com-petitive abilities.
Most annual cropping systems present conditions of high disturbance because of frequent cultivation and harvest, but have relatively low stress since conditions have been optimized through agricultural inputs and crop system
design. Ruderals are highly favored under these conditions, where the characteristics of short life span, high seed pro-duction, and ability to colonize open environments have such advantage. Most plants that fall into the ruderal cat-egory—annual weeds, for example—can also be catego-rized as r-selected.
Degraded agroecosystems, such as eroded hillsides in wet environments, or heavily cropped grain systems in dry-farmed areas that suffer periodic drought stress and wind erosion, favor the growth of stress tolerators. Noncrop spe-cies that are tolerant of these conditions may become the dominant feature of the landscape; examples are Imperata grasses in the wet tropics of Southeast Asia and cheatgrass (Bromus tectorum) in the Great Basin rangelands of the Western United States. Since stress tolerators have been selected to endure the environmental stress characteristic of highly degraded and altered environments, they can estab-lish and maintain dominance even though the environment in which they occur is relatively unproductive.
Many natural ecosystems, as well as perennial cropping systems, support the competitor category of plants. These plants have developed characteristics that maximize the cap-ture of resources under relatively undisturbed conditions, but are not tolerant of heavy biomass removal. Excessive distur-bance through harvest would open the system up to the inva-sion of weedy ruderals, whereas increased intensity of stress, such as that which would accompany overextraction of soil nutrients or water, would open the system to invasion from stress-tolerating organisms. When a forest system made up primarily of competitor species is clear-cut and the soil eco-system is left intact, recolonization by stress-tolerant early successional species is an initial problem, but tree species can usually reestablish and eventually recolonize the site and exclude them. But if fire periodically removes vegeta-tive cover following tree harvest, the intensity of disturbance opens the system to invasion and dominance by shorter-lived and aggressive ruderals that greatly retard the recovery of the forest species.
Both r- and K-selection theory and stress/disturbance-intensity theory provide opportunities for combining our understanding of the environment with our understanding of the population dynamics of the organisms we are dealing with. By focusing this knowledge on both crop and noncrop species, we can plan our agricultural activities accordingly.
ECOLOGICAL NICHE
The concept of life history strategy helps us understand how a population maintains a place in an ecosystem over time. An additional conceptual framework is required for understanding what that place is and what the species’
ecological specialization might be. This is the concept of ecological niche.
An organism’s ecological niche is defined in terms of both its place and its function in the environment. Niche com-prises the organism’s physical location in the environment, its trophic role, its limits and tolerances for environmental TABLE 14.1
Life History Strategies Based on Stress and Disturbance Levels in the Environment
High Stress Low Stress High disturbance Plant mortality Ruderals (R) Low disturbance Stress tolerators (S) Competitors (C) Source: Adapted from Grime, J.P., Am. Nat., 111, 1169, 1977.
168 Agroecology: The Ecology of Sustainable Food Systems conditions, and its relationship to other organisms. The
con-cept of ecological niche establishes an important foundation for determining the potential impact that a population can have on an environment and the other organisms that are there. It can be of great value in managing the complex inter-actions between populations in an agroecosystem.
conceptualIzatIonsof nIche
The niche concept was first introduced in the pioneering work of Grinnell (1924, 1928) and Elton (1927) as the place of an animal in the environment. By “place”, they meant a species’ maximum possible distribution, controlled only by its structural limits and instincts. Today, this aspect of niche is part of what is termed fundamental or potential niche. Potential niche is contrasted with realized niche, the actual area that a species is able to occupy, as deter-mined by its interactions with other organisms in the envi-ronment (i.e., by the impacts of interference, positive and negative).
Both potential niche and realized niche are built on a conceptualization of niche that has two distinct facets. The
“Grinnellian” focus is on the conditions of the habitat in which the organism occurs; the “Eltonian” focus is on what the organism does in that habitat—its ecological role. The lat-ter facet can be understood as the organism’s “profession,” the way it “makes a living” in the habitat it lives in. An animal’s profession, for example, can be flower feeder, leaf feeder, or insect feeder. A microorganism can be a decomposer or a parasite. Many levels of interaction are involved in defining this ecological specialization aspect of a species’ niche.
An important contribution to the niche concept was made by Gause (1934), who developed a theory now known as Gause’s law: two organisms cannot occupy the same eco-logical niche at the same time. If the niches of two organ-isms in the same habitat are too similar, and there are limited resources, one organism eventually excludes the other through “competitive exclusion.” Competitive exclu-sion, however, is not always the cause of two populations with similar niches not occurring together. Other mecha-nisms may be at work.
The idea of the niche being an organism’s profession is often not adequate. To develop a more complex way of understanding niche, ecologists have focused on defining the separate dimensions that make it up. A set of factor–
response curves (discussed in Chapter 3) is determined for a particular organism. These are then layered over each other to form a matrix of factor responses. In a simple two-factor matrix, the area delineated by the overlapping regions of tolerance can be envisioned as the 2D area of resource space occupied by the organism. With the addi-tion of more factor–response curves, this space takes on multidimensional form. This procedure is the basis for a conceptualization of niche as the “multidimensional hypervolume” that an organism can potentially occupy (Hutchinson 1957). By including biotic interactions in the factor matrix, the hypervolume formed by overlapping
factor–response curves comes close to defining the actual niche that an organism occupies.
nIche aMplItude
When the niche is thought of as a multidimensional space, it becomes apparent that the size and shape of this space is different for each species. A measurement of one or more of its dimensions is termed niche breadth or niche amplitude (Levins 1968; Colwell and Futuyma 1971; Devictor et al.
2010), or niche width (Odum and Barrett 2005). Organisms with a narrow niche and very specialized habitat adaptations and activities are called specialists. Those that have a broader niche are referred to as generalists. Generalists are more adaptable than specialists, can adjust more readily to change in the environment, and use a range of resources. Specialists are much more specific in their distribution and activities, but have the advantage of being able to make better use of an abundant resource when it is available. In some cases, since a generalist is not that thorough in its use of resources in a habitat, it leaves niche space within its niche for specialists.
In other words, there can be several specialist niches inside of a generalist niche.
ecologIcal specIalIzatIonand nIche dIversIty
Natural ecosystems are often characterized by a high degree of species diversity. In such systems, many different species occupy what appear at first glance to be similar ecological niches. If we accept Gause’s law—that two species cannot occupy the same niche at the same time without one exclud-ing the other—then we must conclude that the niches of the similar organisms are in fact distinct in some way, or that some mechanism must be allowing coexistence to occur.
Competitive exclusion appears to be a relatively uncommon phenomenon.
In cropping systems as well, ecologically similar organ-isms occupy simultaneously what appears to be the same niche. In fact, farmers have learned from accumulated expe-rience and constant observation of their fields that there can often be advantages to managing a mixture of crop and noncrop organisms in a cropping system even when many of the constituents of the mixture have similar requirements.
Competitive exclusion rarely occurs; therefore there must be some level of coexistence or avoidance of competition.
This coexistence of outwardly similar organisms in both natural ecosystems and agroecosystems is made possible by some kind of ecological divergence between the species involved. This divergence is referred to as niche diversity or diversification of the niche. Some examples include the following:
• Plants with different rooting depths. Variable crop architecture belowground permits different species to avoid direct interference for nutrients or water while occupying very similar components of the niche aboveground (Figure 14.4).
Population Ecology of Agroecosystems 169
• Plants with different photosynthetic pathways.
When one crop uses the C4 pathway for photosyn-thesis and another uses C3, the first may thrive in full sunlight while the other tolerates the reduced-light environment created by the shade of the emer-gent species. The traditional corn/bean intercrop common in Mesoamerica is a well-known example.
• Insects with different prey preference. Two similar parasitic insects may co-occur in a cropping system, but they parasitize different hosts. Host–parasite specificity may be one way of diversifying the niche so as to allow for coexistence of adult insects else-where in the cropping system.
• Birds with different hunting or nesting behaviors.
Several predatory birds may all feed on similar prey in an agroecosystem, but since they have different nesting habits and sites, or feed at different times of day, they can co-occur in the cropping system and help control pest organisms. Owls and hawks are a good example.
• Plants with different nutritional needs. Mixed pop-ulations of weeds can co-occur in the same habitat due in part to the differential nutritional needs that may have evolved over time in each species as a result of the selective advantage of avoiding com-petitive exclusion. A crop population may suffer less negative interference from a mixed population of weeds than from a population of a single domi-nant weed with niche characteristics similar to that of the crop.
It appears that natural selection acts to create niche differen-tiation by separating some portion of the niche of one popula-tion from that of another. Ecological specializapopula-tion and niche differentiation allow partial overlap of niches to occur with-out exclusion.
The concept of niche, combined with knowledge of the niches of crop and noncrop species, can provide an impor-tant tool for agroecosystem management. A farmer can take advantage of niche diversity to exclude a species that is a detriment to the agroecosystem; similarly, he or she can use niche differentiation to allow a combination of species that is of benefit to the system (Figure 14.4).
APPLICATIONS OF NICHE THEORY TO AGRICULTURE
Farmers are constantly managing aspects of the ecological niches of the organisms that occupy the farming system, even though most never refer directly to the concept. Once it is understood as a useful tool of ecosystem management, how-ever, it can be applied in a variety of ways, from ensuring maximum yield through an understanding of a main crop’s niche to determining whether a noncrop species is likely to cause negative interference with the crop. Some specific examples follow.
proMotIngor InhIbItIng establIshMent of Weedy specIes
Any part of the soil surface not occupied by the crop pop-ulation is subject to invasion by weedy noncrop species.
Specialized for being successful in what can be termed pro-ductive environments (i.e., farm fields), weeds occupy a niche that favors r-selected or ruderal populations of annual herbs.
In cropping systems with less disturbance, where total plant biomass undergoes less disruption or removal, competitive (but still r-selected) biennial or perennial weeds become common. In a sense, weediness is a relatively specialized niche characteristic.
The habitat facet of the niche concept can be used to help guide how the environmental conditions of a farm field are manipulated in order to promote or inhibit the establishment of weedy species. The type of modification will depend on the niche specificity of each species in relation to the crop.
With knowledge of the niche characteristics of a weed spe-cies, we can begin by controlling the conditions of the “safe sites” to the disadvantage of the weed. Additionally, we can look for some critical or susceptible phase in the life cycle of the weed population in which a particular management practice could eliminate or reduce the population. It may also be possible to promote the growth of a weed popula-tion that will inhibit other weeds. For example, wild mustard (Brassica spp.) has little negative effect on crop plants but has the ability to displace, through interference, other weeds that may have a negative influence on the crop. A more detailed discussion of this phenomenon is provided in the case study Broccoli and Lettuce Intercrop.
FIGURE 14.4 Different root architectures permitting niche overlap. The shallow root system of the transplanted broccoli (left) and the deeper tap root system of the direct-seeded wild mustard (right) take resources from different parts of the soil profile, allowing the plants to occupy the same habitat without negative interference.
170 Agroecology: The Ecology of Sustainable Food Systems It is important to remember that most weeds are
coloniz-ers and invadcoloniz-ers and that crop fields that are disturbed annu-ally are just the type of habitats they have been selected for.
The challenge is to find a way to incorporate these ecological concepts into a management plan where planned activities, such as cultivation, are timed or controlled so that the weedy niche may be occupied by more desirable species.
bIologIcal controlof Insect pests
Classical biological control is an excellent example of the use of the niche concept. A beneficial organism is introduced into an agroecosystem for the purpose of having it occupy an empty niche. Most commonly, a predatory or parasitic spe-cies is brought into a crop system from which it was absent in order to put negative pressure on the population of a par-ticular prey or host that has been able to reach pest or disease levels due to the absence of the beneficial organism.
It is hoped that once the beneficial organism is introduced into the cropping system it will be able to complete its entire life cycle and reproduce in large enough numbers to become a permanent resident of the agroecosystem. But often the con-ditions of the niche into which the beneficial species is intro-duced may not meet its requirements for long-term survival
and reproduction, so reintroductions become necessary. This can be especially true in a constantly changing agricultural environment with high disturbance and regular alteration of the characteristics of the niche needed to maintain permanent populations of both the pest and the beneficial. Mitigating this problem is one of the advantages of maintaining high diversity at the landscape level (see Chapter 23).
Another potential use of the niche in biological control is the introduction of another organism that has a niche very similar to that of the pest, but which has a less negative impact on the crop. The introduced herbivore, for example, may feed on a part of the plant that is not of economic signifi-cance. If the introduced herbivore has a niche similar enough to the target pest, it might be able to displace it. There might be similar applications for weeds.
desIgnof IntercroppIng systeMs
When two or more different crop populations are planted together to form an intercropped agroecosystem, and the resulting yields of the combined populations are greater than those of the crops planted separately, it is very likely that the yield increases are a result of complementarity of the niche characteristics of the member populations. For intercropping
CASE STUDY: BROCCOLI AND LETTUCE INTERCROP
An intercrop is successful when the potential competitive interferences between its component crop species are mini-mized. This is accomplished by mixing plants with complementary patterns of resource use or complementary life his-tory strategies.
Two crops that have been shown to combine well in an intercrop are broccoli and lettuce. Studies at the University of California, Santa Cruz farm facility (Aoki et al. 1989) have demonstrated that a mixture of these crops will produce a higher yield than a monoculture of lettuce and a monoculture of broccoli grown on the same area of land. (This result, called overyielding, is explained in greater detail in Chapter 17.)
In the study, broccoli and lettuce were planted together at three different densities and the yields from each compared to yields from monocrops of each crop. The lowest intercrop density was a substitution intercrop, in which the overall planting density was similar to that of a standard monocrop. The highest density intercrop was an addition intercrop, in which broccoli plants were added between lettuce plants planted at a standard density. The monocrops were planted at standard commercial densities, which are designed to avoid intraspecific competition.
All three densities of intercrop produced higher total yields than the monocultures. The yield advantages ranged from a 10% greater yield to a 36% greater yield (for the substitution intercrop). The addition intercrop produced lettuce heads of a slightly lower mean weight than the monoculture lettuce, but the combined production still exceeded the total that was produced by a combination of monocrops on the same amount of land. The intercrops also retained more soil moisture than the monocrops, indicating that the physical arrangement of the two species in the field helps to conserve this resource.
These results indicate that interspecific competitive interference did not negatively impact the plants in the intercrops, even when their density was approximately twice that of either of the monocrops. For this avoidance of competition to have occurred, the broccoli and lettuce must each have been able to utilize resources that were not accessible to the other species.
An examination of the two species’ life histories and niches illuminates the complementarity of their resource use patterns and suggests mechanisms for the observed overyielding. Lettuce matures rapidly, completing nearly all of its growth within 45 days of being transplanted into the field. It also has a relatively shallow root system. Broccoli matures much more slowly and its roots penetrate much deeper into the soil. Therefore, when the two are planted nearly simul-taneously, lettuce receives all the resources it needs to complete its growth well before the broccoli grows very large;
then after the lettuce is harvested, the broccoli can take full advantage of the available resources as it grows to maturity.
Population Ecology of Agroecosystems 171
systems to be successful, each species must have a some-what different niche. Therefore, full knowledge of the niche characteristics of each species is essential. In some inter-crop cases, each species occupies a completely unrelated or otherwise unoccupied niche in the system, leading to niche complementarity. In most cases, however, the niches of the member species overlap, but interference at the interspecific level is less intense than interference at the intraspecific level.
Successful management of crop mixtures, then, depends on knowing each member’s population dynamics, as well as its specific niche characteristics. Such knowledge then forms the basis for management of the intercrop as a community of populations, a level of agroecological management on which we will focus in Chapter 16.
POPULATION ECOLOGY: A CROP PERSPECTIVE In this chapter, the focus has been on populations in the context of their environment. Important similarities and dif-ferences between populations of crop, noncrop, and natural species have been discussed. Some of these characteristics, along with additional relevant ones, are summarized in Table 14.2.
Knowledge of these characteristics becomes especially important when we are trying to find ecologically based management strategies for weedy noncrop species. Weedy species have maintained some of the characteristics of wild, natural ecosystem populations (e.g., dispersability, strong intra- and interspecific interference ability, dormancy), but through a range of adaptations (e.g., high seed viability, even-aged population structure, high reproduction alloca-tion, narrower genetic diversity) have adapted to the con-ditions of disturbance and alteration of the environment common in agroecosystems, especially those systems that
depend on annual crops. The ability of weeds to thrive in agroecosystems poses strong challenges for the agroecosys-tem manager.
Each species has certain strategies for ensuring that individuals of that species successfully complete their life cycles, thus enabling populations of that species to main-tain a presence in a cermain-tain habitat over time. Principles of population ecology, applied agroecologically, help the farmer decide where and how to take advantage of each species’ particular life history strategy to either promote or limit the population growth of the species, depending on its role in the agroecosystem. Agroecosystem manag-ers and researchmanag-ers need to build on population ecology concepts such as safe site, r- and K-strategies, and ecologi-cal niche to further develop techniques and principles for effective and sustainable management of crop and noncrop organisms.
FOOD FOR THOUGHT
1. What might permit coexistence of two very simi-lar crop species that would otherwise be thought to competitively exclude each other if allowed to grow in the same resource space?
2. How might the concept of niche diversity be used to design an alternative management strategy for a particular herbivorous pest in a cropping system?
3. Identify the most sensitive steps in the life cycle of a weed species, and describe how this knowledge might be of value in managing populations of the weed in a sustainable fashion.
4. What aspect of plant demographics have agrono-mists been able to use successfully in their quest for improved crop yields, but which has sacrificed over-all agroecosystem sustainability? What changes TABLE 14.2
Population Characteristics of Crop, Noncrop, and Related Natural Species Populations
Crop Population Noncrop Population Natural Population
Dispersal Little or none Very important Important
In-migration Propagule input decoupled from output Immigration very important Most propagules from local population
Seed viability High High Variable
Seed rain Controlled Relatively homogeneous Patchy
Soil environment Homogeneous Homogeneous Heterogeneous
Seed dormancy None; seed not part of seed bank Variable; seed bank present Common; seed bank present Age relationships Often even aged, synchronous Mostly even aged, synchronous Age variable, mostly asynchronous
Intraspecific interference Reduced Can be intense Can be intense
Seed density Low and controlled Usually quite high Variable
Density-dependent mortality Little or none Significant Significant
Interspecific interference Reduced Very important Important
Reproductive allocation Very high Very high Low
Genetic diversity Usually very uniform Relatively uniform Usually diverse
Life history strategies Modified r-strategists r-, C-, and R-strategists K- and S-strategists
Source: Adapted from Weiner, J., Plant population ecology in agriculture, in: Carroll, C.R., Vandermeer, J.H., and Rossett, P.M. (eds.), Agroecology, McGraw Hill, New York, 1990, pp. 235–262.