Use of herbarium data to evaluate weediness in five congeners

Abstract

It is often desirable to quantify a plant's relative weediness or synanthropy, that is, the degree to which aspecies associates with human-caused disturbance, in order to study and understand the biology, ecology and evolutionof weeds and invasive plants. Herbarium specimens are among the most accessible and verifiable sources of data ondistribution and habitat. However, the habitat distribution of species may not be reflected accurately by herbariumspecimendata, due towell-known biases in plant collection. Here, we assess how well herbariumspecimens reflect species'weediness, when compared with direct field surveys. We used five species of Melampodium (Asteraceae) and classifiedtheir degree ofweediness with amodification of Nuorteva's synanthropy index, based on herbariumspecimens. We thenmodelled the distribution of our focal species inMexico usingMaxEnt and identified a polygon of ∼3000 km2 in the stateof Nayarit, Mexico, where there was a high probability of finding all five species. Systematic field searches in the targetarea documented all visible populations of four species along major and minor roads. Then we, again, classified theirdegree of weediness with the synanthropy index, based now on field data, and compared. We found that herbariumdata were an accurate predictor of a species' weediness relative to its congeners despite the well-documented skewof herbarium data towards natural areas, which our data reflected as well. So, herbarium data can be used to classifyspecies' weediness relative to each other, but not in absolute terms, if the specimens were correctly identified and noneof the species were subject to particular collection bias. This study is the first attempt to compare herbarium and fielddata on this subject and may be relevant for other types of investigations based on herbarium data. Our work also highlightsthe usefulness of distribution models based on herbarium specimens.