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.