ABSTRACT

0213187
Hooper
The search for patterns of community assembly has occupied the last 25 years in
community ecology and remains controversial today. A commonly sought assembly rule is that
species coexistence in communities will be enhanced by differences in traits related to
competition for limiting resources (i.e., that niche overlap is minimized). Put another way, do
species from complementary guilds or functional groups have a better chance of persisting than
those with strong overlap in resource use characteristics? Evidence for the role of
complementarity among plant groups in structuring communities is sporadic. The balance
between rates of external stochastic processes (such as disturbance or priority effects of
colonization) compared to internal community dynamics (such as competition or facilitation)
may be critical in determining the degree to which internal assembly dynamics are manifested.
Gaining further insight into this question is critical for understanding how communities might be
structured, and for the implications of that structure for ecosystem dynamics.
We propose a five-year experiment to assess the roles of initial plant composition and
functional complementarity in structuring communities of serpentine annual grasslands. We
seek to test several hypotheses centered on the following questions, generally grouped into two
themes: 1) Does the initial composition of a community shape its subsequent development? If so,
for how long? Are there different stable endpoints? 2) To what extent does complementary
resource use (i.e., niche differentiation) versus dominance among plant species govern patterns
of community development? Are other factors, such as seed input or climatic conditions more
important in determining community composition? Our goal is not to prove or disprove the
predominance of assembly rules, but rather to better understand the conditions under which they
might strongly influence community composition versus be overwhelmed by other factors.
We will use as our experimental system a series of California serpentine grassland
communities that vary in functional group composition and richness. Functional composition
has been maintained since the plots were established in 1992. In fall 1999 and 2000, seeds of six
additional species (3 species each from two groups) were added to test the effects of functional
characteristics and diversity on invasibility of ecosystems. Thus we now have a system with
treatments that differ in initial functional composition, that have had common seed sources for a
number of additional species, and that will be subject to continuing seed rain from surrounding
grasslands. Our plan is to track changes in community composition over the next five years as
the communities develop from their initially different starting points. Monitoring of vegetation changes in these experimental communities through the next several years provides a unique opportunity to gain insight into interactions among a variety of processes that help shape ecological communities.

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