Hoverfly on blue flower

Native versus non-native plants

This blog post is based around my latest Budcast episode and I decided to put into written words what I speak about in that episode, because if you’re more of a reader than a listener, I’d still like to share my thoughts with you.

I listened to two audio books last week, both about how humans can and do impact upon ecosystems. The first was Bringing Nature Home by Douglas W Tallamy and the second was Darwin Comes to Town by Menno Schilthuizen. Although I loved Bringing Nature Home, I actually read Darwin Comes to Town as an antidote to it, I hoped it would provide some positives after Tallamy’s book made me feel frankly quite depressed. The message Tallamy conveys in his book is terrifying when you stop and consider the implications of it. If you haven’t read it, I thoroughly recommend picking up a copy, even if it is supremely sobering.

I expect most of us saw the recently published figures about the declining populations of the world’s insects. If you didn’t, the headline is that 41% of all species are going into decline and a third of those are going into extinction. Tallamy argues in his book that the replacement of native plants with non-natives is leading to habitat loss and a reduction in biodiversity. His point is that as humans, we always think if we remove a bit of habitat the wildlife will move elsewhere, the problem is, there is no elsewhere any more.

Biodiversity is directly linked to the size of an area; new species come about faster in larger areas, the rate at which species go extinct is slower, the risk is spread across a wider area and there’s built-in resilience. When habitats are fragmented, extinction occurs. When we disturb the landscape and fragment habitats, we make them perilously unstable. 

Tallamy talks about native versus non-native or alien plant species and this subject is not cut and dried. We often talk about naturalised species, that is, plants that are able to exist in the wild without human intervention and have been present in the natural landscape for a long time, usually centuries. As an example here in the UK, snowdrops are not a native plant, they are naturalised, with the first recorded sighting of the plant in the wild in 1778. Although we think of this as a long time, Tallamy cites the example of conifers in the Cupressaceae, Pinaceae and Taxaceae families that were introduced to Britain over 500 years ago, yet in that time, of the 500 species of moth that are available to feed on them, only 50 of those moth species choose to do so, suggesting that even after they become part of the natural landscape, these non-natives are never as useful as their native counterparts as food sources. Tallamy believes that in order to be classed as a native, a plant needs to be able to interact with its neighbours in a an ecosystem in what he refers to as “countless” ways. 

Native planting doesn’t have to be boring!

So why does Tallamy believe native insects won’t eat non-native plants? He thinks there are three reasons. Firstly, the plants we import as ornamentals are likely chosen for the very reason they’re resilient to pests, so they’re not a random sample of the plants available, they’ve been actively sought out for their unpalatability. Secondly, there needs to be long evolutionary periods in order for insects to adapt to the specific chemical mix that signifies a particular plant. It’s not that insects go around eating a bit of every leaf and spitting it out if it’s not suitable, they literally will not recognise certain plant materials as potential food sources and therefore won’t even try them. And thirdly, most insects are specialists and will likely only be able to eat or will try to eat the species they’ve evolved in tandem with. 

An important figure to bear in mind going forward is that 90% of insects are thought to be specialists, i.e. adapted to use say one particular plant species as its food source. These specialist insects that make up 90% of the insect population and the remaining 10% are generalists. What has caught me out in the past is thinking  that because there are bees on my cephelaria and butterflies on my buddleia, everything must be fine. Well, it seems it’s not that straight forward, as I’m only catering for the generalists and not the specialists, who make up the lion’s share of invertebrate biomass.

And how about these native plants versus non-natives? A recent RHS study found that in order to support our invertebrate species, we need to be planting or allowing in a wide range of native plants because they are providing a host for the invertebrates when they are at the larval stage. Cut these plants out of the equation and there’s nowhere for the our pollinators to breed, so it doesn’t matter how many pollen and nectar-rich plants we put in our gardens. 

So as the generalists seem happy enough to feed on non-natives, can’t we just allow the generalists to gradually out-compete and eventually replace those fussy specialists? Tallamy addresses this question in his book and the answer is a definite no. He carried out a field study where comparing the diversity and biomass of insects on the 4 most common natives and the 4 most common non-native plant species on a particular site. He found there was 4 times more herbivore biomass on natives and that they supported 3.2 times as many herbivore species. Most importantly, the two types of insects that make up the majority of birds diets were supported to the tune of 35 times more on native species. So although you may think I’ve been banging on about insects without seeming to care about other species, obviously bird populations will be adversely affected when their insect food supplies are low. 

Even more interestingly, he also found that natives plants produced twice as much biomass as non-natives, biomass being the quantity of insects that visited the site. He found that even though the generalists are eating the non-natives, they can’t benefit from them at the same rate as they can benefit from natives.

As a further nail in the coffin of native vs non-native habitats, Tallamy also cites a 2005 study by John D Lloyd and Thomas E Martin which looked at the reproductive success of a particular bird species in a native vs an exotic grassland and concluded that, quote, “the odds of a nest surviving a given day were 17% lower in the exotic habitat, and that nestlings grew more slowly, and had a smaller final mass in the exotic habitat.” Interestingly, despite having lower reproductive success in the exotic habitat, the researchers found no evidence that the studied species, the Chestnut-collared Longspurs preferred to nest in the native habitat, suggesting that animals who are feeding on the insects which feed on the plants can’t recognise a difference between the native and non-native habitats, they just see what they think is a viable habitat, they don’t always know best and make informed choices regarding their long-term survival. Therefore, we, as observers, need to understand that sometimes our intervention may be necessary and that sometimes, what looks like an outwardly thriving population of a species may not be doing quite so well when you look beneath the surface. And before you think that it’s fine, your garden birds are ok because you put food out all year round, it’s not, because a certain times of the year, such as when they have nestlings, some birds that are happy to eat seeds the rest of the time will need to be feeding insects to their young, for example, so there’s no getting round this need for invertebrates however much we try and prop up the system…

Join me on Tuesday 19th March for Part Two of the episode, where I talk about Menno Schilthuizen’s rebuttal of the idea that habitats should be preserved in aspic and how we can help do our part in our own gardens. You can subscribe in iTunes or Android if you don’t want to miss it. Thanks for reading.

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