Assemblage organization over time prevents invasion success in phytoplankton

Invasion plays a central role in long-term species coexistence, sometimes
rejuvenating and, at other times, collapsing diversity. Community
resistance to invasions depends partly on the community’s species richness
and structure, both of which change with community organization over time.
Here, we study the resistance of phytoplankton assemblages to invasions
based on varied initial species richness and structure, as well as
assemblage organization over time. We hypothesize that invasion success
will be greater in less organized assemblages and lower in more organized
assemblages. To test this, we first experimentally constructed
phytoplankton assemblages by mixing natural assemblages from regional
lakes and manipulated richness along with a dilutional gradient as part of
a large-scale mesocosm experiment. Phytoplankton assemblages from these
mesocosms of varying dilution rate were selected and mixed to create
unorganized assemblages of  high and low richness. We considered
these constructed phytoplankton assemblages unorganized because their
structures were not the result of ecological interactions. For an invader,
we used the green alga Golenkinia radiata, a taxon historically observed
in the region but absent from the natural assemblages at the time of
sampling. We conducted four sequential invasion experiments, each lasting
seven days, initiated with the increasingly organized assemblages of the
mesocosms. We found that assemblages, while still unorganized early in
succession, were vulnerable to invasions. However, as the assemblages
organized with time, they became resistant to invasions. Assemblage
richness, which ranged from 23 to 29 in the high-richness mesocosms and
from 15 to 24 in the low-richness mesocosms, had only a marginal effect on
invasion success. Instead, declining resources, notably phosphorus,
reduced diversity, and the emerging dominance of strong competitors near
the niche of the G. radiata invader explained the decreased success of
invasions as the assemblages organized with time. In the more organized
assemblages, G. radiata encountered formidable competitors in established
populations of the diatom Nitzschia acicularis and the chrysophyte Synura
sp., both of which had higher affinities for phosphorus than G. radiata.
Our study highlights that assemblage organization with time plays a
fundamental role in phytoplankton species coexistence, including a
stronger resistance to invasion as assemblages mature.