Guest post by Anders Kolstad

Herbarium specimens reveal increased herbivory over the past century (2018) Meineke et al, Journal of Ecology, DOI: https://doi.org/10.1002/ecs2.1276

The Crux

Herbivory by insects has an important impact on plant life, but in light of the changing climate there have been few studies on corresponding changes in interactions between insects and plants. Today’s paper investigates changes in herbivory on leaves over the last century. How did they manage to analyse this? By using herbarium specimens. We’ve talked a little on EcolMass in the past on the importance of museum collections in ecology, and this is a fantastic example.

What They Did

The study from Northeastern USA quantified the probability of leaf chewing herbivory on 576 herbarium specimens from four different plant species. The specimens deposited in Harvard University Herbaria were collected over a period of 112 years. 5 randomly selected grid cells on a leaf were checked for signs of chewing by insects. They also used historical data to look at the effect of urbanisation and climate warming on herbivory.

What Did They Find Out?

Results showed that a specimen sampled around year 2000 had a 23% higher probability of having chewing marks on it compared to one sampled around year 1900. Analysis showed the most likely reason behind the observed increase in leaf chewing herbivory was elevated winter temperature. With a warming climate, and warmer winters especially, one can expect both increased survivorship of resident insect species and the facilitation of a northward spread of species that used to only be found further south. Of the 69 herbivore species known to feed on the four target plant species, 66 of them had a higher probability of occurrence if winter temperatures increased.

Problems?

The main discussion point is whether there is some bias in the way that herbarium specimens are collected that make this type of data not suitable to answer ecological questions. The authors of the paper themselves address this issue really well. The fact that botanists seek out pristine individuals for their collections means that herbarium specimens will show a lower probability of chewing damage compared to a random wild plant. There is, however, little grounds for saying that botanists are less or more peculiar about the state of their samples today than before. The data (probability of browsing damage on a leaf) is in other words not very accurate (it likely underestimates the actual herbivory), but it has high precision (we have the same non-accuracy all through the time series).

Did You Know: Shifting Baselines What makes long-term studies like this one so important is that they quantify how our world has changed, and point out changes so subtle we may not even have noticed them until shown the difference between a population now and forty years ago. Changes happen, and we begin to accept some of the results of these changes as the new norm. This shift in the perception of the norm is a shift in the baseline, and although it’s hard to commission a long-term study, it’s important that we do remind ourselves of what our original baseline was, and how damaged an ecosystem might actually be.

So What?

We don’t fully know if plants are better or worse off today than before based on this study alone. But we do see evidence that leaf chewing insects, such as caterpillars, beetles, grasshoppers etc., have had a good century, and will probably continue to thrive and feed as the climate warms. This will probably lead to further increase in leaf chewing herbivory. With only four species studied thus far we should be careful to generalise, but at least it looks like herbarium specimens can help us tackle some of the harder ecological questions, especially those that require long time series.

This study demonstrates the value of natural history collections in ecological sciences. And it’s super cool to see that people are always finding new and innovative ways of doing science.