What is Population Dynamics?

The population dynamics of a weed species is the change in population size (# plants / unit area) over time in response to weed biology, environmental factors, and weed management. The weed population we observe in year 2 (N_t+1) is a function of changes that occurred since the beginning of year 1 (N_t). During that year, additions to the population occurred through births (B), such as seed production, and immigration (I) through dispersal of seeds from outside the population. We can also observe losses from the population through deaths (D), such as seed decay and predation, and emigration (E) when seeds are dispersed away from the population.

N_t+1 = N_t + B – D + I – E

The population of a weed species is also dynamic within the growing season. We can take measurements of the weed population at various growth stages during its lifecycle (identified as states) and then examine how each state responds to an environmental or management factor

For the lifecycle of a summer annual weed species, such as Palmer amaranth (Figure 4), we can observe the number of weed seed found in the soil seedbank through soil sampling. Later in the spring, we can observe the number of seedlings that are present. Towards the middle of the summer, the number of flowering adults can be counted and then we can determine the amount of new seed that is produced by each of those flowering adults.

There are three fates of weed seed in the soil seedbank. First, seed are lost through a diversity of means, such as decay and predation. Second, seed can germinate and emerge to become seedlings once appropriate environmental conditions, such as moisture and temperature, occur. Third, seed can remain in the seedbank until next year or even longer. These seed are dormant, meaning that they are viable but in a state of 'suspended animation', waiting for the right environmental conditions.

Individual adult plants produce seed depending on their population density and size.

Dispersal is the shedding of the new seed back into the soil seedbank at the end of the growing season.

*this animation has no audio*

Rates for germination and emergence, survival, and seed production can range widely depending on the weed species, environmental conditions, and choice of weed control practices.

In March, your crop consultant pulled soil samples for fertility tests and determined that, on average, 150 shattercane seed and 285 waterhemp seed m^-2 were present in this field. Once the corn was planted in mid-April, you asked the crop consultant to scout the field for weed problems and she/he observed 3 shattercane seedlings and 15 waterhemp seedlings m^-2.

• What was the rate of germination and emergence for shattercane?

3 shattercane seedlings m^-2 / 150 shattercane seed m^-2 = 0.02 (or 2%)

• What was the rate of germination and emergence for waterhemp?

15 waterhemp seedlings m^-2 / 285 waterhemp seed m^-2 = 0.053 (or 5.3%).

If a proportion of the seedbank population were resistant to the particular herbicide that was applied, germination and emergence and/or survival of that R-biotype portion would likely be higher than the germination and emergence and/or survival of the S-biotype. A soil-applied herbicide would impact the germination and emergence success of the weed species, and a postemergence herbicide would greatly impact both the transitions of germination and emergence and of survival for the weed species.

We suspect that our shattercane population is resistant to Beacon (primisulfuron, an ALS-inhibitor) herbicide due to repeated application of Beacon during the previous 4 years and poor shattercane control last year. Of the 150 shattercane seed m^-2 measured in the soil seedbank in March, we expect that 30% are the R-biotype and 70% are the S-biotype.

• How many R-biotypes will the seedling population contain if the proportion of germination and emergence is 0.02? How many S-biotypes?
• 30% R-biotype * 150 shattercane seed m^-2 * 0.02 germination and emergence = 0.9 R-biotype seedlings m^-2
• 70% S-biotype * 150 shattercane seed m^-2 * 0.02 germination and emergence = 2.1 S-biotype seedlings m^-2
• If Beacon, a postemergence herbicide, is chosen and has no effect on the R-biotype, all S-biotype seedlings will be controlled and only the 0.9 R-biotype seedlings m^-2 will survive.

A comparison of the growth and competitiveness of S- and R-biotypes for several weed species known to be resistant to triazine herbicides has revealed that the ecological fitness is higher for the S-biotype than the R-biotype when grown together without the herbicide. This has not been demonstrated for weed species identified to be resistant to ALS-inhibiting or other herbicides. The S- and R-biotypes are equally fit. Table 1. Dry matter production (g plant^-1) of biotypes of common groundsel and redroot pigweed, susceptible and resistant to atrazine, when grown under non-competitive conditions. (Conard and Radosevich 1979)  

If there are ecological fitness differences among R- and S-biotypes for a given weed species, differences exist in vegetative growth and number of seed produced by an individual plant. Differences in other life cycle states and transitions, such as dormancy, germination, survival, and seed production, could influence the outcome of population dynamics with mixed R- and S-biotype populations. This is important for weed species with triazine R-biotypes, but has not been identified for R-biotypes to other herbicides.