The eastern population of monarch butterflies is declining. We know they need milkweed to thrive… but what size should milkweed patches be? How far apart should patches be? How will planting of milkweed in roadsides and on CRP land affect monarch population growth? Trying to answer these questions with field studies that inform county and state level conservation planning is difficult and cost-prohibitive. But mathematical models based on best available scientific information can help to answer these questions.
Figure 1: Landcover categories in Story County that monarch butterflies must navigate to find milkweed.
Iowa State University postdoctoral research associate Tyler Grant and professor Steve Bradbury, with their colleagues in Australia, Hazel Parry and Myron Zalucki, recently published a paper in Ecological Modelling that describes a model to simulate monarch butterfly movement and egg-laying on agricultural landscapes to help answer these questions.
The model simulates female monarch flight paths across the landscape of Story County, Iowa (Figure 1). With the model, the researchers can visualize what landscape features promote movement (such as additional milkweed patches) and what acts like a barrier (crop fields).
“Monarch butterflies must navigate the agricultural landscape to find milkweed, their host plant, in roadsides, CRP, grasslands, and other habitat,” said Grant. “Because it’s difficult to track monarchs for more than a few hundred yards in the field, we created a model to provide insights on how monarchs are moving across the landscape at distances up to several miles.”
The model simulation can be run with a few butterflies (Figure 2). Or, using advanced computing facilities, the movement of hundreds of thousands of individual monarchs can be simulated.
Figure 2: Simulated movement paths of three monarch butterflies.
As monarch butterflies move across the landscape they periodically stop to lay eggs (Figure 3). The probability of laying an egg depends on the milkweed density of the landcover. By tracking the number of eggs laid in each habitat patch, insights are being gained regarding how different spatial arrangements of habitat patches and how different habitat quality influences the total number of eggs laid in the landscape.
An uncertainty analysis of the model found that the perceptual range (the distance at which a monarch can sense a habitat patch) has a substantial effect on the model’s prediction of how eggs are distributed on the landscape. Current research by the monarch team is already providing information to fill this knowledge gap using radiotelemetry. The model of Grant et al. (2018) will be linked to national scale models to help determine the best locations to place milkweed on the landscape to meet regional or national conservation goals.
The research article is available to download from Ecological Modelling or by contacting the authors.
Figure 3: Egg density after monarchs have laid eggs for 10 days.