Determining the optimal movement strategies in environments with heterogeneously distributed resources and toxicants

Environmental stress forces populations to move away from oppressive regions and look for desirable environments. Different species can respond to the same spatial distributions of resource and toxicant with distinct movement strategies. However, the optimal behavioral strategy may differ when a resource and a stressor occur simultaneously or if they distribute in different patterns. We compared the total abundance of two strains of Caenorhabditis elegans with different locomotion speeds as they forage in various spatial distributions of resource and toxicant. Informed by the experimental observations, we proposed a new two-state population model, wherein nutrient up-take and reproduction are modeled separately, as driven by the spatial distribution of resource and toxicant. We found that fast movers had an advantage when either the toxicant coverage or the overlap between toxicant and resource is increased. Also, to assess the effectiveness of designing refuges to conserve species in stressful cases, we compared different preferences of locations of refuge area according to movement strategies. Our mathematical model explained that fast movement enables individuals to consume resources at one location and reproduce at a separate location to avoid the toxicant-induced reduction in reproduction rate, which underlined its observed advantage in certain experimental settings. This work provided a better model to predict how species with different movement strategies respond to environmental stressors in natural systems.

Methods
Strains. Two strains with random (e.g. with defects in directed) movement were used in this study—MIA471 tax-2(p694) I; egl-4(n478) IV and MIA472 tax-2(p694) I; pdfr-1(ok3425) III. MIA471 is a “roamer” strain that has a faster movement rate than MIA472, which is a “dweller” strain.

Heterogeneous Environments of Resources and Toxicants. We used square petri plates (100 mm x 100 mm) to design a variety of distributions of resources (E. coli) and toxicants (CuSO4·5H2O). 

Population Counting. Initially, three L4 worms were added at the center of each petri dish (between rows C and D, and columns 3 and 4). All the experimental plates were kept in a 20 °C incubator for approximately 120 hours, the amount of time it takes for the majority of the worms to reach a size easily counted under a microscope (above L3 stage) before the food is depleted. The abundance of C. elegans (stage L3 and above) within each of the twelve sections were counted and recorded (n = 280).