Richard Mally: Following the Hidden Logic of Insect Invasions

You have worked in several different countries — Germany, Norway, and now the Czech Republic. Looking back, how would you compare these research environments?

Each environment shaped me in its own way. Looking back, I realise that I seemed to always be at the right place in the right time. In Germany, I received the support that I needed in my early career, and my critical thinking was born there. In Norway, I developed independence, and with this the confidence to advance my own research ideas. Here in the Czech Republic, I arrived in a diverse and international team, providing me with the opportunity to develop cooperations and the skills that come along with them. Prague is a welcoming place, and I made dear friends here, through work as well as through the Czech Socratic Dialogue community.

Your research focuses on insect invasions from a macroecological perspective, linking host plants, herbivores, and large-scale patterns. What continues to surprise you most about insect invasions?

How little has been done in the field of insect invasions! Sure, there is a lot of literature out there on individual insect species and their spread, impact, and how to deal with them. But large-scale synthetic analyses? Not so much. On one hand, this is great, as it gives me the opportunity to do all that research that I am doing! On the other hand, it substantially limits our understanding how insect invasions might work differently from invasions of plants or vertebrates.

Why do you think that is?

I am sure this has to do at least in part with the sheer number of insects. More than a million species are scientifically described, and we know that there are a lot more undescribed species out there. But we don’t even have a grasp what the better estimate for our current total insect diversity is – 2 million or 30 million? And already the numbers of known non-native insect species in the world are staggering: there are about 9,000 of them that we keep track of. This makes insects one of the largest groups of non-natives, second only to plants.

What makes insects different from plants in this respect?

While most plant species were introduced deliberately into new world regions, most insects are unintentional arrivals. While plants are usually well-studied and -described (they don’t run away), many insect species are poorly known. So, it actually shouldn’t surprise me how much still needs to be done in the field of insect invasions.

What conceptual limitations arise if we do not view plant and insect invasions as interconnected and dynamically evolving processes?

I think we will continue to be two or three steps behind in dealing with biological invasions. Always reactive, rarely proactive.

What do you mean by that?

More than half of the non-native insects we know of are plant-feeders – that is a significantly larger proportion than the roughly one third of herbivores among all insect species. We call this ‘invasion disharmony’, meaning that numbers of non-native species in certain taxonomic or functional groups are out of proportion with what we’d expect from looking at the global numbers of species in these taxonomic or functional groups. So, there are far more non-native plant-eating insects than there should be based on the number of global plant-eating insects.

How is this linked to plant invasions?

We know that when global movement of plants increases, the number of non-native insect herbivores follows suit.

If we start treating plant and insect invasions as interconnected processes, the question of predictability becomes central. Which insect traits do you see as most promising for improving predictive invasion models — and where do trait-based approaches still fall short?

Host breadth immediately comes to mind. The wider the selection of things to feed on in the invaded range, the better your chances to establish and flourish. And my feeling is that many of those polyphagous insects are also fast reproducers, and good spreaders – “the fast and the furious” among invaders. This is something we will soon be looking into in more detail.

And where do trait-based approaches reach their limits?

Where a trait-based approach might fall short is predicting the impact a non-native species might have. Most ecosystems feature species that have coevolved to a point of balance, where in plants mechanisms to fend off herbivores evolve, and in the herbivores mechanisms to overcome those defence mechanisms.

What happens when this balance is disrupted?

We know of several examples where this balance was severely compromised by insect herbivores that overpowered plants in the invaded range because they could easily overcome the plant defences. The box tree moth Cydalima perspectalis or the Emerald ash borer Agrilus planipennis are prominent examples.

Your analyses suggest that some trophic groups are more invasion-prone than others. Do you think this reflects the ecological roles these insects play, or deeper biological characteristics shared within those groups?

For one, this tells us about the quality and strength of past and present invasion pathways. Their strength varies with the intensity of global trade of goods, and their quality with cargo hygiene, the transport duration and the type of goods that are traded.

So some groups are simply better prepared for this kind of human-mediated movement?

I am convinced that some groups are indeed better pre-adapted for human-mediated invasions – think of all those synanthropic detritivores like cockroaches, carpet beetles or silverfish that are now virtually cosmopolitan. Others, like thrips and aphids, can quickly build up large populations through asexual reproduction or inbreeding, giving even tiny invading populations a fighting chance for establishment.

But how much of this pattern might simply reflect the way we collect data?

Our analyses are only as good as the underlying data. And we know that these data contain biases. For example, herbivores are of higher concern to agriculture, forestry and other plant-related economies than other trophic insect groups, which means they get special attention in monitoring efforts and are therefore probably over-reported compared to other groups like parasitoids or detritivores, who often go unnoticed. It seems as if there’s no money in a species (either bad as “pest” or good as potentially beneficial), it is not worth putting time and effort in studying it.

In many large-scale analyses, human infrastructure and activity emerge as strong predictors of invasion patterns. How do you interpret these anthropogenic factors – as ecological mechanisms in their own right, or primarily as proxies for propagule pressure and movement pathways?

Human-influenced environments often come with disturbance that is beneficial for the establishment of certain species, as it provides an unoccupied niche or an enemy-free space, or both.

And what about the role of transportation itself?

Anything that speeds up human transportation also speeds up the spread of species, turning human highways into invasion highways. Add to this the sheer number of cars and trucks – each a potential ‘transport quantum’, capable of moving unintentional hitchhiker species hundreds to thousands of kilometres within days. For small-bodied organisms like insects, this is like natural spread “on steroids”. As such, human infrastructure and activity are the scaffolding for species invasions, and are ecological mechanisms in their own right.

When we consider species traits, ecological roles, and human activity together, invasion systems become highly complex. How can invasion ecology move beyond identifying patterns to better understanding the mechanisms behind them?

The one thing I constantly remind myself of is this: biological invasions are 100% human-made, and they take place in a world that is continuously terraformed by us humans. We collect plants in one part of the world, and because they taste good or look good, we bring them home to our part of the world to grow them there. We move goods in vast quantities across the world, and along with them we inadvertently move hitchhikers. So, there is one clear baseline to biological invasions: human activities on a global scale.

Does that perspective simplify the problem for you?

Reminding myself of this takes some of the complexity out of the equation. We are lost though once a non-native species invades a new ecosystem.

Why does it become so difficult at that point?

Because this is where it quickly gets very complex. We can understand and describe simple ecosystems, but just add a few more species, and the complexity exponentiates. And with just a slight change in species abundances or abiotic factors, we have a different ecosystem that won’t adhere to our previous predictions. Ecosystem invasions will always remain complex and predictable only to a certain degree, but we can improve our understanding by filling all those vast knowledge gaps we have on insects, fungi, microorganisms, and all their interactions with plants, the soil and with each other.

So where can we realistically make progress

We’ve made progress for some groups of insects. The research that went into the invasive harlequin beetle Harmonia axyridis is a beautiful example: we now know of the harmonine and of the microsporidian parasites, and of their toxicity to lady beetles in the invaded ecosystems, helping us understand the mechanisms behind the invasion success of this insect.

However, the large majority of non-native insect species does not seem to have any obvious impact on the invaded ecosystems, at least where human interest is concerned. But for all we know, silent extinctions could take place there. And these are the knowledge gaps that I’d love to see filled.

What do you enjoy most about working on large-scale invasion syntheses?

Two things: the big datasets, and the synthesis.

Where did that fascination with data begin?

From a young age I was big into data. I vividly remember spending way too much time of my teenage years in front of my very first computer, compiling long lists of species from all the sources I could get my hands on. One month it was about fossil whales, the next it was ferns and horsetails or some obscure group of marine worms. I was shocked when I found out that there appeared to be no global list of tardigrades, throwing me into a work frenzy for nights on end to put together a database for the water bears of the world. This passion came with a desire to connect the species in these lists with other meaningful data: stratigraphic units in the case of fossils, distribution ranges, host data. Back then I did this for the sheer joy of it.

Does that still drive you?

Nowadays this passion fuels my everyday work. And it goes hand in hand with my other role as the synthesis centre’s Data Steward.

What unresolved question in insect invasion ecology do you think has the greatest potential to reshape the field in the coming years?

I would love to see big advances in our understanding of the genetics in insect invasions.

Why genetics in particular?

I think a large-scale study on dozens or even hundreds of invasive species, comparing the genomes of the invading populations with their populations in the native range, has the potential to bring a lot of fundamental insights and raise important new questions. And genome sequencing is now such a common and affordable procedure that the bottleneck for such a project would probably be the sampling – nothing a well-organised international team couldn’t handle.