Jessica Gurevitch, Samuel M. Scheiner, and Gordon A. Fox - The Ecology of Plants - 2002 / The Ecology of Plants Jessica Gurevitch, Samuel M. Scheiner, and Gordon A. Fox; 2002 / Ch 20

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tions being anaerobic food webs), the diversity of animals, fungi, and microbes is often linked to plant diversity. Thus, the issues discussed in this chapter have practical as well as fundamental implications.

Large-Scale Patterns of Diversity

Scientists first became aware of large-scale patterns in species diversity as a result of the voyages of exploration and colonization undertaken by Europeans in the eighteenth and nineteenth centuries. Their ships often carried botanists and zoologists as part of the crew, the most famous example being the voyage of the Beagle (1831–1836), on which Charles Darwin served as ship’s naturalist in. These naturalists made records of the plants and animals they found, brought back specimens, and added them to the growing catalog of described species. In the eighteenth century, Carl Linnaeus named over 9000 terrestrial plant species; today there are approximately 250,000 named plant species.

From these records, it soon became apparent that tropical regions were typically very rich in species, polar regions were very species-poor, and temperate regions had intermediate numbers of species. For example, Brazil has over 56,000 named plant species, the United States has about 18,000, while Canada has about 4200. Species richness in tropical regions is probably underestimated because many of these areas have still not been thoroughly explored . Even in North America, which has been studied by botanists for hundreds of years, new plant species are being named at a rate of about 60 per year. These plants are occasionally found right under one’s feet—for instance, in 1997, a new species, the mustard Lesquerella vicina (Brassicaceae), was discovered by the botanist James Reveal growing in his neighbor’s yard in Montrose, Colorado.

An explanation for the general latitudinal gradient in species richness may be the oldest major ecological hypothesis (Hawkins 2001). Between 1799 and 1804, Baron Alexander von Humboldt traveled through Mexico, Central America, and northwestern South America. He subsequently published a series of essays under the title Ansichten der Natur (“Views of Nature”), in which he described a global gradient in species diversity. He postulated that this pattern was due to differences in climate, specifically winter temperatures and the effects of freezing.

Besides the latitudinal gradient, many other patterns of diversity have been observed at regional and global scales. The regional diversity of a particular taxonomic group may be very similar to or extremely different from the diversity of that group in similar habitats in other parts of the globe (Schluter and Ricklefs 1993). Comparisons among Northern Hemisphere regions with similar conditions show that there are more plant species in east-

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Figure 20.1

Plant species richness along an elevational gradient in the Santa Catalina Mountains in southern Arizona. Both the lowand high-elevation communities—the Sonoran Desert and boreal forest, respectively—are in extreme environments, resulting in a peak in diversity at intermediate elevations. (After Brown 1988; data from Whittaker and Niering 1975.)

ern Asia than in Europe, while North America has an intermediate number. In high mountain ranges, there are typically more species at mid-elevations than elsewhere (Figure 20.1). Within continents, there are longitudinal gradients of diversity; in the boreal forest of North America, species richness within sites is higher in central Canada than it is in either western or eastern regions (Figure 20.2). In the temperate zone, between approximately 35o and 60o latitude, there is a peak of within-community species diversity (Figure 20.3). In this chapter we explore some of the hypotheses that have proposed to explain these patterns of terrestrial plant species diversity.

Levels of Explanation

How have ecologists attempted to explain the causes of variation in species diversity along geographic or environmental gradients? The processes that we use to explain a pattern change with the temporal and spatial scale of that pattern. At short to medium time scales— up to centuries or millennia—ecological processes dominate. The physiological and ecological properties of each species result in species sorting among communities within a region. Over longer time periods, these properties, together with the history of each species, help determine the distributions of species at regional and continental scales. At even longer time scales, ecological and historical factors affect the distributions of species across the globe. At the longest time scales—from millennia to hundreds of thousands or millions of years—

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Figure 20.2

Number of species (mean ± 1 standard error) in 30 × 30 m2 plots in boreal forests of Canada. Species richness is higher in the central region, especially in white spruce communities. Bars with different capital letters indicate statistically significant differences among regions within community types. Bars with different small letters indicate statistically significant differences within regions among community types. (Data from Qian et al. 1998.)

Regional and Global Diversity 407

allows it to occupy a new habitat. At regional scales, these processes of adaptation can lead to the formation of new species. If the new species is different enough, it may coexist in the same habitat or landscape with the species from which it evolved, increasing species richness in that landscape. At continental to global scales, as new species appear, they may migrate to new regions, so that species richness increases over an entire set of regions.

Some proposed explanations for patterns of species diversity assume that species distributions are in equilibrium with current ecological conditions, while other explanations do not make that assumption. Whether a given explanation assumes that diversityis at equilibrium or not depends, in part, on scale. Within a single community, for example, disturbance can increase diversity (see Chapter 14); this is a nonequilibrium explanation for diversity. But at a larger scale, across communities in a landscape or region, disturbance in particular communities balanced by migration among communities may determine diversity (see Chapter 17). This is an equilibrium explanation for diversity, although the equilibrium is a very dynamic one.

evolutionary processes (adaptation, isolation, speciation, and extinction) predominate. Some processes, including range expansions and contractions and extinctions of populations and species, may happen over long periods of time or very quickly. Humans and their effects on the environment have greatly accelerated these processes for some species (see Chapter 22).

The mix of processes governing diversity will also differ depending on spatial scale. At local scales—with- in communities—interactions among species are of primary importance in determining species richness. These interactions include competition, herbivory, and mutualisms (see Chapters 10 and 11). As our scale expands to the level of the landscape or sets of communities, metapopulation processes may become increasingly important. These processes include migration among communities and extinction within communities (see Chapter 17). Finally, at continental to global scales, migration and extinction in response to climate change become important. We discuss one example, migration following the most recent glacial retreat, below and in Chapter 21.

Evolutionary processes also operate at all of these scales, although over longer time spans. At small spatial scales, local adaptation can occur (see Chapter 6). In response to competition with other species, a population may evolve to occupy a different niche, thereby reducing competition and increasing local diversity. Similarly, a population may evolve a defense against an herbivore that

Figure 20.3

A latitudinal pattern in vascular plant species richness. Each point represents the mean number of species per site averaged across sites in a particular landscape. Species richness of tropical landscapes is underestimated in this analysis. Latitude is given in absolute degrees so that landscapes in the Northern and Southern Hemispheres are superimposed. The latitudinal gradient is very weak because many factors affect large-scale patterns of diversity (see text). (After Scheiner and Rey-Benayas 1994.)

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Explanations for Latitudinal Gradients

A natural tendency in the face of the strong patterns of latitudinal diversity is to seek simple, unifying explanations for those patterns. If, for example, we can account for the temperate-tropical diversity gradient on a number of continents with a single explanation, we are likely to have a deeper understanding than if we try to explain the patterns on different continents separately by invoking details of the floras and histories of each continent. Several theories have been put forward to explain latitudinal diversity patterns (Table 20.1), some of which we delve into here.

A useful starting point for evaluating hypotheses is to build a null model. Used throughout the sciences, null models describe the patterns that would be observed if only random processes were operating. One such null model is the bounded ranges hypothesis (Colwell and Hurtt 1994; Willig and Lyons 1998). Developed as a possible explanation for the latitudinal gradient of animal diversity, this hypothesis uses a simple geometric argument. Species ranges are bounded by the North and South Poles; otherwise, species can appear any-

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where. Random placement of species on a uniform globe would result in many ranges overlapping along the equator, thus creating a peak in diversity in tropical regions and decreases in diversity toward the poles.

Because the bounded ranges model includes no ecological processes—only geometry—it serves as a useful baseline against which to compare observed patterns. For example, the model was found to account for some, but not all, of the latitudinal pattern of diversity in New World bats (Lyons and Willig 1999). Actual species diversity was higher in the Tropics and lower at higher latitudes than predicted by the model. Other biological processes, therefore, are needed to explain the entire pattern. Thus, a null model provides a useful starting point for discovering where interesting patterns lie and what about those patterns requires explanation. No analysis of this kind has yet been done with plants.

A related idea depends on the observation that species’ range sizes tend to increase with latitude and elevation (Rapoport 1982; Stevens 1992) . This pattern is known as Rapoport’s rule. Because temperate-zone species have larger ranges than tropical species, temperate species’ ranges are more likely to extend into the

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Regional and Global Diversity 409

Tropics than tropical species’ ranges are to extend into the temperate zones. The result will be more species in the Tropics—species that live only in the Tropics plus those that have overlapping ranges from neighboring temperate regions. Fine (2001) tested this hypothesis using data for North American trees. He found that few tropical species are also found in extratropical areas, and concluded that Rapoport’s rule cannot explain the latitudinal diversity gradient of plants.

Another simple explanation for latitudinal diversity patterns, especially for the most extreme environments, is differences in opportunities for growth. At very high elevations and latitudes, conditions are so unfavorable that few species are physiologically capable of persisting there. At the summit of the Mauna Kea volcano in Hawaii (4700 m elevation), there is one vascular plant species present, a grass, with only a few individuals in an entire hectare. The climate, with very cold air and little precipitation, means that the soil is poorly developed. Not surprisingly, there are few organisms of any kind in this environment, while plant species richness is very high on the lower slopes of the mountain, especially on the wet side of the island (see Figure 18.11).

An analogous argument explains why species diversity is so low in cold, arid Patagonia, Argentina, in southernmost South America, and so much greater in the Amazon basin near the equator, and why diversity is lower in the Canadian tundra than in the rainforests of Central America. Unfortunately, this simple explanation does little to explain the rest of the latitudinal (or elevational) diversity gradient. More complex explanations are needed.

One major nonequilibrium hypothesis for the increase in diversity toward the equator is that communities in the Tropics are older than those closer to the poles. If communities in the Tropics have remained intact for a longer period of time, then the species in those communities should have had more opportunity to diversify into many species. Given enough time, it has been proposed, there might be more speciation in temperate and polar communities, or more migration of species from low to high latitudes, and the latitudinal gradient might slowly disappear. A related idea is that tropical communities have experienced fewer extinctions because they have experienced fewer climatic fluctuations, and in particular have never been glaciated.

Explanations such as these that invoke time and climatic stability have a number of serious weaknesses. For example, there is much evidence that there has been sufficient time for plant species to migrate into temperate and polar regions (see Chapter 21). In addition, a great deal of evidence makes it clear that tropical communities were strongly affected by recent glaciations, even though they were not covered by glaciers themselves. Similarly, the existence of elevational gradients, for

which migrational lag cannot be the explanation, weakens this hypothesis.

In contrast to the notion that climatic stability in the Tropics has led to increased diversity, some ecologists have proposed the opposite argument, suggesting that speciation and species coexistence are most likely to occur with more frequent disturbances. One reason to believe that such a relationship might exist is that disturbance may reduce the chance of competitive exclusion. However, there is no evidence that tropical communities differ from temperate ones in frequency of disturbance. In addition, as originally formulated, this hypothesis was unclear as to the scale of the disturbance effects involved. That is, it might explain differences in local diversity, but not the large difference in species diversity between Brazil and the United States.

Disturbance might also increase diversity through its evolutionary effects. For example, speciation rates might have been greater in the Tropics because of the repeated contractions and expansions of the tropical rainforest during repeated glaciations. Geographic isolation, which would have been heightened by these shifts, is known to promote speciation (see Chapter 6). However, both temperate and tropical species should have been subjected to similar kinds of isolation during recurrent periods of glaciation and warming. The distribution of species in northern boreal and polar regions show evidence for this pattern of speciation (Figure 20.4). In Chapter 21, we will look at general patterns of speciation and diversity as affected by long-term climate changes.

An old and influential equilibrium explanation for the latitudinal diversity gradient is the available energy, or productivity, hypothesis (Preston 1962). The notion is simple: increasing productivity means that more energy is available for growth and reproduction and that more individuals can therefore live in the same amount of area (see Chapter 14). Because a sustainable population requires some minimum number of individuals (see Chapter 7), more individuals in an area means that they can be divided up among more species, resulting in higher species richness.

On an evolutionary time scale, larger populations reduce the chance of extinction, again resulting in higher species richness. Analysis of the latitudinal pattern shown in Figure 20.3 revealed that productivity differences were associated with about 25% of the total variation in species richness and accounted for most of the latitudinal pattern (Scheiner and Rey Benayas 1994). However, while productivity is an important explana- tion—indeed, it may be the most important single factor in the latitudinal gradient—it is certainly not the only factor causing the gradient. The effects of productivity on diversity also vary with scale, as we will see later in this chapter.

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Figure 20.4

Distribution of the Lathyrus maritimus (Fabaceae) species complex around the Northern Hemisphere polar region. Populations of this plant in different regions differ morphologically, particularly in whether and where leaves have hairs. The different morphological types may be different species or incipient species. The circumpolar distribution of this species complex is typical of many polar and boreal species and genera.

A simple and currently controversial equilibrium theory is often called the area hypothesis. It proposes that the Tropics are more species-rich because the total tropical land area is greater than the total temperate land area (Rosenzweig 1995). This hypothesis contains both ecological and evolutionary components. Ecologically, one can reason that as area increases, spatial heterogeneity also increases. Because the Tropics encompasses more area, it contains more habitat types, and therefore more species. Furthermore, over evolutionary time, a larger area affords greater opportunities for geographic isolation, and therefore promotes greater rates of allopatric speciation. Michael Rosenzweig and Klaus Rohde have conducted a vigorous debate over this issue (Rohde 1992; Rosenzweig 1995; Rohde 1997; Rosenzweig

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Lathyrus maritimus complex

Essentially glabrous specimens (leaves are hairless)

Pubescent specimens (leaf hairs present) Pubescence unknown

and Sandlin 1997; Rohde 1998). While the two ecologists agree that area has some effect, they have been unable to agree on how important the area effect is relative to other factors such as productivity. While it is relatively easy to identify which factors are important in patterns of diversity, assigning exact levels of importance is much more difficult.

Continental Differences

If we compare similar regions in eastern Asia, North America, and Europe, we find that eastern Asia has the most plant species, while Europe has the fewest. This pattern holds over a range of taxa. For instance, if we look at the number of tree species across all plant families,

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eastern Asia has the greatest number of species, North America has an intermediate number, and Europe has the fewest. These patterns are also robust across higher taxonomic levels: the same relationships hold for orders, families, and genera of trees (Latham and Ricklefs 1993).

What can explain these patterns? The first explanation relies on differences in species extinctions. A key difference between Europe and the other two regions has to do with an accident of topography. The Alps, the main mountain range in Europe, runs in an east-west direction, while most or all of the principal ranges in eastern Asia and North America run north-south. During periods of glaciation, climate zones are pushed southward. At the height of the most recent glaciation in North America, for example, the edge of the ice sheet was in Wisconsin and boreal forests extended as far south as the Gulf Coast (see Chapter 21). As climate zones moved southward, the plants followed them, and the southern portions of the United States provided refugia for many species.

In Europe, however, the Alps created a barrier to this southward migration. Species that were adapted to warmer conditions were unable to migrate over the tops

Table 20.2 The number of woody species of various genera that went extinct in Europe during the Pleistocene but are still found in

North America

Source: Niemelä and Mattson 1996.

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of the mountains to escape from the cold, and went extinct. As the glaciers retreated, the species in North America and Asia that had retreated southward migrated northward again. But in Europe, those species had disappeared and were not available to recolonize the newly ice-free temperate areas. The end result of these events, repeated during each of the Pleistocene glaciation and warming periods, is the greatly reduced species diversity of Europe (Table 20.2). North-temperate and boreal communities in North America contain about 50% more vascular plant species than equivalent European ones (approximately 18,000 versus 12,000: Niemelä and Mattson 1996). Fossil evidence supports this explanation: European forests before the Pleistocene had much greater species diversity than they do now (Latham and Ricklefs 1993). This explanation depends on historical contingency: the pattern of diversity is the result of a particular sequence of historical events in a particular place, not of a general or fundamental process that explains all such patterns.

The difference between the floras of North America (particularly the eastern part of the continent) and eastern Asia is also thought to be the result of a particular history, but one operating on a longer time scale and depending on rates of evolution—the rise of orders, families, and species—rather than on extinctions (Latham and Ricklefs 1993; Qian and Ricklefs 1999). China and the United States are approximately the same size and contain a similar range of ecological conditions. Yet China contains 60% more species of vascular plants (29,188 vs. 17,997). Much of this difference is caused by the fact that plant families in China tend to have more species than the same families in the United States.

Hong Qian and Robert Ricklefs (1999) hypothesize that these differences in diversity are largely a result of continental-scale geographic relationships. The southern United States is separated from tropical regions by water to the east and south and by very dry conditions to the west. In contrast, southern China is directly connected with tropical and subtropical regions. The temperate regions, Qian and Ricklefs hypothesize, were repeatedly invaded by species from adjacent subtropical and tropical areas. These species then underwent extensive adaptive radiation in the temperate regions. What makes this argument especially compelling is the finding that the greatest differences in diversity between the two continents are found in older taxonomic groups with strong tropical affinities. Europe also suffers from the same limitation as North America in this regard because the Mediterranean Sea and the arid expanse of North Africa create a barrier between it and tropical and subtropical floras. Thus, a happenstance of geography appears to have been largely responsible for differences in species evolution over very long time periods as well as extinction rates over a shorter ecological time scale.

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Figure 20.6

Tree species richness as measured in 2.5° × 2.5° quadrats across North America. The contour lines connect points with the same approximate number of species per quadrat. (After Currie and Paquin 1987.)

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20

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tures—and west—because of decreasing precipitation (Figure 20.6). This pattern contrasts with the pattern for the entire flora, which shows a peak around the Great Lakes (see Figure 20.1). The mid-latitude peak does not exist for trees, presumably because seasonal niche partitioning occurs among species with different growth forms, and so would not be expected to hold for trees, which belong more or less to cies richness can differ between the entire plant community and a single functional group.

Endemism, Centers of Diversification,

and Isolation

As with global patterns, different continental patterns of diversity exist for different subsets of plants. For trees in North America, the highest number of species is found in the southeastern United States, with steady decreases to the north—because of decreasing tempera-

All of the patterns we have discussed so far involve gradual changes in species diversity over large spatial scales. Diversity can also vary in patchy patterns, in which some regions

have many species compared with adjacent regions. Often these differences are a result of evolutionary processes. For example, high species diversity in a region may occur because a particular lineage had a large evolutionary radiation in that region. Cacti, for example, are especially diverse in central and

southern Mexico because many species originated there. Geographic isolation promotes speciation, as we saw in Chapter 6. Islands are an obvious case of such isola-

tion. Madagascar is a very large island off the southeastern coast of Africa that became separated from that continent 90 million years ago. During that time, many species evolved in isolation there. Madagascar is now one of the most floristically diverse places on Earth, with approximately 10,000 native plant species, about 80% of which are endemic (found nowhere else). Similarly, the Pacific islands of Hawaii, New Zealand, and New Caledonia have all been isolated for millions of years, and all have very high species richness and very high proportions of endemic plants (Table 20.3).

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Even continental areas may have endemic species. Rare endemic plants may be restricted to unusual habitats, such as serpentine barrens (see Box 15A). In some cases a species may be endemic to an area because it is the product of a recent speciation event and has not yet spread. Another possibility is that species may be prevented from spreading from their place of origin. If speciation rates are high in a region and species cannot spread, that region will become species-rich and have a large number of endemic species

The general problem of explaining variation in speciation rates is still one of the greatest challenges in evolutionary biology. Other than geographic isolation, there is little agreement about the mechanisms of speciation. Factors that have been hypothesized to cause high levels of speciation or endemism in plants include protection from herbivores, low productivity, the existence of many local topographic barriers, habitat diversity, a low frequency of major disturbances, and isolation. The case of the astounding levels of plant diversity found in the fynbos of the Cape region of South Africa presents many of these issues and the difficulties in explaining them (Box 20A). Such centers of diversification are also important in conservation, as we will see in Chapter 22.

Relationships between Regional and Local Diversity

Most studies of the causes of diversity have focused either on local patterns of diversity (see Chapter 14) or on regional or global patterns, with few studies seeking to connect the two scales. Recently ecologists have started to make this connection, although the roles of large-scale and small-scale processes and how they interact to determine local and regional diversity are still not well understood.

One question we can ask is whether local diversity is set by the number of species available in the regional species pool, or whether it is determined by processes acting locally. We can test these two hypotheses by sampling a series of regions, measuring both the total number of species in each region and the number of species in a small subset of the region, and then plotting local species richness as a function of regional species richness (Figure 20.7A). If local species richness is determined by the size of the regional species pool, then we should see a straight-line relationship, in which a larger regional species pool leads to more species locally. On the other hand, if local processes, such as competition, limit the local number of species, the relationship should be concave; at some point, adding more species to the regional pool should have little or no effect on local species richness. In an analysis of plant communities in Estonia, Meelis Pärtel and colleagues (1996) found that local diversity was a function of regional diversity, with

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(A)

Local species richness

Regional species richness

(B)

Figure 20.7

(A) Two possible relationships between the size of the regional species pool and local species richness. If local richness depends only on the size of the regional pool, then local richness should increase with regional richness (black line). If local richness depends in addition on local processes, then at some point, adding species to the regional pool should not result in any more species locally (green line). (B) The relationship between regional and local species richness for 14 plant communities in Estonia, including grasslands, shrublands, and forests. (After Pärtel et al. 1996.)

no leveling off of the relationship at the highest regional diversity (Figure 20.7B). As of yet, few other studies have quantified this relationship, and a general answer to this question awaits more research.

Some ecologists are attempting to examine the mechanisms that link local and regional diversity. Jonathan Shurin and Emily Allen (2001) modeled the conditions under which local species interactions (competition and predation) contrast with regional patterns