Teamwork in the Tropics: Pollinators and Frugivores Are Less Choosy at the Equator

The diet of the bright crimson Andean Cock-of-the-rock (Rupicola peruviana) includes fruits of over 100 different plants of the tropical montane rainforest. 

The bright crimson Andean Cock-of-the-rock eats the fruits of over 100 plant species and disperses their seeds. It is in good company, since other seed-dispersing birds and pollinating insects in the tropics are also -- contrary to prior doctrine -- less specialised on individual plant species than their temperate counterparts. This is the outcome of a study conducted by an international research group, which is published September 14 in the journal Current Biology. This suggests that ecosystem functions such as pollination and seed dispersal in the tropics have a higher tolerance against extirpations of individual species than in the temperate communities.

It is a win-win business for bees and plants: bees forage on plant nectar, and in return they pollinate the next flower they visit. Virtually the same is true for fruit-eating birds, which by the way disperse the seeds of plants. A large number of such mutualistic interactions between species exist in an ecosystem, which together form a complex network. Scientists have now analysed the "Who with whom?" in a worldwide study and have discovered that the specialization of pollinators and seed disperses on individual plant species decreases towards the equator.

Surprising results: Specialists tend to be in the temperate zones

That is somewhat unexpected; after all, since Darwin it has been assumed that many pollinating insects and seed-dispersing birds in the tropics were specialised on a small part of the available plant species. Until now this co-evolution of reciprocal specialization has been an important explanation for the higher plant diversity in the tropics compared to temperate latitudes. "The results of our global analysis contradict the assumption that ecological communities in the tropics are generally more specialised than those in the temperate zones," say Matthias Schleuning and Jochen Fründ, the lead authors of the study, from the Biodiversity and Climate Research Centre (BiK-F) and the University of Göttingen.

"Our results show that specialization between animal and plant species tend to be rather a consequence of the available resources than the result of long-term adaptation processes," explains Schleuning. This is also supported by a further finding of the study, according to which contemporary climate and the plant diversity in an ecosystem are more closely related to the interactions between animals and plants than past climate stability. "A simple explanation for this could be that the high tropical plant diversity provides many different resources to animals in a low density. "Whoever is not especially choosy is at an advantage, because then the next food source is not very far away, making foraging more efficient," says Fründ.

Ecosystem functions in the tropics are probably more robust

The lower specialization in the tropics also provides advantages for the plants, because they are better insured against species extirpations -- plants interacting with a number of animal species have a lower risk of extinction if individual species of pollinators or seed dispersers disappear or decline in number. "We therefore suppose that certain ecosystem functions such as pollination and seed dispersal are less susceptible to disruption in the tropics than in the temperate zones. Due to the generalised relationships and the greater diversity, more species can replace the functions of individual declining species," says Nico Blüthgen, the initiator of the study of TU Darmstadt. Such failures in the relationship between animals and plants can even have a considerable economic impact. This is demonstrated by the current massive collapse of bee colonies in the US, which leads to particularly high costs in those places where there is a lack of alternative pollinators.