1 Introduction
Simultaneously understanding species habitat suitability and niches can be useful for the conservation of rare species in the face of global change (Quiroga and Souto 2022), especially when introduced competitors are present (Pascual-Rico et al. 2020). Habitat suitability models often address different aspects of niche theory (Hirzel and Le Lay 2008). Grinnell (1917) established the connection between niches and habitat suitability, where he recognized a species niche is determined by factors regarding habitat quality, such as climate, food, and soil. This connection between niche theory and habitat suitability is the foundation for many species distribution modelling techniques (Richardson and Whittaker 2010), having applications in species conservation (Quiroga and Souto 2022), predicting niche changes in response to changing climate conditions (Petitpierre et al. 2012), and predicting invasion of non-native species (Santamarina et al. 2023, Ramirez et al. 2024). Species can share both geographic and environmental space leading to overlap in niches and habitat suitability. Quantifying the overlap in both environmental and geographic space can aid native species conservation.
Despite consistent conservation efforts over the last few decades, the rare New England cottontail (Sylvilagus transitionlis ) is still declining due to habitat loss and co-occurrence with an introduced competitor, the eastern cottontail (Sylvilagus floridanus ; Litvaitis et al. 2008; Kovach et al. 2022). New England cottontail was once found throughout eastern New York and New England wherever young forest, shrubland, and dense understory habitats were abundant (Nelson 1909). However, due to forest maturation and forest loss to urban development, young forest and shrubland habitat declined substantially during the 1900s (Brooks 2003, Litvaitis 2003, Lorimer and White 2003) and subsequently extirpations have occurred at state (Vermont, Rhode Island) and population levels throughout the remaining range (Litvaitis et al. 2006, Fenderson et al. 2011, 2014, Brubaker et al. 2014, Rittenhouse and Kovach 2020).
Conservation efforts for New England cottontail have included population augmentation from wild and captive populations (Bauer et al. 2020, Ferry 2023), translocation of wild populations (Eline et al. 2023a ), and using vegetation management to create and maintain suitable habitat (Kovach et al. 2022, Eline et al. 2023b ). Population augmentation and translocation have increased the genetic diversity of the five genetically isolated regional populations (Fenderson et al. 2011) and increased patch abundance and dispersal (Bauer et al. 2020). Maintaining young forest, shrubland, and mature forest with dense understory are crucial for New England cottontail because the high stem density provides cover and protection from predators (Barbour and Litvaitis 1993, Litvaitis 1993, 2001, Cheeseman et al. 2018, 2019b , 2021) and shrubland with higher vegetation height increases New England cottontail occupancy probability while decreasing eastern cottontail occupancy probability (Bischoff et al. 2023a ). Concurrent with landscape-level changes in habitat, range expansion of eastern cottontail into the New England cottontail range has complicated New England cottontail management efforts by displacing New England cottontail from young forest and shrubland habitat (Cheeseman et al. 2021) through interspecific competition (Probert and Litvaitis 1996, Cheeseman et al. 2018, Bischoff et al. 2023b ).
EC invading the range of the New England cottontail is an extraordinary case of invasion effects on rare species. Prior to 1899, the only cottontail found in New England was New England cottontail (Johnston 1972). The first eastern cottontail was introduced to mainland New England in the early 1900s, and subsequently private hunting clubs and state agencies continued to introduce eastern cottontail for several decades (Johnston 1972). In less than 100 years since the first confirmed eastern cottontail occurrence, eastern cottontail has become ubiquitous across most of the New England cottontail range (McGreevy et al. 2021). As the introduced competitor’s range expanded, the New England cottontail range and occupancy within its range declined (Litvaitis et al. 2006, Rittenhouse and Kovach 2020). The conservation concern surrounding New England cottontail allowed New England cottontail to be a highly researched species, and thus, habitat associations for both species in the northeastern United States are largely understood. The factors that encourage New England cottontail occupancy and survival while potentially discouraging eastern cottontail populations include vegetation height above 0.5 m (O’Connor 2015, Cheeseman et al. 2018, 2021, Bischoff et al. 2023a , b ) and moderate canopy closure (Buffum et al. 2015). However, these habitat conditions are ephemeral and dynamic across the landscape, thus the location of where these habitat conditions currently occur or might be created through vegetation management has yet to be determined.
Given the rapid expansion of eastern cottontail and concurrent decline of New England cottontail, we used habitat suitability models and niche overlap analyses to investigate overlap between the native New England cottontail and introduced eastern cottontail spatially (i.e., suitable habitat) and environmentally (i.e., niches). Our goal was to identify spatial and environmental areas suitable for New England cottontail but not eastern cottontail. To do this, we assessed how similar or equivalent the two species’ niches were and habitat uniquely suitable for each species. The specific hypotheses we tested for niche overlap and niche dynamics (changes in niches) included: 1) New England and eastern cottontail niche are more equivalent and similar than random, 2) the proportion of eastern cottontail niche without New England cottontail niche (expansion) is higher, 3) New England cottontail niche without eastern cottontail niche is lower (unfilling), and 4) eastern cottontail niche including New England cottontail niche (stability) is higher than if the two species’ niches were random. Spatially predicting overlap allowed us to identify future areas of invasion for eastern cottontail as well as new areas for New England cottontail sampling and habitat conservation.