Convergence of arthropod hydration across US cities with divergent climate
Maintaining hydration is a key challenge for terrestrial organisms. Chronic dehydration has rapid and harsh consequences, including decreases in growth rate and reproduction, as well as increased mortality. Thus, maintaining water balance is of utmost importance to the ecology of most terrestrial organisms.
Variation in rainfall, distribution of water bodies, temperature, humidity, and wind may all influence hydration state. Recent evidence is accumulating that many of these environmental factors trend towards each other in urbanized environments in contrasting climate regions, a phenomenon known as urban homogenization or convergence. We hypothesized that convergence of environmental factors should lead to convergence of arthropod hydration states. We tested this hypothesis by sweep sampling arthropods from trees and shrubs in high and low impervious surface areas of Raleigh, NC, Phoenix, AZ, and Orlando, FL, as well as sampling undeveloped areas outside of each city, during the spring/summer of 2013. Here we focus on testing our a priori graphical predictions about convergence in average hydration between Raleigh (located in the mesic southeast) and Phoenix (located in the xeric southwest), but also will discuss generalized patterns of responses across all three cities.
We found strong congruence between our graphical a priori predictions of hydration convergence and our results. Specifically, a decline in hydration of arthropods in highly impervious parts of urban Raleigh and an increase in residential parks in urban Phoenix meant that despite lower hydration in and around Phoenix, the magnitude of the difference between cities was considerably smaller within urban areas (10% water content, t = 2.18, df = 12, p = 0.03) than in nearby undeveloped areas (16% water content, t = 2.78, df = 4, p = 0.01). The difference within vs outside cities of 6% water content may not seem large, but previous research found decreased growth in crickets chronically held at 1.4% lower water content. Thus, the magnitude of the difference in water content we observed is likely to be physiologically and ecologically important. These results suggest that urban environments in the mesic eastern US are more like deserts than surrounding areas (aka “desertification”) and urban environments in the xeric southwest are more mesic than surrounding areas (aka “riparianization”). Previous work suggests that these differences in hydration may have direct consequences for trophic interactions, population dynamics, and rates of herbivory and pest damage.