As well as having more solar energy per unit area, the tropics also have this energy distributed relatively evenly across the year so avoiding the cold winters of higher latitudes. The reason why this may be relevant to our question of 'Why are the tropics so diverse' can be seen by considering birds. Many high-latitude countries, such as Canada and Britain, have bird species that feed exclusively on insects during the summer, a good example being the Barn Swallow. This species feeds almost exclusively on invertebrates it catches while in flight; such food is effectively unavailable during the northern winter and the swallows migrate south to areas where this food is still plentiful.81 So while higher-latitude sites can have a high biomass of insects in the summer (indeed there can be a greater insect biomass than in many tropical forests), they are unavailable as food for much of the year, thus restricting the diversity of specialist insectivores.82 There is a similar situation with fruit-eating birds: fruit is available year round in many parts of the tropics allowing specialist fruit-eating birds to evolve there.83 However, in a higher-latitude country such as Britain, there are very few fruits available from the late winter until mid-summer,84 making it impossible to be a resident specialist fruit-eating bird. A recent analysis of seasonal variation in bird species richness in North and Central America showed that, because of migration, patterns of diversity tracked changes in temperature, precipitation, and vegetation over seasonal time sales, paralleling a similar relationship on a spatial scale.85 Incidentally, this ability to understand temporal variation in species richness and the close relationship between temporal and spatial variation gives important support to the idea that climate variables play an important role in influencing species richness.
The bird examples we have just discussed also help to make a more general point. Clearly climate often limits species distributions, as illustrated by a multitude of gardening texts describing which plants can (and cannot) be grown in a certain climate. Since more species may be able to tolerate local conditions in the tropics year round, then this may lead to a correlation between climate and/or NPP and species richness. The exact reasons why more species should have tolerances for tropical conditions than temperate conditions are varied. One reason may be that it simply arises out of some form of fundamental physiological constraint—most species tolerate conditions in some places (warm and wet) better than others (cold and dry). However, taking an evolutionary perspective, we find ourselves coming back to a hypothesis we introduced earlier—more species may have tolerances to the tropics because of niche conservatism (many major taxa arose in the humid tropics, and what starts in the tropics tends to stay in the tropics). The relative lack of seasonality in tropical environments may also play a role, promoting increased specialization under these conditions which reduces competition and thereby allows more species to be packed in (see also Rapoport's rule later).
Nevertheless, there are problems with the tolerance idea, particularly when tolerances are viewed as some form of immutable constraint. As David Currie and colleagues63 have pointed out, the fact that 'many species are often absent from areas whose climate they can tolerate, and to which they apparently could disperse', suggesting that tolerances do not fully dictate distributions, and cannot therefore provide a full explanation for the patterns of latitudinal species richness. Possible examples of this are the shrub Rhododendron along with sycamore trees in parts of Europe. Both of the aforementioned species are native to some areas of Europe but, as human introductions have demonstrated, they are able to thrive in many parts of the continent outside their natural range.86 Rhododendron, in particular, appears unlikely to have been limited by dispersal as it naturally reached many of the areas, where it is now a human introduction, in previous interglacial periods.86,87
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