Geology topics

Susan Cordell

Publications and source records attributed to Susan Cordell.

10 recordsLinked to original sources

USGS2025

Photographic guide to the leaf litter arthropod community of the lowland wet forest ecosystem of the Island of Hawaiʻi

Leaf litter arthropods are important components of the food web in forests, and their presence and diversity can provide information on forest health. There has been very little documentation of the leaf litter arthropods in Hawaiian forest ecosystems. This technical report is a photographic guide to some common arthropods collected from forest leaf litter at the Liko Nā Pilina Hybrid Ecosystems Project study site, a lowland wet forest in Hilo, Island of Hawaiʻi, USA. Leaf litter samples were collected from plots of invaded and experimental restoration communities using two complementary methods (litterbags and quadrats), and arthropods were extracted using Berlese funnels. The field site contained many morphospecies that were rare and locally distributed across plots, and only a few that were very common and widely distributed. The majority of the morphospecies identified were mites. This photoguide is designed to help identify arthropods found in plant litter in Hawaiian lowland forests and it may assist with research and education efforts concerned with the diversity, ecology, or conservation of litter arthropods across the Hawaiian archipelago and other Pacific islands.

Hawaii↗

USGS2024

Invasive-dominated grasslands in Hawaiʻi are resilient to disturbance

Non-native-dominated landscapes may arise from invasion by competitive plant species, disturbance and invasion of early-colonizing species, or some combination of these. Without knowing site history, however, it is difficult to predict how native or non-native communities will reassemble after disturbance events. Given increasing disturbance levels across anthropogenically impacted landscapes, predictive understanding of these patterns is important. We asked how disturbance affected community assembly in six invaded habitat types common in dryland, grazed landscapes on Island of Hawai‘i. We mechanically disturbed 100 m 2 plots in six vegetation types dominated by one of four invasive perennial grasses ( Cenchrus ciliaris , Cenchrus clandestinus , Cenchrus setaceus , or Melinis repens ), a native shrub ( Dodonaea viscosa ), or a native perennial bunchgrass ( Eragrostis atropioides ). We censused vegetation before disturbance and monitored woody plant colonization and herbaceous cover for 21 months following the disturbance, categorizing species as competitors, colonizers, or a combination, based on recovery patterns. In addition, we planted individuals of the native shrub and bunchgrass and monitored survival to overcome dispersal limitation of native species when exploring these patterns. We found that the dominant vegetation types showed variation in post-disturbance syndrome, and that the variation in colonizer versus competitor syndrome occurred both between species, but also within species among different vegetation types. Although there were flushes of native shrub seedlings, these did not survive to 21 months within invaded habitats, probably due to regrowth by competitive invasive grasses. Similarly, survival of planted native individuals was related to the rate of regrowth by dominant species. Regardless of colonization/competitor syndrome, however, all dominant vegetation types were relatively resilient to change. Our results highlight that the altered post-agricultural, invaded grassland landscapes in Hawaiʻi are stable states. More generally, they point to the importance of resident communities and their effects on species interactions and seed availability in shaping plant community response to disturbance.

Hawaii↗

USGS2022

The role of microtopography and resident species in post-disturbance recovery of arid habitats in Hawaiʻi

Habitat-suitability indices (HSI) have been employed in restoration to identify optimal sites for planting native species. Often, HSI are based on abiotic variables and do not include biotic interactions, even though similar abiotic conditions can favor both native and nonnative species. Biotic interactions such as competition may be especially important in invader-dominated habitats because invasive species often have fast growth rates and can exploit resources quickly. In this study, we test the utility of an HSI of microtopography derived from airborne LiDAR to predict post-disturbance recovery and native planting success in native shrub-dominated and nonnative, invasive grass-dominated dryland habitats in Hawaiʻi. The HSI uses high-resolution digital terrain models to classify sites' microtopography as high, medium, or low suitability, based on wind exposure and topographic position. We used a split-plot before-after-control-impact design to implement a disturbance experiment within native shrub ( Dodonaea viscosa ) and nonnative, invasive grass ( Cenchrus clandestinus )-dominated ecosystems across three microtopography categories. In contrast to previous studies using the same HSI, we found that microtopography was a poor predictor of pre-disturbance conditions for soil nutrients, organic matter content, or foliar C:N, within both Dodonaea and Cenchrus vegetation types. In invader-dominated Cenchrus plots, microtopography helped predict cover, but not as expected (i.e., highest cover would be in high-suitability plots): D. viscosa had the greatest cover in low-suitability and C. clandestinus had the greatest cover in medium-suitability plots. Similarly, in native-dominated Dodonaea plots, microtopography was a poor predictor of D. viscosa , C. clandestinus , and total plant cover. Although we found some evidence that microtopography helped inform post-disturbance plant recovery of D. viscosa and total plant cover, vegetation type was a more important predictor. Important for considering the success of plantings, percent cover of D. viscosa decreased while percent cover of C. clandestinus increased within both vegetation types 20 months after disturbance. Our results are evidence that HSIs based on topographic features may prove most useful for choosing planting sites in harsh habitats or those already dominated by native species. In more productive habitats, competition from resident species may offset any benefits gained from “better” suitability sites.

Hawaii↗

USGS2022

Rapid ʻŌhiʻa Death in Hawaiʻi

Ceratocystis lukuohia and Ceratocystis huliohia are two newly recognized fungi that have arrived in Hawai‘i and are causing a serious vascular wilt and canker disease, respectively, of ‘ōhi‘a trees ( Metrosideros polymorpha ), the most common and important tree species in Hawai‘i. Management of these diseases has presented challenges due to unique etiological aspects and the exceptionally pathogenic nature of one of these fungi ( C. lukuohia ) once it gains access to the tree’s vascular tissue. Careful study of the spread of the pathogens has resulted in an understanding of the role of ambrosia beetles and the frass they produce that carries the pathogen, as well as the wounding of trees by many different agents by which the pathogen can access and infect the vascular tissue. A variety control measures are being used. These include a state-of-the-art monitoring program to detect diseased and recently killed trees and molecular biology approaches that can confirm if a given tree was infected by Ceratocystis . Based on monitoring more than one million trees have been estimated as killed by the diseases to date. A major part of the program includes the deployment of a field crew that seeks out and fells large infected ‘ōhi‘a trees as these trees are the main source of most of the infective Ceratocystis -laden frass. Long-term control measures also include fencing of some forests to reduce the amount of wounding to ‘ōhi‘a trees by feral cattle and pigs that allows entry of the fungi and quarantine restrictions to ensure there will be no inter-island movement of the pathogens in ‘ōhi‘a products. Finally, methods are also being developed to restore ‘ōhi‘a forests affected by these diseases by determining effective regeneration practices and developing genetically resistant ‘ōhi‘a stock. Hawai‘i has an active extension program dedicated to providing information on how residents and visitors can contribute to protecting ‘ōhi‘a trees from these diseases. More than 500,000 people have participated in this program. Hawai‘i residents have a very deep appreciation for this tree species and do what they can to help prevent these diseases from destroying more of their most treasured tree species.

Hawai'i↗

USGS2021

Hawai‘i forest review: Synthesizing the ecology, evolution, and conservation of a model system

As the most remote archipelago in the world, the Hawaiian Islands are home to a highly endemic and disharmonic biota that has fascinated biologists for centuries. Forests are the dominant terrestrial biome in Hawai‘i, spanning complex, heterogeneous climates across substrates that vary tremendously in age, soil structure, and nutrient availability. Species richness is low in Hawaiian forests compared to other tropical forests, as a consequence of dispersal limitation from continents and adaptive radiations in only some lineages, and forests are dominated by the widespread Metrosideros species complex. Low species richness provides a relatively tractable model system for studies of community assembly, local adaptation, and species interactions. Moreover, Hawaiian forests provide insights into predicted patterns of evolution on islands, revealing that while some evidence supports “island syndromes,” there are exceptions to them all. For example, Hawaiian plants are not as a whole less defended against herbivores, less dispersible, more conservative in resource use, or more slow-growing than their continental relatives. Clearly, more work is needed to understand the drivers, sources, and constraints on phenotypic variation among Hawaiian species, including both widespread and rare species, and to understand the role of this variation for ecological and evolutionary processes, which will further contribute to conservation of this unique biota. Today, Hawaiian forests are among the most threatened globally. Resource management failures – the proliferation of non-native species in particular – have led to devastating declines in native taxa and resulted in dominance by novel species assemblages. Conservation and restoration of Hawaiian forests now rely on managing threats including climate change, ongoing species introductions, novel pathogens, lost mutualists, and altered ecosystem dynamics through the use of diverse tools and strategies grounded in basic ecological, evolutionary, and biocultural principles. The future of Hawaiian forests thus depends on the synthesis of ecological and evolutionary research, which will continue to inform future conservation and restoration practices.

Hawaii↗

USGS2020

Linking plant and animal functional diversity with an experimental community restoration in a Hawaiian lowland wet forest

Testing how plant restoration influences animal taxonomic and functional diversity can shift restoration projects beyond mainly plant community considerations. We incorporated multi-trophic interactions into restoration by describing an ongoing functional trait-based restoration experiment in Hawaiian lowland tropical wet forest (Liko Nā Pilina Experiment), where litter arthropods are examined from a functional perspective thereby linking plants and higher trophic levels. We hypothesized that (1) communities with greater plant functional trait diversity would have cascading effects through food webs, increasing animal diversity and network complexity, and (2) increases in animal species and network complexity would be stronger for restoration efforts in plant communities with more complementary functional traits than those with more redundant traits. We examined experimental treatments of planted communities with the same species richness but with different plant functional trait profiles based on (1) rates of expected carbon turnover (slow or moderate), and (2) the similarity of their functional trait measurements (redundant or complementary), as determined by functional dispersion calculations. Initial data on arthropod communities and leaf litter decomposition rates revealed linkages between plant functional traits and arthropod community diversity. Overall, we argue that a more comprehensive evaluation of restoration accounts for both functional diversity and the multi-trophic nature of animal and plant communities. Developing restoration projects based on plant functional traits that influence both plant and invertebrate species provides a new paradigm, and the incorporation of both native and non-native (but non-invasive) plants shows promise in restoring ecosystem function in disturbed lowland tropical forests.

Hawaii↗

USGS2017

Plant diversity increases with the strength of negative density dependence at the global scale

Theory predicts that higher biodiversity in the tropics is maintained by specialized interactions among plants and their natural enemies that result in conspecific negative density dependence (CNDD). By using more than 3000 species and nearly 2.4 million trees across 24 forest plots worldwide, we show that global patterns in tree species diversity reflect not only stronger CNDD at tropical versus temperate latitudes but also a latitudinal shift in the relationship between CNDD and species abundance. CNDD was stronger for rare species at tropical versus temperate latitudes, potentially causing the persistence of greater numbers of rare species in the tropics. Our study reveals fundamental differences in the nature of local-scale biotic interactions that contribute to the maintenance of species diversity across temperate and tropical communities.

Science↗

USGS2017

Interactions among invasive plants: Lessons from Hawai‘i

Most ecosystems have multiple-plant invaders rather than single-plant invaders, yet ecological studies and management actions focus largely on single invader species. There is a need for general principles regarding invader interactions across varying environmental conditions, so that secondary invasions can be anticipated and managers can allocate resources toward pretreatment or postremoval actions. By reviewing removal experiments conducted in three Hawaiian ecosystems (a dry tropical forest, a seasonally dry mesic forest, and a lowland wet forest), we evaluate the roles environmental harshness, priority effects, productivity potential, and species interactions have in influencing secondary invasions, defined here as invasions that are influenced either positively (facilitation) or negatively (inhibition/priority effects) by existing invaders. We generate a conceptual model with a surprise index to describe whether long-term plant invader composition and dominance is predictable or stochastic after a system perturbation such as a removal experiment. Under extremely low resource availability, the surprise index is low, whereas under intermediate-level resource environments, invader dominance is more stochastic and the surprise index is high. At high resource levels, the surprise index is intermediate: Invaders are likely abundant in the environment but their response to a perturbation is more predictable than at intermediate resource levels. We suggest further testing across environmental gradients to determine key variables that dictate the predictability of postremoval invader composition.

Hawai'i↗

USGS2015

Home range use and movement patterns of non-native feral goats in a tropical island montane dry landscape

Advances in wildlife telemetry and remote sensing technology facilitate studies of broad-scale movements of ungulates in relation to phenological shifts in vegetation. In tropical island dry landscapes, home range use and movements of non-native feral goats ( Capra hircus ) are largely unknown, yet this information is important to help guide the conservation and restoration of some of the world’s most critically endangered ecosystems. We hypothesized that feral goats would respond to resource pulses in vegetation by traveling to areas of recent green-up. To address this hypothesis, we fitted six male and seven female feral goats with Global Positioning System (GPS) collars equipped with an Argos satellite upload link to examine goat movements in relation to the plant phenology using the Normalized Difference Vegetation Index (NDVI). Movement patterns of 50% of males and 40% of females suggested conditional movement between non-overlapping home ranges throughout the year. A shift in NDVI values corresponded with movement between primary and secondary ranges of goats that exhibited long-distance movement, suggesting that vegetation phenology as captured by NDVI is a good indicator of the habitat and movement patterns of feral goats in tropical island dry landscapes. In the context of conservation and restoration of tropical island landscapes, the results of our study identify how non-native feral goats use resources across a broad landscape to sustain their populations and facilitate invasion of native plant communities.

Hawai‘i↗

USGS2013

Biology and impacts of Pacific island invasive species 9. Capra hircus , the feral goat, (Mammalia: Bovidae)

Domestic goats, Capra hircus , were intentionally introduced to numerous oceanic islands beginning in the sixteenth century. The remarkable ability of C. hircus to survive in a variety of conditions has enabled this animal to become feral and impact native ecosystems on islands throughout the world. Direct ecological impacts include consumption and trampling of native plants, leading to plant community modification and transformation of ecosystem structure. While the negative impacts of feral goats are well-known and effective management strategies have been developed to control this invasive species, large populations persist on many islands. This review summarizes the impacts of feral goats on Pacific island ecosystems, and the management strategies available to control this invasive species.

Pacific Science↗