No abstract

Streams and adjacent riparian habitats represent linked terrestrial and aquatic ecosystems that exchange materials and energy. Recognized relationships among apex predators and ecosystem biodiversity led us to hypothesize that these predators in riparian‐stream systems were more likely to be found in sites with high stream quality, defined as increased ecosystem function and integrity. In our freshwater study system, river otter Lontra canadensis and mink Neovison vison play critical roles as apex predators. We used multi‐season occupancy modelling across three sampling years (2012–2014) to compare aspects of the stream communities that explain occupancy dynamics of river otter and mink, including their interactions with other semi‐aquatic mammals. We surveyed for semi‐aquatic mammals at 77 sites in 12 major watersheds in southern Illinois, USA (44 526 km2). Naïve occupancy differed among years but generally increased for river otter, and remained high (≥93.5%) for mink. Increasing substrate availability increased detectability of river otter, whereas mink detection varied by survey period. Occupancy of river otter during the initial survey period was higher in sites closer to reintroduction points. Probability of colonization of river otter was positively associated with macroinvertebrate index of biotic integrity, fish species richness, and beaver presence. Sites with high species richness of fish families preferred by river otter also had increased river otter persistence. Mink occupied sites with increased fish richness, muskrat presence and mussel community index. Taken together, our results show occupancy of both mink and river otter were predicted by aspects of prey diversity and presence, indicating the importance of community composition in occupancy dynamics of riparian predators. Ultimately, these relationships suggest that habitat heterogeneity and system stability are important to apex predator site use. However, the relative role of bottom–up and top–down forcing in stream systems remains to be resolved.

Streams and local riparian habitats are a product of the landscapes they drain. Consequently, aspects of the landscape, as well as local stream morphology, are likely to affect riparian and stream communities, including occupancy of semi‐aquatic mammals. To gain a better understanding of how changing landscapes may affect the occupancy of semi‐aquatic mammals, we sought to identify aspects of the riparian system and stream structure at multiple scales that relate to the presence of beaver (Castor canadensis) and muskrat (Ondatra zibethicus), including their interactions with mink (Neovison vison) and river otter (Lontra canadensis). We estimated multi‐season occupancy using 103 sites sampled over six seasons (2012–2014) in southern Illinois, USA. We hypothesized beaver and muskrat occupancy was related to multiple aspects of landscape and local habitat including land‐cover, water availability, human disturbance, stream characteristics, and the presence of other semi‐aquatic mammals. Beaver occupancy varied over time and was predicted by stream size, stream density, and land‐cover at the landscape scale. Muskrat occupancy also varied over time and was related to local aspects of habitat including percentage of forest in the riparian area, channel incision, land‐cover, and aspects of sediment chemistry. Our results indicate the importance of spatial and temporal variation, scale, and food and water availability for these semi‐aquatic species. Beaver and muskrat are most likely to be affected by changes to water regimes including availability of permanent water sources due to changing landscapes.

The wax and wane of the eelgrass (Zostera marina L.) population in Grevelingen lagoon (East Atlantic; The Netherlands) has been documented for over 25 years, together with quantitative and semi-quantitative data on environmental variables. The population expanded after the closure of the Grevelingen estuary in 1971, but declined from 4600 ha surface area in 1978 to less than 100 ha in 1993. There is little causal evidence which factors are responsible for the observed dynamics of the population. The incomplete picture emerging from the data is that of an extremely impoverished eelgrass population, living under constant oligo-mesotrophic marine conditions. Both the sexual and the vegetative modes of reproduction are severely stressed by environmental variables, most likely a combination of low temperatures, high salinity, low dissolved silicate and low ammonium concentrations. Survival of the population asks for the restoration of moderate estuarine conditions.