More than one step to reach farther ahead

13 February 2014
Comments: 0
13 February 2014, Comments: 0

Seed dispersal is the only opportunity to move that a plant has throughout its life cycle. Many species have adapted their seeds so that they can be easily transported by wind or water. Others ensure their propagation by producing fleshy fruit that serves to award animals: attracted by its food potential the animal ingests the fruit, and the seed will travel through its digestive tract until it ends up being excreted or regurgitated somewhere or other. When dispersal takes place at a great distance, this enables the exchange of seeds between far-flung populations or the colonisation of places where the species was not yet present, thus expanding its range of distribution.

However, the dispersal of seeds can also be a process that involves numerous steps in which different vectors have a role to play. One example might be that of a seed that is dispersed in an initial step by the wind and then in a second step, by ants that carry it to their anthill. The second step is what is referred to as a secondary seed dispersal process. When animals that eat fleshy fruit (frugivores) participate in secondary dispersal, they normally do so in the first step, which means that the seeds are only exposed to digestive treatment. This is the case, for example, with the hazel tree (“avellanillo”). Around the Mediterranean, this tree is normally found in riverbank woodlands where its fruit is mainly eaten by birds. Once the birds deposit the seeds, these can then undergo a secondary transferral by water, when the rivers rise. However, there are cases in which a second animal can intervene in the secondary seed dispersal process, when it eats the original animal which has seeds in its digestive tract.

Secondary seed dispersal in the Canary Islands

In the Canary Islands, fruit-eating lizards of the Gallotia genus abound and are often eaten by predators such as Great Grey Shrikes or the Common Kestrel. In this way, seeds that may lie in the lizard’s digestive tract are removed by the birds to places that are faraway, compared to the places they would have reached if only the lizards had been involved, given that these have a considerably more limited range of movement. In the archipelago, lizards are able to disperse the seeds of more than 50 species of fleshy fruit bearing plants. At the same time, the Common Kestrel is a bird that is found all over the islands, while the Great Grey Shrike inhabits the central and eastern islands (Tenerife, Gran Canaria, Fuerteventura and Lanzarote). This gives us an idea of the magnitude of this secondary seed dispersal process in the Canaries, primarily in the wide-open habitats that both species need if they are to develop their hunting skills, and where lizards are to be found in great numbers.

But how does this double digestive treatment (lizard + bird) affect seeds’ capacity to germinate? In the case of the Great Grey Shrike, its effect is neutral for the majority of species; in other words, it neither improves nor impairs a seed’s capacity to germinate, following the digestive treatment it receives from the lizard in the first step of the dispersal event. In the case of the common Kestrel, as this is a daytime predator with considerable enzymatic activity in its digestive tract, one might expect this would have a negative influence on the survival of the seeds. However, once it has captured the lizard and removed it to a safe place, the kestrel removes its head and its digestive tube before it ingests it. Therefore, most of the seeds actually only receive one single digestive treatment (that of the lizard), and their chances for survival are not altered by the action of the Common Kestrel.

It may seem to us a priori that when a Great Grey Shrike or a Common Kestrel captures a lizard, only one of them is on the winning team. All the same, we should not forget that perhaps running parallel to this predatory process, another seed dispersal process may be taking place; and that other plant species can benefit from it, thereby increasing their chances of germinating and taking root in places that are a long way away from where they originally came from.

Beatriz Rumeu

Recommended reading:
Padilla DP, González-Castro A & Nogales M (2012) «Significance and extent of secondary seed dispersal by predatory birds on oceanic islands: the case of the Canary archipelago.» Journal of Ecology, 100: 416-427.
Padilla DP & Nogales M (2009) «Behaviour of kestrels feeding on frugivorous lizards: implications for secondary seed dispersal.» Behavioural Ecology, 20: 872-877.

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