Species that differ in at least one aspect of their resource use, and thereby experience less intense competitive interactions, allowing them to coexist.
What is overyielding?
Polycultures having a greater yield than the total that their individual components would yield in monocultures due to complementarity or facilitation. Note that not all combinations overyield, grass-legume combinations typically do.
Why greater yield stability in polycultures?
Insurance effect, negative covariance effect
Less pest attacks
Explain the relationship between ecosystems and landscapes?
Agricultural activities can have important effects on natural ecosystems through the movement
of organisms, pesticides, nutrients, and soil. Spillover may positively or negatively affect the landscape
At the same time, the landscape context
can strongly influence ecological processes that occur within
agroecosystems, such as pollination and pest regulation. The
spatial structure of the landscape, including the size, shape,
and connectedness of different ecosystem types can have
dramatic effects on these ecological processes.
What are planned components of agroecosystems?
Spatial or temporal arrangement of domesticated plants or animals that farmers purposefully include in the agroecosystem. It may also include beneficial organisms that are purposefully included in the ecosystem such as nitrogen fixing bacteria or biological control agents.
Crop and livestock selection
Planting densities and arrangement
Tillage (the preparation of land for planting crops)
Nutrient management (manure or fertilizer)
What are unplanned components of agroecosystems?
Organisms that persist in the ecosystem after it’s been converted into an agricultural system or organisms that invade it from the surrounding landscape.
Natural enemies and pest regulation
Soil organisms, decomposition and nutrient cycling
Water flow and purification / contamination
What are the components of the global increase of agriculture?
Increased global cereal production
Increased irrigated land
Increased use of nitrogen and phosphorus fertilizers
Increased global pesticide use and imports
What are the characteristics of larger agroecosystems?
Annual crops rather than perennial crops
Frequent disturbance (you can tell from bare ground)
Low nutrient retention and recycling (resulting from frequent disturbance like harvesting etc.)
High subsidies (fertilizer etc.)
What are the main goals of larger ecosystems?
Maximization of productivity
Minimization of labor
What are the characteristics of smaller agroecosystems?
Moderate to high diversity
A mixture of annuals and perennials
Infrequent, moderate disturbance
High nutrient retention and recycling
What are the main goals of smaller agroecosystems?
Maximization of stability
Minimization of risk
What is the scientific reason to conserve sources of biodiversity?
The genetic information is used to find resistance to certain diseases etc. For example, the hybrid rice created to be resistant to rice blast fungus in Yunnan China. Mixtures of the hybrid rice and glutinous rice show less occurrences of the disease.
Highest to lowest plant diversity in agroecosystems:
Shifting cultivation in humid tropical forests (most diverse while they recover)
Highest to lowest plant diversity in natural ecosystems?
What are the types of diversity in agroecosystems? How are they achieved?
2. Species diversity:
Cover crops / green manures
Pest repellent crops
3. Structural diversity:
Agroforestry (reduced erosion and increased biodiversity by growing trees and shrubs around croplands)
Shade plantations (slightly less productivity for coffee, but far more longevity for the plantation as a whole.
4. Landscape diversity (In the Andes, at different elevations to prevent frost and grow a wider variety of crops adapted to different temperature / humidity conditions)
What are cover crops?
Cover crops are commonly used to suppress weeds, manage soil erosion, help build and improve soil fertility and quality, control diseases and pests, and promote biodiversity. Green manures are the crops that are left to wither.
What are the mechanisms that would allow polycultures to yield more than the component monocultures?
Complementarity, higher yield and higher productivity
More resilience to environmental perturbation
Insurance effect (Under stressful conditions, when one species / genotype suffers, others do well)
Resistance to invasion (less specialist insects, more predator insects)
Negative covariance effect
Better soil health (intraspecific competition is higher than interspecific competition, different species have different phenologies and use different nutrients at different levels of the soil)
Yield stability (due to insurance effects)
Fewer weed problems (due to shading)
Support more unplanned biodiversity and more ecosystem services
Why does pest suppression occur in polycultures?
Herbivores are typically less abundant due to:
More difficulty finding host plants
Move more often and lose their host
Have lower reproductive rates due to limited resources
2. Predators and parasitoids are typically more abundant
Have a greater diversity of foods (greater diversity of herbivores). Higher diversity of insects means higher diversity of predators
Have more options for nest sites
Explain ecosystem services and agriculture
Agriculture is both a consumer and provider of ecosystem services. Agroecosystems are localized in landscapes and influence both the services and disservices produced in the landscape.
In turn, landscapes influence the ecosystem services flowing to the agroecosystems from other ecosystems.
What are agroecosystem disservices (once called externalities and ignored but currently counted and accounted for within the system)
Loss of biodiversity
Loss of wildlife habitat
Sedimentation of waterways
Greenhouse gas emissions
What are agroecosystem provisioning services?
What is landscape simplification?
Expansion of agricultural land
Increase in field size
Loss of field margin vegetation
Loss of biodiversity
What are the effects of landscape simplification
Pest density increases, predator density decreases due to natural enemies not being able to survive in long distances without patches to nest etc.