\nA warmer planet, less nutritious plants and \u2026 fewer grasshoppers?<\/br>
\n2023-04-22 21:46:53<\/br><\/p>\n
It\u2019s tough out there for a hungry grasshopper on the Kansas prairie. Oh, there\u2019s plenty of grass to eat, but this century\u2019s grass isn\u2019t what it used to be. It\u2019s less nutritious, deficient in minerals like iron, potassium and calcium.<\/p>\n
Partly due to that nutrient-deficient diet, there\u2019s been a huge decline in grasshopper numbers<\/a> of late, by about one-third over two decades, according to a 2020 study. The prairie\u2019s not hoppin\u2019 like it used to \u2014 and a major culprit is carbon dioxide, says study author Michael Kaspari, an ecologist at the University of Oklahoma in Norman.<\/p>\n Atmospheric carbon dioxide is at its highest in human history<\/a>. That\u2019s probably fine for plants like the grasses the hoppers munch. They can turn that atmospheric carbon into carbohydrates and build more plants\u2014in fact, plant biologists once thought all that extra carbon dioxide would simply mean better crop yields. But experiments in crops exposed to high carbon dioxide levels indicate that many food plants contain less of other nutrients than under carbon dioxide concentrations of the past. Several studies find that plants\u2019 levels of nitrogen, for example, have fallen, indicating lower plant protein content. And some studies suggest that plants may also be deficient in phosphorus and other trace elements.<\/p>\n The idea that plants grown in today\u2019s carbon dioxide-rich era will contain less of certain other elements\u2014a concept Kaspari categorizes as nutrient dilution\u2014has been well-studied in crop plants. Nutrient dilution in natural ecosystems is less-studied, but scientists have observed it happening in several places, from the woods of Europe<\/a> to the kelp forests off Southern California<\/a>. Now researchers like Kaspari are starting to examine the knock-on effects\u2014to see whether herbivores that eat those plants, such as grasshoppers and grazing mammals, are affected.<\/p>\n The scant data already present suggest nutrient dilution could cause widespread problems. \u201cI think we are in canary-in-a-coal mine territory,\u201d Kaspari says.<\/p>\n It\u2019s clear that rising carbon dioxide levels change plant makeup in a variety of ways. Scientists have done years-long studies in which they pump carbon dioxide over crops to artificially raise their exposure to the gas, then test the plants for nutrient content. One large analysis found that raising carbon dioxide by about 200 parts per million boosted plant mass<\/a> by about 18 percent, but often reduced levels of nitrogen, protein, zinc and iron.<\/p>\n Vegetables like lettuce and tomatoes may be sweeter and tastier<\/a> due to added carbon-rich sugars, but lose out on some 10 percent to 20 percent of the protein, nitrate, magnesium, iron and zinc<\/a> that they have in lower-carbon conditions, according to another large study. On average, plants may lose about 8 percent of their mineral content<\/a>\u00a0in conditions of elevated carbon dioxide. Kaspari likens the effect to trading a nourishing kale salad for a bowl of low-nutrient iceberg lettuce.<\/p>\n When vegetables are grown under elevated levels of carbon dioxide, they typically get bigger and sweeter and may have more of some minerals, such as calcium, an analysis of several different studies found. But quantities of other minerals, including zinc and iron, can go down.<\/p>\n<\/figcaption><\/figure>\n Scientists don\u2019t yet know exactly how extra carbon dioxide leads to changes in all these other nutrients. Kaspari, who discussed the importance of micronutrients such as calcium and iron in ecosystems<\/a> in the 2021 Annual Review of Ecology, Evolution and Systematics, suggests it\u2019s a simple issue of ratios: Carbon goes up but everything else stays the same.<\/p>\n Lewis Ziska, a plant physiologist at the Columbia University Mailman School of Public Health in New York City, thinks it\u2019s more complicated than just ratios. For example, in the vegetable study, elevated carbon dioxide increased the concentration of certain nutrients, such as calcium, even as it limited levels of others.<\/p>\n One contributing factor could be plants\u2019 little openings, called stomata, through which they take up the carbon dioxide they use to make sugars and the rest of their structures. If there\u2019s plenty of carbon dioxide around, they don\u2019t need to open the stomata as often, or for as long. That means plants lose less moisture via evaporation from those openings. The result could be less liquid moving up the stem from the roots, and since that liquid carries elements such as metals from soil, less of those trace elements would reach the stems and leaves.<\/p>\n Scientists have also posited that when carbon dioxide is high, plants are less efficient at taking up minerals<\/a> and other elements because the root molecules that normally pull in these elements are acting at a lower capacity. There are probably multiple processes at play, says Ziska. \u201cIt\u2019s not a one-size-fits-all mechanism.\u201d<\/p>\n Whatever is going on in these well-studied crops, the same thing is presumably occurring in trees and weeds and other non-agricultural species, says Kaspari. \u201cIf it\u2019s happening to the human food supply, it\u2019s happening to everybody else.\u201d<\/p>\nLower-quality food?<\/h2>\n
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