INSECTS AND THE ABUNDANCE OF INSECTIVOROUS BIRDS

There is some evidence from around the world that the widespread use of insecticides is not only impacting significantly on the abundance of insects, but also on vertebrates that are dependent on them for food. The premise is that the fewer insects there are, there is less food for insectivores which, in turn, may affect their individual health and survival, breeding success, and consequent distribution and abundance of their populations. There is also likely to be widespread secondary-poisoning of insectivores that have consumed invertebrates sprayed with insecticides.

Insecticides are likely to be having a significant impact, but we must not lose sight of other causes in the decline of insects and insectivores. For instance, the devastating fires that burnt over 12.6 million hectares of forest, woodland and grassland in Australia during the 2019/20 bushfire season, are a stark reminder that wildfires can impact heavily and instantly on the status of animal and plant species, their habitats and their longer-term recovery.

Habitat clearance and modification is another impact that we need to take into consideration, even if it is partial habitat clearance. For instance, under-scrubbing, where canopy trees are retained, can have a direct major impact on insect abundance and the foraging habitat of insectivorous birds, but there are also indirect impacts. There is a growing body of research that is now exploring these latter impacts, by investigating the relationships of organisms within and between the trophic levels of an ecological community. Thinning of vegetation can alter the soil chemistry by altering the organic content of the soil and variations in soil moisture and soil temperatures. In turn, this can affect the microbial communities that inhabit the soil. Soil microinvertebrates (e.g. mites, small nematodes) that feed directly on the microbes, or benefit directly from the release of nutrients from the breakdown of organic matter by the microbes, are impacted by the changes in the microbial community. Soil macroinvertebrates that are also dependent on the soil nutrients released by microbial action (e.g. earthworms, myriapods, amphipods, cockroaches) or which feed on microinvertebrates or other macroinvertebrates (e.g. arachnids, ants, beetles, true bugs) are also adversely impacted. But these impacts extend beyond the soil layers because many adult insects that are found on tree trunks, logs and vegetation foliage have egg and/or larval stages that develop in the soil or in rotting wood and leaf litter on the ground (e.g. many beetles, cicadas, crickets and grasshoppers). Fewer ground invertebrate larvae mean fewer adult invertebrates above-ground. This potentially results in significantly less food for insectivorous vertebrates (reptiles, frogs, birds and mammals). So altered soil chemistry, resulting from thinning of habitat or increased nutrients (e.g. fertilisers) can have cascading effects on the higher trophic levels of an ecological community, which may be most noticeable as a decreased abundance in bird populations.

Then there are the more complex interactions - altered soil chemistry and moisture content can stress retained vegetation and make it more vulnerable to insect, pathogenic and fungal attacks. This may temporarily increase the abundance of some insects, which attack the tree foliage, which in turn causes more leaves to drop to the ground. But if there are not the right microbial and microinvertebrate balances in the soils and leaf litter, then it is going to take a lot longer for that litter to break down and release nutrients into the soil. A build-up in that litter (ground fuel), as a result of increased rate of leaf-drop and reduced rate of leaf decay, has the potential of making the habitat more prone to wildfires. Increased insect, pathogenic and fungal attacks can eventually kill plants, which ultimately mean fewer insects if there is no ongoing tree recruitment.

The above example is, admittedly, simplistic and does not take into account more complex relationships between organisms, nor does it take into consideration other larger impacts on insect abundance (e.g. climate change, broader landscape management, pollution, invasive species). But it illustrates that the natural world is a complex one and we need to take into account the INDIRECT, as well as the DIRECT ecological impacts, when discussing insect and bird abundances. It also means that we might not have large numbers of insectivorous birds in urban parkland or in street-scapes, especially if those habitats are canopy trees surrounded by lawn or paving, because of the absence, lower abundance or an imbalance of organisms in the lower trophic levels. In Australia, that open habitat structure favours aggressive bird species such as mynas, miners and wattlebirds, which arguably chase smaller or more timid birds away. But these urban habitats are also inadequate in providing enough food to sustain insectivorous bird populations for the reasons described above.