Prioritize Management Efforts Research Articles

Spatial and temporal changes in cumulative human impacts on the world's ocean.

Human pressures on the ocean are thought to be increasing globally, yet we know little about their patterns of cumulative change, which pressures are most responsible for change, and which places are experiencing the greatest increases. Managers and policymakers require such information to make strategic decisions and monitor progress towards management objectives. Here we calculate and map recent change over 5 years in cumulative impacts to marine ecosystems globally from fishing, climate change, and ocean- and land-based stressors. Nearly 66% of the ocean and 77% of national jurisdictions show increased human impact, driven mostly by climate change pressures. Five percent of the ocean is heavily impacted with increasing pressures, requiring management attention. Ten percent has very low impact with decreasing pressures. Our results provide large-scale guidance about where to prioritize management efforts and affirm the importance of addressing climate change to maintain and improve the condition of marine ecosystems.

Citizen science helps predict risk of emerging infectious disease

Engaging citizen scientists is becoming an increasingly popular technique for collecting large amounts of ecological data while also creating an avenue for outreach and public support for research. Here we describe a unique study, in which citizen scientists played a key role in the spatial prediction of an emerging infectious disease. The yearly citizen‐science program called “Sudden Oak Death (SOD) Blitz” engages and educates volunteers in detecting the causal pathogen during peak windows of seasonal disease expression. We used these data – many of which were collected from under‐sampled urban ecosystems – to develop predictive maps of disease risk and to inform stakeholders on where they should prioritize management efforts. We found that continuing the SOD Blitz program over 6 consecutive years improved our understanding of disease dynamics and increased the accuracy of our predictive models. We also found that self‐identified non‐professionals were just as capable of detecting the disease as were professionals. Our results indicate that using long‐term citizen‐science data to predict the risk of emerging infectious plant diseases in urban ecosystems holds substantial promise.

Comparing impacts of alien plants and animals in Europe using a standard scoring system

Summary Alien species can change the recipient environment in various ways, and some of them cause considerable damage. Understanding such impacts is crucial to direct management actions. This study addresses the following questions: Is it possible to quantify impact across higher taxa in a comparative manner? Do impacts differ between taxonomic groups? How are environmental and socio‐economic impacts related? Can impacts be predicted based on those in other regions? To address these questions, we reviewed literature describing the impacts of 300 species from five major taxonomic groups: mammals, birds, fish, terrestrial arthropods and plants. To make very diverse impact measures comparable, we used the semi‐quantitative generic impact scoring system (GISS) which describes environmental and socio‐economic impacts using twelve categories. In each category, scores range from zero (no impact known or detectable) to five (the highest possible impact). Using the same scoring system for taxa as diverse as invertebrates, vertebrates and plants, we found that overall, alien mammals in Europe have the highest impact, while fish have the lowest. Terrestrial arthropods were found to have the lowest environmental impact, while fish had relatively low socio‐economic impact. Overall, the magnitude of environmental and socio‐economic impacts of individual alien species is highly correlated. However, at the species level, major deviations are found. For mammals and birds, the impacts in invaded ranges outside of Europe are broadly similar to those recorded for alien species within Europe, indicating that a consideration of the known impacts of a species in other regions can be generally useful when predicting the impacts of an alien species. However, it should be noted that this pattern is not consistent across all mammal and bird orders, and thus, such information should be considered with caution. Synthesis and applications. Comparing the impacts of alien species across taxa is necessary for prioritizing management efforts and effective allocation of resources. By applying the generic impact scoring system (GISS) to five major taxonomic groups, we provide the basis for a semi‐quantitative cross‐taxa listing process (e.g. ‘black lists’ or 100‐worst‐lists). If more data are collated from different geographical regions and habitats using standard GISS protocols, risk assessments for alien species based on rigorous measures of impact could be improved by taking into account local variation, and context dependence of impacts. This would also allow studies at lower taxonomic levels, and within‐taxon analyses of functional groups and guilds.

Escaped Miscanthus sacchariflorus reduces the richness and diversity of vegetation and the soil seed bank

Prioritizing management efforts and funds for invasive species relies upon knowledge of their ecological impacts. Often, however, novel species have unknown effects and can exhibit long time lags before proliferating, making early management decisions difficult. The grasses Miscanthus sinensis, M. sacchariflorus, and their sterile hybrid M. × giganteus have been introduced to North America and elsewhere for horticultural and agricultural purposes, but little is known about their impacts on resident vegetation upon escape from cultivation. We compared the vegetation communities and soil seed bank of paired areas invaded and uninvaded by escaped M. sacchariflorus at ten sites in Ontario, Canada. Invaded plots had very few species other than M. sacchariflorus and had lower plant species abundance, richness, and diversity than did uninvaded plots, and effects were greater on the vegetation than on the seed bank. Invaded plots also had more live and dead standing biomass, taller vegetation, and greater litter biomass, along with decreased light penetration to the soil surface than in uninvaded plots. However, M. sacchariflorus did not produce a germinable seed bank. Our results suggest that the regional spread of M. sacchariflorus is currently limited by a lack of seed production/viability, but its occurrence along disturbance-prone roadways and drainages facilitates its local vegetative spread. The strong negative effects of M. sacchariflorus on the resident plant community should be considered in risk assessments and management planning for this and other Miscanthus species.

Below‐ground causes and consequences of woodland shrub invasions: a novel paired‐point framework reveals new insights

Summary Confirming the impacts of invasive plants is essential for prioritizing management efforts, but is challenging, especially if impacts occur below‐ground and over long periods as hypothesized for woodland shrub invasions. For this reason, we developed a novel ‘paired‐point’ framework capable of detecting the below‐ground impacts of slow‐growing invasive plants from short‐term data sets in an investigation into the below‐ground consequences of European buckthorn Rhamnus catharticaL. (hereafter buckthorn) invasions into Chicago‐area woodlands. We measured differences between the members of 97 pairs of buckthorn‐invaded and buckthorn‐free points in 10 below‐ground response variables (RVs) that buckthorn is hypothesized to alter (listed below). We then modelled these differences (ΔRVs) in response to the severity of the invasion found at each invaded point. A relationship (linear or nonlinear) between a ΔRV and severity, that is slope ≠ 0, suggests a buckthorn‐induced change. An intercept (value of ΔRV where severity = 0) different from zero suggests a pre‐existing difference. Relating differences to an invasion gradient rather than simply noting their presence provided evidence that the higher levels of moisture, pH, total C, total N, and Ca2+ in invaded soils pre‐date and possibly promote invasion (particularly Ca2+) and that neither earthworm biomass nor soil C : N ratios are associated with buckthorn invasion, all of which suggest buckthorn to be less of a ‘driver’ of below‐ground change than hypothesized. We did, however, find evidence that buckthorn establishes in areas having greater leaf litter mass and higher rates of decomposition, and then proceeds to accelerate decomposition further and to alter spring soil levels. Synthesis and applications. Our findings suggest that decisions regarding regional buckthorn management should be less driven by concerns about buckthorn's below‐ground impacts and that greater consideration of how variation in below‐ground factors relates to invader establishment is needed, rather than simply assuming this variation to be invader‐induced. This latter consideration can help to design better targeted monitoring programmes, limiting the further spread of woodland invaders. These insights illustrate the utility of the paired‐point framework both for investigating below‐ground causes and consequences of slow‐growing invasive plants and for guiding the management of these invasions.

Freshwater hotspots of biological invasion are a function of species–pathway interactions

The introduction and spread of aquatic invasive species constitutes a global epidemic that continues to homogenize the world’s aquatic flora and fauna. Species invasions are occurring at an unprecedented scale, yet few centrally organized distributional databases can accurately display location information used to prioritize management efforts within a watershed, region, or political boundary. For particularly harmful groups of invaders such as some freshwater invertebrates, one consequence is that predictive power is limited because current invasion patterns are unknown. We used the Nonindigenous Aquatic Species database of the US Geological Survey to determine current hotspots of invertebrate invasion in the US and compared those patterns with better-documented fish invasions. Invasive freshwater invertebrates were densely clustered in the Laurentian Great Lakes region compared to invasive fish that were concentrated in Florida and the Southwest. However, temporal patterns of fish and invertebrate invasions did not significantly differ. Ballast water was the most important source of invertebrate introductions, whereas fish were more often introduced by stocking, aquaculture, and aquarium release. A centralized database of current species distributions, ideally at the global scale, would greatly inform prevention and mitigation efforts for harmful species invasions.

Drawing ecological inferences from coincident patterns of population- and community-level biodiversity.

Biodiversity is comprised of genetic and phenotypic variation among individual organisms, which might belong to the same species or to different species. Spatial patterns of biodiversity are of central interest in ecology and evolution for several reasons: to identify general patterns in nature (e.g. species-area relationships, latitudinal gradients), to inform conservation priorities (e.g. identifying hotspots, prioritizing management efforts) and to draw inferences about processes, historical or otherwise (e.g. adaptation, the centre of origin of particular clades). There are long traditions in ecology and evolutionary biology of examining spatial patterns of biodiversity among species (i.e. in multispecies communities) and within species, respectively, and there has been a recent surge of interest in studying these two types of pattern simultaneously. The idea is that examining both levels of diversity can materially advance the above-stated goals and perhaps lead to entirely novel lines of inquiry. Here, we review two broad categories of approach to merging studies of inter- and intraspecific variation: (i) the study of phenotypic trait variation along environmental gradients and (ii) the study of relationships between patterns of molecular genetic variation within species and patterns of distribution and diversity across species. For the latter, we report a new meta-analysis in which we find that correlations between species diversity and genetic diversity are generally positive and significantly stronger in studies with discrete sampling units (e.g. islands, lakes, forest fragments) than in studies with nondiscrete sampling units (e.g. equal-area study plots). For each topic, we summarize the current state of knowledge and key future directions.

Threatened and Endangered Subspecies with Vulnerable Ecological Traits Also Have High Susceptibility to Sea Level Rise and Habitat Fragmentation

The presence of multiple interacting threats to biodiversity and the increasing rate of species extinction make it critical to prioritize management efforts on species and communities that maximize conservation success. We implemented a multi-step approach that coupled vulnerability assessments evaluating threats to Florida taxa such as climate change, sea-level rise, and habitat fragmentation with in-depth literature surveys of taxon-specific ecological traits. The vulnerability, adaptive capacity, and ecological traits of 12 threatened and endangered subspecies were compared to non-listed subspecies of the same parent species. Overall, the threatened and endangered subspecies showed high vulnerability and low adaptive capacity, in particular to sea level rise and habitat fragmentation. They also exhibited larger home ranges and greater dispersal limitation compared to non-endangered subspecies, which may inhibit their ability to track changing climate in fragmented landscapes. There was evidence for lower reproductive capacity in some of the threatened or endangered taxa, but not for most. Taxa located in the Florida Keys or in other low coastal areas were most vulnerable to sea level rise, and also showed low levels of adaptive capacity, indicating they may have a lower probability of conservation success. Our analysis of at-risk subspecies and closely related non-endangered subspecies demonstrates that ecological traits help to explain observed differences in vulnerability and adaptive capacity. This study points to the importance of assessing the relative contributions of multiple threats and evaluating conservation value at the species (or subspecies) level when resources are limited and several factors affect conservation success.

Developing Non-destructive Methods to Determine Natal Origins of Snake River Cutthroat Trout in the Jackson Lake Watershed

Across their native ranges, cutthroat trout populations are imperiled due to habitat loss, habitat alteration, and introduction of non-native species (Liknes and Graham 1988, Behnke 1992, Hitt et al. 2003). These changes have not gone undetected and a great deal of time and money have been invested in conservation and restoration of cutthroat trout populations (Kershner 1995, USDA 1996, Young and Harig 2002, Baker et al. 2008). The success of these projects is tightly linked to the ability of resource managers to prioritize management efforts. Specifically, where should the investments of time and money br focused to yield the greatest impact on conservation and restoration. This study proposes to use a relatively new, proven analytical tool, stable isotope analysis, to identify differences in the stable isotope signatures of tributary streams entering Jackson Lake. These differences are translated into the tissues, specifically otolith bones, of cutthroat trout that use these tributaries during early life stages or upon return for spawning (Kennedy et al. 2002, Muhlfeld et al. 2005, Coghlan et al. 2007, Barnett-Johnson et al. 2008, Walther et al. 2008, Ziegler and Whitledge 2010). The ability to link adult trout back to their natal origins and identify where these adults are returning to spawn will provide the data resource managers need to prioritize conservation and restoration efforts in the upper Snake River watershed, with special emphasis on tributary streams entering Jackson Lake.

Weed Risk Assessment for Aquatic Plants: Modification of a New Zealand System for the United States

We tested the accuracy of an invasive aquatic plant risk assessment system in the United States that we modified from a system originally developed by New Zealand’s Biosecurity Program. The US system is comprised of 38 questions that address biological, historical, and environmental tolerance traits. Values associated with each response are summed to produce a total score for each species that indicates its risk of invasion. To calibrate and test this risk assessment, we identified 39 aquatic plant species that are major invaders in the continental US, 31 species that have naturalized but have no documented impacts (minor invaders), and 60 that have been introduced but have not established. These species represent 55 families and span all aquatic plant growth forms. We found sufficient information to assess all but three of these species. When the results are compared to the known invasiveness of the species, major invaders are distinguished from minor and non-invaders with 91% accuracy. Using this approach, the US aquatic weed risk assessment correctly identifies major invaders 85%, and non-invaders 98%, of the time. Model validation using an additional 10 non-invaders and 10 invaders resulted in 100% accuracy for the former, and 80% accuracy for the latter group. Accuracy was further improved to an average of 91% for all groups when the 17% of species with scores of 31–39 required further evaluation prior to risk classification. The high accuracy with which we can distinguish non-invaders from harmful invaders suggests that this tool provides a feasible, pro-active system for pre-import screening of aquatic plants in the US, and may have additional utility for prioritizing management efforts of established species.

Lake performance differences in response to land use and water quality: Data envelopment analysis

Alsharif KA, Fouad G. 2012. Lake performance differences in response to land use and water quality: Data envelopment analysis. Lake Reserv Manage. 28:130-141. Comparisons of lake conditions to surrounding land use classifications and water quality can be used to assist management of lake resources. This study used Data Envelopment Analysis (DEA), a performance measurement tool used in economics and operations research and management science, to compare lakes based on land use classifications and water quality. Lake performance ratings reflecting land use and water quality conditions were generated by DEA models that assessed surrounding land use and the reciprocal of the chlorophyll a concentration in the Tampa Bay region, Florida, USA. Using the performance continuum established by the DEA, lakes were categorized according to their management demands. Increased catchment, natural land, and agricultural land area were all related to lower performance measurements. It was found that higher DEA performance measurements were estimated for lakes located in smaller catchments that had a lower possibility for runoff and transport of contaminants. Overall, DEA successfully functioned as an assessment tool for comparing lake conditions based on land use in the catchment. The results produced by this novel application of DEA can prioritize management efforts toward lakes that suffer from the least beneficial water quality and land use conditions.

Estimating the financial costs of freshwater invasive species in Great Britain: a standardized approach to invasive species costing

Both ecological and economic impacts factor into invasive alien species (IAS) management considerations; however, economic impacts are often difficult to assess, much less quantify. Studies frequently aggregate identified financial costs as a proxy for IAS economic impacts, but these aggregate figures are often generated in an ad hoc fashion. Such estimates typically sum disparate costs, which might vary with respect to precision, accuracy, and scope. A standardized approach for IAS costing would better enable the comparison of cost estimates between taxa and across studies by controlling for surveying and scaling inconsistencies. This study develops a simple, survey-based approach to generate economic cost estimates for non-native freshwater invasive species (FIS) in Great Britain. The approach scales an average cost for each species by a ratio of management effort, thereby estimating the actual, annual expenditures incurred by a variety of stakeholders. The Great Britain-wide cost of controlling FIS is estimated to be approximately £26.5 million year−1; however, the costs of control could total £43.5 million year−1 if management efforts were undertaken at all FIS infested locations. Cost estimates are highest for Canadian pondweed (Elodea canadensis), a particularly widespread species, and for the zebra mussel (Dreissena polymorpha), which adversely impacts both industrial water users and boaters. This assessment of the relative economic impacts between species provides policy-makers with a monetary basis for rank-ordering species’ economic impacts and prioritizing management efforts. In addition, the cost assessment approach developed in this study could serve as a model for IAS economic impact assessments elsewhere.

Using expert judgment to estimate marine ecosystem vulnerability in the California Current

As resource management and conservation efforts move toward multi-sector, ecosystem-based approaches, we need methods for comparing the varying responses of ecosystems to the impacts of human activities in order to prioritize management efforts, allocate limited resources, and understand cumulative effects. Given the number and variety of human activities affecting ecosystems, relatively few empirical studies are adequately comprehensive to inform these decisions. Consequently, management often turns to expert judgment for information. Drawing on methods from decision science, we offer a method for eliciting expert judgment to (1) quantitatively estimate the relative vulnerability of ecosystems to stressors, (2) help prioritize the management of stressors across multiple ecosystems, (3) evaluate how experts give weight to different criteria to characterize vulnerability of ecosystems to anthropogenic stressors, and (4) identify key knowledge gaps. We applied this method to the California Current region in order to evaluate the relative vulnerability of 19 marine ecosystems to 53 stressors associated with human activities, based on surveys from 107 experts. When judging the relative vulnerability of ecosystems to stressors, we found that experts primarily considered two criteria: the ecosystem's resistance to the stressor and the number of species or trophic levels affected. Four intertidal ecosystems (mudflat, beach, salt marsh, and rocky intertidal) were judged most vulnerable to the suite of human activities evaluated here. The highest vulnerability rankings for coastal ecosystems were invasive species, ocean acidification, sea temperature change, sea level rise, and habitat alteration from coastal engineering, while offshore ecosystems were assessed to be most vulnerable to ocean acidification, demersal destructive fishing, and shipwrecks. These results provide a quantitative, transparent, and repeatable assessment of relative vulnerability across ecosystems to any ongoing or emerging human activity. Combining these results with data on the spatial distribution and intensity of human activities provides a systematic foundation for ecosystem-based management.

Assessing the potential impact of invasive ring-necked parakeets Psittacula krameri on native nuthatches Sitta europeae in Belgium

Summary 1. Biological invasions are a major threat to biodiversity. Given the large number of invasive species, we need to be able to identify invaders with large effects in order to prioritize management efforts. Here, we present a framework using species distribution modelling to predict how abundance of native species will change as a result of competition with an invasive species using ring-necked parakeets Psittacula krameri and nuthatches Sitta europaea as a case study. 2. Ring-necked parakeets are widely introduced throughout Europe and compete with native cavity-nesters. Using the relationship between parakeet abundance and environmental variables in Brussels (Belgium), we predict parakeet abundance across Flanders. A competition coefficient, quantifying the parakeets’ impact on nuthatches, was obtained by a regression on a data set of parakeet and nuthatch abundance. An estimate of the number of nuthatches that will be lost when parakeets have occupied all suitable sites was calculated by superimposing the abundance maps of the two species and applying the competition coefficient. 3. Our results predict a potential population of about 22 000 parakeet pairs, indicating that they could become one of the most numerous cavity-nesters in the region. Parakeet abundance is the highest in older, more fragmented forest in urban areas whereas nuthatches prefer larger, old and oak-dominated forests. 4. Our models indicate that throughout much of their range, and in a variety of habitats, parakeets and nuthatches will compete for nesting cavities, but as the competition strength is only moderate, the total impact of parakeets on nuthatch populations will be limited, with at most one-third of the population at risk. 5. Synthesis and applications. Species distribution models combined with empirical estimates of competition strength can be used as a general tool to make an assessment of the potential impact of established invasive species. Such information is required to make effective decisions on how to prioritize management effort and resources across the multitude of invasive species that currently threaten native ecosystems. For the ring-necked parakeet, our results indicate that there is no compelling evidence indicating that parakeets pose a threat large enough to justify an eradication campaign where they are currently present.

Predicting the potential distribution of invasive ring-necked parakeets Psittacula krameri in northern Belgium using an ecological niche modelling approach

The threat to biodiversity due to invasive alien species is considered second only to that of habitat loss. Given the large number of species that are currently invading ecosystems all over the world, we need to distinguish invaders with minor effects from those with large effects in order to prioritize management efforts. Ecological niche models can be used to predict the potential distribution of an invasive species from occurrence records and environmental data layers. We used the Ecological Niche Factor Analysis (ENFA), a presence-only predictive modelling approach, to describe the invasive ring-necked parakeets’ realized niche and to identify areas suitable for the parakeet in northern Belgium. ENFA proved to be a robust and reliable modelling technique, able to gauge the ecological requirements of an invasive species without the need to include historical information on the starting point of the invasion. ENFA shows that the parakeets tend to occupy relatively rare habitats compared to the main environmental conditions in northern Belgium, although they show some tolerance for environmental conditions inside parks and forests. The general distribution of the ring-necked parakeet is governed primarily by the amount of older forest patches, parks and built-up area in the landscape—reflecting the parakeets’ need for suitable nesting cavities and its reliance upon urban areas to forage. Our resulting habitat suitability maps show that the parakeets have ample room to further increase their range in northern Belgium. Our results indicate some concern for increased competition between parakeets and the nuthatches, native cavity nesters known to suffer from competition with parakeets, as some regions known as nuthatch strongholds are highly likely to be invaded by the parakeets.

Road Development, Housing Growth, And Landscape Fragmentation In Northern Wisconsin: 1937–1999

Roads remove habitat, alter adjacent areas, and interrupt and redirect ecological flows. They subdivide wildlife populations, foster invasive species spread, change the hydrologic network, and increase human use of adjacent areas. At broad scales, these impacts cumulate and define landscape patterns. The goal of this study was to improve our understanding of the dynamics of road networks over time, and their effects on landscape patterns, and identify significant relationships between road changes and other land-use changes. We mapped roads from aerial photographs from five dates between 1937 and 1999 in 17 townships in predominantly forested landscapes in northern Wisconsin, U.S.A. Patch-level landscape metrics were calculated on terrestrial area outside of a 15-m road-effect zone. We used generalized least-squares regression models to relate changes in road density and landscape pattern to concurrent changes in housing density. Rates of change and relationships were compared among three ecological regions. Our results showed substantial increases in both road density and landscape fragmentation during the study period. Road density more than doubled, and median, mean, and largest patch size were reduced by a factor of four, while patch shape became more regular. Increases in road density varied significantly among ecological subsections and were positively related to increases in housing density. Fragmentation was largely driven by increases in road density, but housing density had a significantly positive relationship with largest patch area and patch shape. Without protection of roadless areas, our results suggest road development is likely to continue in the future, even in areas where road construction is constrained by the physical environment. Recognizing the dynamic nature of road networks is important for understanding and predicting their ecological impacts over time and understanding where other types of development are likely to occur in the future. Historical perspectives of development can provide guidance in prioritizing management efforts to defragment landscapes and mitigate the ecological impacts of past road development.

Exotic taxa less related to native species are more invasive.

Some species introduced into new geographical areas from their native ranges wreak ecological and economic havoc in their new environment. Although many studies have searched for either species or habitat characteristics that predict invasiveness of exotic species, the match between characteristics of the invader and those of members of the existing native community may be essential to understanding invasiveness. Here, we find that one metric, the phylogenetic relatedness of an invader to the native community, provides a predictive tool for invasiveness. Using a phylogenetic supertree of all grass species in California, we show that highly invasive grass species are, on average, significantly less related to native grasses than are introduced but noninvasive grasses. The match between the invader and the existing native community may explain why exotic pest species are not uniformly noxious in all novel habitats. Relatedness of invaders to the native biota may be one useful criterion for prioritizing management efforts of exotic species.

Applying Logistic Regression to Determine Regeneration Risk to Stand Replacement Fire on the Kootenai National Forest, Montana

Abstract In 1994, fire managers on the Kootenai National Forest observed that wildfires had produced regeneration loss in some stands but not in others. They questioned what site characteristics and management activities were related to this loss. To address this question and to establish guidelines to “triage” stands and prioritize management efforts, we applied a logistic regression model to data from a set of regeneration stands (n = 135) located on the Libby, Rexford, and Three Rivers Ranger Districts. The occurrence of a stand replacement fire was modeled as a logistic function of Aspect, Habitat Type, Fuel Treatment, and logarithm of trees/ac (log_TPA), with R2 = 0.523 (P < 0.05). Odds ratios derived from logistic regression identified the descriptor characterizing regeneration stands “most at risk” for a stand replacement fire and provided a means to triage stands. Southwest and south aspects had the highest odds ratios (22 and 9) and largest coefficients of variation (3.07 and 2.22) for the Aspect variable. Western hemlock/queencup beadlilly (Tsuga heterophylla/Clintonia uniflora) and western red-cedar/queencup beadlilly (Thuja plicata/Clintonia uniflora), with respective odds ratios of 30 and 17, had the largest coefficients of variation (3.40 and 2.83) for the Habitat Type variable. For the Fuel Treatment variable, the “no fuel treatment” category had the highest odds ratio (11) and coefficient of variation (2.38). Stands with stand replacement fire had a mean log_TPA significantly lower than that of non-stand replacement fire stands (P < 0.001). Competition from understory vegetation may explain these findings. Bracken fern (Pteridium aquilinum), commonly found in cedar-hemlock stands and on southerly aspects, may outcompete tree seedlings and provide a fine-fuel hazard. West. J. Appl. For. 18(3):155–162.

Impact: Toward a Framework for Understanding the Ecological Effects of Invaders

Although ecologists commonly talk about the impacts of nonindigenous species, little formal attention has been given to defining what we mean by impact, or connecting ecological theory with particular measures of impact. The resulting lack of generalizations regarding invasion impacts is more than an academic problem; we need to be able to distinguish invaders with minor effects from those with large effects in order to prioritize management efforts. This paper focuses on defining, evaluating, and comparing a variety of measures of impact drawn from empirical examples and theoretical reasoning. We begin by arguing that the total impact of an invader includes three fundamental dimensions: range, abundance, and the per-capita or per-biomass effect of the invader. Then we summarize previous approaches to measuring impact at different organizational levels, and suggest some new approaches. Reviewing mathematical models of impact, we argue that theoretical studies using community assembly models could act as a basis for better empirical studies and monitoring programs, as well as provide a clearer understanding of the relationship among different types of impact. We then discuss some of the particular challenges that come from the need to prioritize invasive species in a management or policy context. We end with recommendations about how the field of invasion biology might proceed in order to build a general framework for understanding and predicting impacts. In particular, we advocate studies designed to explore the correlations among different measures: Are the results of complex multivariate methods adequately captured by simple composite metrics such as species richness? How well are impacts on native populations correlated with impacts on ecosystem functions? Are there useful bioindicators for invasion impacts? To what extent does the impact of an invasive species depend on the system in which it is measured? Three approaches would provide new insights in this line of inquiry: (1) studies that measure impacts at multiple scales and multiple levels of organization, (2) studies that synthesize currently available data on different response variables, and (3) models designed to guide empirical work and explore generalities.