Evolution of
Animal-Mediated Seed Dispersal Systems
Roman Yukilevich and Henry F. Howe Program in Ecology and Evolution, Department of Biology, University of Illinois-Chicago, Chicago, IL, 60607
We develop a model that describes the conditions for the evolution
of seed dispersal systems involving animal dispersal agents. Six possible
dispersal systems are identified, supplementing the two, which have previously
been the foci of discussion: "specialized" and
"generalized", for small and large dispersal assemblages,
respectively. For purposes of discussion, these are contrasted with a
non-interactive state, analogous to a null hypothesis. It states that in a
specialized system, the plant specializes on a single, "exceptionally
effective dispersal species", while in a generalized system, the plant
generalizes on multiple dispersal species. We then identify environmental
conditions that may be expected to draw the system out of the non-interactive
state. New conditions are proposed to alter previous trends in specialization
in a predictable fashion. Specialized system evolves under four distinct
conditions: The 1) invasion of an equally effective dispersal agent, 2) heavy
and constant infestation by seed or fruit destroying visitors, 3) an
exceptionally effective dispersal agent evolves predatory feeding activities on
fruits or seeds and 4) the extinction of an effective dispersal agent.
Generalized system evolves only under one environmental condition: The invasion
of an effective dispersal agent. In total, we predict three outcomes when
Specialized and Generalized systems evolve: The Specialized system evolves into
either "equally effective dispersal system" or into "reduced
seed establishment system", while Generalized system only evolves into
"effective invader system". After we discuss how environmental
conditions are likely to alter the system of interest, we 1) predict novel
selection pressures on the plant, 2) propose three hypotheses
("convergence, restriction, and attraction hypothesis") as to how a
plant may evolutionarily respond, and 3) predict the dispersal-related plant
traits ("dispersal syndromes") based upon the nature of each
hypothesis. Finally, we discuss possible ecological and historical constraints
on the evolution of seed dispersal systems.
Even though only five systems are likely in our model, each is further
differentiated into plants that have depleting and non-depleting crops.
Transitions from Specialized to Generalized systems and vice versa are
influenced by the extent to which fruit crops are regularly depleted (depleting
plants) or remain superabundant (non-depleting plants). In total, we argue that
the break-up of a non-interactive state under certain environmental conditions
and the subsequent evolutionary response by a plant drive the evolution of seed
dispersal systems.