How do mutualisms in agroecosystems increase the resistance of the entire system to the negative impacts of pests, diseases and weeds?
By contributing beneficial interactions
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.
When cover crops are tilled into the soil, the organic matter added to the soil is called
When cover crops are grown directly in association with other crops they are called
Benefits to the crop community from cover crops:
reduced soil erosion
improved soil structure
enhanced soil fertility
supression of weeds, insects and pathogens
What are some problems that can occur from poorly planned cover crop systems?
If resources are limiting, the could be competitive interference
If allowed to become too dense, some cover crop may be alleopathic to the crop
Damaging herbivores or disease organisms may find the cover crop species to be an ideal alternate hoste, later moving onto the crop
Crop residue may interfere with cultivation, weeding, harvesting or other farming activities
How can weeds be beneficial?
They can protect the soil surface from erosion through root and foliar cover
Take up nutrients that might otherwise be leached from the system
Add organic matter to the soil
Selectively inhibit the development of more noxious species through allelopathy
What conditions are weeds especially well-adapted to?
Simplified, disturbed habitats
When two or more crops are planted together in the same cropping system
In a series of studies of the corn-bean-squash polyculture done in Tabasco, Mexico, it was shown that corn yields could be stimulated as much as 50% beyond monoculture yields when planted with beans and squash. What happened to the yield of the other two crops?
There was significant yield reduction for the other two species, but the total yields for the three crops together were higher than what would have been obtained in an equivalent area planted to monocultures of the three crops
a broadened and complexified concept of diversity, it includes dimmensions such as:
Once diversity is generated, it tends to be self-reinforcing. Explain?
Greater species diversity leads to greater differentiation of habitats and greater productivity, which in turn allow even greater diversity
species diversity in a single location
species diversity across communities or habitats
a measurement of the species diversity of a region such as a mountain range or river valley
T/F Studies of natural ecosystems in early stages of development or following disturbance have shown that all of the dimensions of diversity tend to decrease over time.
False. They increase
the ecosystem's recovery process after a disturbance
the system begins to restore the diversity of species, interactions and processes that existed before disturbance
the successional condition in which the full potential for energy flow, nutrient cycling and population dynamics in that physical environment can be realized
the structural and functional diversity of the ecosystem at maturity provides resistance to change in the face of further minor disturbance
In what stage does an ecosystem reach its highest level of species diversity?
As a system approaches maturity
biomass continues to increase at maturity, though at a slower rate
Why are agroecosytems considered ecologically unstable?
Loss of diversity weakens the functional links between species that characterize natural ecosystems
Nutrient cycling rates and efficiency change
energy flow is altered
dependence on human interference and inputs increase
What are some options and alternatives available for adding the benefits of diversity to the agricultural landscape?
1. adding new species to existing cropping systems
2. reorganizing or restructuring species already present
3. adding diversity-enhancing practices or inputs
4. eliminating dieversity-reducing or diversity-restricting inputs or practices
A primary and direct way of increasing alpha diversity of an agroecosystem
grow two or more crops together in mixtures that allow interaction between the individuals of the different crops
Intercropping is a form of multiple cropping - adds temporal diversity through sequential planting of different crops during the same season, the presence of more than one crop adds horizontal, vertical, structural and functional diversity
plant different crops in adjacent strips
creates a "polyculture of monocultures"
increases beta diversity instead of alpha diversity
presents fewer management and harvest challenges than multiple cropping
What are some benefits from planting trees or shrubs around the perimeter of fields, along pathways of a farm or to mark boundaries?
protection from wind
exclude or enclose animals
produce tree products (firewood, construction materials, fruit, etc.)
a noncrop species planted in a field to provide soil cover, usually in between cropping cycles
usually involve planting different crops in succession or in a recurring sequence
the greater the differences between the rotated crops in their ecological impact on the soil, the greater the benefits of the method
a variation on the rotation practice
leave the land uncultivated or fallow
allows soil to "rest" - involves secondary succession and recovery of diversity in many parts of the system (esp. the soil)
What impact can animal activity have on the agricultural landscape?
activity such as grazing, crop residue consumption and manure deposition can alter aspects of structural divesity, species dominance and system function
What are the two components of species diversity?
Species richness - the number of species
Species evenness - the evenness of the distribution of the indviduals in the system among different species
What are some examples of importance values for a particular species?
number of individuals
Margalef's index of diversity
Measures number of species in terms of number of individuals
(S is number of species, N is number of indviduals)
limited because it can't distinguish the varying diversity of systems with the same s and N
an application of information theory, based on the idea that greater diversity corresponds to greater uncertainty in picking at random an individual of a particular species
Simpson index of diversity
the inverse of an index of community dominance
Based on the principle that a system is most diverse when none of its component species can be considered any more dominant than any of the others