Human-mediated dispersal as a driver of vegetation dynamics: A conceptual synthesis

Prepared by Gesine Pufal & James M. Bullock

Acaena sp. seeds (bidibids) hitchhiking on a shoe, Auckland Islands, New Zealand. Photo credit: Gesine Pufal.

It is a beautiful summer day, you have been out on a hike through the countryside and have returned home to your house in the city. To keep your house tidy, you clean your shoes and trousers on the doorstep. Voilá, you may have just acted as a human seed disperser for seeds that clung to your clothes during your hike through a meadow and shaking them off in an urban garden where they may form new populations. We call this accidental human-vectored dispersal but humans actually transport seeds in numerous ways – not only from hikes to homes but across the globe. Seeds on wool or in the mud on cars, hitchhiking in packages or flower pots, are moved around accidentally but we also transport seeds with intention – just think of fields planted with maize or soybean all over the world. We also influence seed dispersal in more indirect ways. For example, many animals transport seeds and our hunting of them can disrupt their dispersal of seeds, as do roads that block migration paths of seed-dispersing deer. However, those roads might also serve as so-called dispersal corridors. Try and picture a recent drive on a motorway – do you remember seeing the same plants for kilometres on end? Their seeds are probably dispersed along the road by the wind turbulence from the fast-driving cars. It is quite mind-boggling when you realize just how much and in how many different ways humans affect seed dispersal. We were therefore interested how this human-mediated dispersal actually affects the development of vegetation over time.

The examples we have mentioned illustrate three major ways in which humans might shape seed dispersal. Human-mediated dispersal can be quite similar to natural dispersal, for example by taking over the role of the natural disperser of native plant species (on your shoes and trousers). In this case, the vegetation would not change much in response to our activities. However, the vegetation and plant species composition can change drastically when humans either disrupt dispersal or enhance it compared to natural dispersal by either reducing the number of native species or increasing the arrival of new species. As if this is not complicated enough, the environment in the affected plant communities also plays a major role in how human-mediated dispersal might influence vegetation. For example, we might try and move as many avocado seeds as we can to the high arctic – the strong filter of freezing temperatures will stop the establishment of avocado trees. However, weed seeds we transport accidentally to areas where the soil is frequently disturbed (gardens or allotments) thrive because the competition of established native plants is non-existent.

In our synthesis, we show the different ways in which humans can affect seed dispersal and how the interplay between human-mediated seed dispersal and biotic filters might change the vegetation in different areas. To understand these processes is important because human activities are increasing and changing constantly. Likewise, the various ways of human-mediated dispersal will change and affect vegetation dynamics.

This is a plain language summary for the paper of Bullock and Pufal published in the Journal of Vegetation Science (