Insects and That
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Insects and That

Marauding moths make rare bird homeless

White moth on bamboo plant
Trying to coexist with this moth ain’t easy. Photo from: Fu et al., 2022

The success or failure of one species generally has knock-on effects on others.

This is, admittedly, not particularly dazzling ecological insight: take away a plant favoured by herbivores in an ecosystem and just watch the cascade, for example. Climate change is only enhancing the frequency and severity of swings of success in failure in a variety of habitats.

But let’s consider a more specific scenario. If you were to predict the impact of a boom in a butterfly or moth population on local birds, you might be tempted to assume that they’d simply eat a lot of said lepidopterans and have a great time.

Not all such connections are about food, however, and as such, we’re not always talking about a simple road to a population crash for the ecological ‘losers’ of a change. Sometimes the arrival of one can bring more subtle changes in another — as illustrated by a recent case of insect-bird dynamics in a Chinese nature reserve.

More moths means moved nests

Here, Pantana phyllostachysae Chao, a moth which feeds on bamboo leaves, bred in great numbers between 2013–15 due to notably warmer winters in Southwest China. This meant that the species moved to higher elevations within the Wawushan Nature Reserve than it is typically found, in order to keep up its voracious feeding.

The grey-faced liocichla is a threatened species of bird which has a marked preference for nesting in bamboo, at exactly those elevations. Moth-defoliated bamboo is not much use when it’s trying to raise a family, so when the insects moved into their range, occupying around 90% of suitable habitat, the new pressure meant that they had to consider other options.

Perhaps not surprisingly, these new nesting spots didn’t work out as well as their preferred plant. Surveying an area previously investigated for nesting activity in 2010 (and seven nests found), none at all were discovered in 2015, following the arrival of P. phyllostachysae Chao. By significantly extending the search range, five nests were eventually found.

Ultimately, starting a nest is all well and good — what matters, especially in the case of an endangered species, is whether it produces anything. Three of the seven nests found in 2010 were successful, but alas, none of the five from 2015 were ended up producing a brood. The reasons for this were simple: the nests weren’t concealed well enough among alternative plants, so they fell foul of predators.

Defoliated bamboo
Not ideal nesting habitat. Photo from: Fu et al., 2022

Three-year ‘blip’ or long-term trend?

All in all, this is a new and interesting example of what’s known as behavioural plasticity from the birds — shifting activities and preferences to accommodate new information. The major down side here, of course, being that it didn’t work.

While all this might be dismissed as simply survival of the fittest, the species that are the most fit to survive are shifting at a dramatic pace in our changing climate. In some cases, it seems to be altering the playing field so profoundly that some don’t have a chance.

In this case, the study’s authors suggested that in 2018, numbers of grey-faced liocichla nests may have bounced back from the climatic event-sparked moth boom, meaning that such negative effects could be short-term. And since the birds have shown the ability to change behaviour in the face of insect-decimated bamboo, maybe they’ll make better alternative nesting choices next time and raise a couple of broods through to adulthood.

Climate events can quickly become climate norms, so future prospects still don’t look particularly certain for this diminutive bird. It’s possible, though, that the behavioural plasticity that saw it consider alternative nesting arrangements could also see it widening its diet — specifically to consuming more P. phyllostachysae Chao. It’s rare that things work out so neatly in ecology, but you never know.

The researchers admit that they are unsure whether the bird could or does feed on the insect or its larvae — but it seems that if they are, it’s not currently to an extent that it’s bringing them anything close to net benefit. Perhaps a third party species will step in as an effective natural enemy of the moth, helping our beleaguered birds out?

While this example is just a snapshot, it does serve to illustrate the level of impact that invertebrates can make in ecosystems. The ‘little things that rule the world’ rarely fail to underline their credentials.

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Gary Hartley

Gary Hartley

Writer of different things. Come for the insects, stay for the odd literary works, or vice versa. @garyfromleeds https://medium.com/insectsandthat