Shady dealings threaten a formidable predator: weeds tip the balance in a cryptic trophic cascade

Dr Ruchira Somaweera1,2, Dr Blair Bentley2, Dr Nicola J.  Mitchell2, Dr Bruce Webber1,2

1CSIRO Land and Water, Floreat, Australia, 2University of Western Australia , Crawley, Australia

Species interactions play a significant role in the resilience of natural ecosystems. Through range shifts and introductions, global environmental change means that novel taxa and their interactions are disrupting this community stability. The impacts of vegetation on carnivores is a topic rarely explored, especially with regard to large predators such as crocodylians. In north-western Australia, the introduced weed stinking passionflower (Passiflora foetida) smothers the nesting habitat of the Australian freshwater crocodile (Crocodylus johnstoni), rapidly overgrowing nests during the incubation period. To explore how stinking passionflower cover might influence two key temperature-driven demographic parameters: hatching success and primary sex ratio, we measured temperatures below ground at egg deposition level in sites used for nesting by crocodiles, both in bare ground patches and under adjacent weed cover, over two entire nesting seasons. While the cooling imparted by weed coverage did not push nest temperatures below critical levels, it had a large enough effect to potentially alter nest sex ratios. Soil temperatures differed by 2-3 °C between treatments during the temperature sensitive period, which exceeds the ‘transitional range of temperatures’ across which both sexes are produced. Our work reveals novel indirect interactions between an apex predator and an invading weed, which could have utility for predicting downstream effects on food webs and the population dynamics of species threatened by global change.


Ruchira is a herpetologist with research interests broadly centered around behavioural and evolutionary ecology of reptiles, especially crocodiles and sea snakes. His current research underpins the common theme of the adaptive significance of reptiles to the changes in their environment. He run projects across a range or ecosystems, with a particular focus on the tropical ecosystems of northern Australia and SE Asia, and complements this field-based research with controlled-condition laboratory experiments and mechanistic modelling.

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