The roles of stochasticity and biotic interactions in the spatial patterning of plant species in alpine communities

Questions

Plant community composition can be influenced by multiple biotic, abiotic, and stochastic factors acting on the local species pool to determine their establishment success and abundance and subsequently the diversity of the community. We asked if the influences of biotic interactions on the composition of plant species in communities, as indicated by patterns of plant species spatial associations (independent, positive or negative), vary across a productivity gradient within a single ecosystem type. Do dominant species of communities show spatial patterning suggestive of competitive interactions with interspecific neighbors? Do species that span multiple community types exhibit the same heterospecific interactions with neighbours in each community?

Location

Three alpine communities in the southern Rocky Mountains.

Methods

We measured the occurrence of species in a 1‐cm spatial grid within 2 m × 2 m plots to determine the spatial patterns of species pairs in the three communities. A null model of independent species spatial arrangements was used to determine whether species pairs were positively, negatively or independently associated, and how these patterns differed among the communities across the gradient of resource supply and environmental stress.

Results

Positive associations, indicative of facilitation between species, were most common in the most resource‐poor and least productive community. However negative associations, suggestive of competitive interactions among species, were not more common in the two more resource‐rich, productive communities. The dominant species of these communities did exhibit higher negative than positive associations with neighbours relative to positive patterning. Independent interspecific patterning was equally common relative to positive and negative patterns in all communities. Species that previously were shown to either facilitate other species or compete with neighbours exhibited spatial patterning consistent with the earlier experimental work.

Conclusions

A large number of species exhibit a lack of net biotic interactions, and stochastic factors appear to be as important as competition and facilitation in shaping the structure of the three alpine plant communities we studied.