Why insects are important
There has been increasing concern about the decline in insect populations globally. A paper in the journal Biological Conservation, Worldwide decline of the entomofauna: A review of its drivers, carried out a metaanalysis of 73 historical reports of insect decline. They concluded from data, mainly referring to Europe and North America, that "almost half of the species are rapidly declining and a third are being threatened with extinction". This is unwelcome news for anyone interested in wildlife conservation. After all, insects provide vast amounts of food sources for food webs everywhere. For example, baby birds of all persuasions, including seed and nectar eater, need to be fed insects as babies to acquire adequate protein to build their bodies. In other words, they cannot fly without adequate insect protein to build feathers and the like.
The above report is focused on other continents, so what about Australia? David Yeates from the CSIRO comments on CSIROscope that there have been no formal studies done in Australia but "anecdotal reports from entomologists suggest lower than average populations across a number of species". In the early 1970s, when my father took us somewhere at night in the car we would have to stop a couple of times to clean the smear of dead insects off the windscreen. I had forgotten about this until some years back when reading the Moth Snowstorm by Michael McCarthy. Writing from the UK he recounted similar experiences to mine and labelled these reductions in population numbers the 'great thinning'. The memories came flooding back and the sickening realisation that an enormous biomass of insects is now gone. This abundant food source is no longer available to feed birds, noctural wildlife such as mammals, and amphibians. As insects are down at the base of the food chain, the great thinning of insect numbers flows onto the great thinning of pretty much everything else.
The are multiple reasons behind this discussed in the above report with industrial agriculture playing an important role. However, in this article the focus is on native and exotic plants.
Why native species of plants are critical
Douglas Tallamy, an American ecologist, has written two wonderful books - Bringing Nature Home: How Native Plants Sustain Wildlife in Our Gardens and Nature's Best Hope. These books show the importance of insects to sustaining native wildlife populations and, critically, how native plants support far great diversity and numbers of insects than exotic plants. He uses empirical research to show how how native plants are far superior to exotic plants in this regard. The fundamental reason for this, he maintains, is that native plants and endemic insect species have coevolved. Consequently, native plants are far more palitable to insects than the introduced variety. He shows how the replacement of native plants with exotic plants has large negative effects on the carrying capacity of an area for insects and, by association, bird populations and the like.
Tallamy concedes that some insects do adapt to feeding on exotic plants, but he argues these are exceptions that prove the rule. Some ecologists take exception to his findings. Arthur Shapiro, an entomologist at the University of California, disagrees with Tallamy. This scientific disagreement is discussed in this Smithsonian Magazine article.
The focus of Tallamy's mission is the conversion of the vast tract of lawns in our cities, suburbs and semi-rural areas into native plantings that will boost insect food stocks, amongst other ecological benefits. And, his work is in the United States. Does it apply here? It probably does as basic processes such as co-evolution apply both there and here. But without empirical studies we don't know for sure.
Insects and weeds
Even without hard quantitiative data, his work is nonetheless relevant to those of us interested in wildlife conservation in this country. This relevance is not so much about the need to regenerate or plant natives on our properties. This is a given as we do this anyway. Perhaps the more important question that his work raises for us is whether it can inform our approaches to weeds.
There is an important differentiation to make here. Yes weeds do often provide sources of food at times of the year when food is scarce. For example White-headed Pigeons feed on Camphor Laurel berries at a time of year when food is hard to come by here in the Northern Rivers. But how are these same weeds supporting healthy populations of say, caterpillars, which are important food for baby birds? If Tallamy is right, then very weedy landscapes may limit the food resources available to raising birds from eggs to leaving the nest, which brings us back to the comments above about the great thinning. Such a scenario further strengthens the need for weed control in wildlife conservation work, which is discussed in greater depth here.
This is an American video but he explains beautifully why native plants are so important