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

Abstract

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. 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. 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. 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 out‐competed 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. 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.

The relationship between niche breadth and range size of beech (Fagus) species worldwide

“…The colour spectrum follows the distribution range maps (Figure 1) et Brown, 1984;Kambach et al, 2019;Sheth et al, 2020;Slatyer et al, 2013), we found no significant relationship between range size and the biotic and climatic niche breadth estimates for the species in the globally important tree genus Fagus. or biotic interactions with other species, the regional availability of suitable niche space or differences in landscape heterogeneity, evolutionary processes or historical events, as well as human impacts (Boulangeat et al, 2012;Lambdon, 2008;Sillero, 2011;Slayter et al, 2013;Wandrag et al, 2019).…”

Traits linked with species invasiveness and community invasibility vary with time, stage and indicator of invasion in a long‐term grassland experiment

“…; Wandrag et al . in press). Because a set mass (rather than number) of seeds was added to seeded subplots, the seed mass‐number trade‐off remained intact in our experiment, such that light‐seeded invaders would have had more seeds than heavier seeded species, and thus more opportunities to recruit (Duncan et al .…”

“…; Wandrag et al . in press). The relative advantage of lighter, more numerous seeds was apparent for occupancy (where hierarchical distance was significant), but not abundance, the latter of which was mostly strongly related to seed mass CWM.…”

“…That invader abundance was only linked with seed mass in seeded (cf unseeded) subplots may stem from the high rate of seed addition to seeded subplots (4.5 g m −2 per species): this high seeding rate may have enabled invaders to occupy most, if not all, available safe sites, whereas the unseeded subplots were presumably still seed‐limited (Wandrag et al . in press).…”

Ant preference for seeds without awns increases removal of exotic relative to native grass seeds

“…We conducted a seed removal experiment in Austral summer 2018 in grassland on the University of Canberra campus, ACT. We selected three native ( Bothriochloa macra , Chloris truncata , Rytidosperma caespitosum ) and three exotic ( Dactylis glomerata , Eragrostis curvula , Phalaris aquatica ) grass species that co‐occur locally (Wandrag et al ., 2019). Seeds of the three native species have conspicuous awns (Fig.…”