The unnoticed effect of a top predator on complex mutualistic ecological interactions

Abstract

Vertebrates often mediate seed dispersal systems, essential for the maintenance of biodiversity. Some of these acquire a complex multistep process in island environments, where for example a native predatory bird can predate upon a frugivorous vertebrate, dispersing seeds secondarily. These complex mutualistic processes are really threatened by biological invasions especially on islands, due to the particular and intrinsic traits of their biotas. One of the most pernicious invasive mammals is the feral cat, widely introduced on at least 179,000 islands worldwide. Despite the potential impact of disruption of these complex seed dispersal processes, their ecological effects remain largely unknown. Therefore, the main aim of this contribution is to assess the impact of an invasive vertebrate on seed dispersal effectiveness at the crucial phases of plant recruitment: seed damage, viability, and germination. We designed a laborious captivity experiment with lizards and cats to simulate the four potential ways seeds could be dispersed in the wild: (1) control plants, (2) lizard droppings, (3) cat droppings and (4) cat droppings after consuming frugivorous lizards. We considered those four plant species whose seeds were the most abundant in feral cat droppings in all the main habitats of the Canary Islands. The main results indicated that (1) species with thicker seed coat better resisted abrasion caused by the digestive effect of the invasive cats and, (2) native and endemic species with thinner seed coats, that have not evolved with invasive mammals, suffered from a negative effect on seed effectiveness. To our knowledge, this is the first study reporting the potential disruptive impact of secondary seed dispersal systems caused by an invasive predator and one of the scant contributions evaluating seed dispersal effectiveness.