To solve this problem, we need to identify the characteristic feature of a cropland ecosystem from the given options. Let's analyze each option:$\newline \bullet$Option 1: Absence of soil organisms$\newline$ - Soil organisms are present in cropland ecosystems as they play a crucial role in nutrient cycling and soil fertility.$\newline$ - Therefore, this option is incorrect.$\newline \bullet$Option 2: Least genetic diversity$\newline$ - Cropland ecosystems are typically monocultures, meaning they often consist of a single crop species.$\newline$ - This results in low genetic diversity compared to natural ecosystems.$\newline$ - Therefore, this option is correct.$\newline \bullet$Option 3: Absence of weeds$\newline$ - Weeds are often present in cropland ecosystems, and farmers use various methods to control them.$\newline$ - Therefore, this option is incorrect.$\newline \bullet$Option 4: Ecological succession$\newline$ - Ecological succession is a natural process of change in species structure over time, which is not a characteristic feature of managed cropland ecosystems.$\newline$ - Therefore, this option is incorrect.$\newline$Based on the analysis, the characteristic feature of a cropland ecosystem is:$\newline$Option 2: Least genetic diversity$\newline$This corresponds to the correct option, which is Option 2.