1 INTRODUCTION
All organisms in nature are challenged to live in spatially and temporally variable environments at various scales (Levin 1992). Local environments as well as the landscape positions and histories drive local communities through environmental filtering and dispersal limitation from a regional species pool (Gilbert & Lechowicz, 2004; Thompson & Townsend, 2006). Biological communities can vary both in space and time, and knowing what is driving the spatial and temporal structures of communities in natural landscapes is essential to understand regional community structures (Leibold et al., 2004; Gounand et al., 2018).
Ecotones at margins of two different environments provides unique and precious habitats for various organisms and provide important ecological functions (Risser 1995, Smith et al., 1997). They are characterized as spatial environmental gradient but they are often also temporally dynamic, and integral understanding of the spatial and temporal dynamics should largely help understanding ecosystems at ecotones. In fact, in many ecotones, the environment commonly shifts between two different types temporally, and the environmental gradient along the transition zone is represented by the proportion of time that each type of environment is present (Seasonal ecotone) rather than static environmental gradient in space (traditional ecotone) (Fig. 1). For example, the environmental gradient of the aquatic-terrestrial ecotone may be formed by the duration of submergence (van der Maarel., 1990). Many studies have shown species turnover of the biological communities in the ecotone along with the environmental gradient from one extreme to another at a point in time (Tockner et al., 2000; Peterson et al. 2008). However, taking into account the temporal nature of the environmental gradient in the ecotone, which is primarily driven by variation in the timing or frequency of environmental transitions, the magnitude of the effects of landscape positions on the local communities should vary with time elapsed since the environmental transition. The large seasonality in abiotic and biotic factors in ecotone would cause spatial structures of communities to shift seasonally, which we here define as “seasonal reassembly” (Fig. 1).
Natural floodplains are one of the most representative seasonal ecotones, as transitions between aquatic and terrestrial ecosystems (Junk et al., 1989), and transitions between lotic and lentic ecosystems (Tockner et al., 2000). On a natural floodplain, there are many extant and paleo-side-channels as a result of channel migration and abandonment. These waterbodies represent typical lotic and lentic transitions, as the river water flows through such waterbodies above a threshold river discharge. They have diverse degrees of hydrological connectivity to the river main channel, and the duration or frequency of river water flowing through the waterbodies varies (Amoros & Roux 1988; Tockner et al. 2000). In many temperate floodplains, spring snowmelt causes long-lasting floods, and many biota take advantage of the temporal changes. While water flows through some waterbodies for prolonged periods during the flood season, the period of flow is limited or absent for other waterbodies. Such gradients in connectivity support spatially variable biological assemblages from lotic to lentic extremes with gradual species turnover after flood (Uno et al. 2022). Although there is general prediction that the aquatic communities in such waterbodies vary along with the hydrological connectivity gradient (van den Brink et al., 1994; Morand & Joly, 1995; Tockner et al., 2000; Gallardo et al., 2014), seasonal components and formation processes of spatial structures in the aquatic communities in a floodplain have not been well studied.
In this study, we investigate how biological communities persist in seasonal ecotones across seasons, as represented by floodplain aquatic communities. We hypothesized that the effect of environmental gradient (seasonal hydrological connectivity) on biological communities observed after the seasonal transition are weakened and effects of other local environments are more pronounced with time after some stable (low-water) period, and biological communities are seasonally re-assembled. Seasonally re-assemblage would vary among taxa by the mode of dispersal or life histories of the organisms (Fig. 1). Mobile animals such as fishes may move between habitats as the environment changes (Armstrong et al., 2013). Amphibious animals such as many aquatic insects and amphibians may only seasonally inhabit the floodplain waterbodies and their distribution may correspond to the environment of the specific season (Hamer et al., 2023). Short-lived animals such as planktons may quantitatively respond to the environment locally (Baranyi et al., 2002). By studying how four different biological groups including planktons, benthos, fishes, and amphibians seasonally re-assemble with time after the environmental transitions and comparing them, we aim to synthesize seasonal responses of various animals in seasonal ecotones and identify important environmental characters and habitat connectivity for biological communities in ecotones.