Predators contribute to the flow of nutrients through ecosystems by consuming prey,
assimilating some of the prey nutrients and depositing the remainder. These deposited
nutrients could have locally strong impacts on soil and plant productivity, especially
when predators remain within particular patches of space. Spiders are among the most
abundant and diverse predators in many ecosystems. Further, web-building spiders
often remain at one site for extended periods of time and, hence, could have a large
impact on nutrients at that site relative to nearby sites without spiders. We tested if the
web-building black widow (Latrodectus mactans) contributes to the spatial heterogeneity
of nutrients in soil and plant growth using laboratory mesocosms. Spiders were allowed
to build a web in one corner of an arena and feed for over 2 months. We then removed
the spiders and collected soils from beneath the spider’s refuge, the center of the
mesocosm, and the corner furthers from the spider’s refuge. We measured soil nutrient
content and plant growth in separate subsamples of soil.
Results/Conclusions
We observed that spiders generally captured prey within the web boundaries,
consumed and excreted prey nutrients near their refuge, then deposited the prey
carcasses close to the web perimeter. Consequently, we found that soil respiration and
aboveground plant mass was greatest beneath the spider’s refuge. Soil ammonium and
phosphorus was greatest at the center of the mesocosm, while soil nitrogen and nitrate
did not differ spatially. Leaf percent carbon and non-structural carbohydrates were
higher under the spider. Leaf percent nitrogen and protein did not differ spatially. The
patterns of plant growth and composition did not match the patterns of soil nutrients
present, which suggests that spiders promote productivity in soil communities more than
by simply supplying nutrients. Given their high abundance within many ecosystems, we
propose that spiders are significant contributors to nutrient cycling.