Do deposit-feeders compete? Isotopic niche analysis of an invasion in a species-poor system

Abstract

Successful establishment of invasive species is often related to the existence of vacant niches. Competition occurs when invaders use the same limiting resources as members of the recipient community, which will be reflected in some overlap of their trophic niches. The concept of isotopic niche has been used to study trophic niche partitioning among species. Here, we present a two-year field study comparing isotopic niches of the deposit-feeding community in a naturally species-poor system. The isotopic niche analyses showed no overlap between a recent polychaete invader and any of the native species suggesting that it has occupied a vacant niche. Its narrow isotopic niche suggests specialized feeding, however, the high δ(15)N values compared to natives are most likely due to isotope fractionation effects related to nitrogen recycling and a mismatch between biological stoichiometry of the polychaete and the sediment nitrogen content. Notably, highly overlapping isotopic niches were inferred for the native species, which is surprising in a food-limited system. Therefore, our results demonstrate that invaders may broaden the community trophic diversity and enhance resource utilization, but also raise questions about the congruence between trophic and isotopic niche concepts and call for careful examination of assumptions underlying isotopic niche interpretation.

Figure 1. Species composition and abundance of benthic deposit-feeders at the sampling stations in 2009 and 2010 or 2008 and 2011.

Ma = Monoporeia affinis, Mz = Marenzelleria arctia, Mb = Macoma balthica and Pf = Pontoporeia femorata. Samples at stn Mörkö were collected in October resulting in two year classes of amphipods being present, whereas only one year class was found at the other stations, which are sampled in May.

Figure 2. Stable isotope bi-plots illustrating the isotopic niche of the deposit-feeding community at the sampling stations.

The invader Marenzelleria arctia (red) and the native co-occurring species, Monoporeia affinis (black), Macoma balthica (green), and Pontoporeia femorata (blue) at three stations in 2009 and 2010. All sampling events are pooled within years. The lines enclose the standard ellipse area (SEAc). Overlap between species is reported in Table 1. Sediment isotope value is denoted +, early spring bloom isotope value is denoted ♦ and late spring bloom isotope value is denoted ▪. Seston (stn Mörkö only) is denoted ▴. Resource data are mean ± standard deviation.

Figure 3. Mean standard ellipse area (SEAc) for each species at the sampling stations.

Boxed areas indicates the SEA_B with Bayesian 50, 75 and 95% credible interval (Ma = Monoporeia affinis, Mz = Marenzelleria arctia, Mb = Macoma balthica and Pf = Pontoporeia femorata) at the three stations in 2009 and 2010. Shared common letters denote no significant difference (P > 0.05).

Figure 4. Body condition expressed as C:N ratio and isotope diversity indices.

Top panel: Standard ellipse area, SEAc; mid panels: δ¹⁵N range and δ¹³C range and bottom panel: mean nearest-neighbour distance, MNND) for each species, station and year. Ma = Monoporeia affinis, Mz = Marenzelleria arctia, Mb = Macoma balthica and Pf = Pontoporeia femorata. Results from Pearson product moment correlations are reported in the text.

Figure 5. Negative relationship between consumer enrichment in relation to the sediment δ¹⁵N values and nitrogen content in sediment.

Average values for station and year. Ma = Monoporeia affinis, Mz = Marenzelleria arctia, Mb = Macoma balthica and Pf = Pontoporeia femorata. Results from GLM is reported in Table 2.