In recent years, a holistic ecosystem conceptualization has emerged that structurally and functionally links the river, and its riparian and floodplain zones into an integrated ecological unit - the riverine landscape. The riverine landscape often exhibit lotic water-driven disturbance biophysical complexity (e.g., patchiness) over both fine and broad spatio-temporal scales. However, despite the well-documented importance of river corridors (e.g., as biological refuges in human-modified landscapes), the role of riverine landscape pattern and composition on ecosystem structure and function is largely unknown.
This study investigated the influence of internal (river size, lateral flow connectivity) and external (catchment land use and land cover) factors on site-specific riverine landscape patterns. It then used riparian spiders of the family Tetragnathidae and ants as model organisms to explore the associations between internal riverine landscape patchiness and the distribution, diversity, and trophic dynamics [e.g., trophic position (TP), and dependency on aquatically derived carbon (CA)].
Riverine landscape patchiness was measured using a combination of field (vegetation surveys, canopy photography, shoreline habitat measurements) and remote-sensing approaches [e.g., using a GIS, aerial photos, and Light Detection and Ranging (LiDAR) data]. Ants and spiders were surveyed on each side of the river at each study reach. A suite of analytical methods were used including Analysis of Variance (ANOVA), linear regression, Principal Component Analysis (PCA), Maximum Entropy (MaxEnt) modeling, a model-selection approach using Akaike Information Criterion (AIC) and Non-Metric Multidimensional Scaling (NMDS).
Results indicate that both external and internal factors were associated with riverine landscape pattern (patch area and shape and size) including drainage area (a proxy for ecosystem size), proximity to impoundment (a proxy for lateral flow connectivity), and catchment land use and land cover (e.g., % urban, % agriculture). Agricultural riverine landscapes exhibited lower ant density but elevated ant diversity. Patch area, edge, and shape emerged as important predictors of ant diversity whereas patch composition, as well as patch area, edge, and patch density were strongly related to ant density. However, MaxEnt modeling indicated that patch-type influenced ant habitat choice less than gradients in distance from surface water.
Patch composition was strongly associated, with TP and CA of ants (Formica subsericea) greatest at crop patches (TP = 1.79, CA = 91%). Both terrestrial (habitat) and aquatic (emergent insect food resources) were important environmental determinants of riparian tetragnathid distribution, TP and the capacity of spiders to ecologically link aquatic and terrestrial ecosystems. For example, tetragnathid TP (mean = 2.45) was largely driven by emergent insect density. For both ants and spiders, CA was positively related to TP (R-squared = 0.14 for ants, R-squared = 0.48 for spiders), suggesting that algal-based energy pathways contribute to more complex riparian food webs.
The results contribute to a growing understanding of the impacts of landscape change in river corridors, and suggest that integrating conservation efforts at both broad (e.g., catchment) and fine scales (e.g., the aquatic-terrestrial interface) will be an important step in maintaining diverse, functional river-riparian ecosystems. These findings also lend insight into the utility of landscape ecology to river science.