Too hot to persist – Moth diversity is sensitive to increasing temperatures and drought under climate change

25-year survey of Mediterranean moth communities reveals that hot summers result in subsequent reduced moth species richness.
Too hot to persist – Moth diversity is sensitive to increasing temperatures and drought under climate change

Climate change is recognized as one of the major problems in the 21st century. Altered temperature and precipitation not only have an impact on the different ecosystems but subsequently also affect plants and animals (Bellard et al. 2012). For insects, which seem to face massive declines in biomass and diversity (i.e. “insect decline debate”) (Wagner et al. 2021, Outhwaite et al. 2022, Seibold et al. 2019), climate change might also pose a threat. Yet, the insect decline debate at the moment has a strong focus on the impacts of land use change and related threats like habitat degradation and loss (which undeniably is an important driver of insect diversity losses) (Seibold et al. 2019, Habel et al. 2019). Studies linking insect decline with climate change are scarce and often focus on cold-adapted and montane species (Halsch et al. 2021). One key assumption of those studies is that cold-adapted species are prone to climate change, while thermophilous species tend to “expand” their range northwards. Species like Arctia caja, therefore, face strong declines in many areas – probably because their overwintering caterpillars are prone to fungal infections in warm and wet winters (Conrad et al. 2002). Thermophilous and formerly southern distributed species like the pine processionary moth, on the other hand, expand their range and can nowadays be found also in temperate regions (Netherer & Schopf 2010).

Are there “winners of climate change”?

But do thermophilous species per se really “profit” from climate change and are there really “winners” of climate change? Taking into account the classical niche theory, every species has a temperature optimum. If formerly southern distributed species nowadays appear in more temperate regions, the climate there might have shifted to resemble their temperature optimum. Likewise, the temperatures in their original habitat might exceed the temperature optima of these species. We therefore might be able to observe a direct and negative effect of increasing temperatures on species diversity. In addition, hot and dry summers might affect food plants (Haberstroh et al. 2018, Toscano et al. 2019), reducing food plant quality (Teixeira et al. 2020) and therefore limiting the most important resource for an insect’s growing stage. An indirect, negative effect of climate change on insect diversity might be the result.

Mediterranean climate regions are biodiversity hotspots and, as such, are known for their unique and species-rich ecosystems. Simultaneously, they are known for their sensitivity to climate change effects. In our recent paper published in Scientific Reports, we analyzed if Mediterranean moth communities are sensitive to climate change effects (viz. increasing temperatures and decreasing precipitation rates). While previous studies on climate change most of the time considered temperature during the adult species’ flight as a predictor, we followed a different approach. We assumed that the stage most sensitive to climate change effects is the caterpillar. Other life stages, like the egg, the pupae, and adults were previously found to be able to react to unfavorable conditions with dormancy (Haeler et al. 2014, Yela & Herrera 1993). A caterpillar, in contrast, has the only purpose to feed and grow and therefore is unlikely to spontaneously react to hot temperatures and drought with dormancy. In fact, laboratory experiments have shown that caterpillars are sensitive to changes in temperature, with increasing temperatures resulting in high mortality rates (Mironidis & Savopoulou-Soultani 2008). So, are there effects of temperature during larval development on species diversity of adult moths observed later in the year?

Reduced species richness after warm temperatures during larval development

Our 25-year moth survey in a Mediterranean coastal forest reserve gave us insight how temperatures and precipitation rates during larval development are linked to inter-annual species richness fluctuations. We found that hot temperatures during larval development resulted in reduced species richness in the later appearing adult moth community. Especially species with summer developing larvae seem to be affected by increasing temperatures, with a strong species richness decline after hot temperatures during larval development. Overwintering larvae in our analysis were less affected by warm temperatures, although also here a negative trend was observable.

On the left the correlation between Temperatures during larval development (TLarv) and subsequent species richness can be seen (Seperated into (a) overwintering larvae and (b) summer-developing larvae). The right graph (c) shows how interannual species richness fluctuations over the past 25 years are linked with temperature.
On the left, correlations between temperatures during larval development and subsequently observed species richness can be seen (the graph is separated into (a) overwintering larvae and (b) summer-developing larvae). On the right, the graph shows how inter-annual species richness fluctuations (black points) might be influenced by temperature (red points). Each of the black points is a standardized species richness value, resulting from species interpolation-extrapolation of multiple samples.

For the precipitation rates, we found no correlation, neither for overwintering nor for summer-developing larvae. Most probably, the mean monthly precipitation rate data were not accurate enough. The usage of a precipitation concentration index might provide different results, as for caterpillars and food plants, the frequent and constant delivery of rainwater might be more important than the mere amount of rain. In fact, rain events in the Mediterranean tend to become more extreme, which means that there are fewer moderate rain events and instead heavy rain events and storms occur more frequently.

Future outlook – pretty much research to do

Our results give a hint that climate change does not always positively affect thermophilous species. In fact, it seems that species that now occur in temperate zones are prone to exceeded temperature optima and reduced food plant quality in their original distributional range. We therefore should stop to think about “range expansion” and rather talk about “range shifts”. Globally seen, there might be no winners of climate change, but only species that try to adapt to altered conditions. Yet, we do not know what exactly threatens species more: Is it the direct effect of hot temperatures, or is it the indirect effect through reduced food plant quality? For gaining a better understanding of the mechanisms behind climate change ecology, we urgently need laboratory experiments to disentangle direct from indirect effects. We also might profit from analyzing climate change effects on different functional groups. While our study was focused on the community level (viz. adults sharing the same flight period might have developed at the same time as caterpillar), a more detailed analysis of functional guilds such as species with larvae developing in a certain time of year might further help us to understand which species are especially threatened by climate change.



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