The native versus non-native planting debate

How Justin Bere’s rooftop native wildflower meadow provides important support for the local house sparrow population as well as evidence of the role of native planting in the support of a healthy ecosystem.

May 16th 2009: The local house sparrows have suddenly found my rooftop wildflower meadow very interesting. After the adults have fed themselves on sunflower seeds that they collect from nearby bird feeders, they have taken to flying over to the native wild flower meadow roof where they collect beak-fulls of insects and fly off, presumably carrying them to their fledgling chicks. I have noticed that the sparrows are most interested in the Common Vetch native wild flower which is abundant on the roof thanks to ecologist and green roof expert, Dusty Gedge’s planting advice. Close inspection shows that the Common Vetch is host to an abundant supply of huge juicy aphids; one of the favourite foods for baby sparrows, and essential for the survival of the first of the season’s brood of chicks. Further research (below) indicates that Common Vetch is known to be unusually attractive for aphids and that by supporting healthy aphid populations, the Common Vetch in turn supports the declining house sparrow population. I believe that this may indicate a strong link between native Common Vetch and a healthy sparrow population. It also suggests more broadly that green roofs planted with native wildflowers may provide a particularly high level of support for native ecosystems that may not be immediately obvious to many green roof specifiers.

The debate about native or non-native plant species is interesting. A wild eco-system is complex and dynamic, consisting of a rich range of fungi, flowers, trees, insects, birds and mammals living together in immensely complex but reasonably balanced inter-dependent relationships. Common sense would suggest that native species are more likely to fit back into a healthy local ecosystem from where they came, and that non-native species will by definition be misfits. Moreover there are many well known ecological misfit disasters, such as the Japanese Knot Weed. Nature is not black or white, but multi-coloured, so it seems logical to me that there are, in addition to the well known ecological catastrophes, also many less obvious ecological upsets that go unnoticed.

I have therefore chosen to concentrate on native plants for my own green roofs in order to provide what I believe to be the best opportunity for a natural ecosystem to develop. Bumble bees covered my wildflower meadows last summer and it was a pleasure to see the numbers grow over the weeks and months and to see how the bees got bigger and stronger as the season went on. Each time I worried that one flower was coming to an end, another took over the job of supplying food. I noticed that the bees were particularly fond of cornflowers and poppies. This spring another roof area gave early support to bumble bees with Soloman’s Seal beneath a Hazel coppice and following this another meadow is thickly covered with Foxgloves and Verbascum under a Hawthorn thicket. The latter roof is presently providing the next nectar flow as the bee populations strengthen.

Honey bees are also present on the various green roofs of my London home and an independent bee study is to begin shortly. Britain’s native honey bees were almost wiped out in 1921 by disease. Following this, bees from all over Europe were imported to rebuild stocks. Now queen bees are transported all around the world to satisfy the passion of many beekeepers for experimental breeding. Like many people, I am worried about the risks that might be posed by importing queen bees from around the globe. My fear is that this may upset symbiotic relationships between bees and their native habitats. I suspect that much damage has already been done and that studies of honey bee preferences in the UK are not as relevant as if the majority of UK bee colonies were still of a native or at least European origin.

Sure enough, some imported plants may turn out to be very popular with honey bees. For example, the Echium is imported from the Canary Islands and is very popular as a garden plant to support honey bees. I would be very interested to know if plants like the Echium would have been of interest to the native honey bee. Whatever the answer to this question, I cannot help wondering if the bees that flock to the nectar of the Echium aren’t at the same time ignoring some other important but less attractive native plants, such as native wild flowers that might need pollination in order to support the wider ecosystem.

One such plant might be the Common Vetch, although I don’t know how much this particular plant relies on the honey bee as opposed to the bumble bee or other flying insects for pollination. This hardy annual needs pollination to reproduce. Experience on my green roof suggests that sustaining the Common Vetch may provide critically important support for the dwindling house sparrow population.

Research by (1) Kate Vincent in Leicester and (2) Alexander Mitschke, Hilmar Rathjen and Sven Baumung in Hamburg provides convincing evidence that the drastic decline in house sparrows is largely caused by a failure in the supply of food to chicks. Michael McCarthy in the Independent reports that the Hamburg study shows that for the first brood in April, the house sparrow is almost entirely dependent on aphids to feed the young. For the second brood, the birds use ants which feed on the secretions the aphids produce. For the third brood at the end of the summer the birds use flies to feed their young. Sparrows do not travel more than about 50m from their nests so they require appropriate local food supplies for their chicks. If there is a failure in aphids, the Hamburg study shows that the first brood of sparrows will suffer or fail.

Kate Vincent’s research in the UK also suggests that aphids are an important part of the UK house sparrow chick’s diet. Kate’s research shows that aphids contribute more than any other food supply; a total of 35% of the diet of the birds surveyed. Furthermore Kate has discovered that fledglings with up to 20% plant material in their diet generally manage to survive. However, when there is a severe shortage of insects in the local environment, adults are forced to provide their chicks with a larger proportion of plant material and where this proportion increases to 70% it leads to almost certain chick death.

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Such high-level relationships between species are just the tip of the iceberg and we would be foolish to assume that native plants and animals don’t derive many more subtle support-mechanisms from each other in a healthy eco-system. To illustrate this point, the reason that Common Vetch is of so much interest to aphids is that Common Vetch has extrafloral nectaries on its leaf stipules that attract various beneficial predatory and parasitic insects including the aphid. Plants have a defence mechanism that prevents fluids leaking out of the plant when they are under stress, but aphids overcome this by injecting special saliva into the plant. As a result, Common Vetch is often infested by aphids which in turn serve as prey for not only sparrows, but various beneficial insects. The aphids feed on plant phloem and, consequently, survive on a diet relatively low in essential amino acids. Aphids have exploited this unusual niche by evolving a symbiosis with various bacteria that they carry to supplement the nutrients in phloem fluid and also improve the aphid’s resistance to parasitoids and fungi. Honeydew secretions from aphids are a source of food for both ants and honey bees. The ants later become an important source of food for the second brood of sparrow fledglings after the aphid population has typically crashed later in the summer due to a decline in the quality of available plant food.

So strong is the relationship of the Common Vetch and aphids, that in America, the California Lettuce Research Board is experimenting with planting its own native common vetch in lettuce fields to provide aphid feed for hover-fly larvae that maintain hover-fly populations ready to destroy aphids as soon as they appear on the lettuces. There are surely many other interdependent relationships in this miniscule part of the wider eco system. Specific bacteria are present in the guts of animals to aid digestion (it is thought that the human appendix may act as a store of bacteria to restore the gut quickly to full health after illness). So I cannot help wondering if perhaps some of the bacteria that exist in symbiosis with the aphid, might one day be found to provide a useful role in the gut of the fledgling house sparrow enabling it, like aphids, to derive sufficient nutrition from plant food as it becomes an adult? Perhaps the bacteria that enables the aphids to obtain high nutrition from plants, also serves to make the aphid so nutritious to the sparrow chicks? Perhaps, in an inter-dependent life cycle, adult sparrow droppings act to inter-seasonally replenish the same bacteria that is so essential for aphids?

In the construction industry we are familiar with risk assessments. The purpose of these is to consider every possible risk to health and safety of workers and the public. Each perceived risk is rated for its ‘likelihood’ and its ‘severity’. The most serious risk is one that has both a ‘high likelihood’ and a ‘high severity’. One of the main lessons that I would like to draw out of this little study is that the ‘likelihood’ of symbiotic relationships existing in a local native ecosystem is most certainly ‘high’. The example of the Sparrow, the Common Vetch and the Aphid illustrates that the ‘severity’ of upsetting such symbiotic relationships in a local native ecosystem can also be ‘high’. We would therefore be at best foolish and at worst arrogant if we ignored the broader implications of such a risk assessment concerning the maintenance of natural, local and above all, native eco-systems.

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House sparrow feeding on Common Vetch at Justin Bere’s rooftop meadow, Islington, London

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A pair of house sparrows flying in to feed on Justin Bere’s rooftop meadow, Islington, London

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A pair of house sparrows feeding on Justin Bere’s rooftop meadow, Islington, London

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Common Vetch providing host to aphid food supply on Justin Bere’s rooftop meadow, Islington, London

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