Anuran Habitat Associations and Minimums: Identification, Application, and Implications

Abstract

Anurans (frogs and toads) are model species for habitat conservation in disturbed areas due to their reliance on a range of land covers and sensitivity to fragmentation. As a group of species that are reliant on a landscape permeability to access different habitat throughout the year, their presence across the landscape can provide valuable information about minimum habitat amounts and distributions necessary to maintain habitat connectivity for a range of species. In Ontario, historical landscape disturbance and the availability of a long-term anuran monitoring database provides a study area suitable for exploring critical landscape characteristics and thresholds related to anuran species presence. Like many jurisdictions in North America, natural heritage planning in Ontario has adopted minimum habitat disturbance and buffer recommendations but low anuran occupancy in disturbed areas suggests they are insufficient to ensure long-term species presence. Requirements related to habitat connectivity in particular lack detail regarding the amount, type, placement, and permeability of land covers across the landscape. To address these fundamental gaps, this thesis identified ecological thresholds amongst several habitat connectivity and landscape composition metrics designed to model anuran habitat usage in Southern Ontario. It also discusses the land use implications of implementing biologically informed and ecologically meaningful minimum habitat protections for maintaining anuran habitat connectivity. Data for anuran occupancy was derived from acoustic survey data in the Birds Canada amphibian monitoring database. Due to regular species turnover and challenges with acoustic detection, the number of monitoring years required to account for all species present in a breeding wetland is uncertain. Using a sample of 66 wetlands in Southern Ontario with at least eight years of acoustic monitoring data and four identified anuran species, I constructed species accumulation curves and determined that a minimum of three years of standardized acoustic monitoring data area is required to capture a complete picture of anuran species composition. With this result, 290 wetlands across Southern Ontario with anuran acoustic data of six commonly occurring species were determined to be suitable for further analysis. In addition to several commonly used landscape composition metrics used in amphibian habitat modelling (e.g., proportion of forest/wetland, urban, and vegetated land cover), several other metrics related specifically to anuran habitat connectivity were designed and compared to anuran presence. Using Generalized Additive Models (GAMs), each metrics’ explanatory strength of anuran species presence was compared. I detail the construction and performance of one particular metric that explicitly models the isolated impact of vehicle impact on road crossing survival; accounting for the exponential reductions in crossing survival with increasing traffic intensity in a way that is not captured by least cost, cost distance, or circuit theory modelling approaches. This metric was compared against an unaltered cost distance tool, with three maximum movement distances and three minimum survival cut-offs of 50%, 25%, and 5%. The top-performing road crossing survival metrics explained up to 22% deviance, performed best for the spring peeper and gray treefrog, and performed similarly to the top-performing unaltered resistance models. A model that accurately captures the dynamics of road mortality for amphibians and which can be applied to other taxa is important for effective conservation and land use planning. I follow this with an expanded detailing of the construction of additional habitat connectivity metrics modelling juvenile anuran dispersal and overwintering habitat connectivity, exploring all possible combinations of explanatory metrics to identify additive habitat influences. These metrics were designed to address gaps in connectivity modelling methods which have not accounted for anuran movement behaviours. The juvenile dispersal metric divides the landscape around a source wetland into discrete wedges to simulate the auto-correlated and laterally restricted movement behaviours exhibited by juvenile dispersing anurans and outputs a measure of connectivity that reflects the proportion of directions that have reachable breeding habitat. The overwintering connectivity metric took a more traditional approach, using typical resistance modelling to output a metric quantifying the accessibility of overwintering forest habitat surrounding a breeding wetland. Both metrics used anuran occupancy as a response variable. The explanatory strength of landscape composition and habitat connectivity metric varied substantially between the six individual species, indicating notable differences in life history processes, landscape interactions, and species monitoring. Landscape composition metrics consistently explained the highest amount of variance in single-variable models regardless of species, explaining as much as 37% of deviance in anuran species occurrence using proportion of forest and wetland cover within 3000m. Habitat connectivity metrics related to road crossing survival, juvenile dispersal, and overwintering habitat performed best for the spring peeper and gray treefrog, explaining as much as 26% of variance in occupancy for spring peepers using the overwintering habitat connectivity metric. Various areas for improvement and discussions of the implications of these results are included. The performance of landscape composition and connectivity metrics at different maximum movement distances varied, indicating species-specific differences in the effects of habitat composition, distribution, and permeability across difference spatial scales. In multi-variable GAMs, the top performing models were again for the spring peeper and gray treefrog, with up to 48% of variance in gray treefrog occupancy explained. Habitat connectivity metrics were often included in the top-performing models, suggesting that they contribute additional finer-detail significant explanatory strength to anuran population distributions and with further refinement can be valuable tools for landscape planning. Critical thresholds in landscape composition and habitat connectivity were identified across all six species and spatial scales using a segmented regression approach. Significant thresholds were most commonly identified in the landscape composition metrics and strongest with the natural cover composition. For the spring peeper and gray treefrog which had the strongest threshold-type relationships, thresholds at ~40-60% natural cover, ~5-35% of forest and wetland cover, and ~12-55% of urban cover were identified, varying depending on radial distance. Visual breakpoint estimation differed from statistical breakpoint estimation occasionally, emphasizing the importance of critical interpretations of statistical results. Threshold presence was also moderately consistent with non-linear relationships identified in the GAMs, suggesting that species-landscape relationships require critical examination when determining biological recommendations. The identification of thresholds amongst the various habitat and connectivity metrics refines our understanding of minimum biological requirements for anurans and provides additional evidence for science-based conservation efforts. In the concluding of this thesis, I discuss the financial efficiencies related to infrastructure, health, and planning, as well as societal improvements to quality of life for implementing biologically informed minimum habitat protections for anurans and other species reliant on landscape permeability. Identifying and planning around existing wildlife movement corridors and broader habitat thresholds associated with landscape composition and permeability can encourage planning bodies to produce a denser, more efficient, and more equitable landscape for both humans and the wildlife populations living alongside them.