Herbivory and presence of a dominant competitor interactively affect salt marsh plant diversity

Abstract

Question Do herbivory and the presence of a dominant grass competitor interactively affect herbaceous communities and assembly rules in a SW Atlantic salt marsh? Location Upper salt marsh, Mar Chiquita coastal lagoon, Argentina. Methods We performed a field factorial experiment over 4 yr to evaluate the separate and interactive effects of (1) herbivory and (2) competition with the dominant grass species (i.e. Spartina densiflora) on the salt marsh subordinate plant community. The factorial design includes dominant grass removal and herbivory manipulation. Results Our results show that herbivory and presence of the dominant competitor interactively affect subordinate plant cover and diversity. Results further indicate that, in the presence of the dominant competitor, patch‐to‐patch variation in subordinate species composition is lower than expected at random, a result consistent with the expected outcomes of deterministic exclusion following light competition. Removal of the dominant grass nevertheless led to patch‐to‐patch dissimilarity in subordinate species composition, far from the dissimilarity expected at random, indicating increased importance of deterministic processes that drive communities to diverge. Conclusion Our results show that the conditional effect of herbivory on plant diversity can be determined by the presence of a single plant species. Dominant plant species, in addition, may not only affect plant species diversity by determining the number and identity of subordinate species in a given patch (i.e. α‐diversity) but also by affecting spatial variability through habitat homogenization.

Herbivory and dropping effects by small mammals on salt‐marsh vegetation vary across microhabitats

“…These differences agree with the observed use of short and tall vegetation by wild guinea pig in response to predation risk (lower in the tall vegetation; marsh matrix) and preferred food availability (higher in the short vegetation; open patches). Overall, our results show stronger impacts of small vertebrate herbivores on salt‐marsh vegetation than those reported for European marshes (Kuijper & Bakker, ), and they are similar to those from North American marshes (Cargill & Jefferies, ) and previous work on our study site (Alberti et al., ; Daleo et al., ).…”

“…Trophic effects by herbivores are usually context‐dependent because their strength and direction may change according to abiotic (e.g., physical gradients, Alberti, Escapa, Daleo, Casariego, & Iribarne, ) and biotic conditions such as the presence of top predators (Griffin & Silliman, ) or the presence of dominant plant species (Kuijper, Nijhoff, & Bakker, ). Among abiotic factors that regulate herbivore effects on vegetation, soil nutrient availability can lead to changes in nutritional quality and palatability of plant tissues (Augustine et al., ); however, the presence of dominant competitors (Kuijper & Bakker, 2005) and facilitative neighbors (Milchunas & Noy‐Meir, ) may change the growth and structure of subordinate plant species, driving to changes in the impact of plant–animal interactions (Daleo et al., ). While non‐trophic, context‐dependent, effects of small grazers have been described in many systems (Bakker et al., ; Reichman & Seabloom, ), the relative importance of these effects, such as dropping deposition, compared to their consumption effects has rarely been tested.…”

“…Although non‐trophic effects such as trampling (Howison et al., ) and waste products of large herbivores (Bardgett & Wardle, ) were also reported to be important drivers, we still know little about their relative importance on salt‐marsh structure and function (but see Gedan,Silliman, & Bertness, 2009; Pascual et al., ). Small mammals, for example, can change the structure of high‐marsh vegetation through both direct herbivory (Alberti et al., ; Daleo et al., ) and trampling (Pascual et al., ), but their indirect effects on plant diversity through other potentially important non‐trophic mechanisms such as fertilization by droppings remain largely unexplored.…”

Diversity loss in grasslands due to the increasing dominance of alien and native competitive herbs

“…The observed negative impact of strong native competitors on biodiversity (Bobbink et al 1987;de la Cretaz and Kelty 1999;Ervin and Wetzel 2002;Chmura and Sierka 2007;Daleo et al 2017) has motivated a discussion on how species are classified as invasive. Some experts have proposed that problematic native species can also be classified as invasive (Alpert et al 2000;Valery et al 2008).…”

Linking biodiversity to ecosystems: A task for plant community ecologists

“…We should also mention Daleo et al. (), which was runner‐up in our voting for the Editors’ Award. They used a methodical experimental approach involving manipulation of communities by removal to study over 4 years the interactive effects of plant competition and herbivores in a salt marsh in Argentina.…”