Asymmetric competitive effects during species range expansion: An experimental assessment of interaction strength between “equivalent” grazer species in their range overlap

Abstract

Biotic interactions are central to the development of theory and concepts in community ecology; experimental evidence has shown their strong effects on patterns of population and community organization and dynamics over local spatial scales. The role of competition in determining range limits and preventing invasions at biogeographic scales is more controversial, partly because of the complexity of processes involved in species colonization of novel habitats and the difficulties in performing appropriate manipulations and controls. We examined experimentally whether competition is likely to affect poleward range expansion hindering or facilitating the establishment of the limpet Scurria viridula along the south‐eastern Pacific rocky shore (30°S, Chile) in the region occupied by the congeneric S. zebrina. We also assessed whether competition with the “invader” or range‐expanding species could reduce individual performance of the “native” S. zebrina and depress local populations Geographic field surveys were conducted to characterize the abundance and identity of limpets along the south‐eastern Pacific coast from 18°S to 41°S, and the micro‐scale (few cm) spatial distribution across the range overlap of the two species. Field‐based competition experiments were conducted at the southern leading edge of the range of S. viridula (33°S) and at the northern limit of S. zebrina (30°S). Field surveys showed poleward range expansion of S. viridula of ca. 210 km since year 2000, with an expansion rate of 13.1 km/year. No range shift was detected for S. zebrina. The resident S. zebrina had significant negative effects on the growth rate of the invading juvenile S. viridula, while no effect of the latter was found on S. zebrina. Spatial segregation between species was found at the scale of cms. Our results provide novel evidence of an asymmetric competitive effect of a resident species on an invader, which may hamper further range expansion. No negative effect of the invader on the resident species was detected. This study highlights the complexities of evaluating the role of species interactions in setting range limits of species, but showed how interspecific competition might slow the advance of an invader by reducing individual performance and overall population size at the advancing front.

On the importance of habitat continuity for delimiting biogeographic regions and shaping richness gradients

“…Tyberghein et al 2012), there are currently no databases that include the distribution of coastal rocky substrate. To measure this for the EP, we traced around the contours of the outer mainland coastline at a constant elevation of 5 km above sea level (viewed perpendicularly) in cumulative 48 km (c. 30 mile) bins using the software program Google Earth (v. 7.1.5; similar to Fenberg & Rivadeneira 2011;Mart ınez et al 2017;Aguilera et al 2018). The transect does not cover deep coastal inlets such as river estuaries (e.g.…”

Novel co‐occurrence of functionally redundant consumers induced by range expansion alters community structure

“…The distribution of the focal Scurria species and recent range shifts of S. viridula in central‐northern Chile have been documented in field studies across the region (see Aguilera et al, 2013 b , 2019 a , b ). Long‐term abundance data sets (i.e., 1998–2000, 2003 –2005, and 2009–2015) of Scurria limpets, estimated through positioning 50 × 50 cm quadrats, from higher to lower intertidal levels, at different sites spanning ~1,600 km from north to central Chile, show a well‐defined geographic overlap of both herbivore species between 30° and 32° S (see Fig.…”

“…Such impacts cannot simply be anticipated by the effects in the original community, as the novel environmental conditions encountered by the invader may alter its individual performance. In this study, we take advantage of the recent poleward range expansion of an intertidal rocky shore herbivore, the limpet Scurria viridula (Rivadeneira and Fernández 2005, Aguilera et al 2013 b , 2019 b ). This limpet has recently extended its region of overlap with its closely related congener S. zebrina to form a broad transitional zone, where both species co‐occur, from 30° to 32° S (Aguilera et al 2013 b , 2019 b ).…”

“…In this study, we take advantage of the recent poleward range expansion of an intertidal rocky shore herbivore, the limpet Scurria viridula (Rivadeneira and Fernández 2005, Aguilera et al 2013 b , 2019 b ). This limpet has recently extended its region of overlap with its closely related congener S. zebrina to form a broad transitional zone, where both species co‐occur, from 30° to 32° S (Aguilera et al 2013 b , 2019 b ). Because of the oceanographic conditions around the transition zone, larval arrival of space‐dominant mussel and barnacle species is minimal (Navarrete et al 2005).…”

“…1A). Individual per capita reduction of algal abundance and bare rock provision exerted by Scurria species diminish at the range overlap because of interference competition and/or lower individual performances at species range ends (Broitman et al 2018, Aguilera et al 2019 b ). Thus, the joint effects of S. viridula and S. zebrina at the range overlap could be nonadditive (i.e., their combined effect on algal production will be lower than expected by the addition of their individual effects; green square in Fig.…”