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 species: Old plants in a new place

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.

Art 20for 20science

Clockwise from left - Chromolaena odorata invades dry forest understorey, water hyacinth invades and chokes water bodies, scotchbroom is an invasive in shola grasslands

Dispersal: A biological courier service

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. 

Redventedlantana 20  20copy

An indigenous disperser (red vented bulbul) finds the fruits of an invasive (lantana) very edible

Invasive plants and dispersal networks

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. 

People

Funding

  • Department of Science and Technology

Publications

  • Journal Article
    2017
    Plant-disperser mutualisms in a semi-arid habitat invaded by Lantana camara L.
    Plant Ecology 218 (8): 935-946

    Dispersal is an important ecological process that affects plant population structure and community composition. Invasive plants with fleshy fruits rapidly form associations with native and invasive dispersers, and may affect existing native plant-disperser associations. We asked whether frugivore visitation rate and fruit removal was associated with plant characteristics in a community of fleshy-fruited plants and whether an invasive plant receives more visitation and greater fruit removal than native plants in a semi-arid habitat of Andhra Pradesh, India. Tree-watches were undertaken at individuals of nine native and one invasive shrub species to assess the identity, number and fruit removal by avian frugivores. Network analyses and generalised linear mixed-effects models were used to understand species and community-level patterns. All plants received most number of visits from abundant, generalist avian frugivores. Number of frugivore visits and time spent by frugivores at individual plants was positively associated with fruit crop size, while fruit removal was positively associated with number of frugivore visits and their mean foraging time at individual plants. The invasive shrub, Lantana camara L. (Lantana), had lower average frugivore visit rate than the community of fleshy-fruited plants and received similar average frugivore visits but greater average per-hour fruit removal than two other concurrently fruiting native species. Based on the results of our study, we infer that there is little evidence of competition between native plants and Lantana for the dispersal services of native frugivores and that more data are required to assess the nature of these interactions over the long term. We speculate that plant associations with generalist frugivores may increase the functional redundancy of this frugivory network, buffering it against loss of participating species.

  • Dataset
    2017
    Data from: Plant-disperser mutualisms in a semi-arid habitat invaded by Lantana camara L. Plant Ecology
    Data Dryad. http://dx.doi.org/10.5061/dryad.gc6dm

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