A new study has uncovered the lasting impact of historical shipping practices on the plant diversity of New Jersey. Analysing records of species first reported over 150 years ago, Ryan Schmidt and colleagues at Rutgers University found that solid ballast materials used to stabilise sailing ships introduced over 260 new plant species to the state. Nearly half of these species can still be found in New Jersey today.

During the 19th century, sailing ships travelling across the Atlantic would fill their hulls with heavy materials like stones and sand to stabilise the vessel. This “solid ballast” was often unintentionally contaminated with plant materials like https://botany.one/2021/08/does-investigating-the-seed-coat-in-gnetum-gnemon-reveal-how-fruiting-plants-evolved/”>seeds and fruits from the foreign ports where it was loaded. When ships arrived in New Jersey to pick up cargo, the solid ballast would be offloaded onto the shorelines and waterways of busy shipping hubs like Camden and Jersey City.

In the late 1800s, botanists documented hundreds of new plant species growing on these discarded ballast piles that were clearly not native to New Jersey or North America. Species from Europe, Asia, and Africa took root in the piles of rubble and disturbed soils. For some species, their first collections anywhere in North America came from New Jersey ballast sites.

Botanists of the 19th Century were certainly aware of the arrival of new species, but this problem seems to have been ignored over the next hundred years. Schmidt and colleagues write:

Remarkably, since the 1880s, there has been almost no work studying the floristic impact of solid ballast deposition in northeastern North America. While ballast is often referenced as a potential source of introduction for some non-native plants (e.g., Lythrum salicaria and Artemisia vulgaris; Lindroth, 1957; Stuckey, 1980; Mills et al., 1993; Mack, 2000; Mehrhoff, 2000; Lockwood et al., 2013; Shaw et al., 2021), these studies have focused on a small number of plants which have become prominent invasive species (Stuckey, 1980; Barney, 2006; Mosena, 2018). Little has been done to study ballast species on a broad, inclusive floristic scale and evaluate both the successful and unsuccessful introductions in the context of the contemporary flora.

Schmidt et al. 2023

To uncover the legacy of ballast introductions, Schmidt and colleagues turned to records created by those early botanists who first documented the species growing on ballast sites. Herbarium specimens – pressed and preserved plant samples along with data on the location and date they were collected – provide a rich historical record used by botanists for centuries.

The researchers compiled data on over 260 species recorded at major ballast dumping grounds in New Jersey in the 1800s. They then looked at digitised records of these species from herbaria across the eastern US to track when and where each species was collected over the last 150 years.

By mapping these collections through time, they could visualise the spread of species after their initial introduction. Statistical models helped identify distinct invasion trajectories based on how long each species persisted and how far it spread within New Jersey after being introduced via ballast. This novel use of herbarium data provided a portal into the past, revealing the varied and often lasting impacts of historical species introductions on the New Jersey flora.

The analysis of the herbarium data revealed varied invasion outcomes for the 260+ ballast species introduced to New Jersey. Of these unintended imports, 83 species disappeared quickly after their initial discovery, failing to establish despite the constant new introductions during the sailing ship era. Another 54 species persisted for a few decades before also dying out in the state by the early 1900s.

However, the researchers found much greater establishment success for other species. One hundred and twenty-seven ballast introductions, nearly half of those documented, continued to be collected throughout the 20th century and became incorporated into the native flora. Remarkably, 87 of these new species are still found and collected in New Jersey today, over 150 years after their introduction.

The findings demonstrate the lasting botanical impact of historical species introductions. While many newcomers fail to thrive, some unintentionally introduced organisms can become enduring additions, creating novel ecological communities.

The scientists also investigated how these new species spread across New Jersey over time. Mapping where each species was collected revealed geographic clusters corresponding to major shipping ports and inland rail networks.

The spread through the railroad system is interesting. The plants didn’t choose to travel by rail, though botanists did. As botanists can only find plants where the botanists are, they may have biased surveys. However, the plants’ presence also indicates a secondary use of ships’ ballast. Schmidt and colleagues write:

Since ships’ ballast was occasionally used as the material to create the bed for the ties and tracks of the railroad itself (Wright, 2000; Burström, 2017; Williams, 2022), propagules from these ports were likely moved by people who were intentionally transporting these stones to new areas of railroad expansion. These new railways also provided convenient transport for botanists leading to many herbarium collections near railroads (Daru et al., 2018). Ballast plants also thrived in rocky substrate (ballast) and disturbed areas along railroads; an association that can be seen in today’s railroad floras. This hypothesis is supported by the presence of some ballast species in our data set only having been collected at former ballast dumps and at major railroad hubs (e.g., Verbena bracteata being collected almost exclusively in Camden and Jersey City and at major railroad lines in Glassboro, Haddon, Lakehurst, and Sparta).

Schmidt et al. 2023

In the late 1800s, ballast species were concentrated around the ports of Camden and Jersey City, where they first arrived. But soon after, collections appeared inland along 19th-century railroad lines emanating from these ports. The researchers suggest ships’ ballast was likely reused to construct the railbeds, further dispersing propagules.

This spatial analysis shows the important role of early transportation networks in spreading ballast species far beyond their initial points of introduction. The legacy of these historic shipping and rail routes can still be seen in the distribution of non-native plants across New Jersey today.

To categorise the varied fates of ballast species, the researchers defined four distinct “invasion trajectories” based on how long each species persisted and the extent of spread.

Some species were transient “waifs”, disappearing quickly after introduction. Others persisted for a few decades before dying out (“short-term introductions”). But many became “established” in New Jersey, either with localised spread near ports (“established but limited spread”) or spreading widely across the state (“established and widespread”).

These trajectories provide a framework to compare unintentionally introduced species over long timespans. The study revealed that nearly half of the ballast species became established, and 18% spread widely despite lacking traits sometimes thought to promote invasion success. Analysing the traits, genetics, and ecology underlying each trajectory can improve predictions of which unintended introductions may persist or spread.

Calibrachoa parviflora. Image: Canva.

During field surveys of historic ballast sites, the researchers made some exciting botanical discoveries. They rediscovered the rare species Calibrachoa parviflora, which had not been collected in New Jersey for over a century, still clinging to its limited habitat on former ballast piles. This demonstrates how even localised non-native species can persist undetected for long periods.

More surprisingly, the scientists identified a species entirely new to North America – Verbascum maurum, first described scientifically in Africa in 1923. Historical records had misidentified it as the now-extinct Verbascum virgatum, originally collected from 19th-century ballast sites. This discovery highlights the value of revisiting historical specimens and locations, which can reveal past errors and uncover new arrivals previously mistaken for native plants. Careful sleuthing of botanical collections provides insights that enhance our documentation and understanding of biodiversity.

By shedding light on the lasting influence of historical events like ballast deposition, findings from Schmidt and colleagues demonstrate the power of herbarium collections as a window into the past. Specimens collected by early botanists, along with their precise location and date information, provide an unparalleled record that allowed the researchers to reconstruct invasion histories spanning over 150 years.

Such long-term perspectives are critical for understanding human impacts on biodiversity but impossible to capture from contemporary data alone. Yet Schmidt and colleagues highlight that this does not mean modern collecting of plants should be forgotten. They conclude:

Despite the apparent biases in herbarium data, the herbarium records used in this study proved largely robust to collection biases, providing an opportunity to study plant invasions over the temporal span of their establishment. More specifically, the use of herbarium data to track plant invasion from their introduction to establishment and spread provides insights into the dynamics of plant invasion that are often obscured by studies which only consider contemporary records. The continued utility of herbarium records in answering questions that span large time periods is contingent on continued contemporary collection efforts.

Schmidt et al. 2023

Schmidt, R.J., King, M.R., Aronson, M.F.J. and Struwe, L. (2023) “Hidden cargo: The impact of historical shipping trade on the recent‐past and contemporary non‐native flora of northeastern United States,” American Journal of Botany. Available at: https://doi.org/10.1002/ajb2.16224.

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