LRES 543 - Agroecology

Species Diversity and Stability

Gliessman, S.R. 1998. Chapter 15: Species interactions in crop communities.

Introduction

Emergent qualities = community characteristics that arise from the different interactions among populations of the crop community

Research typically focused on crop population rather than community of which it is a part

Lose ability to consider manipulating the community interactions to benefit cropping system

Only detrimental interactions have been considered (weeds, pest herbivores, disease)

Conventional approach minimizes interactions vs. agroecological approach which attempts to understand species interactions in the context of the larger community

2 types of interference

1) removal - removal of some resource by one or both of the interacting organisms

2) addition - one or both organisms adds some substance or structure to the environment

Advantage of interference approach is that it allows a more complete understanding of the mechanisms of interaction

Ways in which interference may combine to effect crop community, see Fig. 15.1

Interactions are complex and difficult to discern

Grass - clover example

Coexistence:

Populations of similar organisms often share the same habitat even though niches highly overlap

Ecologists widely accept the idea that selection for coexistence may be the rule more than the exception

Many domesticated species have evolved in polycultures

Understanding mechanisms of interference that allow coexistence will help us design multiple crop communities

Combine species with slightly different physiological characteristics or resource needs to promote coexistence

Relatively common in complex natural communities

3 Types:

1) Inhabitational - one mutualist lives wholly or partly inside the other (eg. Rhizobium bacteria and leguminous plants)

2) Exhabitational - organisms are relatively independent physically, but interact directly (eg. flowering plant and its insect pollinator)

3) Indirect - interactions among a set of species modify the environment in which they all live to the benefit of the mixture; involve more than 2 species (eg. polyculture agroecosystem)

Facultative mutualisms = all members can survive alone but benefit from interaction

Often mutualisms help species avoid some negative impact

Increase resistance of entire system to negative impacts of pests, diseases and weeds

SPECIAL TOPIC - History of the study of mutualism

Greeks and Romans recognized existence of beneficial relationships among organisms

1600s - natural theology promoted view that plants and animals selflessly aided each other and contributed to "society" of the natural world; guardian or helper animals

The Origin of Species and Industrial Revolution emphasized competition

1837 - Van Beneden introduced term "mutualism"

1893 - Pound explained mutualism as each organism acting in its own self interest

Mutualism did not emerge as an important area of study until the 1970s

Mutually Beneficial Interferences at Work in Agroecosystems

Beneficial Interferences of Cover Crops:

Cover crop = plant species (usually grasses or legumes) grown in pure or mixed stands to cover the soil of the crop community for part or all of the year

Green manure = cover crop tilled into the soil to add OM

Living mulch = cover crop grown directly with other crops

Reduce soil erosion; improve soil structure; enhance soil fertility; suppress weeds, insects, and pathogens (see Table 15.1 for more benefits)

May be beneficial at some times while detrimental at others (see CASE STUDY rye/bellbeans)

Beneficial Interference of Weeds:

With proper management, weeds can serve role of cover crop

Modification of the Cropping System Environment

Weeds protect soil surface from erosion; take up nutrients that might otherwise be leached, add OM, selectively inhibit development of more noxious species through allelopathy

Control of Insect Pests by Promotion of Beneficial Insects

Certain weeds should be regarded as important components of the crop community because of the positive effects they have on populations of beneficial insects

Intercropping:

Two or more crops planted together may reduce need for external inputs

Mostly used in the tropics

Corn-bean-squash polyculture example

- growing 3 crops together gave higher total yield

- LER>1

Understanding ecological foundation of the interactions in polycultures is key to returning its prominence to agriculture

Using Species Interactions for Sustainability

Challenge for agroecologists is to put ecological understanding into the context of sustainability

What are some of the impediments to convincing conventional farmers of the advantages of implementing multi-species cropping systems?

Gliessman, S.R. 1998. Chapter 16: Agroecosystem Diversity and Stability

The complexity that characterizes natural systems is the basis for the ecological interactions that are a crucial foundation for sustainable agroecosystem design. These interactions are largely a function of the diversity of a system.

Management must work at the level that an agroecosystem is much greater than the sum of its parts to take advantage of beneficial interactions and processes

Agroecology emphasizes the need to study both the parts AND the whole.

Controlling conditions managing conditions

Attempt to control and homogenize results in the elimination of beneficial relationships and interferences, leaving only negative interference and interactions

Increase in external inputs to deal with resulting problems

Building on Diversity:

Central priority in whole-system management is creating complex, diverse agroecosystem because this has highest potential for beneficial interactions (see Fig. 16.1)

Decrease external inputs

Alternative pest management (see Table 16.1)

SPECIAL TOPIC: Rhizobium, legumes and the nitrogen cycle

- Rhizobium capture N and make it usable to plants; plants provide sugars for Rhizobium living in root nodules.

- Legumes have been used in intercropping, cover crops, and green manures to improve soil quality and nitrogen content.

Ecological Diversity

Ecological diversity has a variety of different dimensions. (Table 16.2)

1) species

2) genetic

3) vertical

4) horizontal

5) structural

6) functional

7) temporal

Diversity in Natural Ecosystems:

Diversity seems to be an inherent characteristic in most ecosystems.

Self-reinforcing

Important role in maintaining structure and function

Greater diversity leads to greater resistance to perturbation and disturbance

Size of area being considered has an impact on how diversity is measured.

Alpha diversity = variety of species in a relatively small area of one community

Beta diversity = variety of species from one location to another

Gamma diversity = variety of species of a region

Alpha and beta scales are most useful in agroecosystems.

Successional Processes and Changes in Diversity

Dimensions of diversity tend to increase over time.

Disturbance simplifies dimensions of diversity or sets it back to an earlier stage of development.

Maturity reached when full potential for energy flow, nutrient cycling, and population dynamics is realized. May not be most diverse successional stage.

Diversity and Stability

Disagreement exists regarding whether or not greater diversity leads to greater stability.

Definition of "stability" needs to focus on "robustness" of ecosystem, ability to sustain complex interactions and self-regulating processes of energy flow and material cycling.

Need more research into possible causal relationships among different forms of diversity and specific ecosystem processes and characteristics.

Resist falling into circular reasoningdiversity leads to stability and greater stability leads to greater diversity.

Ecological Diversity in Agroecosystems:

Disturbance occurs frequently in an agroecosystem, therefore it cannot proceed very far in its successional development.

Diversity is difficult to maintain.

Agroecosystem is ecologically unstable.

Agroecosystems CAN be more complex and diverse.

The Value of Agroecosystem Diversity

Diversity is of value in agroecosystems for a variety of reasons:

More microhabitat differentiation, allowing species to become specialists

Opportunities for coexistence and beneficial interference between species leads to greater sustainability

Disturbed environments can be better taken advantage of

Increases chances of beneficial population dynamics between herbivores and their predators

Better resource use efficiency

Decreases risk for farmer

Creates more microclimates that may be occupied by a range of beneficial noncrop organisms

Conservation of biodiversity in surrounding natural ecosystems

Belowground diversity performs a variety of services that have impacts both on and off the farm

Methods of Increasing Diversity in Agricultural Systems

Intercropping

Temporal, horizontal, vertical, structural, and functional diversity increased

Traditional farming systems in rural or developing areas, especially tropics

Strip Cropping

Polyculture of monocultures

Increases beta diversity

Presents few management challenges than intercropping

Hedgerows and Buffer Vegetation

Provide protection

Increase beta diversity

Provide habitat for beneficial organisms

Cover-cropping

Provides soil cover

Improves soil quality (OM, biological activity and diversity, nutrient trapping)

Rotation

Increase temporal diversity

Residues from different crops affect different soil physical qualities

Fallow

"Rest" allows recovery of diversity, especially in soil

Create a mosaic of plots in different successional states

Reduced or minimal tillage

Minimizing disturbance may enhance diversity

High organic matter inputs

Stimulate species diversification of the belowground subsystem

Reduction in use of chemical inputs

Chemicals may kill nontarget organisms or leave detrimental residues which decreases diversity

Multi-step process, very challenging

More work, risk, and uncertainty

To manage diversity effectively, we need ways to measure it and its impact on the performance and functioning of the agroecosystem.

2 Components of Diversity

1) species richness = number of species

2) species evenness = evenness of the distribution of the individuals in the system

Can use biomass or productivity as a measure instead of individuals

Margalef's index

diversity = (s-1)/(log N)

limited it its usefulness because can't distinguish varying diversity between systems with same number of species

Shannon index (see formula in text)

Minimum diversity = 0

Relatively diverse = 3-4

Simpson index

Minimum diversity = 1

Relatively diverse = >5

 

Assessing the Benefits of Intercrop Diversity:

There can be disadvantages to intercropping, but these shouldn't stop a farmer from using it. Rather, the arguments against should be a means for determining where research needs to be focused to avoid problems.

Land Equivalent Ratio (LER)

LER = the yield advantage obtained by growing two or more crops as an intercrop compared to growing the same crops as a separate monoculture = Yp_i/Ym_i

where Yp is the yield of each crop in the polyculture and Ym is the yield of each crop in monoculture

Any value greater than 1 indicates a yield advantage for the intercrop

Application and Interpretation of the LER

An LER higher than 1.0 indicates the presence of positive interferences. Any negative interspecific interference that exists in the mixture is not as intensive as the intraspecific interference that exists in the monocultures.

An LER higher than 1.5 indicates negative interference is minimal in the intercrop system and at least one member will do better when grown in polyculture than in monoculture.

3 situations where LERs may not be appropriate:

1) When combined intercrop yield must exceed the yield of the higher-yielding sole crops

2) When intercropping must give full yield of a "main" crop plus some additional yield of a second

3) When the combined intercrop yield must exceed a combined sole-crop yield

Colonization and Diversity

Consider the crop field as an island surrounded by an ocean that organisms have to cross in order to become part of the species diversity of the agroecosystem.

Island Biogeography Theory:

Many interactions that determine the niche of an organisms after it reaches the island are very different from the conditions of the niche it left behind.

First invaders have potential to evolve characteristics that could allow it to expand into a new niche.

Basic principles:

- smaller the island, the longer it takes for organisms to find it

- further an island from source of colonists, the longer it takes for colonists to find it

- smaller and more distant islands have smaller and more depauperate flora and fauna

- many niches on islands can be unoccupied

- many organisms that reach islands occupy a much broader niche

- early colonizers often arrive ahead of limiting predators and parasites

- as colonization proceeds, changes occur in the niche structure of the island

- the earliest arrivees are mostly r-selected

Agricultural Applications:

Study showed that equilibrium between species of pests and natural enemies that was predicted by island biogeography theory was not realized in soybean field surrounded by natural forest and soybean fields, probably due to short life cycle of the soybean field.

Use "islandness" to slow arrival of pests or hasten the arrival of beneficial organisms

CASE STUDY: Borders around a crop field can play an important role in pest management. Each border can have different effects on different pests.

Diversity, Stability, and Sustainability

The challenge to an agroecologist is to demonstrate advantages that can be gained from introducing diversity into farming systems, incorporating ecosystem components important in nature, and managing diversity for the long term.

We must focus our research on understanding the contribution each species makes and use this knowledge to integrate each species into the system in the optimal time and place.

What are some possible mechanisms for a crop to produce higher yield when planted in polyculture than when in monoculture?

What sort of incentives could be offered for those farmers managing more diverse systems?

What are some of the forms of agroecosystem diversification that will best promote the successful use of IPM?

Kareiva, P. 1996. Diversity and sustainability on the prairie.

Studies are emerging which provide experimental evidence supporting principle that biological diversity begets more robust and productive ecosystems

- Ecotron experiment found that a diverse food web involving annual plants and associated insects and soil fauna was more productive than species-poor communities.

- Tilman et al. (1996) grassland experiment resulted in diverse plots using and retaining nutrients more efficiently than did less diverse plots-less leaching and increased uptake of nitrate resulted in increased productivity (measured by plant cover)

- No differences once species exceeded 10 species (Tilman et al. 1996)

View Tilman's work as a call to develop more experiments of the same sort, especially to study ecosystems like forests that do not lend themselves to elegant manipulation

Many alternative reasons for preserving biodiversity other than productivity, robustness, and efficiency of system

Largest portion of earth's diversity may come from poorly understood phenomenon of the ability of hundreds to thousands of species that seemingly compete for same resources to coexist in seemingly homogeneous sites

Four major classes of theories for local diversity

1) local spatial heterogeneity

2) nonequilibrium conditions

3) interactions among at least 3 trophic levels

4) neighborhood recruitment limitation

- disturbances such as tree fall gaps lead to a predictable successional sequence in which one tree replaces another and culminates in dominance by a canopy tree species

- at intermediate rates of disturbance, there is a range of sites occupied by different successional stages

Recruitment limitation theory in rainforest of Panama (Hubbell et al. 1999)

- many plant species absent because of poor dispersal ability or low local abundance in disturbed areas

- if species is absent, it has no chance of competitive victory at the site; inferior competitors can then dominate site-"victory by default"

- much of evidence gathered by Hubbell supports recruitment limitation over intermediate disturbance theory

Is high local diversity explained by:

- trade-off between recruitment ability vs. competitive ability?

- recruitment limitation allowing local coexistence of species that are capable of regional coexistence?

- recruitment limitation slowing the rate of competitive displacement that high local diversity can be maintained without any trade-offs?

- diversity of explanations for diversity?

Diversity-productivity hypothesis:

- assumes interspecific differences in the use of resources allow more diverse plant communities to more completely utilize limiting resources and attain greater productivity

- nutrient leaching losses are inversely related to diversity because of greater nutrient capture and/or immobilization

- these two assumptions lead to the diversity-sustainability hypothesis: the sustainability of soil nutrient cycles and thus of soil fertility depends on biodiversity

This study randomly drew from a pool of 24 native prairie species, thus results on the single-species plot were not due to attributes of a single species, but rather to the degree of diversity.

Experimental biodiversity gradient (1, 2, 4, 6, 8, 12, or 24 species) established on N-limited soil

Measured productivity (peak standing crop), plant cover, rooting-zone extractable soil NO₃^- and NH₄⁺

Results:

- Greater plant diversity led to greater productivity

- Rooting-zone extractable soil NO₃^- and NH₄⁺ was a decreasing function of species richness

- In summary, plant productivity and resource utilization were significantly greater at high plant diversity

- Similar results in undisturbed native grassland

Compensatory competitive interactions might have played a role in causing experimental results

- 5 species had greater abundance than expected on the basis of proportions seeded

What implications does this study have for the establishment of agroecosystems?

How might this study provide a template for further diversity-sustainability experiments?

How could agroecologists use results from this study to change policies regarding diversity in agriculture?