Temporal patch occupancy dynamics of the Siberian flying squirrel in a boreal forest landscape

Ability to predict species distribution in a landscape is of crucial importance for natural resource management and species conservation. Therefore, the understanding of species habitat requirements and spatio-temporal dynamics in occurrence is needed. We examined patch occupancy patterns of the Siberian flying squirrel Pteromys volans in northern Finland across a seven year study period. Forest patches dominated by mature spruce ( Picea abies ) in a study area (375 km²) were surveyed to monitor the presence or absence of the flying squirrel. The patch occupancy pattern was dynamic: about half of the habitat patches were occupied at least once during the study period and more patches were colonised than were abandoned. Patches that were continuously occupied (i.e. occupied during all sample periods) were typically of high quality (based on habitat and landscape characteristics), continuously unoccupied patches were usually of low quality, and intermediate quality patches were occupied intermittently. The variables explaining patch occupancy were similar each year, and a statistical model based on data from the year 2000 also predicted occupancy in 2004 with similar accuracy. However, data from a single survey were inadequate for identifying patches used intermittently by flying squirrels. Despite inconsistent occupancy, these patches may be important for the local persistence of flying squirrels. The dynamic occupancy pattern may thus affect estimates of suitable habitat area and identification of functional patch networks for landscape planning. These results emphasise the need for follow-up studies to better understand population patterns and processes in time.     

The vulnerability of habitats to plant invasion: disentangling the roles of propagule pressure,time and sampling effort

Aim To quantify the vulnerability of habitats to invasion by alien plants having accounted for the effects of propagule pressure, time and sampling effort. Location New Zealand. Methods We used spatial, temporal and habitat information taken from 9297 herbarium records of 301 alien plant species to examine the vulnerability of 11 terrestrial habitats to plant invasions. A null model that randomized species records across habitats was used to account for variation in sampling effort and to derive a relative measure of invasion based either on all records for a species or only its first record. The relative level of invasion was related to the average distance of each habitat from the nearest conurbation, which was used as a proxy for propagule pressure. The habitat in which a species was first recorded was compared to the habitats encountered for all records of that species to determine whether the initial habitat could predict subsequent habitat occupancy. Results Variation in sampling effort in space and time significantly masked the underlying vulnerability of habitats to plant invasions. Distance from the nearest conurbation had little effect on the relative level of invasion in each habitat, but the number of first records of each species significantly declined with increasing distance. While Urban, Streamside and Coastal habitats were over‐represented as sites of initial invasion, there was no evidence of major invasion hotspots from which alien plants might subsequently spread. Rather, the data suggest that certain habitats (especially Roadsides) readily accumulate alien plants from other habitats. Main conclusions Herbarium records combined with a suitable null model provide a powerful tool for assessing the relative vulnerability of habitats to plant invasion. The first records of alien plants tend to be found near conurbations, but this pattern disappears with subsequent spread. Regardless of the habitat where a species was first recorded, ultimately most alien plants spread to Roadside and Sparse habitats. This information suggests that such habitats may be useful targets for weed surveillance and monitoring.     

A toad more traveled: the heterogeneous invasion dynamics of cane toads in Australia

To predict the spread of invasive species, we need to understand the mechanisms that underlie their range expansion. Assuming random diffusion through homogeneous environments, invasions are expected to progress at a constant rate. However, environmental heterogeneity is expected to alter diffusion rates, especially by slowing invasions as populations encounter suboptimal environmental conditions. Here, we examine how environmental and landscape factors affect the local invasion speeds of cane toads (Chaunus [Bufo] marinus) in Australia. Using high-resolution cane toad data, we demonstrate heterogeneous regional invasion dynamics that include both decelerating and accelerating range expansions. Toad invasion speed increased in regions characterized by high temperatures, heterogeneous topography, low elevations, dense road networks, and high patch connectivity. Regional increases in the toad invasion rate might be caused by environmental conditions that facilitate toad reproduction and movement, by the evolution of long-distance dispersal ability, or by some combination of these factors. In any case, theoretical predictions that neglect environmental influences on dispersal at multiple spatial scales may prove to be inaccurate. Early predictions of cane toad range expansion rates that assumed constant diffusion across homogeneous landscapes already have been proved wrong. Future attempts to predict range dynamics for invasive species should consider heterogeneity in (1) the environmental factors that determine dispersal rates and (2) the mobility of invasive populations because dispersal-relevant traits can evolve in exotic habitats. As an invasive species spreads, it is likely to encounter conditions that influence dispersal rates via one or both of these mechanisms.     

Forces structuring tree squirrel communities in landscapes fragmented by agriculture: species differences in perceptions of forest connectivity and carrying capacity

The North American red squirrel ( Tamiasciurus hudsonicus ) has expanded its range into the central hardwoods of the United States in conjunction with increasing forest fragmentation and declining gray squirrel ( Sciurus carolinensis ) populations. We used translocation experiments and patch occupancy data to test for interspecific differences in mobility and sensitivity to habitat loss and modification by agriculture. We released squirrels in fencerows to test the hypothesis that gray squirrels display inferior mobility relative to red and fox ( S. niger ) squirrels. Elapsed time to movement from fencerows for 76 individuals increased with distance to forest patches and harvesting of crops. Gray and red squirrels took longer to move from fencerows than fox squirrels, and gray squirrels were less successful at moving from fencerows than red and fox squirrels. Ecologically scaled landscape indices revealed the degree to which interspecific differences in mobility and individual area requirements accounted for the occurrence of these species across landscapes. Gray squirrel distribution was constrained both by individual area requirements and dispersal ability. Occurrence of red and fox squirrels was related to patch size but was unaffected by landscape connectivity.     

Economics of invasive species policy and management

This article examines the use of economic analysis to inform bioinvasion management, with particular focus on forest resources. Economics is key for understanding invasion processes, impacts, and decision-making. Biological invasions are driven by and affect economic activities at multiple scales and stages of an invasion. Bioeconomic modeling seeks to inform how resources can be optimally allocated across invasion management activities—including prevention, surveillance programs for early detection and management, and controlling invasion populations and spread—to minimize the long-term costs and damages. Economic analysis facilitates understanding of decisions by public and private decision-makers, gaps between these, and the design of policies to achieve socially desirable outcomes. Private decision-makers may undercontrol invasions relative to socially optimal levels, because they generally account for their own costs and benefits of control but less often for broader ecosystem impacts or future spread across the landscape. Economic analysis considers approaches for increasing private invasion management and evaluates feedbacks between ecological and economic systems that can affect policy outcomes. Future research should continue evaluation and design of control strategies across the biosecurity continuum and across species to enhance cost-effectiveness, better incorporate uncertainty into policy design, increase focus on incentives and behavioral tools to influence private behaviors that affect invasion spread, and incorporate invasive species consideration within broader systems-focused science. In addition, challenges in valuing biodiversity and ecosystem service impacts and the costs and effectiveness of control measures are key data gaps. Greater collaboration between decision-makers and researchers will facilitate development and communication of usable economic research.     

Can Invasive Species Facilitate Native Species? Evidence of How, When, and Why These Impacts Occur

Although the predatory and competitive impacts of biological invasions are well documented, facilitation of native species by non-indigenous species is frequently overlooked. A search through recent ecological literature found that facilitative interactions between invasive and native species occur in a wide range of habitats, can have cascading effects across trophic levels, can re-structure communities, and can lead to evolutionary changes. These are critical findings that, until now, have been mostly absent from analyses of ecological impacts of biological invasions. Here I present evidence for several mechanisms that exemplify how exotic species can facilitate native species. These mechanisms include habitat modification, trophic subsidy, pollination, competitive release, and predatory release. Habitat modification is the most frequently documented mechanism, reflecting the drastic changes generated by the invasion of functionally novel habitat engineers. Further, I predict that facilitative impacts on native species will be most likely when invasive species provide a limiting resource, increase habitat complexity, functionally replace a native species, or ameliorate predation or competition. Finally, three types of facilitation (novel, substitutive, and indirect) define why exotic species can lead to facilitation of native species. It is evident that understanding biological invasions at the community and ecosystem levels will be more accurate if we integrate facilitative interactions into future ecological research. Since facilitative impacts of biological invasions can occur with native endangered species, and can have wide-ranging impacts, these results also have important implications for management, eradication, and restoration.Contribution Number 2293, Bodega Marine Laboratory, University of California at Davis.     

Invasion speeds in fluctuating environments

Biological invasions are increasingly frequent and have dramatic ecological and economic consequences. A key to coping with invasive species is our ability to predict their rates of spread. Traditional models of biological invasions assume that the environment is temporally constant. We examine the consequences for invasion speed of periodic and stochastic fluctuations in population growth rates and in dispersal distributions.     

Heterogeneity in patch quality buffers metapopulations from pathogen impacts

Many wildlife species persist on a network of ephemerally occupied habitat patches connected by dispersal. Provisioning of food and other resources for conservation management or recreation is frequently used to improve local habitat quality and attract wildlife. Resource improvement can also facilitate local pathogen transmission, but the landscape-level consequences of provisioning for pathogen spread and habitat occupancy are poorly understood. Here, we develop a simple metapopulation model to investigate how heterogeneity in patch quality resulting from resource improvement influences long-term metapopulation occupancy in the presence of a virulent pathogen. We derive expressions for equilibrium host–pathogen outcomes in terms of provisioning effects on individual patches (through decreased patch extinction rates) and at the landscape level (the fraction of high-quality, provisioned patches), and highlight two cases of practical concern. First, if occupancy in the unprovisioned metapopulation is sufficiently low, a local maximum in occupancy occurs for mixtures of high- and low-quality patches, such that further increasing the number of high-quality patches both lowers occupancy and allows pathogen invasion. Second, if the pathogen persists in the unprovisioned metapopulation, further provisioning can result in all patches becoming infected and in a global minimum in occupancy. This work highlights the need for more empirical research on landscape-level impacts of local resource provisioning on pathogen dynamics.     

Minimizing invader impacts: striking the right balance between removal and restoration

Invasive species can cause severe damage in their introduced range; this damage often persists even after removal of the invader. In order to efficiently allocate a limited budget between invader removal and restoration of habitat from which the invader has been removed, it is vital to quantify the impacts of the invasion within an economic context. Here we develop optimal management strategies for biological invasions, which minimize both the direct economic costs of removal and restoration, and the ecological costs of present and future damage caused by the invasion. We demonstrate how this can be formulated as a linear programming problem, enabling the fast and efficient computation of optimal solutions. Using a simple example, we outline some general principles for the optimal control of an invader that damages its environment. Notably, we show that the most effective strategies often switch the priority of removal and restoration over time, and outline the conditions under which restoration is prioritized over removal. The proportion of total funds allocated to restoration will depend on the annual budget, the persistence of damage, and the relative costs of damage, removal and restoration.     

Predicting the spread of invasive species in an uncertain world: accommodating multiple vectors and gaps in temporal and spatial data for <Emphasis Type="Italic">Bythotrephes longimanus</Emphasis>

Real-world uncertainties and data limitations make it difficult to predict how, when and where non-indigenous species (NIS) will spread. Typically only a small fraction of sites are sampled during only a few time intervals, such that we know neither the full spatial extent nor the true temporal progress of invasion. Yet, these unsampled locations might affect the invasion dynamics. We extend propagule pressure models to incorporate both human-mediated and natural fluvial dispersal vectors, and develop techniques to incorporate missing spatial and temporal data on invasions. We apply our model to Bythotrephes longimanus, a high-risk aquatic NIS, using a regional-scale 311-lake survey in a popular watershed in Ontario and extending our analysis to 1,300 unsampled lakes. Of 100 model runs with different random subsets of 50 sampled lakes reserved for validation, we were able to obtain an average area under the curve value of 0.89. Human-mediated dispersal accounted for 99.75% of the contribution of propagules to probability of establishment. Although the discovery rate is accelerating, our results suggest the annual rate of lake invasions is decelerating over time. Management efforts controlling recreational boating traffic out of the largest lakes in the system will be the most effective way of slowing the spread of B. longimanus in lakes within this system.     

Patch occupancy by squirrels in fragmented deciduous forest: effects of behavior

We tested whether species-specific behavioral traits could explain patterns of habitat patch occupancy by five different squirrel species in Ontario, Canada: the northern and southern flying squirrel (Glaucomys sabrinus and Glaucomys volans), the North American red squirrel (Tamiasciurus hudsonicus), the eastern chipmunk (Tamias striatus), and the eastern gray squirrel (Sciurus carolinensis). Flying squirrel species exhibit group nesting in winter, which may put them at risk of extirpation in small patches with few individuals to contribute to group nests. Flying squirrels are also volant, potentially making non-treed matrix a barrier. Our surveys revealed that G. sabrinus was most likely to occur in large patches that were embedded in landscapes with low connectivity, and least likely to occur in small patches in highly connected landscapes. Conversely, G. volans was most likely to occur in large, well-connected patches and least likely to occur in small, unconnected patches. Patch occupancy by the cursorial squirrels was not strongly influenced by patch area or isolation. These findings reinforce previous studies suggesting that an understanding of species-specific traits such as behavior is an important consideration when interpreting habitat fragmentation effects.     

Hotspots of plant invasion predicted by propagule pressure and ecosystem characteristics

Aim Biological invasions pose a major conservation threat and are occurring at an unprecedented rate. Disproportionate levels of invasion across the landscape indicate that propagule pressure and ecosystem characteristics can mediate invasion success. However, most invasion predictions relate to species’ characteristics (invasiveness) and habitat requirements. Given myriad invaders and the inability to generalize from single‐species studies, more general predictions about invasion are required. We present a simple new method for characterizing and predicting landscape susceptibility to invasion that is not species‐specific. Location Corangamite catchment (13,340 km²), south‐east Australia. Methods Using spatially referenced data on the locations of non‐native plant species, we modelled their expected proportional cover as a function of a site’s environmental conditions and geographic location. Models were built as boosted regression trees (BRTs). Results On average, the BRTs explained 38% of variation in occupancy and abundance of all exotic species and exotic forbs. Variables indicating propagule pressure, human impacts, abiotic and community characteristics were rated as the top four most influential variables in each model. Presumably reflecting higher propagule pressure and resource availability, invasion was highest near edges of vegetation fragments and areas of human activity. Sites with high vegetation cover had higher probability of occupancy but lower proportional cover of invaders, the latter trend suggesting a form of biotic resistance. Invasion patterns varied little in time despite the data spanning 34 years. Main conclusions To our knowledge, this is the first multispecies model based on occupancy and abundance data used to predict invasion risk at the landscape scale. Our approach is flexible and can be applied in different biomes, at multiple scales and for different taxonomic groups. Quantifying general patterns and processes of plant invasion will increase understanding of invasion and community ecology. Predicting invasion risk enables spatial prioritization of weed surveillance and control.     

Metapopulation dynamics and biological invasions: a spatially explicit model applied to a freshwater snail

The spatial spread of invading organisms is a major contemporary concern. We focus here on invasions in inherently fragmented habitats, such as freshwater systems, and explore the usefulness of metapopulation models in this context. Maximum-likelihood methods allow the estimation of colonization and extinction rates, as functions of habitat patch sizes and positions, from time series of presence/absence data. This framework also provides confidence intervals of these estimates and hypotheses tests. We analyze a previously unpublished 12-year survey of the spread of the introduced snail Tarebia granifera in 47 Martinican rivers. Simple metapopulation models reproduce with reasonable accuracy several quantitative aspects of the invasion, including regional abundance, spatiotemporal structure, and site-by-site colonization dates. Sensitivity analysis reveals that the invasion sequence depended strongly on metapopulation size (number of sites) and spatial structure (distances among sites). The invasion history has also been accelerated by stochastic events, as illustrated by a large, central river that happened to be colonized very early and served as an invasion pool. Finally, we discuss the benefits of this approach for the understanding of invasions in fragmented landscapes.     

Initial conditions and their effect on invasion velocity across heterogeneous landscapes

Accurate, time dependent control options are required to halt biological invasions prior to equilibrium establishment, beyond which control efforts are often impractical. Although invasions have been successfully modeled using diffusion theory, diffusion models are typically confined to providing simple range expansion estimates. In this work, we use a Susceptible/Infected cellular automaton (CA) to simulate diffusion. The CA model is coupled with a network model to track the speed and direction of simulated invasions across heterogeneous landscapes, allowing for identification of locations for targeted control in both time and space. We evaluated the role of the location of initial establishment insofar as it affected the pattern and rate of spread and how these are influenced by patch attributes such as size. Our results show that the location of initial establishment can significantly affect the temporal dynamics of an invasion. Traditional network metrics such as degree and measures of topological distance were insufficient for predicting the direction and speed of the invasion. Our coupled models allow the dynamic tracking of invasions across fragmented landscapes for both theoretical and practical applications.     

Patterns and Pathways in the Post-Establishment Spread of Non-Indigenous Aquatic Species: The Slowing Invasion of North American Inland Lakes by the Zebra Mussel

The zebra mussel, Dreissena polymorpha, has spread through eastern North American aquatic ecosystems during the past 15 years. Whereas spread among navigable waterways was rapid, the invasion of isolated watersheds has progressed more slowly and less predictably. We examined the patterns of overland spread over multiple spatial and temporal extents including individual lake districts, states, and multi-state regions in the USA and found that only a small proportion (<8%) of suitable inland lakes have been invaded, with the rate of invasion appearing to be slowing. Of the 293 lakes known to be invaded, 97% are located in states adjacent to the Laurentian Great Lakes with over half located in Michigan. Only six states have more than 10 invaded lakes and only in Michigan and Indiana have more than 10% of suitable lakes become invaded. At smaller spatial extents, invaded lakes are often clustered within a lake-rich region across southern Michigan and northern Indiana. This clustering appears primarily due to multiple overland invasions originating from the Great Lakes followed to a lesser extent by subsequent secondary overland and downstream dispersal. Downstream spread appears responsible for only one third of the inland invasions. Temporally, invasions peaked in the late 1990s, with only 13 new invasions (0.4% of suitable lakes) reported in 2003 in the four-state region surrounding Lake Michigan. Peak rates of invasion occurred 4–6 years earlier in Michigan relative to Indiana and Wisconsin, but this time lag is likely due to differences in the establishment of Great Lake source populations rather than ‘stepping stone’ dispersal across the landscape.     

What affects the probability of biological invasions in Antarctica? Using an expanded conceptual framework to anticipate the risk of alien species expansion

Successful alien species invasion depends on many factors studied mostly in post invasion habitats, and subsequently summarized in frameworks tailored to describe the studied invasion. We used an existing expanded framework with three groups of contributing factors: habitat invisibility, system context and species invasiveness, to analyze the probability of alien species invasions in terrestrial communities of Maritime Antarctic in the future. We focused on the first two factor groups. We tested if the expanded framework could be used under a different scenario. We chose Point Thomas Oasis on King George Island to perform our analysis. Strong geographical barrier, low potential bioclimatic suitability and resource availability associated with habitat invasibility significantly reduce the likelihood of biological invasion in Antarctica. An almost full enemy release (low pressure of consumers), the high patchiness of the habitat, and the prevalence of open gaps also associated with habitat invasibility increase the possibility of invasion. The dynamics of functional connectivity, propagule pressure and spatio-temporal patterns of propagule arrival associated with human activity and climate change belonging to the system context contribute to an increase in the threat of invasions. Due to the still low land transport activity migration pathways are limited and will reduce the spread of alien terrestrial organisms by land. An effective way of preventing invasions in Antarctica seems to lie in reducing propagule pressure and eliminating alien populations as early as possible. The expanded conceptual framework opens up wider possibilities in analyzing invasions taking place in different systems and with multiple taxa.     

Monitoring Golden-Cheeked Warblers on Private Lands in Texas

ABSTRACT A majority of North American breeding habitat for neotropical migrants exists on private lands, requiring monitoring strategies focused on habitat in these private holdings. We outline study designs and protocols using repeated Presence-Absence surveys across a gradient of patch sizes to develop a range-wide monitoring program for the endangered golden-cheeked warbler (Dendroica chrysoparia) in Texas, USA. We surveyed 200–400 point-count locations across approximately 30 private properties annually from 2005 to 2008. We used data from our surveyed patches (n = 147) and the Ψ (occupancy), p (detection), and γ = 1 - ɛ parameterization to estimate patch dynamics and associated detection probabilities for golden-cheeked warblers. Patch size had a strong association with patch occupancy, and all patches >160 ha were predicted to be occupied. We found no evidence that large golden-cheeked warbler populations located on public lands in the vicinity of our study area influenced occupancy dynamics. We conducted simulations across a range of detection probabilities to evaluate potential sample sizes for both standard- and removal-based occupancy modeling. Simulations using parameter estimates from our analysis indicated that removal-based sampling is superior to standard sampling. Based on our results, surveying golden-cheeked warbler presence in oak-juniper (Quercus-Juniperus) patches under a removal modeling framework should be considered as one alternative for range-wide monitoring programs because patch-level monitoring would be necessary to estimate proportion of range occupied. Large contiguous patches are rare across the species’ range; hence, conservation and management of the mosaic of smaller patches within a landscape context would be required for maintaining species viability. Thus, we recommend the identification of areas where smaller, contiguous patches represent a significant portion of the available habitat within the local landscape and targeting these areas for habitat maintenance and improvement.     

Landscape diversity slows the spread of an invasive forest pest species

According to the associational resistance hypothesis, diverse habitats provide better resistance to biological invasions than monocultures. Host‐plant abundance has been shown to affect the range expansion of invasive pests, but the effect of landscape diversity (i.e. density of host/non‐host patches and diversity of forest habitat patches) on invasions remains largely untested. We used boundary displacement models and boosted regression tree analyses to investigate the effects of landscape diversity on the invasion of Corsica by the maritime pine bast scale Matsucoccus feytaudi over an 18‐yr period. Taking the passive wind dispersal of the scale into account, we showed that open habitats and connectivity between host patches accelerated spread by up to 13%, whereas landscapes with high tree diversity and a high density of non‐host trees decreased scale spread by up to 14%. We suggest a new mechanism for such associational resistance to pest invasion at the landscape level, which we term ‘the pitfall effect’.     

Patch occupancy, population density and dynamics in a fragmented red squirrel Sciurusvulgaris population

We studied population dynamics of red squirrels in a group of small forest fragments, that cover only 6.5% of the total study area (4664 ha) and where distances to the nearest source population were up to 2.2 km. We tested effects of patch size, quality and isolation and supplementary feeding on patch occupation during 1995–99. Larger patches and patches with supplementary feeding had a higher probability of being occupied. No patch <3.5 ha was ever occupied. No effects of isolation were found, suggesting that the forest habitat in the study area is not sufficiently fragmented to influence red squirrel distribution across patches. For medium sized patches (3.7–21 ha), that were occupied some years, there was an increase in patch occupation over the years, even though overall population size tended to decrease. These patches had a high turnover, especially of males. Patches in which the squirrel population went extinct were recolonized within a year. For patches that were at least some years occupied, squirrel density depended on patch quality only. No effects of patch size, isolation and winter temperature on population density were found. These data suggest that in our study area habitat fragmentation has no effect on local squirrel density and that the random sample hypothesis explains the distribution pattern across patches.     

Predicting patch occupancy in fragmented landscapes at the rangewide scale for an endangered species: an example of an American warbler

AIM: Our objective was to identify the distribution of the endangered golden-cheeked warbler (Setophaga chrysoparia) in fragmented oak-juniper woodlands by applying a geoadditive semiparametric occupancy model to better assist decision-makers in identifying suitable habitat across the species breeding range on which conservation or mitigation activities can be focused and thus prioritize management and conservation planning. LOCATION: Texas, USA. METHODS: We used repeated double-observer detection/non-detection surveys of randomly selected (n = 287) patches of potential habitat to evaluate warbler patch-scale presence across the species breeding range. We used a geoadditive semiparametric occupancy model with remotely sensed habitat metrics (patch size and landscape composition) to predict patch-scale occupancy of golden-cheeked warblers in the fragmented oak-juniper woodlands of central Texas, USA. RESULTS: Our spatially explicit model indicated that golden-cheeked warbler patch occupancy declined from south to north within the breeding range concomitant with reductions in the availability of large habitat patches. We found that 59% of woodland patches, primarily in the northern and central portions of the warbler's range, were predicted to have occupancy probabilities ≤0.10 with only 3% of patches predicted to have occupancy probabilities >0.90. Our model exhibited high prediction accuracy (area under curve = 0.91) when validated using independently collected warbler occurrence data. MAIN CONCLUSIONS: We have identified a distinct spatial occurrence gradient for golden-cheeked warblers as well as a relationship between two measurable landscape characteristics. Because habitat-occupancy relationships were key drivers of our model, our results can be used to identify potential areas where conservation actions supporting habitat mitigation can occur and identify areas where conservation of future potential habitat is possible. Additionally, our results can be used to focus resources on maintenance and creation of patches that are more likely to harbour viable local warbler populations.