Temporal Variation in the Environment Influences Competitive Interactions.
When one species is more efficient at exploiting a shared, limiting resource, it may be able to exclude the other species (see Section 13.2). However, when environmental conditions vary through time, the competitive advantages may also change. As a result, no one species reaches sufficient density to displace its competitors. In this manner, environmental variation allows competitors to coexist whereas under constant conditions, one would exclude the other.
The work of Peter Dye of the South African Forestry Research Institute provides an example of shifting competitive ability resulting from temporal variation in resource availability in the grasslands of southern Africa. He examined annual variations in the relative abundance of grass species occupying a savanna community in southwest Zimbabwe. From 1971 to 1981, the dominant grass species shifted from Urochloa mosambicensis to Heteropogon contortus (Figure 13.6a). This observed shift in dominance was a result of yearly variations in rainfall (Figure 13.6b). Rainfall during the 1971–1972 and 1972–1973 rainy seasons was much lower than average. U. mosambicensis can maintain higher rates of survival and growth under dry conditions than can H. contortus, making it a better competitor under conditions of low rainfall. With the return to higher rainfall during the remainder of the decade, H. contortus became the dominant grass species. Annual rainfall in this semiarid region of southern Africa is highly variable, and fluctuations in species composition such as those shown in Figure 13.6 are a common feature of the community.
Peter Adler (Utah State University) and colleagues observed a similar pattern for a prairie grassland site at Hays, Kansas, in the Great Plains region of North America. Adler and colleagues examined the role of interannual climate variability on the relative abundance of prairie grasses over a period of 30 years (1937–1968). The researchers found that year-to-year variations in climate correlated with interannual variations in species performance. The year-to-year variations in the relative competitive abilities of the species functioned to buffer species from competitive exclusion.
Besides shifting the relative competitive abilities of species, variation in climate can function as a density-independent limitation on population growth (see Section 11.13). Periods of drought or extreme temperatures may depress populations below carrying capacity. If these events are frequent enough relative to the time required for the population to recover (approach carrying capacity), resources may be sufficiently abundant during the intervening periods to reduce or even eliminate competition.
13.8 Competition Occurs for Multiple Resources
In many cases, competition between species involves multiple resources and competition for one resource often influences an organism’s ability to access other resources. One such example is the practice of interspecific territoriality, where competition for space influences access to food and nesting sites (see Section 11.10).
A wide variety of bird species in the temperate and tropical regions exhibit interspecific territoriality. Most often, this practice involves the defense of territories against closely related species, such as the gray (Empidonax wrightii) and dusky (Empidonax oberholseri) flycatchers of the western United States. Some bird species, however, defend their territories against a much broader range of potential competitors. For example, the acorn woodpecker (Melanerpes formicivorus) defends territories against jays and squirrels as well as other species of woodpeckers. Strong interspecific territorial disputes likewise occur among brightly colored coral reef fish.
Competition among plants provides many examples of how competition for one resource can influence an individual’s ability to exploit other essential resources, leading to a combined effect on growth and survival. R. H. Groves and J. D. Williams examined competition between populations of subterranean clover (Trifolium subterraneum) and skeletonweed (Chondrilla juncea) in a series of greenhouse experiments. Plants were grown both in monocultures (single populations) and in mixtures (two populations combined). The investigators used a unique experimental design to determine the independent effects of competition for aboveground (light) and belowground (water and nutrients) resources (see Section 11.11). In the monocultures, plants were grown in pots, allowing for the canopies (leaves) and roots to intermingle. In the two-species mixtures (Figure 13.7), three different approaches were used: (1) plants of both species were grown in the same pot, allowing their canopies and roots to intermingle, (2) plants of both species were grown in the same pot allowing their roots to overlap, but with their canopies separated, (3) the plant species were grown in separate pots with their canopies intermingled, but not allowing the roots to overlap.
Clover was not significantly affected by the presence of skeletonweed; however, the skeletonweed was affected in all three treatments where the two populations were grown together. When the roots were allowed to intermingle, the biomass (dry weight of the plant population) of skeletonweed was reduced by 35 percent compared to the biomass of the species when grown as a monoculture. The biomass was reduced by 53 percent when the canopies were intermingled. When both the canopies and roots were intermingled, the biomass was reduced by 69 percent, indicating an interaction in the competition for aboveground and belowground resources. Clover plants were the superior competitors for both aboveground and belowground resources, resulting in a combined effect of competition for these two resources (see Sections 11.11 and 18.4). This type of interaction has been seen in a variety of laboratory and field experiments. The species with the faster growth rate grows taller than the slower-growing species, reducing its available light, growth, and demand for belowground resources. The result is increased access to resources and further growth by the superior competitor.
In a series of field studies, James Cahill of the University of Alberta (Canada) examined the interactions between competition for above- and belowground resources in an old field grassland community in Pennsylvania. With an experimental design in the field similar to that used by Groves and Williams in the greenhouse, Cahill grew individual plants with varying degrees of interaction with the roots of neighboring plants through the use of root exclusion tubes made of PVC pipe. He planted the target plant inside an exclusion tube that was placed vertically into the soil to separate roots of the target plant from the roots of other individuals in the population that naturally surround it. He controlled the degree of belowground competition by drilling varying numbers of holes in the PVC pipe that allowed access to the soil volume from neighboring plants (see Section 11.11 and Figure 11.20 for further description of method). Cahill varied the level of aboveground competition by tying back the aboveground neighboring vegetation. In total, he created 16 combinations of varying intensities of above- and belowground interaction with neighboring plants. This experimental design allowed Cahill to compare the response of individuals exposed to varying combinations of above- and belowground competition to control plants isolated from neighbors. The results of his experiments show a clear pattern of interaction between above- and belowground competition. In general, increased competition for belowground resources functions to reduce growth rates and plant stature, the result of which is reduced competitive ability for light (aboveground resource).
13.9 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.
