Now that we have reviewed species and ecosystem diversity, which reflect biodiversity within communities and across landscapes, let’s consider patterns of biodiversity at larger regional and global scales. In order to make conservation decisions, we need to know something about the biological differences among Earth’s various regions, such as islands and continents, and climatic zones, such as those on cold mountaintops or in the tropics. Such knowledge will help us choose the best places for biological reserves and will inform management of these areas.

The biomes are large geographic areas recognized by their distinctive biological structure—associations of plants, animals, and other organisms—and characteristic vegetation or plant growth forms, such as trees, shrubs, grasses, or vines. For example, the plant and animal species that live in tropical forests and tundra are entirely different, and both support species different from those living in the Mediterranean scrub biome. However, because of their large scale, biomes are made up of many interacting communities and ecosystems. Variation in climate, soils, and other physical factors determine the type of biome covering a geographical region (Figure 4.3 on page 99). For example, the cold climates of the far north are dominated by the low herbs and dwarf trees of the tundra biome, as well as the vast expanses of coniferous trees characteristic of the taiga or boreal forest biome. Meanwhile, tropical forests are found in regions where it is warm year-round and very rainy. Deserts develop where there are dry climates.

As we will see in Chapter 7 (pages 193 and 195), the soils of the terrestrial biomes are also distinctive and support very different types of economic activities: Temperate grasslands support extensive growing of grains, such as wheat and maize, whereas the boreal forests support vast wood harvesting for lumber and wood pulp for paper manufacturing.

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The major marine biomes include the open ocean, ocean floor, coral reefs, and kelp forests (Figure 4.4 on page 100). While coral reefs are limited to warm tropical oceans, kelp forests are found in cool-temperate coastal waters through cold regions, such as the coasts of southern California to Alaska. The diversity of species living in and on intact coral reefs rivals that of tropical forests, and the organisms making up a coral reef community are very different from those inhabiting a freshwater marsh. Aquatic biomes that occur at the transition between marine and terrestrial environments include salt marshes and mangrove forests. Meanwhile, freshwater wetlands occupy the transition between freshwater and terrestrial environments. Because of highly variable physical conditions and greater nutrient availability, these transitional biomes have low biodiversity but exceptionally high production.

Consider that the number of bird species living in tropical Suriname is over 7 times higher than in Newfoundland, although they are roughly the same size (163,820 km² versus 111,390 km²). Other groups of organisms such as mammals, trees, reptiles, fish, and insects also increase in species richness from the poles to the tropics. Similar increases in species richness occur among freshwater and marine organisms, such as fish, mollusks, and algae. As a consequence, regions near the equator contribute disproportionately to global species richness (Figure 4.5 on page 101).

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Why are the tropics so diverse? There are a few possible reasons for this phenomenon. The simplest is that there is simply more room in the tropics. The continents are centered around the equator, and the amount of land area there exceeds that in temperate regions. Other factors may include the amount of sunlight that the equator receives year-round, which contributes to greater levels of plant growth. The tropics also tend to have relatively stable climates, compared with the temperate zone, where seasonal changes limit growth in cold months. However, we do not yet have an entirely satisfactory explanation for this long-known pattern, and ecologists continue their research.

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Although the tropics are generally the most diverse parts of the globe, particular areas seem to be “biodiversity hotspots,” with spectacularly high numbers of species. Conservation International, a nongovernmental conservation organization, has identified 34 biodiversity hotspots on Earth, which they define as regions that have been reduced from their historic area by at least 70% and that support at least 1,500 endemic species of plants—that is, local or regional species found nowhere else on Earth. These biodiversity hotspots represent only 2.3% of Earth’s land area yet support half the world’s plant species (150,000 species) and nearly half the terrestrial vertebrate species. Many of these hotspots are located in the temperate zone, including areas around the Mediterranean Sea and the California region (Figure 4.6).

Islands are natural laboratories for the study of biodiversity. For instance, biologists Robert MacArthur and E. O. Wilson proposed that the number of species on an island remains relatively constant over time but that the composition of species on an island changes. According to their theory, known as the equilibrium model of island biogeography, the number of species on an island is the result of a balance between the rate at which new species immigrate (arrive from somewhere else) there and the rate at which species already on the island become locally extinct (although they may persist elsewhere).

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Let’s look at an example. The birds on Delos, a small Greek island of only 6 km², were surveyed during the middle of the 20th century and then again 35 years later (Figure 4.7). Between surveys, three bird species became extinct on the island, while two new species immigrated. The result was an insignificant decline in the number of bird species from seven to six. The bird community on Delos supports the theory proposed by MacArthur and Wilson.

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MacArthur and Wilson proposed that the area of an island and its distance from the mainland source population determine rates of immigration and extinction. Larger islands tend to have higher species numbers for two reasons. First, they provide a larger target area and therefore receive more immigrants. Second, they have lower extinction rates because they can maintain larger populations of resident species. Now consider islands that are close to the mainland. They receive more immigrants because of close proximity. They also have lower extinction rates because immigration can help sustain the population of a species. Consequently, large islands close to the mainland tend to harbor the most species, whereas small, isolated islands harbor the fewest (Figure 4.8).

The equilibrium model of island biogeography can be broadly applicable to any habitat that occurs as isolated patches, such as remnant patches of woods in a landscape that has been cleared to establish agricultural fields or national parks or wildlife refuges, surrounded by intensively used forest or grazing lands. The implication of the equilibrium model of island biogeography is that larger and less isolated natural habitats will support higher species richness.