Support In a recent field study of prairie plants, the more plant species a prairie plot had, the more vigorously the plants grew and the better the soil retained nutrients. Thus, Conclusion having more plant species improves a prairie's ability to support plant life.
The author concludes that having more plant species causes a prairie to be better able to support plant life. This is based on a study of praire plants, which found a correlation between the number of plant species on a prairie plot and how well plants grew on the plot.
The author assumes the correlation between plant species and how well plants grew is explained by the number of species causing more vigorous growth. This overlooks alternate explanations. For example, maybe the relationship is reversed. Better soil and ability to support plant life might cause a greater number of species to flourish on the plot. Or maybe there’s a third factor that causes both better ability to support plant life and greater numbers of species.
The argument is most vulnerable to criticism on the grounds that it
infers of two correlated phenomena, X and Y, that X causes Y without considering whether Y causes X
The author infers that more plant species (X) causes greater ability to support plant life (Y) based on the correlation observed in the study between more plant species (X) and greater ability to support plant life (Y).
fails to describe the mechanism by which productivity is supposedly increased
The author doesn’t need to describe the causal mechanism. A causal claim can be supported by evidence, even if the author never describes the mechanism underlying the causal relationship.
takes for granted that the characteristics of one prairie plot could reveal something about the characteristics of other prairie plots
There’s nothing flawed about thinking that one prairie plot can reveal something about others; if that plot is similar to other plots, it can still reveal something about others. Also, the study evaluated multiple plots; it wasn’t based on just one plot.
bases a general conclusion on data that is likely to be unrepresentative
We have no reason to think that the plots involved in the study are likely to be unrepresentative of prairie plots.
takes an increase in number to indicate an increase in proportion
The study showed that the more plant species in a plot, the better the plants grew. This doesn’t tell us there was an “increase” in plant species in any plot. In addition, the argument never asserts anything about the proportion of plant species or plants.
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