Relative Competitive Abilities Change along Environmental Gradients.

As environmental conditions change, so do the relative competitive abilities of species. Shifts in competitive ability can result either from changes in the carrying capacities of species (values of K; see Quantifying Ecology 13.1) related to a changing resource base or from changes in the physical environment that interact with resource availability.

Many laboratory and field studies have examined the outcomes of competition among plant species across experimental gradients of resource availability. Mike Austin and colleagues at the Commonwealth Scientific and Industrial Research Organization (CSIRO) research laboratory in Canberra, Australia, have conducted several greenhouse studies to explore the changing nature of interspecific competition among plant species across experimental gradients of nutrient availability. In one such experiment, the researchers examined the response of six species of thistle along a gradient of nutrient availability (application of nutrient solution). Plants were grown both in monoculture (single species) and mixture (all six species) under 11 different nutrient treatments, ranging from 1/64 to 16 times the recommended concentration of standard greenhouse nutrient solution. After 14 weeks, the plants were harvested, and their dry weights were determined. Responses of the six species along the nutrient gradient for monoculture and mixture experiments are shown in Figure 13.8.

Interpreting Ecological Data

1. Q1. Which of the three species of thistle included in the graph had the highest biomass production under the 1/64 nutrient treatment? What does this imply about this species’ competitive ability under low nutrient availability relative to other thistle species?

2. Q2. Using relative biomass production at each treatment level as an indicator of competitive ability, which thistle species is the superior competitor under the standard concentration of nutrient solution (1.0)?

3. Q3. At which nutrient level is the relative biomass of the three species most similar (smallest difference in the biomass of the three species)?

Two important results emerged from the experiment. First, when grown in mixture, the response of each species along the resource gradient differed from the pattern observed when grown in isolation—interspecific competition directly influenced the patterns of growth for each species. Second, the relative competitive abilities of the species changed along the nutrient gradient. This result was easily seen when the response of each species in the mixed-species experiments was expressed on a relative basis. The relative response of each species across the gradient was calculated by dividing the biomass (dry weight) value for each species at each nutrient level by the value of the species that achieved the highest biomass at that level. The relative performance of each species at each nutrient level then ranged from 0 to 1.0. Relative responses of the three dominant thistle species along the nutrient gradient are shown in Figure 13.9. Note that Carthamus lanatus was the superior competitor under low nutrient concentrations, Carduus pycnocephalus at intermediate values, and Silybum marianum at the highest nutrient concentrations.

In a series of field experiments, Richard Flynn and colleagues at the University of KwaZulu-Natal (South Africa) examined trade-offs in competitive ability among five perennial C4 grass species at different levels of soil fertility and disturbance. Soil fertility treatments were established through the application of different levels of fertilizer, whereas varying levels of clipping were used to simulate disturbance resulting from grazing by herbivores. Individuals of the five grass species were grown in both monoculture and mixtures at each treatment level. The results of their experiments show a pattern of changing relative competitive abilities of the species along the gradients of soil fertility and disturbance (Figure  13.10). Moreover, in some of the results there were clear interactions between soil fertility and disturbance on competitive outcomes.

Field studies designed to examine the influence of interspecific competition across an environmental gradient often reveal that multiple environmental factors interact to influence the response of organisms across the landscape. In New England salt marshes, the boundary between frequently flooded low marsh habitats and less frequently flooded high marsh habitats is characterized by striking plant zonation in which monocultures of the cordgrass Spartina alterniflora (smooth cordgrass) dominate low marsh habitats, whereas the high marsh habitat is generally dominated by Spartina patens (Figure 13.11a). The gradient from high to low marsh is characterized by changes in nutrient availability as well as increasing physical stress relating to waterlogging, salinity, and oxygen availability in the soil and sediments. In a series of field experiments, ecologist Mark Bertness of Brown University found that S. patens individuals transplanted into the low marsh zone (dominated by S. alterniflora) were severely stunted with or without S. alterniflora neighbors, that is, with or without competition (Figure 13.11b). In contrast, S. alterniflora transplants grew vigorously in the high marsh (zone dominated by S. patens) when neighbors were removed (without competition), but were excluded from the high marsh when S. patens was present, that is, with competition (Figure  13.11c). Bertness also observed that S. alterniflora rapidly invaded the high marsh habitats in the absence of S. patens. He concluded that S. alterniflora dominates the physically stressful low marsh habitats because of its ability to persist in anoxic (low oxygen) soils, but it is competitively excluded from the high marsh by S. patens. S. patens is limited to high marsh habitats as a result of its inability to tolerate the harsh physical conditions in frequently flooded low marsh habitats.

Chipmunks furnish a striking example of the interaction of competition and tolerance to physical stress in determining species distribution along an environmental gradient. In this case, physiological tolerance, aggressive behavior, and restriction to habitats in which one organism has competitive advantage all play a part. On the eastern slope of the Sierra Nevada live four species of chipmunks: the alpine chipmunk (Tamias alpinus), the lodgepole chipmunk (Tamias speciosus), the yellow-pine chipmunk (Tamias amoenus), and the least chipmunk (Tamias minimus). Each of these species has strongly overlapping food requirements, and each species occupies a different altitudinal zone (Figure 13.12).

The line of contact between chipmunks is determined partly by interspecific aggression. Aggressive behavior by the dominant yellow-pine chipmunk determines the upper range of the least chipmunk. Although the least chipmunk can occupy a full range of habitats from sagebrush desert to alpine fields, it is restricted in the Sierra Nevada to sagebrush habitat. Physiologically, it is more capable of handling heat stress than the others, enabling it to inhabit extremely hot, dry sagebrush. In a series of field experiments, ecologist Mark Chappell of Stanford University found that when the yellow-pine chipmunk is removed from its habitat, the least chipmunk moves into vacated open pine woods. However, if the least chipmunk is removed from the sagebrush habitat, the yellow-pine chipmunk does not invade the habitat. The aggressive behavior of the lodgepole chipmunk in turn determines the upper limit of the yellow-pine chipmunk. The lodgepole chipmunk is restricted to shaded forest habitat because it is vulnerable to heat stress. Most aggressive of the four, the lodgepole chipmunk also may limit the downslope range of the alpine chipmunk. Thus, the range of each chipmunk is determined both by aggressive exclusion and by its ability to survive and reproduce in a habitat hostile to the other species.

Quantifying Ecology 13.1 Competition under Changing Environmental Conditions: Application of the Lotka–Volterra Model

Under any set of environmental conditions, the outcome of interspecific competition reflects the relative abilities of the species involved to gain access and acquire the essential resources required for survival, growth, and reproduction. As we have seen in the analysis of interspecific competition using the Lotka–Volterra equations, two factors interact to influence the outcome of competition—the competition coefficients (α and β), and the carrying capacities of the species involved (K1 and K2). The competition coefficients represent the per capita effect of an individual of one species on the other. These values will be a function of both the overlap in diets and the rates of resource uptake of the two species. These values, therefore, reflect characteristics of the species. In contrast, the carrying capacities are a function of the resource base (availability) for each species in the prevailing environment. Changes in environmental conditions that influence resource availability, therefore, influence the relative carrying capacities of the species and can directly influence the nature of competition.

Consider, for example, two species (species 1 and 2) that draw on the same limiting food resource: seeds. The diets of the two species are shown in Figure 1a. Note that the overlap in diet of the two species is symmetric. If the rate of food intake (seeds eaten per unit time) is the same, we can assume that the competition coefficients are the same. For this example, let us assume a value of 0.5 for both α and β.

Now let’s assume that the size distribution of seeds and their abundance vary as a function of environmental conditions. For example, in Figure 1b the average seed size increases from environment A to B and C. As the size distribution of seeds changes, so will the carrying capacity (K) for each species. Now assume that the carrying capacities of the two species vary as shown in the following table.

13.10 Interspecific Competition Influences the Niche of a Species

Previously, we defined the ecological niche of a species as the range of physical and chemical conditions under which it can persist (survive and reproduce) and the array of essential resources it uses and drew the distinction between the concepts of fundamental and realized niche (Chapter 12, Section 12.6). The fundamental niche is the ecological niche in the absence of interactions with other species, whereas the realized niche is the portion of the fundamental niche that a species actually exploits as a result of interactions with other species. As preceding examples have illustrated, competition may force species to restrict their use of space, range of foods, or other resource-oriented activities. As a result, species do not always occupy that part of their fundamental niche where conditions yield the highest growth rate, reproductive rate, or fitness. The work of Jessica Gurevitch of the University of New York–Stony Brook illustrates this point well. Gurevitch examined the role of interspecific competition on the local distribution of Stipa neomexicana, a C3 perennial grass found in the semiarid grassland communities of southeastern Arizona. Stipa is found only on the dry ridge crests where grass cover is low, rather than in moister, low-lying areas below the ridge crests where grass cover is greater. In a series of experiments, Gurevitch removed neighboring plants from individual Stipa plants in ridge-crest, midslope, and lower-slope habitats. She compared the survival, growth, and reproduction of these plants with control individuals (whose neighboring plants were not removed). Her results clearly show that Stipa has a higher growth rate, produces more flowers per plant, and has higher rates of seedling survival in midslope and lower-slope habitats (Figure  13.13). But its population density in these habitats is limited by competition with more successful grass species. Thus, Stipa distribution (or realized niche) is limited to suboptimal habitats because of interspecific competition.

Get your Custom paper done as per your instructions !

Order Now