FSD 2010 - Schaefer

5th International Symposium-Workshop on Frugivores and Seed Dispersal (1985-2010)

Martin H. Schaefer

Dept. of Evolutionary Biology and Animal Ecology, University of Freiburg, Biology I
Hauptstrasse 1
D-79104 Freiburg

martin.schaefer(a)biologie.uni-freiburg.de

Activities

I am broadly interested in the evolution and ecology of communication between plants and animals. Communication evolves because stationary plants that depend upon pollinators and seed dispersers for reproduction and seed dispersal need to attract these animals to their reproductive organs. In my group we analyse how animals respond to plant traits used in communication to assess how their selective pressure might shape plant traits. Not surprisingly, we found that increasing contrasts in fruits to the background lead to higher visitation rates of birds (Cazetta et al. 2009). Despite this adaptive benefit of highly contrasting cues, plants have not maximised the conspicuousness of fruit displays (Schaefer et al. 2007). This is partly due to distinct signalling strategies where high contents of anthocyanins function as defence against fungi (Schaefer et al. 2008a) and to advertise high contents of antioxidants that increase the immune response of birds (Catoni et al. 2008, Schaefer et al. 2008b). We also found that the communicative function can be transferred from the fruits to accessory structures. Interestingly, the colour of bracts can be strongly influenced by the environment in Sambucus nigra. Red bracts increase the detectability but also the reliability of information transfer because the sugar contents in fruits are correlated to the production of anthocyanins in vegetative tissue (Schaefer and Braun 2009). Birds attend to the informative cues of the bracts suggesting that the environmentally-enforced correlation between visual traits and fruit rewards presents an alternative, and more robust mechanism than true communication which evolves as a coevolutionary process. As such, abiotic factors can strongly influence the form of information transfer between plants and animals.

Abstract

Visual communication between fruits and frugivores

The detection of fruits is a first crucial step in the interaction between fruits, seed dispersers, and fruit predators. The selective pressures acting upon the detection of fruits can be understood under the framework of communication theory. Communication theory expects that traits evolve so that they increase the detection of mutualists while minimising the risk of detection by predators. Visual traits such as the colours of fruits and their accompanying structures also encode information on the quality of fruit rewards. As such fruit-eating animals can impose differential selective pressures upon fruit colours resulting in trade-offs between signal efficacy and signal content that may partly explain the diversity of fruit colouration. For example, fruit-eating birds can select for increased conspicuousness against the background and they can also select for increased contents of pigments as nutritional antioxidant rewards. Fruit colours and their reliability in indicating fruit rewards are partly influenced by the environment. The intensity of surrounding light can influence concomitantly the sugar rewards of fruits and the visual signals of fruit displays. The phenotypic linkage between signal design (pigment production) and sugar quality of fruit rewards are probably widespread resulting in reliable plant–animal communication. Consequently, there are distinct evolutionary and ecological parameters that determine fruit colouration and its relative importance of long-distance detection.

References

Catoni, C., Schaefer, H. M. and Peters, A. 2008. Fruit for health: the effect of anthocyanins on humoral immune response and food selection in a frugivorous bird. Functional Ecology 22: 649-654.

Cazetta, E., Schaefer, H. M. and Galetti, M. 2009. Why are fruits colorful? The relative importance of achromatic and chromatic contrasts for detection by birds.Evolutionary Ecology 23: 233-244.

Schaefer, H. M. and Braun, J. 2009. Reliable cues and signals of fruit quality are contingent on the habitat in black elder (Sambucus nigra). Ecology 90: 1564-1573.

Schaefer, H. M., Schaefer, V. and Vorobyev, M. 2007. Are fruit colors adapted to consumer vision and birds equally efficient in detecting colorful signals? American Naturalist 169: S159-S169.

Schaefer, H. M., Rentzsch, M. and Breuer, M. 2008a. Anthocyanins reduce fungal growth in fruits.Natural Product Communications 3: 1267-1272.

Schaefer, H. M., McGraw, K. and Catoni, C. 2008b. Birds use fruit colour as honest signal of dietary antioxidant rewards. Functional Ecology 22: 303-310.