency for body size among populations of particular species to vary with characteristics of the islands – are actually distinct and scale-dependent phenomena. Patterns within archipelagos reflect the influence of island isolation and area on selective pressures (immigration filters, resource limitation, and intra- and interspecific interactions) within particular species. These patterns contribute to variation about the general trend referred to as the island rule, not the signal for that more general, large-scale pattern. The island rule itself is an emergent pattern resulting from a combination of selective forces whose importance and influence on insular populations vary in a predictable manner along a gradient from relatively small to large species. As a result, body size of insular species tends to converge on a size that is optimal, or fundamental, for a particular bau plan and ecological strategy.

Body size evolution in insular vertebrates: generality of the island rule

Body temperature (T(b)) profoundly affects the fitness of ectotherms. Many ectotherms use behavior to control T(b) within narrow levels. These temperatures are assumed to be optimal and therefore to match body temperatures (Trmax) that maximize fitness (r). We develop an optimality model and find that optimal body temperature (T(o)) should not be centered at Trmax but shifted to a lower temperature. This finding seems paradoxical but results from two considerations relating to Jensen's inequality, which deals with how variance and skew influence integrals of nonlinear functions. First, ectotherms are not perfect thermoregulators and so experience a range of T(b). Second, temperature-fitness curves are asymmetric, such that a T(b) higher than Trmax depresses fitness more than will a T(b) displaced an equivalent amount below Trmax. Our model makes several predictions. The magnitude of the optimal shift (Trmax - To) should increase with the degree of asymmetry of temperature-fitness curves and with T(b) variance. Deviations should be relatively large for thermal specialists but insensitive to whether fitness increases with Trmax ("hotter is better"). Asymmetric (left-skewed) T(b) distributions reduce the magnitude of the optimal shift but do not eliminate it. Comparative data (insects, lizards) support key predictions. Thus, "suboptimal" is optimal.

https://faculty.washington.edu/hueyrb/pdfs/MartinHuey08.pdf

Why “Suboptimal” Is Optimal: Jensen’s Inequality and Ectotherm Thermal Preferences

Ecology and Natural History of Desert Lizards: Analyses of the Ecological Niche and Community Structure, Eric R. Pianka. Princeton University Press, Princeton, New Jersey (1986), x, +208. Price $45.00 hardback, $19.95 paperback

The green anole (Anolis carolinensis) occurs throughout the southeastern United States and its distribution defines the northern limit of the range of this genus of neotropical lizards. The variation in female, egg, and hatchling sizes is quantified for eight populations of the green anole that range from south Florida to the mountains of east Tennessee. Six populations are composed of the typical red-throated (dewlap) morph of this species, one of the gray-throated morph, and one population is representative of a variant of the red-throated morph. Among populations of the red-throated morph, female, egg, and hatchling sizes all increased significantly with increasing latitude. Additionally, females in the north had disproportionately larger eggs and offspring than did females to the south. We propose that increased selection pressure for large hatchling body size associated with decreasing winter temperatures (i.e., in- creasing latitude) results in the observed latitudinal diversification in the sizes of green anole eggs, juveniles, and adults. Whereas egg size was largely independent of female body size in southern popu- lations, there was a significant positive relationship between egg and female sizes in northern populations. The relationships between egg width and pelvic aperture width, relative to female body size, were examined for lizards from three populations that varied in the degree to which egg size was related to female body size. The size of the pelvic girdle aperture appeared to constrain the egg size of northern females to a greater extent than it did in southern females. Taken together, these data suggest that egg size may be optimized by natural selection in southern populations, but not in northern populations. Optimization of egg size may not be possible for lizards in northern populations due to this apparent morphological constraint on egg size, which is consistent with predictions of the pelvic constraint model. The environ- mental, physiological, and morphological factors that may combine to limit the continued northward range expansion of this successful colonizer are discussed. The genus Anolis is an ecologically and mor- phologically diverse group of approximately 300 species and subspecies of neotropical lizards in the family Iguanidae. Most species of Anolis are arboreal, but several are either saxicolous, ter- restrial, or semi-aquatic. Maximum adult female body size may be as little as 2 g in some species to over 50 g in one Puerto Rican anole (Andrews and Rand, 1974). Whereas species of Anolis have diverged in body size and habitat preference,

Geographic Variation in the Life History of the Lizard Anolis carolinensis and Support for the Pelvic Constraint Model

Since its introduction, Anolis sagrei (Sauria: Polychrotidae) has been replacing native A. carolinensis in Florida and native A. conspersus in Grand Cayman Island as the common anole of urban environments and other open habitats. To assess the likelihood that predation of juvenile native anoles by A. sagrei adults is an important interaction in this process, the propensities for intraguild predation and cannibalism were assessed for A. sagrei and A. carolinensis in Florida and for A. sagrei and A. conspersus in Grand Cayman. Predation experiments were conducted in cages, using freshly captured lizards, in which adult males of each species were presented with conspecific and heterospecific juveniles. Adult A. sagrei were (1) significantly more likely to eat juveniles than were adult A. carolinensis or A. conspersus, and (2) significantly more likely to eat heterospecific than conspecific juveniles, whereas adult A. carolinensis and A. conspersus were not. Thus, the propensity for intraguild predation is asymmetrical in favor of introduced A. sagrei in Florida and Grand Cayman. Further study is needed, however, to determine the importance of intraguild predation under field conditions.

Evidence for asymmetrical intraguild predation between native and introduced Anolis lizards.

Introduced species usually fail to establish, but when they succeed, may undergo character release and rapid evolutionary divergence in novel environments. We collected brown anoles (Anolis sagrei: Lacertilia: Iguanidae) from a single Florida population and released them onto two ecologically different dredge-spoil islands in central Florida (forested and non-forested) and measured differences in population growth, individual growth, body size, and condition over four years. The population on the non-forested island expanded twice as fast as the forested island population and reached a density of ca. 12,000 lizards ha−1 and a biomass of ca. 43.3 kg ha−1, among the largest values recorded for non-aggregated terrestrial vertebrates. First-year progeny grew larger than their surviving parents on both islands, indicating character release occurred in early stages of both invasions. However, in subsequent years, lizards became larger on the forested island, but smaller on the non-forested island. Body condition declined over time on both islands, but the effect was most dramatic on the non-forested island. Lizards on the forested island had the lowest survival rates and highest tail autotomy frequencies. These results were attributed to differences in abiotic and biotic conditions on the two islands. Brown anoles are generally larger on islands where they have been introduced than on their native Caribbean islands, and are much larger on mainlands than on islands, indicating character release occurred at larger geographic scales as well. Habitat influences the morphology of introduced species possessing the ability to rapidly adapt to local conditions, presenting invasive species managers with ‘moving targets’.

Introduced species as moving targets: changes in body sizes of introduced lizards following experimental introductions and historical invasions

The literature on Eastern Box Turtles (Terrapene carolina carolina: Emydidae) often characterizes them as terrestrial with only anecdotal references to their aquatic habits. The rapid decline in many populations of box turtles suggests an urgent need to assess all aspects of their habitat requirements. In this study, trailing devices and radio transmitters were used to determine seasonal movements and the extent to which turtles used aquatic habitats within their home ranges. Home range sizes averaged 1.88 ± 0.49 ha via minimum convex polygon analysis, and 2.26 ± 0.76 ha via 95% fixed kernel analysis. Kernel estimators were more effective at representing uneven home range use, often demonstrating the extensive use of water. A total of 131 turtles were found in two small temporary ponds from June through August. Individuals remained in the ponds for up to 23 consecutive days. As many as 32 turtles were found at one time in one of these ponds. Turtles often extended their home ranges by means of ab...

Arthur C. Echternacht

Papers

Ecology and Natural History of Desert Lizards: Analyses of the Ecological Niche and Community Structure, Eric R. Pianka. Princeton University Press, Princeton, New Jersey (1986), x, +208. Price $45.00 hardback, $19.95 paperback

Geographic Variation in the Life History of the Lizard Anolis carolinensis and Support for the Pelvic Constraint Model

Evidence for asymmetrical intraguild predation between native and introduced Anolis lizards.

Introduced species as moving targets: changes in body sizes of introduced lizards following experimental introductions and historical invasions

Aquatic Habitat Use Relative to Home Range and Seasonal Movement of Eastern Box Turtles (Terrapene carolina carolina: Emydidae) in Eastern Tennessee