Agroecosystem = a site of agricultural production –a farm. Defining the farm as an ecosystem allows analysis of the food production system as a whole, including the complex set of inputs and outputs and the interconnections of component parts.

• biotic factors (organisms)
• abiotic factors (soil, light, moisture, temperature)

• Species diversity 

1. Species richness (total number of species):

Scale dependence

        Alpha (a ), Beta (b ) and Gamma (g ) diversity

         Intercropping                Strip cropping            Monoculture

The scale for Alpha diversity is selected to identify processes between individual organisms (e.g. competition for resources required for growth in shared resource pools) that determine the degree of diversity and the maintenance of diversity.

The scale for Beta diversity is selected to identify yet another set of processes that operate between populations or patches (metapopulations) to determine degree of diversity and maintenance of diversity.  An example of this would be the process of seed dispersal for each of the species in a community.  Plant species with low dispersal would tend to be more patchy. This may best pertain to weeds in agroecosystems.

In an agroecosystem diversity can be managed by planting crops in different patterns and changes of crops over time.

Soybeans grown with a smother crop of mustard.

Gamma diversity = landscape variability and yet another set of processes like climate and soil parent material factors determining the variation in species number over broad areas or regions.

• Dominance, Evenness, and Relative abundance    

Diversity indices take relative abundance into account

(refer to the description from your "M" field trip assignment)

• Vegetative structure

1. horizontal
2. vertical       

• Trophic structure (describes how nutritive needs of different species are met)
  1. producers
  2. consumers
     • herbivores
     • predators and parasites
     • parasitoids
  3. detritivores (decomposers)

Do food webs exist in agroecosystems?

How might food webs be different in agroecosystems than in more natural "undisturbed" ecosystems?

Agroecosystems tend to decrease the organisms in food webs which decreases the number of links which in turn may decrease the number of mechanisms that regulate population growth.  In a way, this deregulation is done intentionally in agriculture so that the crop can grow unregulated.  Unfortunately, pests also respond to the lack of regulation and their populations grow unregulated. It is also quite likely that many of the microorganisms (decomposers and consumers) under high frequency soil disturbance in agroecosystems, become deregulated. Those organisms that can take advantage of the agricultural inputs (fertilizers and pesticides) and high frequency disturbance prosper at the expense of others. 

So...the question becomes...Can we recreate enough of a food web in an agroecosystem to provide maximum regulation without a major impact on crop production?

Important information that we must have:

• Are all linkages in the food web equal? Are some more important for system regulation?

Keystone species has a narrow and more broad (modern) definition.

Narrow defn. = a species that preferentially consumes and thus holds in check (regulates) another species that would otherwise dominate the system (Paine, 1969).

Broad defn. = a species whose impact on its community or ecosystem is large, and disproportionately large relative to its abundance (Power et al., 1996).

• How will we know when we have enough food web structure to accomplish our goal of a maximum of natural regulation?

Demonstration of concept with a wild oat study: 

[See wild oat seed bank graphs]

Bioindicators = multiple measures of organism health in the presence of environmental stressors which include several levels of biological organization and time scales of response.

 

• Stability (describes how much the species diversity, dominance structure, vegetative structure and trophic structure within a community remains the same over time)

Stability = decreased risk for producer (especially if system is monoculture)

Can a system be too stable?

Crop mimicry in weeds...

• Resiliency to disturbance.