The first real wave of cold weather is rolling through the Midwest, and that is the monarch butterfly's cue to migrate. In autumn, monarch butterflies make massive group migrations from the northern United States all the way to Mexico, and they pass through much of the United States on the way. They are definitely visible in Illinois, and now that migrations have started, they can be observed flying in groups, and their flight pattern changes to "directional flight," flight that is in a definite southern direction. Monarch butterflies rest in immense groups at night called roosts.
However large the clusters of butterflies on trees seem to be, there is still a decline in monarch population due to human interference and land development. Monarch butterflies feed and lay their eggs exclusively on milkweed plants, and milkweed plants are often destroyed when humans cut down natural areas to build on top of them. Milkweed is especially prevalent in Illinois and the rest of the Midwest. Prairies were once the main staple of land in Illinois, before the state began to develop, and milkweed is a native prairie plant. Milkweed still grows wherever it can, but it is becoming much less common, since humans are building more and more.
This year, monarch butterfly roosts are even smaller than usual, and are fewer in number. According to learner.org's Monarch Butterfly Tracking Project
data, only 43 roosts have been reported so far this year, in contrast to 156 at the same point last year. The roosts have also been more short-lived than usual, lasting only one or two days before they dissipate. In years past at this point in the migration season, monarch roosts tended to contain a few thousand butterflies, but the largest one recorded this year only numbered 1,500. The rest of them this year numbered only in the hundreds.
Humans aren't the only contributor
Monarch butterflies also suffer losses because of birds that pick through them while they are roosting in groups. Monarch butterflies are poisonous to birds (due to a chemical in the milkweed they consume as caterpillars), but hungry birds still try to pick through groups of butterflies to find the least poisonous ones. These losses don't impact the butterfly population nearly as much as the effect of human interference, but are still worth noting.
Another factor that may have been involved in the decrease of monarchs is the stretching of warm weather into later months this year (for instance, in my hometown, we had temperatures of 99+ in the beginning of September, which I have not seen for a very long time). Weather has also fluctuated wildly in Illinois this year, often with changes of 40 or 50 degrees up or down within a few days, which may confuse the butterflies and upset their natural rhythms.
The decrease of monarch butterflies will surely have more impacts on the environment than we already see, as is the case whenever a major part of an ecosystem is disrupted. Monarchs are pollinators, so a decrease in their population will affect all plants that need to be pollinated, including ones that are grown commercially on farms. Other pollinators such as bees (which are also on the decline) may also be affected by the same factors affecting monarchs, so monarchs are an indicator of underlying changes in the climate that need to be noticed. Efforts are being made to support monarch populations, such as initiatives to plant more milkweed, but their decline is definitely obvious, and we are already seeing the impacts.