Ant‐mediated seed dispersal does not facilitate niche expansion

Abstract

Summary1. Whereas classic niche theory is based on the contraction of the niche via negative interactions, facilitative niche theory suggests that mutualisms can expand the niche via positive interactions. Specifically, animal-mediated seed dispersal can expand the utilization of physical space by plants and allow greater access to resources and other environmental requirements. Ant-mediated dispersal of plant propagules (myrmecochory) is a common mutualism throughout the world, particularly in the deciduous forests of the eastern United States where this research is conducted. 2. We examine two facets of niche expansion via ant-mediated seed dispersal: (1) increased utilization of resources along resource gradients and (2) escape from unfavourable density-dependent conditions. 3. We test these assumptions by introducing Hexastylis arifolia seeds in cafeteria-style bait stations along abiotic gradients (moisture, temperature, light) for removal by key seed dispersers from the ant genus Aphaenogaster. We also examine plant aggregation along the same gradients. 4. Ant-mediated dispersal services decrease significantly with increasing soil moisture and ultimately fail at levels that are demonstratively within the plant's niche optima; further, the decline in dispersal services is correlated with increasing plant aggregation, suggesting that enemy escape also falters at relatively high levels of soil moisture. 5. Synthesis. Facilitated propagule dispersal fails to expand the Hexastylis arifolia niche in either enhanced resource utilization or decreased density dependence as the niche requirements for the ant disperser are nested within those for the plant. The strength of this interaction varies across space and time, and in doing so may undermine attempts to predict future distributions. Further, given that myrmecochores are typically poor dispersers, the incomplete niche overlap between the plant and its facilitator makes this plant guild particularly susceptible to climatic change if each participant responds individually to shifting environmental conditions.

The mixed effects of experimental ant removal on seedling distribution, belowground invertebrates, and soil nutrients

“…The experimental removal of ants from a southeastern temperate forest resulted in the aggregation of seedlings around H. arifolia maternal plants in the ant removal plots, likely due to the exclusion of A. rudis, the most common and important seed dispersing ant species in many eastern deciduous forests (Ness et al 2009, Warren et al 2010, Zelikova et al 2008). This result is consistent with the idea that ants (and A. rudis in particular) play a major role in seed dispersal in southwestern deciduous forests.…”

“…In our study, we did not find evidence that increased aggregation significantly decreased seedling survival, but it may be that the effects of competition are greater as plants mature and shade each other. Because H. arifolia does not mature to reproductive stage for seven years, we were unable to directly examine the effects of aggregation on life-time demographic factors other than seedling survival, though work by Giladi (2004) and Warren et al (2010) indicate that an increase in local density decreases survival and growth of H. arifolia seedlings and adults. Our study was limited to three years and many of the factors that exert selective pressure on plant survival, such as density dependence and pathogen accumulation, take longer than 3 years to emerge.…”

“…Ants may also deposit seeds into unfavorable germination sites (Rice andWestoby 1986, Christian andStanton 2004) such as logs or move seeds into marginal habitat (Giladi 2004). The distribution of H. arifolia in the study area is patchy, with distinct boundaries between occupied and unoccupied habitats (Giladi 2004, Warren et al 2010, suggesting that the distribution of suitable habitat is not continuous. However, ants, especially A. rudis, generally disperse seeds short distances (Giladi 2004, Zelikova et al 2008.…”

“…In southeastern temperate forests, Aphaenogaster rudis disperse the majority of ant-dispersed seeds and are considered to be keystone mutualists for myrmecochorous plants (Giladi 2004, Ness et al 2009, Zelikova et al 2008, Warren et al 2010. Indeed, in one recent study (Mitchell et al 2002), the single best correlate of myrmecochore abundance was the abundance of A. rudis, suggesting that these ants have a strong effect on myrmecochore fitness.…”

The shape of things to come: woodland herb niche contraction begins during recruitment in mesic forest microhabitat

“…Conversely, positive biotic interactions can extend the fundamental niche beyond that of the realized niche by enhancing access to limiting resources [9][10][11]. This requires, however, that resource conditions are optimal for both mutualists, and failed or limited facilitation can result in contracted response patterns similar to those created by competitive interactions [12,13]. Connell [14] suggested that species experience physical stress where resources are low-though this can be ameliorated by facilitation [9,15]-and biological stress where resources are high, but fundamental to realized niche contractions have been tested rarely since Connell's studies [16 -18].…”