Invasive species and dispersal networks

Invasive plants come from far-off places and lodge themselves quite firmly and irrevocably in their new ranges. How do these plants affect the relationship between indigenous species and their dispersers? 

Invasive plants are very fascinating. Mostly because invasive plants were never found in places where they are now pestilential! Over hundreds of years, plants have been taken from places that they have evolved in and have been introduced to new places either purposefully or accidentally. In their new ranges, some plants became invasive when they spread far and wide (thousands of kilometres away from where they were introduced) in large numbers and eventually start to affect other plants that were already growing in that area. Invasive plants are known to affect a whole range of ecosystem attributes – from altering soil nitrogen to increasing the frequency of fire occurrences; from decreasing indigenous plant diversity to altering local hydrology; from competing for pollinators to secreting chemicals that are toxic to other plants. Yet, invasive plant problems are unique to the places they colonise and there are often no general remedies to these problems.

A lot of invasive plants package their seeds in sugary, colourful fruits that certain animals find irresistible as food. Birds and mammals that swallow these fruits, carry them in their guts and void seeds in new places far away from the parent plant. Plants rely upon this process of dispersal in order to reach new places that have conditions conducive for growth and reproduction, while (hopefully) escaping the predators and parasites that affect parent plants. Dispersal can thus shape the populations of plant species and eventually the composition of plant communities. Upon arrival in a new range, invasive plants rapidly form cooperative links with dispersers that can spread their seeds. Depending on how far the dispersers move, and how long they can retain seeds in their guts, invasive plants will reach new areas, establish and proliferate, only to start the cycle of dispersal and spread once more. 

The formation of links between invasive species and indigenous dispersers means that some of the links between indigenous plants and their dispersers are thrown off balance. Some dispersers may forsake the plants they were earlier dependent upon and switch to invasive plants instead. Switching from one fruit source to another will determine how many seeds get removed from indigenous and invasive plants, eventually affecting how far these plants get. But the behaviour of dispersers is dependent on how much fruit is available (dispersers might visit fruit-laden plants instead of sparsely-fruiting plants more often), how close together fruiting plants are (dispersers might prefer going to fruiting plants that are clumped together so that they don't have to spend too much energy looking for food) and how nutritious the food source is. These three conditions could not only vary in space, but also in time. 

This project aims at understanding how an invasive plant changes the way the seeds of native plants are dispersed in space and time. Invasive species may offer larger quantities of more nutritious fruits, thus 'stealing' dispersers away from indigenous species. Or, they may attract more dispersers to indigenous plants fruiting close to invasive plants. These patterns may be different at different times of the year. The observations from this project will help in developing predictions for what plant communities will look like in the future in the presence of an invasive plant.