Abstract:
Evaluating the relative influences of positive and negative interactions in
shaping communities is a major topic in modern ecology. Facilitative interactions
between basal species are important in habitats with intense predation pressure or
severe abiotic stresses. However, few studies address the potential for positive
interactions between trophic levels to influence community structure by altering
patterns of predation.
I investigated whether the association between the canopy-forming alga
Hedophyllum sessile and the herbivorous chiton Katharine tunicata was due to the
provision of a refuge from predation, a preference for Hedophyllum as a food item,
or amelioration of abiotic conditions. In a field experiment, Katharine were not
affected by predation or Hedophyllum thalli, but showed a strong behavioral
selection for shaded areas during summertime low tides. By providing shade,
Hedophyllum controls the distribution of the system's major herbivore.
In a second field experiment I evaluated the relative effects of shade and
Katharina on the rest of the community. Shade had strong positive effects on a
suite of consumers, increasing abundances of seven animal groups relative to
unshaded areas. Shade and Katharina had quantitatively equal negative effects
on the abundance of basal species, but their effects were qualitatively very
different. The positive interaction between Hedophyllum and Katharina affects the
entire community by altering patterns of herbivory. Such complex networks of
positive, negative, direct and indirect interactions can produce deceptively simple
patterns in natural systems.
I used field experiments and laboratory analyses to evaluate potential
physiological benefits of this positive interaction on Katharina. Levels of heat
shock protein 70 isoforms in field populations were greater in summer than in
winter, suggesting that Katharina are experiencing seasonal sub-lethal stress.
Although shade did not affect Hsp7O levels in Katharina maintained in field
enclosures, amelioration of abiotic stresses through positive biotic interactions
could have direct physiological consequences for beneficiary species.
These studies provide strong evidence that positive interactions between
trophic levels can profoundly affect the physiology of individuals, the distribution
and abundance of populations, and the structure of communities. I present a
conceptual model to summarize predictions of the importance of these multi-level
positive interactions in structuring communities.
