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 10

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Figure 10.17

Diagram of two transects, A and C, in the fynbos of southern Africa, showing the locations of the three experimental study sites along each transect. Dominant species and plants of some other growth forms are also shown. (After Richards et al. 1997.)

limestone bedstraw grew normally, as expected. Heath bedstraw survived, but grew slowly and had yellowish leaves, indicating nutrient deficiency. When grown alone in sandy soil, heath bedstraw grew vigorously, while limestone bedstraw survived, but grew poorly. When the two species were grown together in calcareous soil, limestone bedstraw overtopped heath bedstraw and eliminated it from the mixture. In sandy soil, heath bedstraw became dominant, but did not completely eliminate limestone bedstraw during the course of the experiment. Tansley concluded that while each species appeared to be adapted to the soil in which it lived in nature, competition also played an important role in determining the restriction of the two species to different soil types.

A recent study in a different system found quite different results. M. B. Richards and his colleagues at the university of Cape Town in South Africa (Richards et al. 1997) examined the relative importance of competition and adaptation to soil type among six shrubs belonging to the Proteaceae. These species grow in the fynbos of southern Africa, one of the most species-rich places on Earth for plants (see Box 20A). The fynbos is dominated by an astonishing diversity of shrubs as well as other growth forms. The environment is characterized by nutrient-poor soils, summer drought, and rolling, dissected terrain. Large numbers of apparently ecologically similar species coexist in the fynbos, and there is great species variation

among communities. What determines the typically sharp discontinuities among communities dominated by different species, and what is the role of competition in determining these boundaries?

Richards and his colleagues chose three transects, each of which crossed a sharp community boundary with a transition from one distinct soil type to another. Each of the transects contained a different species pair in which one dominant species replaced the other (Figure 10.17). In a three-year experiment to compare the influences of soil type and interspecific competition, seeds of both species were planted in monoculture and in mixture at three sites along each transect. Interspecific competition consistently decreased growth, but the magnitude of its effect was small compared with that of soil type. Adaptation to soil conditions strongly affected both seedling growth and survival at two of the three sites (Figures 10.18 and 10.19) and the researchers suggest that soil type, not competition, may be the critical factor determining species distribution.

Competitive interactions may change over time, and the ultimate outcome of competition may be different than initial observations suggest. Lythrum salicaria (purple loosestrife, Lythraceae) is an invasive species in North America that appears to be displacing native wetland species (see Chapter 11 and Figure 14.3). Some ecologists have argued, however, that evidence that Lythrum is actually displacing native species is weak. Mal and col-

Figure 10.18

Mean survival (%) and biomass (g dry weight,

± 1 standard error; error bars omitted where too small to be visible) after three years of experimental seedlings of (A) Leucadendron meridianum (Proteaceae) and L. coniferum at transect A and (B) Protea susannae (Proteaceae) and P. compacta at transect C when grown at sites within their natural distribution (green bars) or outside of it (gray bars). In transect A, survival and biomass were both far greater within the natural distributions of both species than outside them. In transect C, survival and biomass were lowest outside the natural distributions of both species for most sites. (After Richards et al. 1997.)

leagues (Mal et al. 1997) carried out a four-year field competition experiment between Lythrum and Typha angustifolia (cattail, Typhaceae), a dominant native wetland species. Typha was initially competitively superior, but in the second and third years of the experiment, the species were relatively evenly matched. By the fourth year of the study, Lythrum, the invader, became competitively dom-

inant, displacing Typha. The researchers attribute this result to differences in life history strategy between the two species. Typha has large rhizomes with substantial stored resources, which might give it an initial competitive advantage, but the high costs of producing new ramets and the strong suppressive effects of Lythrum led to the eventual competitive replacement of Typha.

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Figure 10.19

(A) Mean survival (%) and (B) biomass (g dry weight, ± 1 standard error) after 3 years for experimental seedlings of L. meridianum planted at three densities, in monoculture and in mixture, at three sites along transect A. Competition did not appear to have much effect on biomass or survival, or may have had inconsistent effects that were far smaller than the effects of being within or outside the natural distribution. L. meridianum did not decrease in survival at increased densities, and decreased in biomass at increased densities only at the Limestone site. Performance in mixture (green bars) was generally better than performance in monoculture (gray bars). Patterns were similar for other species and transects. (After Richards et al. 1997.)

Competition along Environmental Gradients

One of the most critical and contentious issues concerning the importance of competition in plant communities is whether there are particular kinds of habitats in which competition is predictably strong, determining community composition, or predictably weak and unimportant. Ecologists agree that competition is intense in productive, nutrient-rich habitats, at least when disturbance and herbivory are low. In these environments, plants are able to develop large canopies quickly, and competition is thought to be primarily for light. The relative intensity and importance of competition in productive and unproductive habitats, however, remain matters of debate.

Conceptual Models of Competition in Habitats with Differing Productivities

Grime (1977, 1979) proposed that competition is unimportant in unproductive environments, and that success in these environments is dependent largely on ability to tolerate abiotic stress (low nutrients, drought, or cold, for example), rather than on competitive ability. He further argued that in environments where disturbance frequently reduces plant mass, competitive exclusion should be prevented. The dominant plants in such environments should not be competitively superior, but rather should possess traits that allow them to withstand disturbance or recolonize rapidly following disturbance.

Most subsequent discussion has focused on unproductive environments. Newman (1973) disagreed with Grime’s characterization, arguing that competition is important in low-resource as well as high-resource environments, but that the resources for which plants compete differ—light in productive environments, nutrients and water in unproductive environments. Later work by Tilman (1987) reinforced and developed Newman’s ideas. Tilman argued that competition in low-productiv- ity environments would be for belowground resources (primarily nitrogen), whereas competition in high-pro- ductivity environments would be primarily for aboveground resources (light). Aerts (1999) reiterated Grime’s argument in part, maintaining that selection in nutrientpoor habitats would favor traits that reduce nutrient losses rather than those that enhance the ability to compete for nutrients, resulting in slow growth rates.

tion, applied to the distributions of a number of wetland species in Ontario, Canada. Typha latifolia (cattail, Typaceae) occupies core habitat, while other species become more prominent as one moves toward more extreme conditions in more peripheral habitats. (After Keddy 1990.)

Cladium

Xyris

Drosera

Sabatia

Gravel lake shores

500

250

0

g/m 2

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Sediment organic matter content (%)

Figure 10.21

Competitive abilities of six wetland plant species and the locations where they are naturally found along a gradient from exposed, nutrient-poor to sheltered, nutrient-rich shores, corresponding to a gradient in the percentage of organic matter contained in the sediment. (A) Competitive abilities expressed as target scores, defined as the mean relative growth (increase in dry mass) of the target species when grown in the presence of all neighbor species; this score is similar to competitive response. (B) Competitive ability expressed as neighbor scores, defined as the mean relative growth (increase in dry mass of all neighbor species in the presence of the target species; this score is similar to competitive effect. (After Wilson and Keddy 1986.)

with low competitive ability were displaced to disturbed sites with poor soils, where competitive exclusion was prevented by wave action and low soil nutrients.

Gurevitch (1986) carried out a field study of competition along an environmental gradient in southeastern Arizona. She hypothesized that Hesperostipa neomexicana, a C3 grass, was limited to arid ridgetops by competitively superior C4 grasses. This is precisely the opposite of what one would expect if physiology were determining species distributions, as C4 species should be better able to tolerate the unfavorably hot, dry conditions on the ridge crests. She removed neighbors from

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around target Hesperostipa individuals at three sites along a gradient from a ridge crest to a moister lower slope. Competition affected growth of mature plants, flowering, seedling establishment, and survival.

When competitors were removed, growth and flowering for the mature Heterostipa plants were greatest on the lower slope, where its abundance was lowest. Competition had the smallest effect on estimated population growth rates on the ridge tops, where Heterostipa was most abundant, and increasingly greater effects downslope. The largest effects were at the lowest sites where Heterostipa was present (Figure 10.22). These results strongly suggested that competition was a major factor in determining the distribution of Hesperostipa along this gradient of productivity and environmental favorability.

Theodose and Bowman (1997) suggested the existence of the opposite pattern, in which competition prevented a species from a more productive area from growing in a resource-poor site. The perennial Deschampsia caespitosa (hair grass, Poaceae) is common in moist alpine meadows in the tundra of the Front Range of Colorado, but is rare in dry meadows. The dry meadows are dominated by a sedge, Kobresia myosuroides (Cyperaceae). The authors hypothesized that Deschampsia was prevented from growing in the dry environment by competition with Kobresia. An earlier study had demonstrated that Kobresia was kept out of the moist meadows by deep winter snow.

Theodose and Bowman transplanted individuals of each species, as well as two-species pairs of plants to a dry meadow, either clipping the existing vegetation (largely Kobresia) at ground level or leaving it intact. Deschampsia had a greater increase in survival in response to vegetation clipping than did Kobresia, and soil moisture was substantially depressed in plots with intact vegetation compared with those in which vegetation was clipped (Figure 10.23). The researchers concluded that interspecific competition with Kobresia excluded Deschampsia from the dry meadows by depressing soil moisture below the drought tolerance of Deschampsia. Kobresia, with greater tolerance of drought, was able to survive during periods of low moisture.

One problem with this conclusion, however, is that the mortality of Kobresia (28%) was actually higher than the mortality of Deschampsia (15%) in intact vegetation. Furthermore, most of the Deschampsia plants survived in the intact vegetation of the dry meadow. Therefore, it is difficult to argue convincingly that competitive exclusion was the result of high mortality in Deschampsia. The growth of Deschampsia in intact vegetation was also greater than the growth of Kobresia, and the growth of both species was about equally affected by vegetation clipping. Nothing is known about the effects of competition on reproduction or establishment for these species in these environments. So, although this study clearly

Figure 10.22

Results of a removal experiment at three topographic positions in a southeastern Arizona grassland (elevation 1400 m). The table at the top shows means for biomass and cover (N = 40; confidence limits available in Gurevitch 1986) at the three experimental sites (ridge crest, midslope, and lower slope) and at a lower topographic position below them (wash). The lower table gives experimental results for Control (neighbors not removed) and Removal (mature neighbors removed) treatments at the three experimental sites. Means for seedlings/m2 (in 1980 plus 1981), mature plant growth, and number of flowers produced by mature plants (N = 20) over the 20 months of the experiment are shown. Seedling survival was based on the total number of seedlings surviving, and population growth was estimated as described in the original paper. The graph at right shows soil water potential at the top (ridge crest) and bottom (wash) of the topographic gradient at the 15–20 cm depth (where the bulk of the roots were located) over a drying and rewetting cycle of 6 months in 1980. (After Gurevitch 1986.)

demonstrated intense and statistically significant effects of competition in this resource-poor habitat, more work needs to be done to conclusively demonstrate that competition leads to the exclusion of Deschampsia in the dry meadows.

Evidence from Research Syntheses

While the results of individual studies are apparently contradictory, there have been several attempts to gain a better overview of competition along environmental gradients. In a cross-continental set of field experiments, the intensity of competition was compared for transplanted Poa pratensis (bluegrass, Poaceae) individuals at

Figure 10.23

Mortality of Deschampsia caespitosa (Poaceae) and Kobresia myosuroides (Cyperaceae) in a dry meadow when transplanted into intact vegetation (gray bars) and with neighbors clipped (green bars). Values are mean mortality (± 1 standard error) based on four plots, each of which had 10 experimental plants per species. (After Theodose and Bowman 1997.)

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