Abstract: In many ecosystems, community dynamics follow a repetitive pattern of seasonal succession, but the interannual predictability of this pattern varies widely from system to system, as abiotic factors driving seasonal succession vary among years. If all species in a system shift their phenology similarly in response, the same general pattern will be maintained, though the absolute timing may change. However, differential influences on or responses by different species may introduce greater relative variability into seasonal succession. Migratory species, for example, must use local cues to predict conditions at their destination, reducing their ability to adjust the timing of migration to those conditions in a given year.
We investigated how the phenology of migratory and resident species interact to influence the predictability of seasonal succession in lakes. We analyzed an 18-year dataset of biweekly zooplankton community composition collected from spring to fall in six lakes in coastal Connecticut. Two lakes support native populations of migratory alewife, a keystone zooplanktivorous fish; three lakes contain resident alewife populations; and one has no alewife. We identified the dates of key seasonal events, such as the spring peak and summer minimum of zooplankton species, in each year in each lake. We then used these to compute the relative timing of seasonal events (i.e., the interval between spring peaks and summer minima). The timing of elimination of daphnia in lakes with migratory alewife, an indicator of migratory alewife impact, was highly predictable, varying within about a 2 week window. However, the relative timing of seasonal events was significantly more variable in lakes with migratory alewife (sd = 22.7 days) than in lakes with resident alewife (sd = 13.59 days) or no alewife (sd = 19.31 days) (p < .001). These results indicate that the impact of migratory species can create greater variability in the timing of seasonal succession. As many migrants have strong seasonal impacts on their destination ecosystems, this disconnect between the timing of migrants and residents may have widespread implications for seasonal community dynamics in systems with migratory species.
