The perks and perils of roads

 

November 2022
Elia Pirtle, Local Landcare Facilitator

 

Spring is an amazing time in our region. The wildflower displays are stunning, the birds are singing, and we get the return of cool sunny days perfect for a drink on a patio with friends. But as we sadly know, springtime is also roadkill season. It is hard not to notice the many squished magpies, shinglebacks and turtles on our roads right now.

 

Why exactly do we see such high rates of roadkill this time of the year? 

 

The simple answer is that we are not the only ones feeling the irresistable draw of the warm sun on a cool day – early spring is the start of ‘reptile basking’ season, as our scaley neighbours leave their winter hides to find places to warm up. And what about all the magpies out and about right now?  It is of course ‘baby season’ for many animals, magpies included, and parents are still teaching their young how to safely find food.

 

The more complicated answer is that roads are notorious for attracting wildlife, in a phenomenon scientists call an “ecological trap.” This phenomenon contributes to wildlife deaths on our roads all year long, but with a particular rise in the spring time. Read on to learn more.

 

Roads can be very attractive to our local birds and reptiles, but it is a trap!

 

What is an “ecological trap”?

Animals are very good at looking for environmental cues to find places they can live, thrive and reproduce successfully. These cues are formed from tens of thousands of years of evolution – they are deeply hardwired into an animal’s instincts. And all that time, the association was strong between the presence of these cues and an animal’s survival.

 

However, in the last 200 years (which is an absolutely tiny fraction in the evolutionary history of many species), humans have added some strange new features to the environment. Some of these features have meant that long-established cues of good quality habitat have suddenly become very unreliable. 

 

A classic example is the presence of artificial light at night, which is an extremely new phenomenon to navigate for many species (see an example involving sea turtles included at the end of this article).

 

 

The perks of roads

Roads are yet another new feature in our environment that causes previously reliable cues to become “ecological traps”.  

 

Road sides tend to be very productive sites, and are thus often very attractive to wildlife as they contain many cues of good quality foraging and breeding grounds. As Dr. Alisa Coffin from the University of Florida discussed in her comprehensive review of the impact of roads on wildlife, roads can:

 

  • have very productive vegetation due to abundant light, runoff water and nutrients, thus supporting abundant insect populations;
  • provide abundant food sources for small predators such as insectivorous birds and mammals attracted by abundant insect populations;
  • act as convenient movement routes for wide ranging species like dingos and bats, or animals on the move looking for new breeding grounds or dispersing from their parent’s home ranges;
  • act as an easy foraging ground for larger predators (due to it being a common movement corridor) and scavengers (due to accumulation of roadkill);
  • and act as sources where water will pool after rain events attracting animals looking for a drink.

 

 

In Australia, roads have an additional draw for wildlife – due to the dramatic clearing that grassland and woodland habitats in south eastern Australia has experiences, roadsides often act as last remnant sites of native habitat.

 

For our “cold-blooded” or more accurately “ectothermic” neighbours, as Denim Jochimsen from the Idaho Fish and Game Department discusses in his comprehensive review of the effects of roads on reptiles and amphibians, there is an additional attraction for roads.  At this time of year, many reptiles are coming out of winter hibernations, as the days start creeping up in temperature. Many are starting to migrate around in search of new foraging or breeding grounds.

 

But in the spring season, when mornings can still be quite cool, they can’t just pop out of their burrow or rock hide and get straight into the days activities. They have some minimum body temperature they must first achieve before they can get out and about foraging effectively. So they need to start their day ‘warming up’.

 

A rock amongst the grass is usually the sort of spot they like to choose, as the surface of the rock captures more of the sun’s energy and warms up much quicker than the damp soil, while also creating a break in the grass canopy so the reptile can sit directly under the sun’s rays. So if a lizard gets excited about a good basking rock, you can imagine how attractive the expansive warm, dark black roads look to a chilly lizard.

 

Right now, we are getting many days where the air is still cool but we are getting more sunny days which can warm up the ground. They cannot resist the draw of a warm belly on the road pavement. 

 

 

The perils of roads

Of course, we know that roads are actually quite a bad place for wildlife to hang out due to the great risk of mortality caused by car collisions. The problem is, many species attracted to the productive features of roadside habitats have not had even close to enough time to evolve an instinctual association between the roads and high ‘car predation’.

 

The result is roadkill. Researchers from the University of Western Sydney calculated that 9 million kangaroos and wallabies may be killed on Australian roads every year. According to research from peri-urban roads near Sydney, as well as rural roads in the Australian interior, about 10% of local populations of swamp wallabies and kangaroos respectively were being killed on roads yearly (reviewed in Burgan and Brainwood 2008).

 

The numbers are likely much worse for smaller species like magpies and stumpy tail lizards, but their deaths are simply not recorded to the degree of larger mammals. One study in the United States observed a 100% mortality rate for snakes attempting to cross a highway in Florida. Another saw particularly high levels of autumn roadkill among snake species which regularly migrate to find winter burrows. Autumn is the time when sportsman activity (and thus car traffic) was highest in the wilderness area in question (reviewed in Jochimsem 2004).

 

This, thankfully not squished, sand goanna lived to bask another day, but it was playing it risky laying out in the middle of a road near Stawell

 

What about animals that can learn?

Animals may not have had the time to evolve an strong instinctive aversion to roads yet, but some species are quite good learners and may be able to learn how to navigate roads more safely within their own lifetime.

 

However, even for clever species, like our magpie ravens, this learning takes time, putting young birds at highest risk of car related deaths. This has been shown by a great deal of research conducted in Europe, with some studies showing that up to 90% of bird deaths between late spring and early autumn were of juvenile birds, while the rates of juvenile to adult deaths then tended to equalize into the winter (reviewed in Erritzoe et al. 2003). 

 

Around this time every year, it’s hard not to notice all the baby magpie ravens hanging out along our roadsides. They are learning from their parents that road sides are a great place to find food, in the form of basking lizards, insects and roadkill, but they are also still learning how to avoid speeding cars. As we can sadly see clearly each day, many of the babies don’t learn the lesson in time.

 


This little magpie juvenile in the backyard wasn’t getting my message that the ground is probably not a great place to spend the night, but with some persuasion eventually fluttered up into the tree

 

Take it easy on the roads!

The roadkill rates tend to drop again as the summer progresses, as the babies learn the rules of the road, and also as the ground temperatures warm and roads are no longer the by far best place for a chilly reptile to warm up. 

 

But right now, our baby birds and chilly reptiles seeking warm bellies need a bit of extra help, so please take it just a bit slower on the roads, to give them the best chance of making it through the spring!

 

 

 

 

A classic example of an “ecological trap”

 

Artificial light at night creates a classic “ecological trap” for many species that rely on suble reflections of light to identify breeding and feeding grounds at night. Sea turtles are one of these species.

 

Sea turtles have evolved to use a light based cue as soon as they emerge from their egg. When baby turtles hatch on the beach at night, they need to quickly get into the ocean to avoid being eaten. So they look for the sparkle of lights on the horizon, formed by the reflection of the moon and stars on the water! Once they see these lights, they head straight towards the brightest ones. For hundreds of thousands of years, the only night time lights these baby turtles would see would reliably lead them into the ocean. 

 

Of course this is not at all the case now – the nights are lit up with all manner of artificial lights in our towns and cities. This is bad news for baby turtles, who still have a deep instinct to crawl towards sparkling lights, trusting they would soon find themselves in the ocean. However now when they look for the brightest light, instead of it coming from the moon, it may come from street lights, luring the baby turtle towards human infrastructure. This is the opposite direction they need to go, often to their ultimate end. Some of the early research bringing this problem to light in Florida, USA was finding near complete mortality at some nest sites due to disorientation caused by artificial lights (reviewed in Witherington 1986).

 

 

Things are looking up for the baby turtles now, after the discovery of this light attraction led to a massive amount of research aimed at understanding and then minimizing artificial light driven mortality. Now sea turtle conservation plans, including at Australian breeding sites in QLD and WA take into account the need to minimize artificial city lights in the vicinity of known sea turtle hatch sites, to improve the survival success of the hatchlings.

 

References

Alisa W. Coffin. (2007). “From roadkill to road ecology: a review of the ecological effects of roads.” Published in the peer reviewed scientific journal: Journal of transport Geography: Volume 15(5).

Blair E. Witherington, (1986). “Human and natural causes of marine turtle clutch and hatchling mortality and their relationship to hatchling production on an important Florida nesting beach.” Dissertation available through the University of Florida.

Shelley Burgin and Meredith Brainwood. (2008). “Comparison of road kills in peri-urban and regional areas of New South Wales (Australia) and factors influencing deaths.” Published in the Royal Zoological Society of New South Wales book: Too close for comfort: Contentious issues in human-wildlife encounters.

Denim Jochimsen, et al. (2004). “A literature review of the effects of roads on amphibians and reptiles and the measures used to minimize those effects. Published by Idaho Fish and Game Department, USDA Forest Service.

Johannes Erritzoe et al. (2003). “Bird casualties on European roads—a review.” Published in the peer reviewed scientific journal: Acta Ornithologica, Volume 38(2).