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, 282 (1800), 20142846

Mutualism Between Co-Introduced Species Facilitates Invasion and Alters Plant Community Structure

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Mutualism Between Co-Introduced Species Facilitates Invasion and Alters Plant Community Structure

Kirsten M Prior et al. Proc Biol Sci.

Abstract

Generalized mutualisms are often predicted to be resilient to changes in partner identity. Variation in mutualism-related traits between native and invasive species however, can exacerbate the spread of invasive species ('invasional meltdown') if invasive partners strongly interact. Here we show how invasion by a seed-dispersing ant (Myrmica rubra) promotes recruitment of a co-introduced invasive over native ant-dispersed (myrmecochorous) plants. We created experimental communities of invasive (M. rubra) or native ants (Aphaenogaster rudis) and invasive and native plants and measured seed dispersal and plant recruitment. In our mesocosms, and in laboratory and field trials, M. rubra acted as a superior seed disperser relative to the native ant. By contrast, previous studies have found that invasive ants are often poor seed dispersers compared with native ants. Despite belonging to the same behavioural guild, seed-dispersing ants were not functionally redundant. Instead, native and invasive ants had strongly divergent effects on plant communities: the invasive plant dominated in the presence of the invasive ant and the native plants dominated in the presence of the native ant. Community changes were not due to preferences for coevolved partners: variation in functional traits of linked partners drove differences. Here, we show that strongly interacting introduced mutualists can be major drivers of ecological change.

Keywords: co-introduced species; invasional meltdown; myrmecochory; seed dispersal mutualisms.

Figures

Figure 1.
Figure 1.
Experimental design. (a) Mesocosms were set up in a 6 × 7 array in a red pine plantation at KSR. Mesh enclosures prevented ants and seeds from entering or escaping the mesocosms. (b) Each mesocosm contained two adult plants of four myrmecochore species planted along a central line in the middle of the mesocosm: A. acutiloba (position A), A. canadense (position B), S. canadensis (position C) and C. majus (position D). Each individual plant was planted offset from these positions in different directions along the central line (see circles in figure). We placed wire rings (10 cm in diameter) around the base of each adult plant to estimate dispersal. Two pieces of rotting red pine wood (approx. 10 × 20 cm) were placed at either end of the mesocosms to act as ant nests (N1 and N2). (Online version in colour.)
Figure 2.
Figure 2.
Seed dispersal. Mean (±s.e.m.) proportion of dispersed seeds of the (a) invasive plant, C. majus, and the (b) native plants 3 days after seeds were added to mesocosms (n = 7). Open circles represent low-density ant treatments (one colony), and filled circles represent high-density ant treatments (two colonies). Different letters indicate statistically significant differences between ant species. Density treatments were also significantly different (native plants p = 0.008; invasive plant p = 0.029).
Figure 3.
Figure 3.
Plant recruitment. Mean (±s.e.m.) log response ratio (ln RR) of ant effects on plant recruitment of the (a) invasive plant C. majus (seedlings and established plants), and the (b) native plants (seedlings) 1 year after seeds were added to mesocosms (n = 7). Open circles represent low-density ant treatments (one colony), and filled circles represent high-density ant treatments (two colonies). For the invasive plant, the effect sizes are similar for the low- and high-density M. rubra ant treatments (i.e. the symbols are overlapping). Different letters indicate statistically significant differences between ant species. A positive effect size represents a facilitative effect of ants on plants, a negative effect size an antagonistic effect.
Figure 4.
Figure 4.
Ant behavioural traits. (a) Mean (±s.e.m.) proportion of dispersed seeds of all plant species in the laboratory trial for A. rudis (open circles) and M. rubra (filled circles) colonies (n = 10). (Top inset) Mean (±s.e.m.) number of ants in contact with seeds within the first 50 min of the laboratory trial. (Bottom inset) Mean (±s.e.m.) proportion of dispersed seeds out of all dispersed seeds of C. majus (black), A. acutiloba (grey), A. canadense (white) and S. canadensis (hatched). (b) Mean (±s.e.m.) proportion of dispersed seeds in a field trial conducted on August 22, 2012 (n = 10). (Inset) Mean (±s.e.m.) proportion of dispersed seeds during three separate time periods in the field; filled bars, M. rubra; open bars, A. rudis.

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