Philosophical justifications for the extirpation of non-indigenous species: the case of the grasshopper Schistocerca nitens (Orthoptera) on the Island of Nihoa, Hawaii

The case of Schistocerca nitens’ establishment on the island of Nihoa presents a challenging case for conservation biologists with respect to the justification for human efforts to extirpate the insect. In justifying our actions we seek reasons that are ecologically plausible (i.e., consistent with empirical and theoretical understandings of science), ethically compelling (i.e., based on sound reasoning from well-established moral principles), and logically consistent (i.e., avoiding fallacies and contradictions). Our analysis shows that the conventional arguments for conservation programs do not meet these criteria in the case of S. nitens. The following reasons fail on the basis of ecological, ethical, or logical standards: the protection of biodiversity, avoidance of ecological harm, biological qualities of the invasive species (herbivory, fecundity, mobility, recency, and functional integration), anthropogenic basis of arrival, harmfulness or unnaturalness of human agency, the interference with ecological processes, or disturbance of equilibria. Rather, we suggest that an aesthetic argument provides an ecologically, ethically, and logically sound basis for conservation biologists to justify taking action against the grasshopper. The aesthetic concept of a “thick sense” of beauty gives rise to a compelling moral case for extirpation based on virtue ethics in a manner similar to the argument against tolerating roadside litter.     

Long-term population trends of endangered Hawaiian waterbirds

We analyzed long-term winter survey data (1956–2007) for three endangered waterbirds endemic to the Hawaiian Islands, the Hawaiian moorhen (Gallinula chloropus sandvicensis), Hawaiian coot (Fulica alai), and Hawaiian stilt (Himantopus mexicanus knudseni). Time series were analyzed by species–island combinations using generalized additive models, with alternative models compared using Akaike information criterion (AIC). The best model included three smoothers, one for each species. Our analyses show that all three of the endangered Hawaiian waterbirds have increased in population size over the past three decades. The Hawaiian moorhen increase has been slower in more recent years than earlier in the survey period, but Hawaiian coot and stilt numbers still exhibit steep increases. The patterns of population size increase also varied by island, although this effect was less influential than that between species. In contrast to earlier studies, we found no evidence that rainfall affects counts of the target species. Significant population increases were found on islands where most wetland protection has occurred (Oahu, Kauai), while weak or no increases were found on islands with few wetlands or less protection (Hawaii, Maui). Increased protection and management, especially on Maui where potential is greatest, would likely result in continued population gains, increasing the potential for meeting population recovery goals.     

Different prey resources suggest little competition between non-native frogs and insectivorous birds despite isotopic niche overlap

Non-native amphibians often compete with native amphibians in their introduced range, but their competitive effects on other vertebrates are less well known. The Puerto Rican coqui frog (Eleutherodactylus coqui) has colonized the island of Hawaii, and has been hypothesized to compete with insectivorous birds and bats. To address if the coqui could compete with these vertebrates, we used stable isotope analyses to compare the trophic position and isotopic niche overlap between the coqui, three insectivorous bird species, and the Hawaiian hoary bat. Coquis shared similar trophic position to Hawaii amakihi, Japanese white-eye, and red-billed leiothrix. Coquis were about 3 ‰ less enriched in δ¹⁵N than the Hawaiian hoary bat, suggesting the bats feed at a higher trophic level than coquis. Analyses of potential diet sources between coquis and each of the three bird species indicate that there was more dietary overlap between bird species than any of the birds and the coqui. Results suggest that Acari, Amphipoda, and Blattodea made up >90% of coqui diet, while Araneae made up only 2% of coqui diet, but approximately 25% of amakihi and white-eye diet. The three bird species shared similar proportions of Lepidoptera larvae, which were ~25% of their diet. Results suggest that coquis share few food resources with insectivorous birds, but occupy a similar trophic position, which could indicate weak competition. However, resource competition may not be the only way coquis impact insectivorous birds, and future research should examine whether coqui invasions are associated with changes in bird abundance.     

Hawaiian biogeography and the islands' freshwater fish fauna

Aim This paper describes known patterns in the distributions and relationships of Hawaiian freshwater fishes, and compares these patterns with those exhibited by Hawaii's terrestrial biota. Location The study is based in Hawaii, and seeks patterns across the tropical and subtropical Indo‐west Pacific. Methods The study is based primarily on literature analysis. Results The Hawaiian freshwater fish fauna comprises five species of goby in five different genera (Gobiidae). Four species are Hawaiian endemics, the fifth shared with islands in the western tropical Pacific Ocean. All genera are represented widely across the Indo‐west Pacific. All five species are present on all of the major Hawaiian islands. All five species are amphidromous – their larval and early juvenile life being spent in the sea. Although there has been some local phyletic evolution to produce Hawaiian endemics, there has been no local radiation to produce single‐island endemics across the archipelago. Nor is there evidence for genetic structuring among populations in the various islands. Main conclusions In this regard, the freshwater fish fauna of Hawaii differs from the well‐known patterns of local evolution and radiation in Hawaiian Island terrestrial taxa. Amphidromy probably explains the biogeographical idiosyncrasies of the fish fauna – dispersal through the sea initially brought the fish species to Hawaii, and gene flow among populations, across the archipelago, has hitherto inhibited the evolution of local island endemics, apparently even retarding genetic structuring on individual islands.     

Small-scale genetic structure in an endangered wetland specialist: possible effects of landscape change and population recovery

The effects of anthropogenic landscape change on genetic population structure are well studied, but the temporal and spatial scales at which genetic structure can develop, especially in taxa with high dispersal capabilities like birds, are less well understood. We investigated population structure in the Hawaiian gallinule (Gallinula galeata sandvicensis), an endangered wetland specialist bird on the island of O`ahu (Hawai`i, USA). Hawaiian gallinules have experienced a gradual population recovery from near extinction in the 1950s, and have recolonized wetlands on O`ahu in the context of a rapidly urbanizing landscape. We genotyped 152 Hawaiian gallinules at 12 microsatellite loci and sequenced a 520 base-pair fragment of the ND2 region of mitochondrial DNA (mtDNA) from individuals captured at 13 wetland locations on O`ahu in 2014–2016. We observed moderate to high genetic structuring (overall microsatellite F_ST?=?0.098, mtDNA F_ST?=?0.248) among populations of Hawaiian gallinules occupying wetlands at very close geographic proximity (e.g., 1.5–55 km). Asymmetry in gene flow estimates suggests that Hawaiian gallinules may have persisted in 2–3 strongholds which served as source populations that recolonized more recently restored habitats currently supporting large numbers of birds. Our results highlight that genetic structure can develop in taxa that are expanding their range after severe population decline, and that biologically significant structuring can occur over small geographic distances, even in avian taxa.     

Differential parasitism of seed-feeding Cydia (Lepidoptera: Tortricidae) by native and alien wasp species relative to elevation in subalpine Sophora (Fabaceae) forests on Mauna Kea, Hawaii

Alien parasitic wasps, including accidental introductions and purposefully released biological control agents, have been implicated in the decline of native Hawaiian Lepidoptera. Understanding the potential impacts of alien wasps requires knowledge of ecological parameters that influence parasitism rates for species in their new environment. Sophora seed-feeding Cydia spp. (Lepidoptera: Tortricidae) were surveyed for larval parasitoids to determine how native and alien wasps are partitioned over an elevation gradient (2200–2800 m) on Hawaii Island, Hawaii. Parasitism rate of native Euderus metallicus (Eulophidae) increased with increased elevation, while parasitism rate by immigrant Calliephialtes grapholithae (Ichneumonidae) decreased. Parasitism by Pristomerus hawaiiensis (Ichneumonidae), origins uncertain, also decreased with increased elevation. Two other species, Diadegma blackburni (Ichneumonidae), origins uncertain, and Brasema cushmani (Eupelmidae), a purposefully introduced biological control agent for pepper weevil, did not vary significantly with elevation. Results are contrasted with a previous study of this system with implications for the conservation of an endangered bird species that feed on Cydia larvae. Interpretation of results is hindered by lack of knowledge of autecology of moths and wasps, origins, phylogeny, systematics, competitive ability, and physiological limitations of each wasp species. These factors should be incorporated into risk analysis for biological control introductions and invasive species programs.     

Effects of founder events on the genetic variation of translocated island populations: implications for conservation management of the northern quoll

Translocation is a strategy commonly used to maximize the persistence of threatened species, but it may sometimes lead to undesirable genetic consequences. The northern quoll (Dasyurus hallucatus) is a carnivorous marsupial that is critically endangered in Australia’s Northern Territory due to rapid population declines in areas recently colonized by the exotic cane toad Chaunus [Bufo] marinus. In 2003, 64 quolls were translocated to two offshore islands to establish insurance populations and reduce the species’ risk of extinction. In this study, we assessed genetic diversity at five microsatellite loci in the translocated populations, two endemic islands and three mainland populations. In the short-term (three generations), the translocated populations showed a slight but non-significant reduction in genetic diversity (A = 4.1–4.2; H _e = 0.56–0.59) compared to the mainland source populations (A = 5.0–8.4; H _e = 0.56–0.71). In comparison, high genetic erosion was observed in the endemic island populations (A = 1.5–2.9; H _e = 0.11–0.34). Genetic bottlenecks were detected on both endemic islands and in one mainland population, indicating recent reductions in population size. Our results are consistent with previous studies describing greater losses of genetic diversity on islands compared to mainland populations. Divergence from ancestral allele frequencies in the translocated populations also suggests effects due to founder events. This study, although short-term, highlights the importance of continued monitoring for detecting changes in genetic diversity over time and makes a significant contribution to our understanding of the effects of founder events on island populations.     

Patterns of endemic extinctions among island bird species

The relationship between island biogeography and the vulnerability of island biota to extinction as a result of human activities was examined. In particular, this study analyzed whether island area, maximum elevation of an island, isolation from the nearest continental landmass, or date of human colonization had statistically significant relationships with the proportion of endemic island bird species that have become endangered or extinct. The study examined islands or island groups with endemic bird species, and which have never been connected to a continental landmass. Both modern and fossil bird species were incorporated into the analysis. Islands that were colonized by humans earliest had the lowest proportion of modern species alone, and modern and fossil species combined, that have gone extinct. However, date of human arrival was not correlated with the proportion of modern species that are endangered. Maximum elevation of an island was negatively correlated with the proportion of modern species that are extinct, and was positively correlated with the proportion of modern species that are endangered. Area was negatively correlated with the proportion of modern species that are endangered. Isolation of islands was not significantly correlated with the proportion of modern species extinct or endangered, but was positively correlated with the proportion of modern and fossil species combined that have gone extinct. These results indicate that the initial spasm of island bird extinctions due to human contact may have, in part, passed. They also indicate that bird species on islands colonized earliest by humans may have had more time to adapt to the presence of man and his commensal species, resulting in reduced extinction rates.     

Ecological impacts of feral pigs in the Hawaiian Islands

The foraging habits of exotic ungulate species can directly and indirectly affect native plant and animal distribution and abundance patterns. Most of the studies on feral pig interactions with other biota in the Hawaiian Islands have been published as difficult to access reports to governmental and nongovernmental organizations, graduate student theses, and a few in peer reviewed journals. In this paper we discuss the origins of pig introductions to Hawaii, their feralization process, population expansion, and interactions with native and non-native biota. We also consider the environmental effects triggered by pigs on local ecosystems and biotic communities. Feral pig activities can reduce the abundance of native plant species, enhance conditions for the establishment of invasive non-indigenous plants, and perhaps indirectly negatively impact native forest bird species. Pig foraging and traveling patterns may also lead to physical alteration of ecosystems by increasing soil erosion that may lead to watershed degradation. However, much remains to be learned about the strength and significance of aforementioned interactions and their long-term effects on Hawaiian biota and ecosystems due to some confounding events. Elucidating the dynamics and long-term ecological effects generated by pigs is a crucial step towards increasing our understanding of and more effectively managing biotic interactions.     

Floristic biogeography of the Hawaiian Islands: influences of area, environment and paleogeography

Aim A detailed database of distributions and phylogenetic relationships of native Hawaiian flowering plant species is used to weigh the relative influences of environmental and historical factors on species numbers and endemism. Location The Hawaiian Islands are isolated in the North Pacific Ocean nearly 4000 km from the nearest continent and nearly as distant from the closest high islands, the Marquesas. The range of island sizes, environments, and geological histories within an extremely isolated archipelago make the Hawaiian Islands an ideal system in which to study spatial variation in species distributions and diversity. Because the biota is derived from colonization followed by extensive speciation, the role of evolution in shaping the regional species assemblage can be readily examined. Methods For whole islands and regions of each major habitat, species–area relationships were assessed. Residuals of species–area relationships were subjected to correlation analysis with measures of endemism, isolation, elevation and island age. Putative groups of descendents of each colonist from outside the Hawaiian Islands were considered phylogenetic lineages whose distributions were included in analyses. Results The species–area relationship is a prominent pattern among islands and among regions of each given habitat. Species number in each case correlates positively with number of endemics, number of lineages and number of species per lineage. For mesic and wet habitat regions, island age is more influential than area on species numbers, with older islands having more species, more single‐island endemics, and higher species : lineage ratios than their areas alone would predict. Main conclusions Because species numbers and endemism are closely tied to speciation in the Hawaiian flora, particularly in the most species‐rich phylogenetic lineages, individual islands’ histories are central in shaping their biota. The Maui Nui complex of islands (Maui, Moloka‘i, Lāna‘i and Kaho‘olawe), which formed a single large landmass during most of its history, is best viewed in terms of either the age or area of the complex as a whole, rather than the individual islands existing today.     

Evolution and biogeography of native Hawaiian Hylaeus bees (Hymenoptera: Colletidae)

The only bees native to the Hawaiian Islands form a single clade of 60 species in the genus Hylaeus. The group is understudied and relatively poorly known. A data set consisting of 1201 base pairs of the mitochondrial genes cytochrome oxidase I and II and tRNA‐Leucine, and 14 morphological characters was used to construct a phylogenetic tree for 48 of the 60 known species. Genetic variation was high, including amino acid changes, and a number of species showed evidence of heteroplasmy. Tree support was low due to high levels of homoplasy. Biogeographical analysis using DIVA indicates that early radiation took place on the island of Hawaii. This places an upper age limit of only 0.4–0.7 Myr for the group, an unusually short time for such a large radiation. Moreover, it is an unusual biogeographical pattern among the Hawaiian biota. © The Willi Hennig Society 2006.     

Patterns of genetic connectivity among anchialine habitats: a case study of the endemic Hawaiian shrimp Halocaridina rubra on the island of Hawaii

Anchialine habitats, landlocked bodies of mixohaline water that fluctuate with the tides but have no surface connection to the sea, are known from around the world. Many anchialine organisms have widespread distributions and it has been hypothesized that high levels of gene flow and low levels of genetic differentiation are characteristic of populations from these habitats. However, the generality of this hypothesis requires further assessment, particularly in light of the significant negative impact these habitats and their biota have experienced from anthropogenic causes. This study investigated the population structure and demography of an endemic Hawaiian anchialine species, the atyid shrimp Halocaridina rubra, using mitochondrial cytochrome c oxidase subunit I (COI) gene sequences. A survey of 305 individuals from 16 populations collected on the island of Hawaii revealed 135 haplotypes. These haplotypes belonged to one of two divergent (2.7-4.9%) lineages; notably, no haplotypes were shared between the two coasts of the island. Along each coast, strong subdivision and little to no gene flow occurs between populations separated by > 30 km. The population structure and demography of H. rubra on Hawaii are influenced by regional hydrology, geology, volcanism and two distinct colonization events of the island. Thus, H. rubra on Hawaii demonstrates that populations of endemic anchialine organisms may exhibit significant levels of genetic structure and restricted levels of gene flow over limited geographic scales. This report brings novel insight into the biology of anchialine organisms and has important implications for the future management of these habitats and their biota.     

Estimating density of calling male Eleutherodactylus coqui in Hawaii from audio recordings of the nighttime frog chorus

Analysis of sound pressure data obtained from audio recordings of night-time Eleutherodactylus coqui chorusing was used to estimate population density of calling male coqui frogs in the major Hawaiian island populations. Study plots 400 m² in size (20 by 20 m; n = 32) were established on the islands of Hawaii and Maui to provide venues for field counting calling male frogs to investigate the ground truth relationship with the recorded sound pressure data. Male coqui frogs emit a two-note ‘co-qui’ advertisement call at regular intervals. Exploiting the temperature-related and relatively consistent male coqui call repetition rate and the average emitted ‘co’ call sound pressure level (70.4 dB at 1 m), cue counts obtained from a correlation process available in the program Raven were also used to estimate the density of calling male coqui. The study methodology included comparing linear regression results between the field-counted frogs and the number of frogs obtained from the cue count method. Both methods revealed a linear relationship between calling male population density and associated average sound pressure of the resulting chorus.     

Spawning seasonality and body sizes at sexual maturity in the bluespine unicornfish,Naso unicornis (Acanthuridae)

We herein evaluate several reproductive metrics of Hawaiian Archipelagic populations of the bluespine unicornfish (Naso unicornis), an economically and ecologically important, broadly distributed tropical Pacific reef fish, based on multi-year, fishery-dependent and fishery-independent collections. Sex-specific spawning seasonality was characterized for fish collected mostly from Oahu (Main Hawaiian Islands, MHI) using a gonadosomatic index. Histological slides preparations were used to score gonad developmental phase and to classify individuals of either sex as immature or mature. Sex-specific median body lengths at maturity (L₅₀) were estimated by logistic fits of proportion mature versus length class. Spawning was highly seasonal in Hawaii, with a single brief (May–June) peak spawning period. Proportionate gonad-to-body weight values were relatively low, averaging only about 0.1 % and 0.6 % across all months of year and 0.16 % and 1.03 % during May–June for males and females, respectively. Median lengths at sexual maturity differed between the sexes. L₅₀ values for fish collected throughout all months of year were 30.1 ± 0.5 (standard error) cm Fork Length (FL) for males and 35.5 ± 0.7 cm FL for females. Spawning seasonality and L₅₀ estimates for bluespine unicornfish in Hawaii suggest that the species spawns several months earlier in the calendar year and matures at larger body lengths in Hawaii versus Guam, the Northern Mariana Islands, and Pohnpei in the Federated States of Micronesia. Estimated lengths at sexual maturity are compared to the minimum length (14 inches or 35.6 cm FL) mandated for this species in Hawaii: median size at maturity occurs at a length appreciably less than (males) or approximately equal to (females) minimum legal size. A likely disproportionately large contribution of old females to population replenishment is discussed relative to the minimum size limit.     

Molecular phylogeny of Banza (Orthoptera: Tettigoniidae), the endemic katydids of the Hawaiian Archipelago

The extant endemic katydids (Orthoptera: Tettigoniidae) of the Hawaiian Archipelago include one to three species per high island and a single species on Nihoa, all currently placed in the genus Banza. These acoustic insects provide an excellent opportunity for investigating the evolution of reproductive isolation and speciation, but such studies require an understanding of phylogenetic relationships within the group. We use maximum parsimony, likelihood-based Bayesian inference, and maximum likelihood to infer phylogenetic relationships among these taxa, based on approximately 2kb of mitochondrial cytochrome oxidase I and cytochrome b. Our results strongly support two distinct high island clades: one clade ("Clade I") composed of species from Kauai, Oahu, Molokai, and Lanai and another clade ("Clade II") composed of species from Maui and Hawaii (Banza unica, from Oahu, may be basal to both these clades, but its placement is not well resolved). Within these clades, some inferred relationships are strongly supported, such as the sister status of B. kauaiensis (Kauai) and B. parvula (Oahu) within Clade I, but other relationships remain more ambiguous, such as the relative position of B. brunnea (Maui) within Clade II. Although a detailed reconstruction of the historical biogeography of the Hawaiian katydids is difficult, we use our genetic data combined with the known geological history of the Hawaiian Islands to set limits on plausible historical scenarios for diversification of this group. Beyond these historical biogeographic inferences, our results indicate possible cryptic speciation on both Oahu and Hawaii, as well as what may be unusually high average rates of nucleotide substitution. The present work sets the stage for future genetic and experimental investigations of this group.     

Response of native Hawaiian woody species to lava-ignited wildfires in tropical forests and shrublands

Wildfires are rare in the disturbance history of Hawaiian forests but may increase in prevalence due to invasive species and global climate change. We documented survival rates and adaptations facilitating persistence of native woody species following 2002–2003 wildfires in Hawaii Volcanoes National Park, Hawaii. Fires occurred during an El Niño drought and were ignited by lava flows. They burned across an environmental gradient occupied by two drier shrub-dominated communities and three mesic/wet Metrosideros forest communities. All the 19 native tree, shrub, and tree fern species demonstrated some capacity of postfire persistence. While greater than 95% of the dominant Metrosideros trees were top-killed, more than half survived fires via basal sprouting. Metrosideros trees with diameters >20 cm sprouted in lower percentages than smaller trees. At least 17 of 29 native woody species colonized the postfire environment via seedling establishment. Although the native biota possess adaptations facilitating persistence following wildfire, the presence of highly competitive invasive plants and ungulates will likely alter postfire succession.     

Managing Conservation Reliant Species: Hawai'i's Endangered Endemic Waterbirds

Hawai''I''s coastal plain wetlands are inhabited by five endangered endemic waterbird species. These include the Hawaiian Coot (''alae ke''oke''o), Hawaiian Duck (koloa maoli), Hawaiian Stilt (ae''o), Hawaiian Gallinule (Moorhen) (''alae ''ula), and Hawaiian Goose (nēnē). All five species are categorized as being “conservation reliant.” The current strategy to recover these endangered birds includes land protection and active management of wetlands. To assess the effectiveness of the current management paradigm, we compared species population trends across the state to those on six actively managed wetland national wildlife refuges (Refuges) thought to be critical for the survival of these endangered species. To perform the evaluation we relied on systematic semiannual population counts that have been conducted across most wetlands in the state and monthly population counts that have occurred on Refuges during the same time period. We found that statewide and Refuge populations of the Hawaiian Coot, Stilt and Gallinule have rebounded from historic lows and over the last 20 years have slowly increased or remained stable. We also documented that Refuges are important to each species year-round and that a disproportionately larger percentage of the population for each species is found on them. Understanding of why Refuges successfully house a disproportionate percentage of these “conservation reliant” species can inform current and future conservation efforts as well as ensure long-term population viability for these species.     

Microevolutionary patterns and processes of the Native Hawaiian colonizing fern Odontosoria chinensis (Lindsaeaceae)

Abstract.— The vascular‐plant flora of the Hawaiian Islands is characterized by one of the highest rates of species endemism in the world. Among flowering plants, approximately 89% of species are endemic, and among pteridophytes, about 76% are endemic. At the single‐island level, however, rates of species endemism vary dramatically between these two groups with 80% of angiosperms and only 6% of pteridophytes being single‐island endemics. Thus, in many groups of Hawaiian angiosperms, it is possible to link studies of phylogeny, evolution, and biogeographic history at the interspecific and interisland levels. In contrast, the low level of single‐island species endemism among Hawaiian pteridophytes makes similar interspecific and interisland studies nearly impossible. Higher levels of interisland gene flow may account for the different levels of single‐island endemism in Hawaiian pteridophytes relative to angiosperms. The primary question we addressed in the present study was: Can we infer microevolutionary patterns and processes among populations within widespread species of Hawaiian pteridophytes wherein gene flow is probably common? To address this broad question, we conducted a population genetic study of the native Hawaiian colonizing species Odontosoria chinensis. Data from allozyme analyses allowed us to infer: (1) significant genetic differentiation among populations from different islands; (2) historical patterns of dispersal between particular pairs of islands; (3) archipelago‐level patterns of dispersal and colonization; (4) founder effects among populations on the youngest island of Hawaii; and, (5) that this species primarily reproduces via outcrossing, but may possess a mixed‐mating system.     

Mid-ocean isolation and the evolution of Hawaiian reef fishes

The Hawaiian fish fauna has close affinities with the fauna of the Indo-west Pacific from which it is derived, but is depauperate. It is characterized by a large number of endemic species (30% of inshore fishes), which are often the most abundant species in their families in Hawaii. Although there is evidence of local adaptation, there has been no radiation of species within the island chain, as occurs in the terrestrial biota of isolated islands. Three major factors have contributed to these trends: (1) the geographic isolation of the islands, and oceanographic features, especially current patterns; (2) the life history characteristics of the fishes, especially their dispersal capabilities; and (3) the extent of adaptive differentiation to environmental conditions after they reached Hawaii.     

High genetic structure of the Cozumel Harvest mice,a critically endangered island endemic: conservation implications

We assessed the genetic structure and diversity of Reithrodontomys spectabilis, a critically endangered, endemic rodent from Cozumel Island, México. A total of 90 individuals were trapped from September 2001 to January 2005. Microsatellite data analysis revealed high genetic diversity values: a total of 113 alleles (average 12.5 per locus), H _o  = 0.78, H _e  = 0.80. These high values can be related to Cozumel’s size (478 km²) and extensive native vegetation cover, factors that could be promoting a suitable population size, high heterozygosity and the persistence of rare alleles in the species, as well as some long-term movement of individuals between sampling localities. A strong genetic structure was also observed, with at least four genetic groups, associated with a pattern of isolation by distance. We found a strong allelic and genetic differentiation shown between localities, with negligible recent gene flow and low inbreeding coefficients. The species life history and ecological characteristics—being nocturnal, semi-terrestrial, a good tree climber, having lunar phobia and significant edge effect—are likely affecting its genetic structure and differentiation. The high genetic diversity and population structure award R. spectabilis a significant conservation value. Our results can serve as a basis for future research and conservation of the species, particularly considering the problems the island is facing from habitat perturbation, urbanization and introduction of exotic species. In view of the structure and genetic variability observed, it is essential to establish and reinforce protected areas and management programs for the conservation of the endemic and endangered Cozumel Harvest mice.