Data from: Quantifying niche availability, niche overlap and competition for recruitment sites in plant populations without explicit knowledge of niche axes.

1. Niche availability, niche overlap and competitive ability are key
determinants of the distribution and abundance of species. However,
quantifying each of these components is difficult because it is not always
possible to identify or measure relevant environmental gradients (niche
axes) along which species might partition or compete for niche space. 2.
We describe a method that uses seed addition experiments to quantify the
number of ‘safe-sites’ (microsites suitable for a species to recruit from
seed) at a location and show how this method can be used to quantify niche
availability, niche overlap and competitive ability. We illustrate our
approach using two seed addition experiments in grassland. 3. In the first
experiment, we added seeds of one native and two exotic grass species,
alone and in mixture, to plots that were arrayed along a gradient of soil
moisture availability. We show that the three species partitioned
safe-sites, implying that all three species could locally co-occur through
niche partitioning, in part due to different responses to moisture
availability. 4. In the second experiment, we added seeds of three
commonly co-occurring native grass species, alone and in mixture, to plots
with no obvious environmental gradients. One species outcompeted two
others for site occupancy, allowing us to quantify both the degree of
niche overlap and the relative ability of each species to compete for
safe-site occupancy without a priori knowledge of the niche axes. Results
from both experiments demonstrate the fine-scales at which species can
partition niches to facilitate co-occurrence. 5. Synthesis. By
conceptualising a plot of ground as containing a limited number of
microsites that are safe for the recruitment of a given species, and using
seed addition experiments to measure both the number of safe-sites and
degree of safe-site overlap among species, we show how niche availability,
niche overlap and competitive ability can be quantified at fine-scales
without a priori knowledge of niche axes. Our approach allows questions
about niche availability and competition for shared niche space to be
empirically tested, and to examine how these processes vary along
environmental gradients to shape species distributions and patterns of
co-occurrence.