It’s a well-known fact: human disturbance has extremely negative effects on not only the environment, but also on biodiversity, the variety of life forms in the world. From building dams to deforestation, and from hunting to overexploitation, we thought that the full extent of human disruption of nature had been reached. However, a new study conducted by a number of collaborating biologists from the Biodiversity and Climate Research Centre (Frankfurt, Germany), Muséum National d’Histoire Naturelle (Brunoy, France), Integrative Ecology Group (Sevilla, Spain), the Department of Ecological Anthropology at University of San Antonio (San Antonio Texas), Institut Mediterrani d’Estudis Avançats (Mallorca, Spain), Smithsonian Tropical Research Institute (Balboa, Republic of Panama), and the Department of Biological Sciences at Johann Wolfgang Goethe-Universität Frankfurt (Frankfurt, Germany) has brought to light the severity of human disturbance on nature in another way. In a study published in the journal, Conservation Biology, lead authors Julia Markl and Matthias Scheuning, both from the Biodiversity and Climate Research Centre in Frankfurt, have discovered that human disturbance has significant negative effects on an essential biological process in nature: animal-mediated seed dispersal.
Animal-mediated seed dispersal is the biological process by which animals spread plant seeds. This process can occur in several ways; the seed can be transported in the fur of vertebrate animals, through ingestion, and dispersal by ants, just to name a few. Seed dispersal is a significant process because it is the means by which plant species not only migrate, but is also a key way to maintain plant biodiversity according to three hypotheses. The escape hypothesis proposes that the further seedlings are from their parent, the higher their competitive advantage. This is because they are further away from the pathogens and predators that prey on their parent. The second hypothesis, known as the colonization hypothesis, suggests that seed dispersal increases the chances of seeds reaching new sites to settle and grow. Lastly, the direct dispersal hypothesis states that the seed dispersal agents deposit the seeds where they have the best chance of germination. All three of these hypotheses lead to the same conclusion: seed dispersal is a critical process for maintaining genetic diversity and plant viability. However, as the new study in Conservation Biology shows, this process is not only crucial, but a sensitive one as well.
In this study, a collaboration between the many prominent research institutes mentioned above, researchers identified 35 previous research articles that compared 83 examples of animal-mediated seed dispersal between tropical/subtropical forests that had been disturbed by humans and ones that had not been disturbed. Criteria that were considered in determining the negative effects on seed dispersal were visitation rates of animals, seed removal, and seed-dispersal distance. Then, using formal meta-analyses, the study assessed the effects that forest fragmentation, hunting, and selective logging (the practice of cutting down a few trees while leaving the rest intact) had on seed dispersal. Furthermore, the researchers evaluated whether larger seeds were affected in the same way as small seeds by these human disturbances.
After researching case studies that compared the aforementioned types human disturbances, through formal meta-analyses, the study concluded that human disturbance reduced the number of frugivores, which are the primary type of animal responsible for animal-mediated seed dispersal, by hunting and by modifying the habitat structure by selective logging. Fragmentation affected the frugivore community only indirectly, but selective logging and hunting directly reduced the potential for animal-mediated seed dispersal. Furthermore, large-seeded plant species were more sensitive to human disturbance than the small-seeded plants, particularly when larger frugivores were decreased.
This study has major implications. As the results show, hunting and selective logging negatively affect the animal-mediated seed dispersal necessary to maintain plant viability and diversity. Also, since the large-seeded plants were more vulnerable to the effects human disturbance had on animal-mediated seed dispersal, the disruption of this crucial process could lead to a change in species composition, a consequence that is detrimental to plant species richness in a habitat. This study sheds light on just how many different, yet significant ways human disturbances have a negative impact on nature. If we do not realize how severe the ramifications of disturbing these natural processes are, and do not implement any measures to raise awareness of these consequences, it is very possible that the diversity we know and cherish today in plant species could be lost; a homogenized set of plants would take their place, impacting the biodiversity of not only plant, but animal species as well.
Markl, J.S., Schleuning, M., Forget, P.M., Jordano, P., Lambert, J.E., Traveset, A., Wright, S.J. and Böhning-Gaese, K. (2012),Meta-Analysis of the Effects of Human Disturbance on Seed Dispersal by Animals. Conservation Biology. doi: 10.1111/j.1523-1739.2012.01927.x
Luhta, Jorma. Bohemian Waxwing feeding (Bombycilla garrulus) on berries. Photograph. BBC Nature. BBC News. Web. <http://www.bbc.co.uk/nature/adaptations/Seed_dispersal>.