Internal and External Dispersal of Plants by Animals: An Aquatic Perspective on Alien Interference

Abstract

Many alien plants use animal vectors for dispersal of their diaspores (zoochory). If alien plants interact with native disperser animals, this can interfere with animal-mediated dispersal of native diaspores. Interference by alien species is known for frugivorous animals dispersing fruits of terrestrial plants by ingestion, transport and egestion (endozoochory). However, less attention has been paid to possible interference of alien plants with dispersal of diaspores via external attachment (ectozoochory, epizoochory or exozoochory), interference in aquatic ecosystems, or positive effects of alien plants on dispersal of native plants. This literature study addresses the following hypotheses: (1) alien plants may interfere with both internal and external animal-mediated dispersal of native diaspores; (2) interference also occurs in aquatic ecosystems; (3) interference of alien plants can have both negative and positive effects on native plants. The studied literature revealed that alien species can comprise large proportions of both internally and externally transported diaspores. Because animals have limited space for ingested and adhering diaspores, alien species affect both internal and external transport of native diaspores. Alien plant species also form large proportions of all dispersed diaspores in aquatic systems and interfere with dispersal of native aquatic plants. Alien interference can be either negative (e.g., through competition with native plants) or positive (e.g., increased abundance of native dispersers, changed disperser behavior or attracting additional disperser species). I propose many future research directions, because understanding whether alien plant species disrupt or facilitate animal-mediated dispersal of native plants is crucial for targeted conservation of invaded (aquatic) plant communities.

Epizoochory in Parrots as an Overlooked Yet Widespread Plant–Animal Mutualism

“…In this study, we found that epizoochory in parrots adds a new example of understudied animal-mediated dispersal mode [ 15 ] that challenges our understanding of this dispersal syndrome in several ways. First, epizoochory is generally considered infrequent compared to other zoochory mechanisms such as endozoochory (i.e., dispersal of viable seeds after gut passage), as it is mostly caused by the eventual attachment of propagules to the body of mammals or some bird species [ 11 , 16 , 17 , 34 ]. These plant–animal interactions are not considered mutualistic, as adhered fruits do not provide dispersal agents with a nutritional reward [ 11 , 16 ], with the only exception of some extremely rare events of epizoochory in songbirds [ 21 , 22 ].…”

“…However, the unique fruit manipulation abilities of parrots, using their large and mobile beaks, their tongues, and their feet for feeding [ 25 ], may facilitate high rates of seed attachment through pulp residues of fleshy fruits, viscid mucilages, and seeds with specialized structures for anemochory that easily adhere on their body surfaces. Finally, coinciding with typical epizoochory syndromes [ 11 , 16 ], parrots may perform directed dispersal by moving seeds far from the mother plant to similar habitats where germination is feasible.…”

“…Among different seed dispersal mechanisms, epizoochory (i.e., the dispersal of animal and plant propagules adhered to the body surface of animals, [ 11 ]) is one of the zoochory syndromes less studied, as we have important knowledge gaps on its frequency, efficiency and the extent of its effects on plant communities [ 12 , 13 , 14 , 15 ]. Plants with epizoochory syndromes usually show specialized structures in their seeds such as hooks or barbs that facilitate the adherence and attachment of propagules [ 11 , 12 , 13 , 16 ], although there are examples of seeds that stick to animal body parts when mixed with mud [ 17 ]. Effective attachment mechanisms can assist long retention times on the disperser, increasing the likelihood to disperse over long distances.…”

Endozoochory by mallard in New Zealand: what seeds are dispersed and how far?

“…In New Zealand, introduced blackbirds ( Turdus merula ) and song thrushes ( T. philomelos ) dispersed more seeds than native frugivorous avifauna within secondary forested areas ( Wyman & Kelly, 2017 ; but see Macfarlane, Kelly & Briskie, 2016 ). However, alien plants can also disperse by endozoochory and this could have negative consequences for native biodiversity and ecosystem functions ( Reynolds, Miranda & Cumming, 2015 ; Green, 2016 ; Van Leeuwen, 2018 ).…”

Seeds and the City: The Interdependence of Zoochory and Ecosystem Dynamics in Urban Environments

“…For example, the invasive antdispersed South African shrub Polygala myrtifolia outcompeted native plant species for seed dispersal services by ants (Rowles and O'Dowd, 2009). However, competition for seed dispersal services may not necessarily result in negative outcomes for native species if these benefit from the additional dispersers brought into the area by the resources provided by alien plant species (van Leeuwen, 2018).…”