[MUSIC] I have previously talked a lot about how the flowering plants have evolved in concert with the pollinators, and how floral traits and pollination mechanisms are correlated. But of course there are many other biotic and abiotic factors affecting evolution of the floral traits, as well as the vegetative traits of the flowering plants. Let's first take a look at some of the other plant-animal interactions, than pollination. An important aspect in the life cycle of a flowering plant is seed dispersal. Many plants simply drop their seeds on the ground, or use the wind for shorter or longer dispersal, or a few even use water. Plants that have wind dispersal will usually have dry fruits, often capsules like these, with many small and light seeds. The capsules are dry, they are already opened, and most of the seeds have fallen out. But here are a few, small and light enough to be carried a short distance by the wind. Other plants rely on animals to disperse the seeds. So they need something to make them attractive. Most commonly they enclose the seeds in fleshy fruits like these different berries. We can also see some red berries of some of the cacti. And here is one. Many fleshy fruits become colorful when the seeds are ripe to signal now is time to be eaten. Usually the bird or mammal will eat not only the fruit, but the seeds too. But they are adapted to pass unharmed through the gut. An alternative way of attracting animals, is to make the seeds themselves edible as in many larger nuts. But this procedure obviously only works if not all the seeds end up being eaten. Whereas insects are the most frequent pollinators of flowering plants, vertebrates, mostly birds and mammals, are the most frequent seed dispersal agents. However, there are some insects that have a major function in seed dispersal. The best example is ants. Ants are relatively modern insects having developed in late Cretaceous, and ant dispersal is a rather advanced and specialized trait in the evolution of seed dispersal. But in general the co-evolutionary link between seed dispersing animals and flowering plants, is not as tight as between the plants and their pollinators. Both pollination and dispersal usually take place to a clear mutual benefit for the flowering plants and the animals. But another association between plants and animals would seem very much to the favor only of the animals is herbivory. Herbivores feed not only on fruits or seeds, but on foliage, or other plant parts. In general it's obviously not good for the plant to be eaten. So to avoid being eaten, plants have developed different types of defense strategies. Spines is a classical example of physical defense. Here in the succulent collection we see lots of examples on that. Not just on the many cacti, but also on other succulents. Defense towards herbivores may also be chemical and many plants contain different types of poisonous compounds, directed towards herbivores. The euphorbias are also examples of that. They contain a more or less poisonous milky sap. Any defense mechanism evolving in the plants, will, however, be challenged by new inventions in the animals. So herbivores and plants live in an everlasting arms race. On a larger scale, whole plant communities develop in concert with the animals inhabiting the communities. Open grasslands and savannahs depend on large herbivores, and at the same time the herbivores depend on the open grassland. Going back in evolutionary times to the Cretaceous, there is a correlation between the spread and increase in numbers of flowering plants, and the decline in numbers of high browsing dinosaurs. At the same time, low browsing dinosaurs increased in numbers. As the early flowering plants were generally rather small, and had a different leaf anatomy than their seed plant ancestors, the theory is that the spread and diversification of the flowering plants, acted as a driver for the evolution of the smaller dinosaurs as ornithopods and ceratopsians and their particular jaw morphology. Later on in the evolution of the flowering plants, a similar theory was proposed for the evolution of larger mammal herbivores and their jaw morphology, as a response to the spread and diversification of the grasses which have a high content of silica in their leaves. So we see that there is an intimate connection between the evolution of the flowering plants and the animals inhabiting the world. But other factors affecting evolution are the abiotic ones, like temperature and humidity, just to mention a few. Abiotic factors may have some influence on floral morphology, but a much more pronounced influence on vegetative traits. Large thin leaves are obviously only suited for a humid environment, whereas in dry environments, as imitated in the succulent house, plants develop different strategies to keep the water. To reduce evaporation, they can reduce their surface like these cacti, and other succulents. They can make an insulation of dense hairs, or develop organs of tissue adapted for storing the water. Another very well-known response to an abiotic factor, temperature, is broad-leaved trees in cold climates becoming deciduous. During the entire Cretaceous, the Earth was a much warmer and more humid place than it is today. The sea level was substantially higher than now and the level of carbon dioxide in the atmosphere was also much higher, due to volcanic activity. So the flowering plants originated and evolved, in a much different world than the one we know today. When we combine all the evidence that we have from fossils, and from phylogenetics, the evidence suggests that the first flowering plants were small wetland shrubs or herbs, maybe even true aquatic plants. Some abiotic changes did of course, occur during the Cretaceous, and not least the breakup of land masses, led to substantial habitat diversification. The flowering plants undoubtedly responded to such changes, but the most drastic change must have occurred at the K-P boundary. The asteroid impact which ended the Cretaceous, became a game changer for most life on Earth. The Earth became cooler, light was reduced, and many flowering plants became extinct. Local extinction rates up to 70% have been reported. However, some members of all the major lineages of plants that had evolved during the Cretaceous survived this event, and diversified substantially during the subsequent Cenozoic. Two groups were particularly successful, the monocots, and the eudicots. Today, the monocots account for some 25% of all species of flowering plants, and the eudicots for more than 70%. Among the monocots, two groups are particularly successful. They are the grasses and the orchids. Grasses here, orchids there. The orchids are the second largest of all flowering plant families, with more than 21,000 species described so far. But despite the very high number of species, they don't have a major ecological impact. No plant community would be likely to collapse even if all orchids disappeared. Quite the opposite with the grasses. The grass family includes some 10,000 species, but the success lies not as much in numbers of species as in ecosystem dominance. Grasses dominate the vegetation of some 20 to 30% of the terrestrial surface of the Earth. And many herbivores are entirely dependent on the grasses. This level of dominance has been achieved by no other family of flowering plants. But the largest family of flowering plants on Earth today, must be found among the eudicots. But I don't think that many actually know which family it is. With some 23,000 species the Asteraceae, that is the composite or sunflower family, is the largest. Here you can see a few. Some species can be important elements in local floras. But just as the orchids, the composites have not gained ecosystem dominance. But today flowering plants in general dominate most terrestrial ecosystem. Just think about tropical rainforests, temperate forests of broad-leaved trees, the African savannah, or all grasslands. They're all dominated by flowering plants. Even deserts are more abundant in flowering plants, than any other group of plants. So are there ecosystems left where the flowering plants do not dominate? I actually indirectly eluded to one of them just before, by mentioning temperate forests of broad-leaved trees. But in the temperate climate zone and in mountain areas elsewhere, we also have the conifer forests. That is an ecosystem dominated by another kind of seed plants. And there are also the peat bogs dominated by sphagnum mosses. Even if your eye at first catches all the other plants, it is the tiniest sphagnum mosses that build up the entire bog. But whereas flowering plants do dominate most terrestrial ecosystems, the situation is entirely different in the marine environment. Only at low depths in coastal areas, a few flowering plants have adapted themselves to a life in sea. On a global scale, we are talking about no more than some 60 species of flowering plants, known as sea grasses. Even if they may be locally dominant, and have huge importance for marine life, the overwhelming amount of plant biomass in the ocean comes from algae. To end the story about the flowering plants, it may be relevant to ask the questions. Why are we so interested in knowing how the flowering plants evolved? And why is past diversity relevant at all? The overarching answer to these questions, is that only by knowing the past, can we have a chance to look into the future. The more detailed we can describe past evolution, the more detailed can we predict future evolution. The pertinent questions these days relate to global warming. So how will the flowering plants cope with the world getting warmer? All other things aside, probably quite good. We know that they originated and diversified in a much warmer climate than now. And we also see that the ecosystems which are not dominated by flowering plants, are now mostly located in cooler climates. So regardless of many local exceptions that may cause species extinction, the likely scenario is a world with even more dominance of flowering plants. [MUSIC]
