Patterns in species richness

When the ranges of individual species are superimposed on one another and counted up, striking patterns in the total numbers of species become clear. Species richness, as it is called, tends to be greater at the warmer end of each biome in the mid and high latitudes, and in the wetter parts in the tropics. In general, there is a strong trend towards more species of trees in forests at lower latitudes. This trend is most obvious in eastern Asia where the climate is uniformly moist from north to south and the only

Figure 2.25a. The cross-leaved heath (Erica tetralix), a plant with an oceanic distribution. Source: Christian Fischer.

Figure 2.25b.

The cross-leaved heath (Erica tetralix) shows a typical "oceanic" range, along the western side of Europe next to the Atlantic Ocean.

Erica Tetralix Range

Figure 2.26. Rhododendron ponticum.

major trend in climate is in terms of temperature (Figure 2.27). Some areas of the world show trends related to both temperature and rainfall: for example, the species richness of the deciduous forest in eastern North America which increases towards the south but also decreases into the dry interior of the US (Figure 2.28).

No-one is quite sure why species richness tends to be higher in warmer and moister environments. A range of hypotheses have been put forward during the last 150 years, but each of them starts to look paradoxical when examined in detail.

One popular idea amongst ecologists notes that the latitudinal difference in tree species richness correlates strongly with net primary productivity, the growth rate of vegetation. According to this idea, if there is a bigger "cake" of resources enabling and resulting from faster growth, there is more chance for species each to take their own "slice" (or niche). However, when we look in detail there is not really much evidence that species are on average more specialized in species-rich environments than in species-poor environments.

Another idea suggests that, because the world was nearly all warm and moist around 50 to 60 million years ago when the flowering plants were busy diversifying,

Figure 2.27. Tree species richness map of parts of eastern Asia (eastern Russia, Japan, Taiwan). These are the numbers of wild tree species occurring per cell in a geographical sampling grid, based on published tree species range data. There is a very strong latitudinal gradient. Source: Redrawn from Author.

Figure 2.27. Tree species richness map of parts of eastern Asia (eastern Russia, Japan, Taiwan). These are the numbers of wild tree species occurring per cell in a geographical sampling grid, based on published tree species range data. There is a very strong latitudinal gradient. Source: Redrawn from Author.

most lineages became fundamentally adapted to living in the tropics. 0ver more recent time, the cold and dry environments that have become much more widespread have presented a new challenge that few lineages of plants have been able to adapt to. If this is the case, surely we would expect to see the levels of botanical richness of the high latitudes increasing in the fossil record, as more groups of plants overcame these barriers. Also, the earliest groups of plants that made it out into colder and drier environments should have been busy diversifying into more and more forms over

Tree Richness
Figure 2.28. Wild tree species richness for North America. From Adams (2009).

time. Yet. in the fossil record we see almost no signs of such a build-up in diversity. Essentially the same groups of plants have been important for the past 30-40 million years in the colder temperate forests, with nothing much added. Contrary to the expectations of this hypothesis, diversity in the temperate forests has if anything declined somewhat over the past few million years (see below). Essentially, then, the causes of these grand geographical gradients in richness remain a mystery to ecologists. For a lot more discussion on the topic, see my other book Species Richness (Adams, 2009).

Even if we cannot really explain latitudinal gradients, certain other broad-scale patterns in species richness can be explained more convincingly in terms of past events which destroyed most species of plants in some places but left many more to survive in others. For example, the tree flora of temperate eastern Asia is a lot richer in species than climatically similar parts of Europe and North America, even though all three areas show a strong underlying trend in species richness which parallels the average temperature. The reason for this difference between the regions may be the fact that during glacial phases over the past 2 million years, the climate in parts of east Asia stayed a lot moister than anywhere in Europe or the eastern USA. Pollen evidence from ancient lakes shows signs that moist, closed forest persisted in eastern Asia. In North America, and Europe especially, the climate was much drier with open woodland, scrub or grassland. Many drought and cold-sensitive types of trees that existed in all three regions before about 3 million years ago would have been able to survive in Asia, whereas they died out in Europe and North America.

In this chapter we have considered how vegetation is shaped by climate in a relatively static sense. Even when including the extremes of its changes, we have merely touched upon the ways in which plants might have moved from one place to another when climate shifted. Chapter 3 is devoted to these transformations in vegetation in response to climate.

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