1. Evolution of species interactions in Mediterranean
annual plant communities
Rachel M. Germain and Benjamin Gilbert; Dept. of Ecology & Evolutionary Biology, University of Toronto
Background Biogeographic history influences species interactions Experimental work
Because more closely related species often share ecologically- How do species differences and biogeography combine to
important traits, ecologists initially predicted that competitive regulate phylogeny-coexistence relationships?
exclusion would occur more frequently between recently diverged
species1. In nature, however, phylogeny-coexistence relationships Study system
are often weak at best, or lack generality2. Most work attributes
this apparent disconnect between theory and reality to the lack of • 32 species native to the mediterranean grasslands of California
conservatism in the underlying traits3. or Spain, selected to capture a wide range of the taxonomic
diversity found in both regions
An additional but rarely examined problem is that trait differences
can both promote (niche differences) and deter (fitness
differences) coexistence (fig. 1). As such, closely related species A
A B Experimental setup
Exotic frequency (t + 1)
Exotic frequency (t + 1)
1:1 1:1
can be either more or less likely to coexist, depending on the • Greenhouse experiment (fig. 4) from January to August 2012
relative strength of niche and fitness differences4. However, it
remains unknown what determines the relative strength niche • Niche differences: plants grown at a constant density at six
versus fitness differences and how these differences contribute to Exotic frequency (t) Exotic frequency (t) frequencies of species A to species B
phylogeny-coexistence relationships.
• Fitness differences: each species grown alone at low densities to
NICHE DIFFERENCES FITNESS DIFFERENCES estimate seed production in the absence of competition
PROMOTE COEXISTENCE DETER COEXISTENCE
• Mathematical models are used to compare the population
1:1 1:1
growth rates (estimated via seed production) when competitors
originate from the same and different biogeographic regions
Relative frequency (t + 1)
Relative frequency (t + 1)
I predict that stable coexistence should occur more frequently
between distantly related species that have coevolved through
evolutionary time (fig. 2A).
C D
Exotic frequency (t + 1)
Exotic frequency (t + 1)
1:1 1:1
Relative frequency (t) Relative frequency (t)
Figure 1. Trait differences can both promote (left) and deter (right) coexistence. The 1:1 line indicates the Exotic frequency (t) Exotic frequency (t)
boundary between positive and negative population growth in one time step.
Figure 3. Population dynamics of exotic species (orange) when introduced to a native community (black) in a new region, according to four scenarios: (A) no niche or fitness differences, (B) no niche
Trait evolution but fitness differences, (C) niche but no fitness differences, and (D) both niche and fitness differences. Dashed lines in drawing represent distinct spatial niches. Plant drawings modified from F. L.
Pérez 2011.
Although most contemporary work acknowledges that niche and Biogeographic history may shape the evolutionary trajectories of competitive exclusion6,7 (fig. 3B,D). This latter phenomenon is the
fitness differences both contribute to species coexistence, their niche and fitness differences between species. When competitors product of differences both in the underlying environmental
evolutionary trajectories and corresponding implications for originate from the same biogeographic region, niche differences conditions and in community-wide fitness among species in their
community structure remain unknown. Trait differences should be should evolve faster than fitness differences (fig. 2A) to explain home and introduced ranges. For example, the invasion of
minimal immediately after speciation, and increase over the high diversity in nature, and promote coexistence between European grasses across California landscapes has been
evolutionary time as species diverge (fig. 2). Because it seems more distantly related species. This is possible because attributed to the long history of grazing pressure in their home
unlikely that niche and fitness differences evolve at the same competition is a well-known driver of niche divergence between range8, selecting for traits that accelerate growth and provide a
rate, the probability of coexistence can either increase (fig. 2A) or co-occurring species5, but may simultaneously cause species to competitive advantage in regions where grazing pressure is
Figure 4. Photos of greenhouse experiment, which includes 16 species pairs that span a broad range
decrease (fig. 2B) over evolutionary time depending on which trait- become more similar than different in fitness as poor competitors relaxed. The unpredictability associated with species from of divergence times and originate from either the same or different biogeographic regions.
type evolves faster. are excluded from the community. When competitors originate different biogeographic regions is consistent with observed
from different biogeographic regions, rates of niche divergence patterns of species invasions, where some species have
A Figure 2. Evolutionary trajectories of are unpredictable with respect to evolutionary relationships, with spectacularly negative impacts on native diversity while others
ACKNOWLEDGMENTS
Coexistence metric
niche (α) to fitness (λ) differences the potential for fitness differences to accumulate and drive fail to invade altogether9.
COEXISTENCE and corresponding implications for
species coexistence. In scenario
(Δα/Δλ)
ZONE
1 A, coexistence occurs because niche
References: 1. Elton CS, 1946, J. of Anim. Ecol. 15: 54-68; 2. Swenson NG, Enquist BJ, 2009, Ecology 90: 2161-2170; 3. Cavender-Bares J, Keen A, Miles B, 2006, Ecology 87: S109-122; 4. Mayfield MM, Levine
EXCLUSION differences evolve faster than fitness
JM, 2010, Ecol. Lett. 13: 1085-1093; 5. Dayan T, Simberloff D, 2005, Ecol. Lett. 8: 875-894; 6. MacDougall AS, Gilbert B, Levine JM, 2009, J. of Ecol. 97: 609-615; 7. Strauss SY, Webb CO, Salamin N, 2006, PNAS 103:
differences (i.e., Δα/Δλ > 1). In
ZONE scenario B, exclusion occurs because
5841-5845; 8. Mitchell CE, Agrawal AA, Bever JD, Gilbert GS, Hufbauer RA, Klironomos JN, Maron JL, Morris WF, Parker IM, Power AG, Seabloom EW, Torchin ME, Vazquez DP, 2006, Ecol. Lett. 9: 726-740; 9. Williamson
M, Fitter A, 1996, Ecology 77: 1661-1666.
B niche differences evolve slower than
fitness differences (i.e., Δα/Δλ < 1).
Time since divergence