Abstract
Biological control (the importation of enemies from an invader's native range) is often considered our best chance of controlling the most widespread invaders. Ideally, the agent reduces invader abundance to some acceptably low level, and the two coexist at low density with the agent providing continuous control over the long-term. But the outcome may be complicated when the agent is attacked by native predators and parasites. We used a spatially explicit, discrete-time, individual-based, coupled plant-seed predator-parasitoid model to estimate the impact of the biocontrol agent Eustenopus villosus (a seed predator) on the invasive, annual weed Centaurea solstitialis, both with and without the generalist parasitoid Pyemotes tritici. We estimated the agent's ability to reduce plant density, spread rate, and population growth rate over 50 years. We used long-term demographic data from two sites in central California, USA, to parameterize the model and assess how populations in different climatic zones might respond differently to the agent and the parasitoid. We found that the biocontrol agent reduced plant density (relative to predictions for an uncontrolled invasion), but its impact on the invader's spread rate was modest and inconsistent. The agent had no long-term impact on population growth rate (lambda). Parasitism caused a trophic cascade, the strength of which varied between sites. At our coastal site, the parasitoid entirely eliminated the impact of the agent on the plant. At our Central Valley site, even when parasitized, the agent significantly reduced plant density and spread rate over several decades (although to a lesser degree than when it was not parasitized), but not invader lambda. Surprisingly, we also found that the length of time the invader was allowed to spread across the landscape prior to introducing the agent (5, 25, or 50 years) had little influence over its ability to control the weed in the long-term. This is encouraging news for land managers attempting to control invasive plants that have already established widespread, high-density populations. Unfortunately, our results also show that attack by the native generalist parasitoid had a larger influence over how effectively the agent reduced invader performance.
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Claims 9
Claim · model-written summary
In the simulation, the biocontrol agent reduced Centaurea solstitialis plant density relative to predictions for an uncontrolled invasion.
- Taxon
- Centaurea solstitialis
Centaurea solstitialise0 - Treatment
- biocontrol agente1
- Interactor
- Eustenopus villosus
Eustenopus villosuse0biocontrol_agent - Outcome
- plant densitye1
- Direction
- reducede1
- Comparator
- uncontrolled invasione1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the biocontrol agent Eustenopus villosus (a seed predator) on the invasive, annual weed Centaurea solstitialis, both with and without the generalist parasitoid Pyemotes tritici”
e1“We found that the biocontrol agent reduced plant density (relative to predictions for an uncontrolled invasion)”
Claim · model-written summary
The biocontrol agent had a modest and inconsistent impact on the invader's spread rate in the simulation.
- Taxon
- Centaurea solstitialisCentaurea solstitialise0
- Treatment
- biocontrol agente0
- Interactor
- Eustenopus villosusEustenopus villosuse0biocontrol_agent
- Outcome
- spread ratee1
- Qualifier
- modest and inconsistente1
Not reportedComparatorDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the biocontrol agent Eustenopus villosus (a seed predator) on the invasive, annual weed Centaurea solstitialis, both with and without the generalist parasitoid Pyemotes tritici”
e1“its impact on the invader's spread rate was modest and inconsistent”
Claim · model-written summary
The biocontrol agent had no long-term impact on the population growth rate (lambda) of the weed in the simulation.
- Taxon
- Centaurea solstitialisCentaurea solstitialise0
- Treatment
- agente1
- Interactor
- Eustenopus villosusEustenopus villosuse0biocontrol_agent
- Outcome
- population growth rate (lambda)e1
- Direction
- no_changeinferrede1
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the biocontrol agent Eustenopus villosus (a seed predator) on the invasive, annual weed Centaurea solstitialis, both with and without the generalist parasitoid Pyemotes tritici”
e1“The agent had no long-term impact on population growth rate (lambda)”
Claim · model-written summary
Parasitism by the generalist parasitoid caused a trophic cascade whose strength varied between sites.
- Taxon
- Centaurea solstitialisCentaurea solstitialise0
- Interactor
- Eustenopus villosusEustenopus villosuse0biocontrol_agent
- Second interactor
- Pyemotes tritici
Pyemotes triticie0other - Outcome
- trophic cascadee1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the biocontrol agent Eustenopus villosus (a seed predator) on the invasive, annual weed Centaurea solstitialis, both with and without the generalist parasitoid Pyemotes tritici”
e1“Parasitism caused a trophic cascade, the strength of which varied between sites”
Claim · model-written summary
At the coastal site, the parasitoid entirely eliminated the impact of the agent on the plant.
- Taxon
- Centaurea solstitialisCentaurea solstitialise0
- Treatment
- the parasitoide1
- Interactor
- Eustenopus villosusEustenopus villosuse0biocontrol_agent
- Second interactor
- Pyemotes triticiPyemotes triticie0other
- Outcome
- the impact of the agent on the plante1
- Direction
- eliminatede1
- Setting
- coastal sitee1
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the biocontrol agent Eustenopus villosus (a seed predator) on the invasive, annual weed Centaurea solstitialis, both with and without the generalist parasitoid Pyemotes tritici”
e1“At our coastal site, the parasitoid entirely eliminated the impact of the agent on the plant”
Claim · model-written summary
At the Central Valley site, even when parasitized, the agent significantly reduced plant density and spread rate over several decades.
- Taxon
- Centaurea solstitialisCentaurea solstitialise0
- Treatment
- the agente1
- Interactor
- Eustenopus villosusEustenopus villosuse0biocontrol_agent
- Second interactor
- Pyemotes triticiPyemotes triticie0other
- Outcome
- plant density and spread ratee1
- Direction
- reducede1
- Setting
- Central Valley sitee1
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the biocontrol agent Eustenopus villosus (a seed predator) on the invasive, annual weed Centaurea solstitialis, both with and without the generalist parasitoid Pyemotes tritici”
e1“At our Central Valley site, even when parasitized, the agent significantly reduced plant density and spread rate over several decades”
Claim · model-written summary
At the Central Valley site, the agent did not significantly reduce the invader's population growth rate (lambda), even though density and spread were reduced.
- Taxon
- Centaurea solstitialisCentaurea solstitialise0
- Treatment
- the agente1
- Interactor
- Eustenopus villosusEustenopus villosuse0biocontrol_agent
- Second interactor
- Pyemotes triticiPyemotes triticie0other
- Outcome
- invader lambdae1
- Direction
- no_changeinferrede1
- Setting
- Central Valley sitee1
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the biocontrol agent Eustenopus villosus (a seed predator) on the invasive, annual weed Centaurea solstitialis, both with and without the generalist parasitoid Pyemotes tritici”
e1“At our Central Valley site, even when parasitized, the agent significantly reduced plant density and spread rate over several decades (although to a lesser degree than when it was not parasitized), but not invader lambda”
Claim · model-written summary
The length of time the invader spread before the agent was introduced (5, 25 or 50 years) had little influence on the agent's long-term ability to control the weed.
- Taxon
- Centaurea solstitialisCentaurea solstitialise0
- Treatment
- the length of time the invader was allowed to spread across the landscape prior to introducing the agente1
- Interactor
- Eustenopus villosusEustenopus villosuse0biocontrol_agent
- Outcome
- ability to control the weede1
- Direction
- no_changeinferrede1
- Qualifier
- 5, 25, or 50 yearse1
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the biocontrol agent Eustenopus villosus (a seed predator) on the invasive, annual weed Centaurea solstitialis, both with and without the generalist parasitoid Pyemotes tritici”
e1“the length of time the invader was allowed to spread across the landscape prior to introducing the agent (5, 25, or 50 years) had little influence over its ability to control the weed in the long-term”
Claim · model-written summary
Attack by the native generalist parasitoid had a larger influence on how effectively the agent reduced invader performance.
- Taxon
- Centaurea solstitialisCentaurea solstitialise0
- Treatment
- attack by the native generalist parasitoide1
- Interactor
- Eustenopus villosusEustenopus villosuse0biocontrol_agent
- Second interactor
- Pyemotes triticiPyemotes triticie0other
- Outcome
- how effectively the agent reduced invader performancee1
- Direction
- largere1
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the biocontrol agent Eustenopus villosus (a seed predator) on the invasive, annual weed Centaurea solstitialis, both with and without the generalist parasitoid Pyemotes tritici”
e1“attack by the native generalist parasitoid had a larger influence over how effectively the agent reduced invader performance”