Main Club, 5th August 2022
In the absence of our scheduled speaker, the breach was kindly filled by Damian Lettoof, who was to give the short focus talk. Instead, he gave a full-length talk on his PhD research at Curtin University about how urbanisation has affected Tiger Snakes in and around the Perth metropolitan area.
The Tiger Snake Notechis scutatus is a common snake of wetland ecosystems in the SW of WA. It is a top predator, meaning that it has no predators itself (Only the juveniles do). It eats almost anything that moves, but frogs are its favourite. It lives for ten to fifteen years in the wild. In and around Perth, the Tiger Snake is restricted to wetland “islands” and is very common at Herdsman Lake. It can serve as a “bio-indicator” in that it accumulates contaminants. This bio-accumulation of pollutants can be measured in snakes sampled to indicate the wetland’s health.
Damian chose four locations for his research: Yanchep, Joondalup, Herdsman Lake, and Bibra Lake. Only the Yanchep site was outside the urban fringe. He aimed to measure the impact of urbanisation and look at pollutants and parasites as health parameters. He found that the snakes carried several parasites: ticks which carry pathogens, skin worms, trematodes (flukes) in the mouth and body, and nematodes in great quantities in the stomach, which eat the snake’s food.
He found that the snakes accumulate a suite of metals and metalloids. He measured these elements in their livers and scales.
Because of a lack of available data, he also had to measure these pollutants in the lake sediments. The 16 metals he found included arsenic, barium, cadmium, cobalt, copper, mercury, molybdenum, tin, and selenium. Those metals in high concentrations in the sediment were also high in the snake livers. Though not dangerously high, they might speed up the snakes’ metabolism. Dredging and drainage (as at Herdsman) can release metals from the sediment. The amounts were very site-specific. Herdsman had the highest concentration of metals overall, but surprisingly, the Yanchep site had the highest concentration of some. Significantly, almost every snake contained rat poison – and Bobtails too.
Surprisingly, parasite load had little correlation with the health of the snakes. And if there was one parasite species, there were likely more. Damian talked about co-evolution, with a snake being a natural habitat for parasites. However, pollutant load did affect body condition.
Damian also looked at the genetic relationships of Tiger Snakes between sites, including some outside his study sites. Those on Carnac Island introduced there a century ago were the most isolated. There seemed to be gene flow between Bibra Lake and Black Swan Lake, south of Rockingham, but not between the sites north of the river.
Damian concluded that urbanisation has introduced many metal contaminants to food webs in metropolitan wetlands, but not at dangerously high levels for Tiger Snakes.
Mike Gregson