Changes in the Composition of Plant Communities

Plant ecologists tell us that the species composition of plant communities is going to change as climates become warmer, as carbon dioxide (CO2) levels increase, as rainfall becomes more erratic, and as nitrification (due to the use of fertilizers and the burning of fossil fuels) intensifies.

What does this mean for monarchs? We aren’t sure but there are some indications that the total impact of these factors could be quite negative for the milkweeds on which monarchs depend. Increasing nitrification tends to favor fast growing plants (C3 photosynthesis), particularly cold season grasses such as broome and fescue. Most of these grasses are species introduced from Europe. Warm season prairie grasses, such as big and little blue stem and Indian grass (C4 photosynthesis) will be at a disadvantage; similar results are expected for increases in CO2. Erratic rainfall is tolerated better by some species than others and which species are favored doesn’t appear to be predictable but seedling establishment by perennials such as milkweeds is likely to decline due to drying off under these conditions. Perennial grasses and forbs (flowering plants such as milkweeds) seem certain to decline in areas with long droughts and steadily decreasing rainfall over long periods.

In general, the predictions are that grasses, mostly fast-growing introduced species, will increase and that flowering plants will decrease resulting in an overall decrease in species diversity. Further, with increasing temperatures the habitat “niche” for many milkweeds will shift northward. Whether milkweeds will be able to follow these shifts and extend their distributions into new favorable areas is an open question. Generalist species and good colonizers such as the common milkweed should be able to extend their distributions. The distributions of many of the more specialized milkweeds, e.g., those that are confined to prairies or specialized habitats, are likely to shrink. Specialized species will be at a distinct disadvantage since habitat fragmentation is now so extensive that it is becoming less and less likely that seed dispersal by these species will be sufficient to allow step by step advances to new habitats that may arise due to warming to the north of the present distributions.