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.