Mice, rats and other rodents are a major threat to global food production and act as reservoirs for disease. Worldwide, there are about 2000 species of rodents, but only 5-10% are major pest species in agricultural and urban environments.
Some of these consume substantial amounts of agricultural produce, and in many developing countries, farmers consider rodents the main impediment to higher yields. In Tanzania alone, damage due to rodents causes an estimated annual yield loss of 5-15% of maize, corresponding to about $45million, and food which could feed about 2 million people.
Obviously, we need better pest control strategies than we have today.
Rodent management or control actions can have potentially harmful impacts on small mammals and other wildlife, especially where non-selective methods such as poisons are applied. However, without rodent pest management, native species may be at risk from aggressively invasive species such as the black rat, Rattus rattus, and from pathogens carried by the pest rodents.
In such contexts, proactive rodent management activities might be beneficial not only for humans through reduced crop losses and human health risks, but also for native wildlife and natural ecosystems through reduced risks of invasion by exotics and their pathogens.
An important ecosystem service provided by biodiversity is natural pest control. This is also one of the ecosystem services threatened by human activities.
Ecosystem services are the benefits that humans derive from ecosystems: provisioning services (e.g. food production); supporting services (e.g. soil retention and formation); cultural services (e.g. recreation); and regulatory services such as pollination and pest control – the focus of this article. Biodiversity is at the core of ecosystem services humans derive from the planet.
Biodiversity conservation is therefore part of the equation for sustaining human livelihoods. Global environmental changes threaten many ecosystems and are predicted to cause significant changes to the supply of ecosystem services that are vital for human well-being. A primary reason for concern over the current accelerated loss of species is the associated loss of ecological functions.
A first step toward attaining the goal of retaining these invaluable services is to identify ecosystem service providers – biodiversity components that are linked to and provide a specific ecosystem service. This is a complex process as it incorporates many role players. In this case, however, we focus on a single service: biological control of rodent pests in agricultural ecosystems.
The Benefits of Natural Predators
Managing your farm so that it can harbour populations of predators such as snakes and birds is an effective as well as a sophisticated technique. Reductions in the numbers of these animals has been shown to increase pest abundance in agricultural lands worldwide and moves towards an Integrated Pest Management (IPM) approach is gaining in popularity. Although data on the specific benefits in a South African context are lacking, it seems reasonable to assume that, given the dominance of rodents in many bird of prey and snake species, that these predators have a major agricultural benefit. And it is free.
Climate change models predict that South Africa will become hotter and drier in future. In a study done on European vertebrates that provide pest control services, researchers found that ecosystem service providers in areas suitable for agriculture will likely suffer species loss while cooler climates will increase in species diversity and number.
Other studies show similar results, predicting substantial losses of vertebrate species (amphibian, reptile, birds and mammals) which naturally prey on invertebrates and rodents. These losses are often associated with increased temperature and drought, which may push many species to environmental conditions that are beyond their critical tolerances. These very same conditions are also likely to increase rodent populations to a level that gives mice and rats a competitive advantage over humans. In short, hotter and drier climates – like in South Africa – are most vulnerable.
The Problem with Current Methods
The goal for any control programme of wild fauna is to obtain the effect wanted by man with a minimum of risk to himself and a minimum of impact to the ecosystem. Reductions in vertebrate abundance and species can be expected to decrease natural pest control in agricultural crops. This is problematic because natural pest-control services are becoming increasingly important as rodents begin to develop resistance to chemicals as use is curbed by environmental regulations and consumer trends.
It is well known that routine applications of toxic substances against field rodents affect non-target wildlife and pollute the environment. The effectiveness of treatment is often questionable. And while chemical control like the use of rodenticides may appear to work – it is often temporary in nature with populations restabilising shortly after treatment. Chemical methods used in isolation are unsustainable, expensive and ultimately diminish the biodiversity which we rely on to provide these services.
Integrated Pest Management
Given the diversity among rodent pests and the agro-ecosystems where they occur, a number of management strategies have been designed in the past. Most of them have been very successful under specific conditions and this has encouraged people to also try to apply them elsewhere in very different ecosystems. However, there is no “one-size-fits-all” approach. Rodent management strategies need to take into consideration the species in question and local conditions.
Options such as: rodenticides, trap-barrier systems, diversionary feeding, fertility control and outbreak forecasting each have a role and are better used in combination with a good understanding of the enemy one wishes to conquer. The biology of rodents is diverse and complex and so is the ecology of the fields in which they live. Specific approaches must be identified, based on the ecological characteristics of the agro-ecosystem and the pest species involved. Different techniques must be combined and fine-tuned to work optimally in each system.
Regardless of which other techniques are suitable, sustainable pest control can only be achieved by combining ecological methods that promote biodiversity and the pest control services they provide. The best way to do this is to promote the habitat conditions of their predators – ecological pest control.
We already know that predators reduce rodent numbers by feeding on them, but studies have also shown that high intensities of predators act as a signal, serving to reduce the reproduction rates of their prey and decreasing litter sizes and number of live pups.
Bio-control is the managed use of one organism to limit the numbers or spread of another. In essence, it involves the habitat promotion and manipulation of the indigenous predator populations by the supplementation of otherwise limiting resources – such as nesting sites in the case of barn owls or cover objects for snakes.
Snakes and owls are an excellent combination because they act as facilitating agents for one another. Owls drive rodents to cover, the preferred habitat of the snake. Such effects lead to predation on rodents even though the raptor itself is not directly responsible for it. Rodents shifting habitat to avoid either predator may inadvertently land in the clutches of the other.
Perhaps most importantly, the bio-control approach avoids the severe and systemic environmental costs of repeated doses of toxins: other rodent predators are unaffected, properties remain ecologically functional, and farmland is left healthy and productive. In the face of climate change, providing suitable habitat in artificial agro-ecosystems for the predators which we rely on may have far greater benefits than are currently realised.
Promoting Barn Owls
An optimal array of owl nest boxes in this area requires about one nest box per 25 ha of cropland, with each box spaced at least 500m from its nearest neighbour. While the capital outlay required to set up such an array of boxes can be more or less the same as the cost of poison needed to control rodents on an average farm in an average year (depending on the building materials used), maintenance costs for an existing nest box scheme are probably less than 5% of the cost of another application of poison.
Public opinion of snakes has been changing in recent years. Our past aversion to snakes has changed as many people now realize that snakes are important. Snakes play a complex role of both predator and prey in the natural environment. In an agricultural setting they can be beneficial for growers as they consume large numbers of rodents that impact crops.
A method to encourage their presence is to construct artificial snake habitat by providing cover objects for snakes to use, especially during the hot summers. During this time, for snakes like the puff adder, most of their hunting is done at night, when rodents are most active. During the day, they will conceal themselves where there is shade and security. Many agricultural areas have few cover objects and therefore snakes tend to be more visible and vulnerable, and therefore encountered more often.
Providing cover will reduce encounters with people and increase the habitat from which snakes can provide their pest control service from.
Snakes avoid conflict with people at all costs. A natural concern is that with more snakes come more snake encounters. Snakes are a fact of any rural area and, as we need them, preventative approaches are best.
That’s where we come in; we provide educational services such as Snake Awareness and Frist Aid courses as well as supply Snake Guardz lower leg protection.
We are also able to survey your property for snakes and make recommendations on ecological pest control methods specific to your area.
Biodiversity sustains us. Natural predators form part of this diversity. It is in all of our best interests to find ecologically friendly ways to manage pests and live in harmony with ecosystem service providers like snakes which assist us in their control, for free.