Understanding Factors Affecting the Decline of the Samoan Swallowtail Butterfly
The Samoan swallowtail is a large and strikingly marked butterfly endemic to the Samoan Archipelago. Once widespread and common, its populations have declined dramatically. The Samoan swallowtail is a possible candidate as an indicator of ecosystem health for this oceanic island ecosystem.
Overview:
The Samoan swallowtail (Papilio godeffroyi) is a striking butterfly endemic to the Samoan archipelago. It is mostly restricted to the island of Tutuila, an area representing approximately 5% of its former range. Few insects are commonly thought of as indicators of ecosystem health, but the Samoan swallowtail represents a possible candidate for this role in an oceanic island ecosystem where forests are generally dominated by native species and relatively few threats from invasive species have been identified. The species also could serve as a model for valuing and conserving butterflies in the Polynesian-Micronesia biodiversity hotspot.
The National Park of American Samoa (NPSA) protects much of the intact rainforest habitat of the butterfly, but managers are hampered in developing conservation strategies by a lack of information about its basic life history and the factors driving its decline. To fill this need, our study identified and mapped distributions of host plants of the butterfly within NPSA; determined environmental factors that influence distributions of both larvae and their host plants; quantified the influence of parasitoid enemies on egg, larval, and pupal stages; and developed strategies for enhancing populations. This study provided park managers and interpreters with a rare opportunity to understand the role of native insects in the structure, function, and health of remote island ecosystems and to connect local people with a sometimes-overlooked element of their natural heritage.
Project Objectives:
The primary objectives were to identify environmental factors that affect the Samoan swallowtail and to provide NPSA managers with information to help them protect and enhance their populations. As the first ecological study of this rare butterfly, results increased the knowledge and appreciation of a charismatic Samoan insect. The study identified host plants used by the butterfly to evaluate how their availability may limit populations and to provide a sound basis for monitoring. Habitat variables that may influence the distribution of the butterfly and its host plants were evaluated to understand factors limiting the species at the landscape level, identify management opportunities, and facilitate modeling the potential range of the species. Identifying parasitoids of the butterfly and determining rates of parasitism provide baselines for monitoring changes in the parasitoid community and in species that pose the greatest threats as well as for biosecurity screening and interdiction. Results can provide guidance to managers on ways to monitor and possibly enhance populations of the Samoan swallowtail and its host plants.
Study goals aligned with priorities of the USGS Ecosystems Science Strategy in that research was conducted on the abundance, distribution, productivity, and health of a rare species in order to aid its recovery. As a rare, declining species, the Samoan swallowtail is a likely candidate for federal protection, and information about changes in its habitat associations inform resource management and agency decision-making. Key research elements also focused on identifying threats and ecological characteristics of invasive species that may impact the butterfly as well as on developing measures for their control. The research also evaluated environmental factors that potentially aid managers to assess tradeoffs involved in conservation and land use strategies. NPSA managers protect natural resources while also accommodating traditional agricultural practices, and this study was designed to provide them with a scientific basis for designing strategies to protect, sustain, and restore the ecosystem functions and services of habitats harboring the swallowtail.
Highlights and Key Findings:
We assessed abundances of swallowtail eggs, larvae, and pupae on 117 trees of its host plant, Micromelum minutum, during 2013–2014. Swallowtails were not observed using other tree species. Eggs made up 90% of all detections, followed by larvae and pupae. Tree use was widespread with 89% of all trees yielding swallowtails at least once, but only 10% of the trees yielded swallowtails 90% or more of the time. Egg abundance was positively related to leaf biomass of individual trees but not of the whole stand, suggesting that other factors may be affecting swallowtail distributions.
In the lab, we found no evidence for parasitism of larvae or pupae, but 81% of all viable eggs were parasitized by a species of encyrtid wasp identified as Ooencyrtus pitosina. Parasitism rates differed little among sites suggesting the impact of this wasp was similar over the landscape. Other natural enemies (for example, pathogens and predatory geckos, ants, and birds) were not evaluated, but they likely also influence swallowtail mortality rates and may have contributed to its decline across the archipelago.
Our findings suggest that the Samoan swallowtail is a good candidate for conservation management, likely responding well to habitat restoration focused on increasing low-density stands of M. minutum, its host plant.
The Samoan swallowtail is a large and strikingly marked butterfly endemic to the Samoan Archipelago. Once widespread and common, its populations have declined dramatically. The Samoan swallowtail is a possible candidate as an indicator of ecosystem health for this oceanic island ecosystem.
Overview:
The Samoan swallowtail (Papilio godeffroyi) is a striking butterfly endemic to the Samoan archipelago. It is mostly restricted to the island of Tutuila, an area representing approximately 5% of its former range. Few insects are commonly thought of as indicators of ecosystem health, but the Samoan swallowtail represents a possible candidate for this role in an oceanic island ecosystem where forests are generally dominated by native species and relatively few threats from invasive species have been identified. The species also could serve as a model for valuing and conserving butterflies in the Polynesian-Micronesia biodiversity hotspot.
The National Park of American Samoa (NPSA) protects much of the intact rainforest habitat of the butterfly, but managers are hampered in developing conservation strategies by a lack of information about its basic life history and the factors driving its decline. To fill this need, our study identified and mapped distributions of host plants of the butterfly within NPSA; determined environmental factors that influence distributions of both larvae and their host plants; quantified the influence of parasitoid enemies on egg, larval, and pupal stages; and developed strategies for enhancing populations. This study provided park managers and interpreters with a rare opportunity to understand the role of native insects in the structure, function, and health of remote island ecosystems and to connect local people with a sometimes-overlooked element of their natural heritage.
Project Objectives:
The primary objectives were to identify environmental factors that affect the Samoan swallowtail and to provide NPSA managers with information to help them protect and enhance their populations. As the first ecological study of this rare butterfly, results increased the knowledge and appreciation of a charismatic Samoan insect. The study identified host plants used by the butterfly to evaluate how their availability may limit populations and to provide a sound basis for monitoring. Habitat variables that may influence the distribution of the butterfly and its host plants were evaluated to understand factors limiting the species at the landscape level, identify management opportunities, and facilitate modeling the potential range of the species. Identifying parasitoids of the butterfly and determining rates of parasitism provide baselines for monitoring changes in the parasitoid community and in species that pose the greatest threats as well as for biosecurity screening and interdiction. Results can provide guidance to managers on ways to monitor and possibly enhance populations of the Samoan swallowtail and its host plants.
Study goals aligned with priorities of the USGS Ecosystems Science Strategy in that research was conducted on the abundance, distribution, productivity, and health of a rare species in order to aid its recovery. As a rare, declining species, the Samoan swallowtail is a likely candidate for federal protection, and information about changes in its habitat associations inform resource management and agency decision-making. Key research elements also focused on identifying threats and ecological characteristics of invasive species that may impact the butterfly as well as on developing measures for their control. The research also evaluated environmental factors that potentially aid managers to assess tradeoffs involved in conservation and land use strategies. NPSA managers protect natural resources while also accommodating traditional agricultural practices, and this study was designed to provide them with a scientific basis for designing strategies to protect, sustain, and restore the ecosystem functions and services of habitats harboring the swallowtail.
Highlights and Key Findings:
We assessed abundances of swallowtail eggs, larvae, and pupae on 117 trees of its host plant, Micromelum minutum, during 2013–2014. Swallowtails were not observed using other tree species. Eggs made up 90% of all detections, followed by larvae and pupae. Tree use was widespread with 89% of all trees yielding swallowtails at least once, but only 10% of the trees yielded swallowtails 90% or more of the time. Egg abundance was positively related to leaf biomass of individual trees but not of the whole stand, suggesting that other factors may be affecting swallowtail distributions.
In the lab, we found no evidence for parasitism of larvae or pupae, but 81% of all viable eggs were parasitized by a species of encyrtid wasp identified as Ooencyrtus pitosina. Parasitism rates differed little among sites suggesting the impact of this wasp was similar over the landscape. Other natural enemies (for example, pathogens and predatory geckos, ants, and birds) were not evaluated, but they likely also influence swallowtail mortality rates and may have contributed to its decline across the archipelago.
Our findings suggest that the Samoan swallowtail is a good candidate for conservation management, likely responding well to habitat restoration focused on increasing low-density stands of M. minutum, its host plant.