Invasive congeners differ in successional impacts across space and time

Abstract

Invasive species can alter the succession of ecological communities because they are often adapted to the disturbed conditions that initiate succession. The extent to which this occurs may depend on how widely they are distributed across environmental gradients and how long they persist over the course of succession. We focus on plant communities of the USA Pacific Northwest coastal dunes, where disturbance is characterized by changes in sediment supply, and the plant community is dominated by two introduced grasses--the long-established Ammophila arenaria and the currently invading A. breviligulata. Previous studies showed that A. breviligulata has replaced A. arenaria and reduced community diversity. We hypothesize that this is largely due to A. breviligulata occupying a wider distribution across spatial environmental gradients and persisting in later-successional habitat than A. arenaria. We used multi-decadal chronosequences and a resurvey study spanning 2 decades to characterize distributions of both species across space and time, and investigated how these distributions were associated with changes in the plant community. The invading A. breviligulata persisted longer and occupied a wider spatial distribution across the dune, and this corresponded with a reduction in plant species richness and native cover. Furthermore, backdunes previously dominated by A. arenaria switched to being dominated by A. breviligulata, forest, or developed land over a 23-yr period. Ammophila breviligulata likely invades by displacing A. arenaria, and reduces plant diversity by maintaining its dominance into later successional backdunes. Our results suggest distinct roles in succession, with A. arenaria playing a more classically facilitative role and A. breviligulata a more inhibitory role. Differential abilities of closely-related invasive species to persist through time and occupy heterogeneous environments allows for distinct impacts on communities during succession.

Fig 1. Foredune cross-section schematics.

(A) Foredunes contain both spatial and temporal gradients along their cross-sections. Foredune consists of toe, crest, and heel moving from left to right. For the spatial dune gradient, quadrats were assigned values between-1 and 1 for their placement along the dune cross-section. Chronosequence age increased from the dune toe to heel (see main text for details). Shoreline and backdune are shown for orientation. (B) Historical dune consisting of only Ammophila arenaria (gray). (C) Invasion of historical dune by A. breviligulata (black) through preemptive colonization of a new dune. Ammophila arenaria retains its original population, and a backdune invasion boundary delineates the two species’ distributions along the dune. (D) Invasion of historical dune by A. breviligulata through displacement of A. arenaria. Note that there is no longer a backdune invasion boundary because the A. arenaria has been locally extirpated.

Fig 2. Contour plot of Ammophila cover shows distinct species distributions across chronosequence ages and dune gradient.

Contours show areas of increasing cover across both gradients. Contours were created using predicted values of generalized linear models (see main text for details). Chronosequence ages were calculated for each quadrat using aerial photos and shoreline change rates (see main text for details). Dune gradient was calculated for each quadrat based on its standardized location along dune cross sections with the toe, crest, and heel at-1, 0, and 1, respectively. Each point on figure refers to the chronosequence age and dune gradient for an individual quadrat. Sites dominated by A. arenaria are shown in red, and those from A. breviligulata sites shown in blue.

Fig 3. Contour plot of (A) native cover and (B) species richness shows differences in distribution across across chronosequence ages and dune gradient in dunes of different Ammophila dominance.

Contours show areas of increasing cover or richness across both gradients. Quadrats from sites dominated by A. arenaria are shown in red, those from A. breviligulata sites shown in blue. See main text and Fig. 2 caption for additional details.

Fig 4. Changes in Ammophila cover and plant richness over a 21-year period from 1988 to 2009.

‘Switch’ refers to transects that had substantial A. arenaria in 1988 and were dominated by A. breviligulata in 2009. “Estab.’ refers to transects that were dominated by A. breviligulata in both 1988 and 2009. (A) Mean relative Ammophila cover ± S.E.; (B) Relative Ammophila cover across the dune gradient; (C) Mean plant richness ± S.E.; (D) Plant richness across the dune gradient. Light red denotes switched transects in 1988; light blue denotes established transects in 1988; dark red denotes switched transects in 2009; dark blue denotes established transects in 2009. Three separate linear contrasts shown at different heights in (A) and (C), stars signify significance (p < 0.05), NS signifies not significant. For (B), there was a significant difference in Ammophila cover between switched and established transects in 1988 but not in 2009. For (D), there were no significant differences in richness along the dune gradient within either 1988 or 2009. See Table 1 for full models.