Size evolution in island lizards

Aim  The island rule, small animal gigantism and large animal dwarfism on islands, is a topic of much recent debate. While size evolution of insular lizards has been widely studied, whether or not they follow the island rule has never been investigated. I examined whether lizards show patterns consistent with the island rule.
Location  Islands worldwide.
Methods  I used literature data on the sizes of island–mainland population pairs in 59 species of lizards, spanning the entire size range of the group, and tested whether small insular lizards are larger than their mainland conspecifics and large insular lizards are smaller. I examined the influence of island area, island isolation, and dietary preferences on lizard size evolution.
Results  Using mean snout–vent length as an index of body size, I found that small lizards on islands become smaller than their mainland conspecifics, while large ones become larger still, opposite to predictions of the island rule. This was especially strong in carnivorous lizards; omnivorous and herbivorous species showed a pattern consistent with the island rule but this result was not statistically significant. No trends consistent with the island rule were found when maximum snout–vent length was used. Island area had, at best, a weak effect on body size. Using maximum snout–vent length as an index of body size resulted in most lizard populations appearing to be dwarfed on islands, but no such pattern was revealed when mean snout–vent length was used as a size index.
Main conclusions  I suggest that lizard body size is mostly influenced by resource availability, with large size allowing some lizard populations to exploit resources that are unavailable on the mainland. Lizards do not follow the island rule. Maximum snout–vent length may be biased by sampling effort, which should be taken into account when one uses this size index.     

Hematology and serum chemistry of the island spotted skunk on Santa Cruz Island

We determined serum biochemistry and hematologic values for island spotted skunks (Spilogale gracilis amphiala) on Santa Cruz Island (California, USA). Samples were collected from island spotted skunks chemically restrained with ketamine hydrochloride and acepromazine in August 1999 (dry season) and from skunks manually restrained in August 2000 (dry season) and January 2001 (wet season). One parameter, glucose, significantly differed with season, with higher levels during the wet season. Serum chemistry and hematologic profiles suggest that method of restraint (manual or chemical), as well as other methodologic details, may influence blood characteristics in the island spotted skunk.     

Ectoparasites of the island fox on Santa Cruz Island

The ectoparasite fauna for island foxes (Urocyon littoralis) on Santa Cruz Island (California, USA) in April (wet season) and September (dry season) 1998 was evaluated. Three taxa of ectoparasites were identified. These were fleas (Pulex irritans), lice (Neotrichodectes mephitidis), and ticks (Ixodes pacificus). Ectoparasite abundances varied seasonally. Typical of insular endemic species, island foxes may be especially vulnerable to the introduction of novel disease organisms and their vectors.     

Size changes in island plants: independent trait evolution in Alyxia ruscifolia (Apocynaceae) on Lord Howe Island

Species that are endemic to isolated islands often differ dramatically in size from their mainland relatives, for reasons that are poorly understood. While decades of research have sought to better understand insular size changes in animals, far fewer studies have investigated insular size changes in plants. Here, I test for changes in plant stature, seed size and leaf area in a woody shrub (Alyxia ruscifolia, Apocynaceae), which inhabits both the continent of Australia, and Lord Howe Island, a subtropical island located 600 km off Australia's east coast. Results showed that island plants became reproductively mature at earlier stages of ontogeny than mainland plants, and that mature plants were taller on the mainland, providing a rare example of dwarfism in plants. Conversely, island plants produced larger seeds, which might make them more competitive as seedlings. Seeds produced by island plants were also less circular and more oblong in shape than their mainland counterparts, perhaps to facilitate their dispersal by avian frugivores with limited gape sizes. Lastly, island and mainland plants had similar average leaf sizes. However, juvenile plants on the mainland produced smaller, more needle‐shaped leaves with larger terminal spines relative to adult plants, which may help protect them against large, ground‐dwelling herbivores. On the other hand, island plants showed weaker ontogenetic shifts in leaf morphology in the absence of large herbivores. When interpreted jointly, results indicate that stature, seed size and leaf area are on separate evolutionary trajectories in A. ruscifolia, which appear to be determined by a complex suite of disparate selection pressures between Lord Howe Island and the mainland.     

Body size of insular carnivores: little support for the island rule

Large mammals are thought to evolve to be smaller on islands, whereas small mammals grow larger. A negative correlation between relative size of island individuals and body mass is termed the "island rule." Several mechanisms--mainly competitive release, resource limitation, dispersal ability, and lighter predation pressure on islands, as well as a general physiological advantage of modal size--have been advanced to explain this pattern. We measured skulls and teeth of terrestrial members of the order Carnivora in order to analyze patterns of body size evolution between insular populations and their near mainland conspecifics. No correlations were found between the size ratios of insular/mainland carnivore species and body mass. Only little support for the island rule is found when individual populations rather than species are considered. Our data are at odds with those advanced in support of theories of optimal body size. Carnivore size is subjected to a host of selective pressures that do not vary uniformly from place to place. Mass alone cannot account for the patterns in body size of insular carnivores.     

Sexual size dimorphism in island plants: the niche variation hypothesis and insular size changes

The niche variation hypothesis predicts insular populations exhibit increased sexual size dimorphism (SSD), to minimize intraspecific competition. Although many animal taxa conform to this prediction, insular patterns of SSD have yet to be investigated in plants. Here, we tested for differences in SSD of dioecious plants that colonised four island groups (Kermadec, Three Kings, Chatham and Auckland Islands) from New Zealand. Using herbarium collections, we quantified leaf and stem sizes of 263 individuals from 28 dioecious taxa. We developed a novel analytical technique to explore changes in the direction of SSD on islands. Lastly, we tested for evolutionary size changes of male and female plants on islands. The degree of SSD did not vary predictably between insular and mainland taxa, contrary to predictions of the niche variation hypothesis. Furthermore, the direction of SSD was not predictable on islands, while it was consistently female biased on the mainland. Our results suggest that selection favours increased size of both sexes on islands and that SSD is unpredictable for insular plants.     

Parasite island syndromes in the context of nidicolous ectoparasites: Fleas (Insecta: Siphonaptera) in wild passerine birds from Azores Archipelago

Island syndrome, previously established for isolation process of insular vertebrates' populations, have been adapted to insular parasites communities, termed parasite island syndromes. In this work, were studied for the first time the insular syndromes for nidicolous ectoparasites of the bird species, Turdus merula, Sylvia atricapilla, Fringilla coelebs and Erithacus rubecula from Azores and the mainland Portugal. Flea species were only recorded on Azorean birds, namely Dasypsyllus gallinulae and Ctenocephalides felis felis, known as not host-specific parasites. In the absence of shared flea species between mainland and islands birds, a comparison among our fleas prevalence to Azores Islands and mainland fleas prevalence, recorded to others European studies, showed that Azorean host populations undergo higher prevalence than the mainland one. This result was consistent with parasite island syndromes predictions recorded to ectoparasites, hippoboscid flies and chewing lice, that fleas have higher prevalence on the Azores Islands compared to mainland Portugal. However, our results provide a new perspective to parasite island syndromes assumptions, namely in the context of nidicolous ectoparasites that spend only brief periods on the hosts' body.     

Insular shifts in body size of rice frogs in the Zhoushan Archipelago, China

1. Differences in body size between mainland and island populations have been reported for reptiles, birds and mammals. Despite widespread recognition of insular shifts in body size in these taxa, there have been no reports of such body size shifts in amphibians. 2. We provide the first evidence of an insular shift in body size for an amphibian species, the rice frog Rana limnocharis. We found significant increases in body size of rice frogs on most sampled islands in the Zhoushan archipelago when compared with neighbouring mainland China. 3. Large body size in rice frogs on islands was significantly related to increased population density, in both breeding and non-breeding seasons. Increases in rice frog density were significantly related to higher resource availability on islands. Increased resource availability on islands has led to higher carrying capacities, which has subsequently facilitated higher densities and individual growth rates, resulting in larger body size in rice frogs. We also suggest that large body size has evolved on islands, as larger individuals are competitively superior under conditions of harsh intraspecific competition at high densities. 4. Increases in body size in rice frogs were not related to several factors that have been implicated previously in insular shifts in body size in other taxa. We found no significant relationships between body size of rice frogs and prey size, number of larger or smaller frog species, island area or distance of islands from the mainland. 5. Our findings contribute to the formation of a broad, repeatable ecological generality for insular shifts in body size across a range of terrestrial vertebrate taxa, and provide support for recent theoretical work concerning the importance of resource availability for insular shifts in body size.     

Convergence in reduced body size,head size,and blood glucose in three island reptiles

Many oceanic islands harbor diverse species that differ markedly from their mainland relatives with respect to morphology, behavior, and physiology. A particularly common morphological change exhibited by a wide range of species on islands worldwide involves either a reduction in body size, termed island dwarfism, or an increase in body size, termed island gigantism. While numerous instances of dwarfism and gigantism have been well documented, documentation of other morphological changes on islands remains limited. Furthermore, we lack a basic understanding of the physiological mechanisms that underlie these changes, and whether they are convergent. A major hypothesis for the repeated evolution of dwarfism posits selection for smaller, more efficient body sizes in the context of low resource availability. Under this hypothesis, we would expect the physiological mechanisms known to be downregulated in model organisms exhibiting small body sizes due to dietary restriction or artificial selection would also be downregulated in wild species exhibiting dwarfism on islands. We measured body size, relative head size, and circulating blood glucose in three species of reptiles—two snakes and one lizard—in the California Channel Islands relative to mainland populations. Collating data from 6 years of study, we found that relative to mainland population the island populations had smaller body size (i.e., island dwarfism), smaller head sizes relative to body size, and lower levels of blood glucose, although with some variation by sex and year. These findings suggest that the island populations of these three species have independently evolved convergent physiological changes (lower glucose set point) corresponding to convergent changes in morphology that are consistent with a scenario of reduced resource availability and/or changes in prey size on the islands. This provides a powerful system to further investigate ecological, physiological, and genetic variables to elucidate the mechanisms underlying convergent changes in life history on islands.     

Herbivory and the evolution of divaricate plants: Structural defences lost on an offshore island

Many island plants are characterized by unique morphology. For example, the high branching angles and small leaves of divaricate plants are a common feature of the New Zealand flora. The divaricate growth form may be an adaptation to deter browsing by extinct avian herbivores (moa); alternatively aspects of the insular climate may be responsible. However, our understanding of the selective pressures responsible for the high branching angles and small leaves of divaricate plants is incomplete. Here, I tested for differences in traits associated with the divaricate growth form between plants from Chatham Island and the New Zealand mainland. Moa never reached the Chatham Islands and its flora is derived from plants on mainland New Zealand. Therefore, I predicted Chatham Island plants to have lost morphological adaptations that may have deterred moa herbivory. Traits were quantified on 316 individuals in the field, allowing for 12 island‐mainland taxonomic comparisons. Chatham Island plants consistently produced smaller branching angles, larger leaves, shorter internodes and larger stems than related mainland plants. Results are therefore consistent with the hypothesis that selection for small leaves and high angled branching may be relaxed on the Chatham Islands due to an absence of moa. Smaller branching angles and larger leaves may offer a competitive advantage to Chatham Island plants.     

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

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.     

Human activity reduces niche partitioning among three widespread mesocarnivores

Anthropogenic disturbances can constrain the realized niche space of wildlife by inducing avoidance behaviors and altering community dynamics. Human activity might contribute to reduced partitioning of niche space by carnivores that consume similar resources, both by promoting tolerant species while also altering behavior of species (e.g. activity patterns). We investigated the influence of anthropogenic disturbance on habitat and dietary niche breadth and overlap among competing carnivores, and explored if altered resource partitioning could be explained by human‐induced activity shifts. To describe the diets of coyotes, bobcat, and gray foxes, we designed a citizen science program to collect carnivore scat samples in low‐ (‘wildland’) and high‐ (‘interface’) human‐use open space preserves, and obtained diet estimates using a DNA metabarcoding approach. Habitat use was determined at scat locations. We found that coyotes expanded habitat and dietary niche breadth in interface preserves, whereas bobcats and foxes narrowed both niche breadth measures. High human use was related to increased dietary niche overlap among all mesocarnivore pairs, increased coyote habitat overlap with bobcats and foxes, and a small reduction in habitat overlap between bobcats and foxes. The strongest increase in diet overlap was among coyotes and foxes, which was smaller in magnitude than their habitat overlap increase. Finally, coyote scats were more likely to contain nocturnal prey in interface preserves, whereas foxes appeared to reduce consumption of nocturnal prey. Our results suggest that dominant and generalist mesocarnivores may encroach on the niche space of subordinate mesocarnivores in areas with high human activity, and that patterns in resource use may be related to human‐induced activity shifts.     

Ecology and reproductive patterns of the agamid lizard Japalura swinhonis on an east Asian island, with comments on the small clutch sizes of island lizards

I describe the habitat use, diet, and the male and female reproductive cycles of Japalura swinhonis, an oviparous agamid lizard inhabiting Orchid Island, a tropical island off the southeastern coast of Taiwan. Ninety percent of lizards (n=126) were observed on tree trunks or at the forest edge. The diet of J. swinhonis on Orchid Island consisted mostly of hymenopterans (53.33%) and orthopterans (16.67%). The mean snout-vent length (SVL) of adult males was 74.58 (n=89) and that of females was 69.31 (n=37) mm. Females exhibited a long vitellogenic period from November to February, with parturition occurring from March to October. The onset of vitellogenesis did not correlate with the mass of the female fat bodies. Females produced two to five eggs per clutch, and clutch size was not correlated with SVL. Two clutches were recorded during a single year in some individuals. Clutch size in J. swinhonis was compared with that in other Japalura species. Clutch sizes of Japalura species are larger in mainland China than on insular Taiwan. Clutch size is also mainly affected by environmental constraints, and smaller clutch sizes are probably affected by predators on Orchid Island.     

Biogeographical kinetics on mainland and island volcanoes

Aim To compare post‐eruption biotic recolonization times on mainland and island volcanoes. Location The research involved the study of the recolonization kinetics of Mt Vesuvius (a mainland volcano in southern Italy) and the Island of Vulcano (southern Italy). Comparisons were also made with Jorullo Volcano (Mexico) and Mount St Helens (USA) (two mainland volcanoes) and with Krakatau (Indonesia) (an island volcano). Methods Island volcanoes are expected to possess inherently impoverished faunas and floras, and recolonization after eruption is expected to occur to a low level. In comparison, the recolonization kinetics for a mainland volcano should be characterized by a higher plateau of species, and by species with low dispersal ability. To test this model, recolonization times after the small‐scale Plinian eruption of 1631 were calculated for various insect groups of Mt Vesuvius and compared with recolonization times calculated for the Island of Vulcano, which erupted dramatically in 1888. For this purpose, thorough insect checklists, based on exhaustive samplings, were extracted from the literature. Results obtained from Mt Vesuvius and Vulcano were also compared with recolonization times calculated for biotas of other mainland and island volcanoes. Results Insect recolonization times from the 1631 Vesuvius eruption varied according to the ecology of the animal group considered and appeared very long when compared with those obtained for the island volcano of Vulcano. Results obtained from Vulcano also suggest the possibility that this island hosts more species than expected at equilibrium, a state of affairs also found for the butterflies of Krakatau. Main conclusions In keeping with the predictions of multi‐phase models developed in island biogeography theory, island volcanoes have a lower species richness at equilibrium than do mainland volcanoes, but might host, in a first phase of recolonization, more species than expected at equilibrium, because ecological space is unsaturated and inter‐specific interactions are limited. The lower isolation of mainland volcanoes (allowing higher immigration rates) leads to higher rates and longer periods of recolonization and hence to higher species richness at equilibrium.     

Slaying dragons: limited evidence for unusual body size evolution on islands

Aim Island taxa often attain forms outside the range achieved by mainland relatives. Body size evolution of vertebrates on islands has therefore received much attention, with two seemingly conflicting patterns thought to prevail: (1) islands harbour animals of extreme size, and (2) islands promote evolution towards medium body size (‘the island rule’). We test both hypotheses using body size distributions of mammal, lizard and bird species. Location World‐wide. Methods We assembled body size and insularity datasets for the world’s lizards, birds and mammals. We compared the frequencies with which the largest or smallest member of a group is insular with the frequencies expected if insularity is randomly assigned within groups. We tested whether size extremes on islands considered across mammalian phylogeny depart from a null expectation under a Brownian motion model. We tested the island rule by comparing insular and mainland members of (1) a taxonomic level and (2) mammalian sister species, to determine if large insular animals tend to evolve smaller body sizes while small ones evolve larger sizes. Results The smallest species in a taxon (order, family or genus) are insular no more often than would be expected by chance in all groups. The largest species within lizard families and bird genera (but no other taxonomic levels) are insular more often than expected. The incidence of extreme sizes in insular mammals never departs from the null, except among extant genera, where gigantism is marginally less common than expected under a Brownian motion null. Mammals follow the island rule at the genus level and when comparing sister species and clades. This appears to be driven mainly by insular dwarfing in large‐bodied lineages. A similar pattern in birds is apparent for species within orders. However, lizards follow the converse pattern. Main conclusions The popular misconception that islands have more than their fair share of size extremes may stem from a greater tendency to notice gigantism and dwarfism when they occur on islands. There is compelling evidence for insular dwarfing in large mammals, but not in other taxa, and little evidence for the second component of the island rule – gigantism in small‐bodied taxa.     

The blue lizard spandrel and the island syndrome

Background  

Many small vertebrates on islands grow larger, mature later, lay smaller clutches/litters, and are less sexually dimorphic and aggressive than their mainland relatives. This set of observations is referred to as the 'Island Syndrome'. The syndrome is linked to high population density on islands. We predicted that when population density is low and/or fluctuating insular vertebrates may evolve correlated trait shifts running opposite to the Island Syndrome, which we collectively refer to as the 'reversed island syndrome' (RIS) hypothesis. On the proximate level, we hypothesized that RIS is caused by increased activity levels in melanocortin receptors. Melanocortins are postranslational products of the proopiomelanocortin gene, which controls pleiotropically pigmentation, aggressiveness, sexual activity, and food intake in vertebrates.     

Factors determining the number of land bird species on Islands around South-Western Australia

Summary New evidence from the passerine faunas of islands off Southwestern Australia agrees with the hypothesis that the passerine faunas of Australian and New Zealand islands are impoverished because most passerine species are poor colonizers. Dispersal of landbirds onto Carnac Island near Perth was infrequent, and many of those species that arrived were represented by single birds. Comparison of similarly structured island and mainland habitats showed that island habitats still have fewer passerine bird species than mainland habitats. Island bird faunas are more stable over short periods of time than over long periods; this is contrary to island avifaunas in the Northern Hemisphere.The following features typify the avifaunas of Australian islands: immigration of species of land birds occurs infrequently; (natural) extinction is rare; and the degree of saturation of the avifaunas is low. Without more direct evidence, competitive interactions should not be invoked to account for the species poverty of these insular avifaunas.     

Hematology and serum chemistry of the island fox on Santa Cruz Island

Serum and hematologic biochemistry values for island foxes (Urocyon littoralis) on Santa Cruz Island (California, USA) in April (wet season) and September (dry season) 1998 were evaluated. Serum chemistry of island foxes generally varied seasonally; 10 (40%) of the 25 serum characteristics were higher in the wet season, and three (12%) of the 25 serum characteristics were higher in the dry season. No hematologic parameters varied between seasons, although some measures varied between sexes. Blood analytes also varied with age; fox pups had higher values than adults for one hematologic and four serum parameters, whereas adult foxes had higher values for five hematologic characteristics. The information on blood chemistry provides baseline data useful in the monitoring of this threatened insular endemic carnivore.     

Acoustic structure of songs in island populations of the Japanese bush warbler, Cettia diphone, in relation to sexual selection

Bird songs in island populations have often been reported to be simplified, in that island birds have a smaller number of song types and song-element types compared to mainland birds. However, there is less information on the characteristics of acoustic structure in island songs. I investigated song structure of one mainland and three island populations of Japanese bush warblers, Cettia diphone, and found that island songs had an acoustically simple structure. The frequency-modulated (FM) portions of the songs were shorter and had fewer frequency inflections in the insular populations than in the mainland population, while the number of FM notes, the frequency range of these notes, and the song repertoire sizes of males did not differ between the islands and the mainland. I also investigated whether the song complexity is related to sexual selection pressure using the degree of sexual size dimorphism as a proxy for the latter. The degree of dimorphism in body mass was larger on the mainland. Thus, weakened sexual selection on islands is a possible factor in the formation of simple songs. Further studies related to male–male competition and female choice on islands are required.