Impacts of Lake Organisms on Terrestrial Systems

Trophic Cascades ACROSS Ecosystems How do organisms with complex life histories tie together aquatic & terrestrial systems? Can these organisms cause trophic cascades across ecosystems? Dragonflies nymphs are aquatic but adult forms are terrestrial Nymphs are important food for fish & adults are important predators of pollinators

Proposed Trophic Cascade

Methods Katharine Ordway Preserve, Florida 4 fishless & 4 fish-filled ponds Box-sampled dragonfly nymphs in ponds

Point-counted adult dragonflies Methods Point-counted adult dragonflies Compared # & sizes of dragonflies present between ponds Observed dragonflies predations & identified prey

Pollination Constructed net cages, 1 with an adult dragonfly, the other without & recorded # and identities of pollinators (Hymenoptera, Diptera & Lepidoptera) that visited each cage & the # of flowers that pollinators visited Also observed pollinators in the open around ponds at St. John’s Wort shrubs Determined if shrubs were pollen-limited

Dragonfly Size & Abundance Black = large Grey = medium White = small Fish limit abundance & size distribution of dragonflies = less & smaller dragonflies near fish ponds

Less Pollination near Fishless Ponds Black = Diptera Grey = Lepidoptera White = Hymenoptera 1st graph shows less pollinators near fishless ponds with a lot less bees (hymenoptera) 2nd graph shows shrubs near fishless ponds 2x more pollen-limited than shrubs near fish ponds

Trophic Cascades Fish indirectly facilitate terrestrial plant reproduction

When Prey Strike Back! What about the effects of predation on adult dragonflies (for example birds)? What about reverse situations?

Implications Conditions in aquatic ecosystems can have major effects on life cycles & reproductive success of terrestrial organisms & vice versa Understanding interactions across ecosystems crucial for gauging effects of anthropogenic-induced change Lakes AND surrounding terrestrial vegetation should be well protected in conservation management

Effects of Aquatic Insect Emergence on Terrestrial Insects via Birds How does the interaction of birds and aquatic insects affect the abundance of terrestrial insects in a riparian deciduous forest?

Predictions Predict denser bird populations in riparian zones due to aquatic insect emergence thus also causing a depression in terrestrial herbivore abundance

Methods Japanese lilac dominating shrub, buds early around same time as aquatic insect emergence 6 plots, 3 in riparian area, 3 upland (200m from stream) Same tree densities in riparian & upland plots Lepidoptera leaf rollers dominant herbivores on lilac, important early terrestrial spring prey for many birds

Methods Great Tit Calculated visiting frequency of 3 dominant bird species to each plot Recorded prey types of bird species Prey types classified as - Leaf rollers - Other terrestrial prey - Aquatic prey Marsh Tit Crowned Willow Warbler

Methods Bird Exclusion Experiment: Randomly selected trees in each plot & covered some with a mesh to keep birds out others used as a control to identify effects of bird removal on abundance of leaf rollers Used malaise trap to find abundance of flying insects (many aquatically-derived) & identified insects to order (or family) to distinguish origin then weighed to calculate biomass

More Riparian Insects = More Birds? 1st graph shows a significantly higher aquatic prey biomass in riparian areas 2nd graph indicates that there appears to be more bird visitation in riparian opposed to upland areas

Bird Diets Shows that in upland forests birds rarely prey on emerged aquatic insects Found predation on leaf rollers higher overall in riparian areas due to denser bird population Activity of great tits & willow warblers much higher in riparian areas

Results Aquatic insect emergence negatively & indirectly affected leaf rolling Lepidoptera larvae through forest birds Also, they suggested that birds had a strong impact on abundance of aquatic insect population

Implications Many predators (eg. birds & fish) rely on both terrestrial & aquatic prey inputs These “outside” prey inputs can positively or negatively affect other prey populations Nakano and Murakami 2001 PNAS 98: 166-170

Importance of These Studies Human interference rapidly altering spatial patterns of habitat interfaces on a broad scale Increasing destruction of stream banks can seriously impair energy exchange between habitats Loss/ degradation of one habitat may have more detrimental effects on neighbouring habitats than previously thought

References Knight, T. M., McCoy, M. W., Chase, J. M., McCoy, K. A., Holt, R. D. (2005). Trophic cascades across ecosystems. Nature 473, 880-883. Murakami, M., Nakano, S. (2002) Indirect effect of aquatic insect emergence on a terrestrial insect population through predation by birds. Ecology Letters 5, 333-337 Nakano, S. & Murakami, M. (2001). Reciprocal subsidies: dynamic interdependence between terrestrial and aquatic food webs. Proceedings of the National Academy of Science USA, 98, 166-170.