Overall, we found the general patterns of rodent catches, rodent activity and rodent related residents’ complaints to be consistent across measurements (Tables 1, 2 and 3; Fig 1). All three measurements were highest during the lockdown period and lowest post-lockdown (Fig 1). Rodent catches were slowly declining pre-lockdown, while an abrupt spike in catches during lockdown was seen, with an almost immediate crash and a slow recovery during post-lockdown (Table 1, Fig 2A). In contrast, rodent activity seemed to have been on the rise pre-lockdown, with lockdown triggering a steep decline in activity that continued post-lockdown (Table 2, Fig 2B). We found no temporal changes in the number of rodent related complaints received by the council (Table 3, Fig. 4). We found no spatial changes for any of the measurements between pre-lockdown, lockdown and post-lockdown periods (Fig. 4). The spatial distribution of multi-catch rodent stations seemed to have changed during lockdown and returned to the pre-lockdown distribution in the post-lockdown. Multi-catch and bait stations data pre-lockdown appear related, but this relationship seemed to have been disrupted during lockdown and continued to be disrupted post-lockdown.
Despite the higher levels of rodent catches, activity and sighting during lockdown is consistent with rodent population observations after hurricanes 22,23, whereas the rapid decline at the end of the lockdown period is not. We hypothesize that this might be due to the undeniable temporal and physical differences between a climatic event and a pandemic. Hurricanes are short-lived, with vast physical effects on the landscape, while the COVID-19 pandemic has had a long-term effect on human behavior and no tangible physical consequences in terms of infrastructure. Hurricanes might cause a shift in habitat characteristics, potentially increasing landscape heterogeneity thus encouraging rodent abundance 2,47. In contrast, social restriction, which involves closure of restaurants, cafes and other food venues 48, might have only reduced or eliminated human-derived food resources where they have been plentiful before. Commensal rodents show high levels of neophobia 49,50 and taste aversion 51,52 resulting in high levels of ‘trap-shyness’ 53,54 and low bait acceptance 55,56. However, a reduction in food resources might have driven animals to engage in “bold” behaviors during foraging whilst in a lower physiological state 57-60. These hunger-driven behaviors might explain reports of rats feeding in close proximity to humans 31,35,36 as well cannibalism 31,37. Ultimately, hunger might have caused these animals to overcome their neophobia and taste aversion, resulting in a decrease in trap shyness and higher bait acceptance, driving an increase in mortality by electrocution or poisoning.
A higher mortality by lethal traps and rodenticide, as well as the decrease in carrying capacity by the reduction of food resources, can explain the steep decline in catches, activity and sightings that we found after the lockdown period 61,62. Moreover, it is highly unlikely that the peaks in rodent activity, catches and complaints would be due to an increase in the rodent population, given that the lockdown period lasted only 45 days. Even rats, which are known for their prolific reproduction 63,64, would not be able to reproduce and mature in such a limited time frame. The recovery in the population is therefore expected to be more gradual, like the steady but slow increase in activity and rodent catches we found in the post-lockdown period. Remarkably, rodent related residents’ complaints seem to mirror rodent activity and tradability, similarly to what has been reported during periods of no disturbance 46. This is regardless of the potential for cognitive biases in residents perceptions that have been reported during COVID-19 41-43.
Given that our data did not cover several years, we were unable to account for natural seasonal cycles in the rodent population. Several studies have shown that urban rodent populations follow a seasonal gradient that reflect both human changes in behaviors and temperature 19,46,65,66. Colder months seem to trigger lower rodent activity, that then increase towards spring and peaks in summer 46,65,66. Our pre-lockdown multi-catch station data seems to support this, but not so our bait station data. It is possible that in a subtropical City such as Sydney (average minimum temperature 15.7◦C 67) the effect of seasonal changes in temperature might not be as strong as that detected in laboratory studies 65,66 and more temperate cities 46. Additionally, it has been well documented that cities are “heat islands” that experience significantly milder winters than surrounding areas 68. This might be more pronounced in coastal cities like Sydney. Moreover, the expected seasonal changes in rodent activity cannot explain the abrupt increase and decline in catches, nor the abrupt decline in rodent activity during lockdown. Therefore, we argue that the effects we report are solely due to the changes in human behaviors, and unintended effects on the rodent population, elicited by the COVID-19 restrictions.
Interestingly, we found no evidence of spatial changes driven by the lockdown. This supports the findings Parsons et. al 2020 reported from Warsaw, Poland but contrast with their results from New York City and Tokyo 45. They suggested that COVID-19 lockdown measures trigger an increase in rodent movement and potential massive migrations, based on the increased association of rats and food service establishments and the formation of new hotspots of rat sightings in New York City 45. Our data suggest the contrary and based on the well-known site fidelity pest rodents species show 69, it is difficult to reconcile that the effects measured are not localized. In the case of Tokyo and New York City, the result may reflect cognitive bias where residents are spending more time at home during social distancing measure are more likely to see rodents in a different area, and thus report and call pest controllers. Parsons et. al 2020, does suggest that this pattern of movement was not recorded in Warsaw, potentially because of the lack of restaurant clusters in that city, a situation that maybe be similar to the one in Sydney.
If the peak in rodent activity was indeed due to abnormal foraging behavior caused by starvation followed by a population crash, it is highly possible that the lockdown acted as a genetic bottleneck. The City of Sydney Council currently has 942 rodent bait stations deployed, versus a maximum of 60 multi-catch rodent stations deployed on any one day. It follows that rodents would be significantly more likely to encounter a bait station than a multi-catch trap. Thus, an increase in acceptance of rodenticide baits could be the main cause of a population crash, driven by the reduction of human-derived food resources during lockdown. Similarly to other reported cases, after such a mortality event, the genetic variation within the remaining population could be up to 90% lower than the original population 70. Australia is currently one of the only countries in the world where rodenticide resistance has not yet been detected. Thus far a limited number of studies have looked into the subject, either through feeding trials 71,72 or genetic detection of reported mutations in the VKORC1 gene responsible for conferring such resistance 73. Thus, there is a high possibility some individuals inhabiting Australian urban areas may carry mutations, either novel or previously reported 74, conferring on them rodenticide resistance. It follows that a higher proportion of the lockdown surviving individuals could be genetically resistant to rodenticides 75. These remaining individuals would then become the “founding gene pool” that would give rise to a genetically distinct and potentially rodenticide resistant population. This “new” population would not take long to recover and repopulate the area 64, evident by the rapid increase in rodent catches and activity we report post-lockdown. This is a very different interpretation to that offered by Parsons et. al 2020, where the assumption of the mass movement of rats might drive an increase in genetic variation due to interbreeding between not previously connected populations 76,77.
Although the risks of commensal rodents to be infected or transmit SARS-CoV-2 are low 78, we know that these animals pose other health risks 2,7-14. Thus, an increase in rodent-human interactions has the potential to place further strains on health systems around the world. This could become even more pronounced if rodenticide resistance in these pests become widespread 79. It is possible that the onset of COVID-19 might have disrupted not only human behavior, but also commensal rodent populations, with profound implications for the future management of these species.