The effects of canopy cover and leaf litter species on macroinvertebrate diversity in vernal pools

Background/Question/Methods

Vernal pools are temporary wetlands that provide critical habitat for amphibians, macroinvertebrates, and plants because their seasonal drying prevents the establishment of predatory fish.  Although highly valued for amphibian conservation, over 100 macroinvertebrate species use vernal pools and are important predators, prey, and nutrient cyclers.  Hydroperiod and water chemistry may influence invertebrate diversity, but canopy cover and leaf litter also have bottom-up effects on their communities.  Specifically, canopy cover restricts algal and macrophyte resources for herbivores, while leaf litter chemistry affects wetland nutrients and the presence of tannins and other secondary compounds.  We investigated how canopy cover and leaf litter species affect macroinvertebrate diversity in 416L mesocosm wetlands.  Each mesocosm received 180g of aged litter from pin oaks, silver maples, or a mixture of these species and was assigned to a low (30%) or high (73%) canopy cover treatment simulated with shade cloth.  All wetlands contained identical initial macroinvertebrate communities, which simulated assemblages observed in previous field studies.  Throughout summer 2011, we recorded monthly measures of conductivity, dissolved oxygen, pH, temperature, invertebrate abundance, richness, and Shannon diversity, zooplankton abundance, algal productivity, and leaf litter decomposition.  Additionally, water samples were preserved for later nutrient analysis (carbon, nitrogen, and phosphorous).

Results/Conclusions

Fingernail clams, snails, and diving beetles began reproducing in the mesocosms within the first two weeks.  Repeated measures ANOVA indicated the treatments were similar in June, but invertebrate abundance increased fastest in oak tanks and was generally larger in 30% canopy throughout the season.  Similarly, macroinvertebrate family richness differed among the treatments and was greatest in 30% canopy/oak litter tanks.  Underwater light was also greatest in this treatment, but light availability gradually increased in all 30% canopy treatments.  Conductivity was highest in mesocosms with maple litter and generally larger under 30% canopy within litter treatments.  Tanks with only oak litter had the most dissolved oxygen, but oxygen increased in maple treatments throughout the season.  Our results suggest that terrestrial habitat is an important regulator of wetland communities, as canopy cover limits wetland productivity, while leaf litter’s impact on water chemistry limits underwater light availability for autotrophs.  Nutrient-rich, but lignin-poor litter, such as silver maple, may facilitate microbial communities and deplete dissolved oxygen for macroinvertebrates.  Conversely, oak litter decomposes slowly, favoring algal-driven productivity and greater oxygen availability for macroinvertebrates.  Ultimately, vernal pool preservation efforts must emphasize both terrestrial and aquatic habitat to protect wetland biodiversity.