Predicting the competitive interactions and trophic niche consequences of a globally invasive fish with threatened native species

1. Novel trophic interactions between invasive and native species
potentially increase levels of inter-specific competition in the receiving
environment. However, theory on the trophic impacts of invasive fauna on
native competitors is ambiguous, as while increased inter-specific
competition can result in the species having constricted and diverged
trophic niches, the species might instead increase their niche sizes,
especially in omnivorous species. 2. The competitive interactions between
an omnivorous invasive fish, common carp Cyprinus carpio, and a tropically
analogous native and threatened fish, crucian carp Carassius carassius,
were tested using comparative functional responses (CFRs). A natural pond
experiment then presented the species in allopatry and sympatry,
determining the changes in their trophic (isotopic) niche sizes and
positions over four years. These predictive approaches were complemented
by assessing their trophic relationships in wild populations. 3. CFRs
revealed that compared to crucian carp, carp had a significantly higher
maximum consumption rate. Coupled with a previous cohabitation growth
study, these results predicted that competition between the species is
asymmetric, with carp the superior competitor. 4. The pond experiment used
stable isotope metrics to quantify shifts in the trophic (isotopic) niche
sizes of the fishes. In allopatry, the isotopic niches of the two species
were similar sized and diverged. Conversely, in sympatry, carp isotopic
niches were always considerably larger than those of crucian carp and were
strongly partitioned. Sympatric crucian carp had larger isotopic niches
than allopatric conspecifics, a likely response to asymmetric competition
from carp. However, carp isotopic niches were also larger in sympatry than
allopatry. In the wild populations, the carp isotopic niches were always
larger than crucian carp niches, and were highly divergent. 5. The
superior competitive abilities of carp predicted in aquaria experiments
were considered to be a process involved in sympatric crucian carp having
larger isotopic niches than in allopatry. However, as sympatric carp also
had larger niches than in allopatry, this suggests other ecological
processes were also likely to be involved, such as those relating to fish
prey resources. These results highlight the inherent complexity in
determining how omnivorous invasive species integrate into food-webs and
alter their structure.

1. 入侵物种与本地物种间的新型营养相互作用，可能会提升入侵接收生境中的种间竞争水平。然而，关于入侵动物对本地竞争者的营养影响的相关理论仍存在歧义：一方面，增强的种间竞争可能会导致物种的营养生态位收缩并发生分化；另一方面，物种反而可能扩大其生态位幅度，这一现象在杂食性物种中尤为显著。
2. 本研究以杂食性入侵鱼类——鲤（Cyprinus carpio, common carp），以及生态位相似的濒危本地鱼类——鲫（Carassius carassius, crucian carp）为研究对象，采用比较功能响应（comparative functional responses, CFRs）实验探究二者间的竞争相互作用。随后开展自然池塘实验，设置异域分布（allopatry）与同域分布（sympatry）两组处理，在四年的周期内监测二者的营养（同位素）生态位幅度与位置变化。此外，本研究还通过野外种群调查验证上述营养相互关系，以补充前述预测性实验的结果。
3. 比较功能响应实验结果显示，相较于鲫，鲤的最大摄食率显著更高。结合此前的共养生长实验结果，本研究推测二者间的竞争具有不对称性，鲤为优势竞争者。
4. 池塘实验通过稳定同位素指标量化了鱼类营养（同位素）生态位的幅度变化。在异域分布组中，两个物种的同位素生态位幅度相近且发生了分化；而在同域分布组中，鲤的同位素生态位始终显著大于鲫，且二者的生态位存在明显的分区现象。同域分布的鲫的同位素生态位幅度大于异域组的同种个体，这很可能是其应对鲤带来的不对称竞争的响应。但与此同时，同域组的鲤的同位素生态位幅度也大于异域组的同种个体。在野生种群中，鲤的同位素生态位始终大于鲫，且二者生态位高度分化。
5. 实验室实验预测的鲤具有优势竞争能力，被认为是导致同域分布鲫的同位素生态位幅度大于异域组个体的潜在过程之一。但由于同域组的鲤的生态位幅度同样大于异域组，这提示还存在其他生态过程参与其中，例如与鱼类猎物资源相关的过程。本研究结果凸显了确定杂食性入侵物种如何融入食物网并改变其结构这一问题的内在复杂性。