Macro-ecological patterns in seed removal by animals

Abstract

This thesis aims to improve our understanding of several long-held ideas concerning seed removal by animals across large-scale gradients. By assembling a database of 13,135 animal-seed interactions across all vertebrate taxa, I provided the first broad test of the idea that large animals ingest large seeds. Surprisingly, I found that the size of ingested seeds was significantly negatively correlated with animal body weight. This negative relationship was driven by large animals, particularly ungulates, ingesting small seeds. The results show that the loss of large animals could have negative effects on the dispersal of small-seeded plants, in addition to the more widely acknowledged impacts on large-seeded plants. Next, I used data for 4008 Australian species to provide the first quantitative analysis of the idea that fleshy fruits are more prevalent towards the tropics. Plants were more likely to bear fleshy fruits at low latitudes, and in regions with warm, wet and stable climates. Fruit type was more strongly affected by conditions during the parts of the year in which they grow than by conditions during the harshest parts of the year, suggesting that some current theories on plant traits may focus on the wrong aspects of climate. Finally, I performed a field study across 25 sites spanning 28 degree of latitude along the east coast of Australia, to provide the first empirical test of the idea that seed predation and seed defense are greater towards the tropics. Contrary to traditional expectations, neither seed predation nor seed physical defence was more intense at low latitudes. In fact, pre-dispersal predation and defence were greater at higher latitudes. My results are consistent with recent findings on latitudinal gradients in herbivory and defences in leaves. My findings cast further doubt on the generality of latitudinal gradients in biotic interactions, and suggest that increased seed/seedling mortality as predicted by the Janzen-Connell hypothesis does not provide a plausible explanation for the greater diversity of tropical ecosystems. My thesis has tested several well-accepted ideas on seed removal by animals, and shown that our understanding of the factors that shape global patterns in biodiversity needs to be reshaped