Testing the facilitation-competition paradigm under the stress-gradient hypothesis: decoupling multiple stress factors

Abstract

While the facilitation-competition paradigm under the stress-gradient hypothesis has received recent attention, its rigorous testing is yet to be explored. Most of the studies have considered a switch in the net interactions from competition to facilitation with increasing environmental stress as primary evidence supporting the hypothesis, though few studies examined changes in interaction along a full range of a stress gradient. Here, we have conceptualized possible variations in the patterns of change in interaction strength along such gradient. Based on this, we empirically evaluated the temporal shift in the interaction between two marine sessile animals, goose barnacles (Capitulum mitella) and mussels (Septifer virgatus), under multiple stress factors. The net effect of goose barnacles on mussel survivorship was positively related to the total stress gradient encompassing two stress factors, physical disturbance and thermal stress, while no negative value occurred even under mild conditions. When the two stress factors were treated separately, however, the net effect demonstrated apparently different patterns: monotonic increase with physical disturbance versus a quasi-asymptotic pattern (no change over a wide range) with thermal stress. These variable situations have not previously been recognized in this discipline, and the present study emphasizes the importance of an integrative and mechanistic approach to testing and deciphering the facilitation-competition paradigm.

Figure 1

Schematic showing the possible relationships between an environmental stress gradient and the strength of species interaction: (a) facilitation–competition shift, (b) facilitation-only and (c) competition-only, with (i) monotonic increase, (ii) monotonic decrease, (iii) positively peaked and (iv) negatively peaked.

Figure 2

Mortality of Septifer in the transplant experiments for six experimental periods (n=8 for each treatment). Values are untransformed means+1 s.e. Stress factors involved are shown in parentheses.

Figure 3

Relationships between the stress gradients and the net effect of Capitulum on the survivorship of Septifer. (a) Total stress expressed as the mortality of Septifer in the Septifer-only treatment versus the net effect of Capitulum (y=0.229−0.008x+0.00027x², R²=0.99, p=0.0008). (b) Physical disturbance expressed as the mortality of Septifer in the shaded treatment versus the net effect of Capitulum (y=0.093+0.008x+0.00014x², R²=0.93, p=0.0202). (c) Thermal stress expressed as the mortality of Septifer in the barrier treatment versus the net effect of Capitulum (a dashed line fitted by eye).

Figure 4

Schematic showing possible variation in the relationship between an environmental stress gradient and the strength of species interaction under harsh environmental conditions. Facilitative effect may (a) reach an asymptote, (b) decline to a neutral level or (c) turn into a negative value (competitive effect) with increasing environmental stress.