Quantifying the impacts of an invasive weed on habitat quality and prey availability for tiger snakes (Notechis scutatus) in urban wetlands

Abstract

Invasive plants are a threat to natural ecosystems worldwide with urban wetlands being some of the most susceptible and highly modified environments of all. The tiger snake (Notechis scutatus) is a top predator that persists in urban wetlands in south-western Australia, many of which have been degraded by introduced kikuyu grass (Cenchrus clandestinus). To evaluate the potential impact of kikuyu grass on habitat quality for western tiger snakes we quantified the structural features of habitats within wetlands degraded by kikuyu grass and compared them to wetlands with native vegetation, examined tiger snake prey availability, assessed predation risk for juvenile snakes using clay models, and measured the thermal quality of the vegetation. Proliferation of kikuyu grass reduced habitat structural heterogeneity by reducing available bare ground and increasing vegetation density. This homogenisation of habitat structure had little effect on the predation risk for juveniles or the thermal properties of tiger snake shelter sites; however, one key snake prey species, the motorbike frog, had significantly lower abundance in the most impacted habitat. Habitat types with more structural complexity also offered tiger snakes more stable thermal regimes and lower predation risk. These findings indicate that the current extent of kikuyu grass invasion offers overall similar habitat quality for tiger snakes and may contribute to their persistence in urban wetlands, but they, along with their anuran prey, may befit from some increased habitat structural complexity in open areas.

Stuck in the weeds: Invasive grasses reduce tiger snake movement

“…For tiger snakes, sites dominated by kikuyu and native vegetation did not differ substantially with respect to prey availability, temperature or predation pressure for tiger snakes (Cornelis et al 2022), so we suspect these factors are not directly driving the observed differences in tiger snake movements.…”

“…Our findings add to an increased understanding of how changes in habitat composition, driven by invasive vegetation, can affect space use by animals. Our current findings suggest that management plans for urban wetlands in the Perth urban area should improve structural complexity of homogenous kikuyu habitat by planting native species to support the predators in these ecosystems (Cornelis et al 2022). However, it remains to be determined if and how this grass affects reproductive success and long-term survival of tiger snakes.…”

“…Herdsman Lake (HS; 31.92°S, 115.80°E) and Kogolup Lake (KL; 32.12°S, 115.83°E) is dominated by invasive kikuyu grass. Loch McNess in Yanchep National Park (Y; 31.54°S, 115.68°E) and Black Swan Lake (BS; 32.47°S, 115.77°E) are dominated by native vegetation, predominantly Schoenoplectus spp in open habitats; Ghania decomposita and Lepidosperma longitudinale in sedgelands close to water; and Banksia , Melaleuca and Eucalyptus spp in woodlands (Cornelis et al 2022; Fig. 1).…”

“…Invasive weed species in wetlands (Grella et al 2018) can replace native plant species, causing compositional and structural changes to the vegetation (Braithwaite et al 1989; Reed et al 2005). Habitat homogenisation by invasive plants can alter the structural complexity of microhabitats (Lambdon et al 2008; Cornelis et al 2022) and the availability of resources for animals (Schirmel et al 2016) including the thermal quality of the vegetation (Hacking et al 2015) and opportunities for camouflage (Valentine et al 2007). Moreover, structural components of the vegetation can make mobility (Newbold 2005) and foraging (Maerz et al 2005) more difficult, which can impact animal movement and behaviour (Doherty et al 2019, Stewart et al 2021).…”

“…The inter-plant distance of invasive grasses is often lower compared to native grasses, reducing the amount of bare ground that, together with limited plant diversity, results in reduced environmental heterogeneity (Litt and Steidl 2011; Lindsay and Cunningham 2012, Abom et al 2015). Consequently, increased vegetation density and reduced availability of bare ground are two key structural features of kikuyu grass that differentiates it from the native riparian grass/tussock vegetation that occurs naturally in these wetlands (Cornelis et al 2022).…”