Habitat corridors alter relative trophic position of fire ants

Habitat fragmentation disrupts species movement, leading to local extinctions and altered community structure. Habitat corridors, which connect isolated patches of habitat and facilitate movement between patches, provide a potential solution to these negative impacts. However, most studies to date have examined the movement of species alone without considering emergent effects on the community (e.g., altered trophic structure). We use large-scale, experimental landscapes and nitrogen stable isotopes ratios (delta-N-15) of a common generalist consumer (the fire ant, Solenopsis invicta) to determine how corridors affect trophic structure. Thus, because the fire ant is a species whose trophic position is flexible and whose diet typically reflects local prey availability, we assume that shifts in fire ants' trophic position between connected and isolated patches are likely to reflect shifts in patch trophic structure. We found that colonies in isolated patches had lower means and ranges of delta-N-15 than colonies in otherwise similar connected patches, suggesting that corridors may increase fire ants' trophic position and breadth, respectively. Previous work in our landscapes documented higher species richness of plants in connected than unconnected patches. Patch means of ant delta-N-15 were positively correlated with plant richness, suggesting that increased plant richness may influence the observed responses in fire ant delta-N-15. Together these results suggest that fragmentation may reduce trophic position and narrow trophic breadth of dietary generalists such as the fire ant. These shifts likely reflect an alteration of food webs in isolated patches. Our results suggest that corridors may be effective in preventing or reducing such alterations.生境破碎化（Habitat fragmentation）会干扰物种移动，进而引发局部灭绝与群落结构改变。连接孤立生境斑块并促进斑块间物种移动的生境廊道（Habitat corridors），为缓解此类负面影响提供了可行解决方案。然而，迄今为止绝大多数相关研究仅聚焦于单一物种的移动行为，未考虑其对群落的涌现效应（emergent effects）——例如营养结构（trophic structure）的改变。本研究依托大规模实验景观，以及常见广食性消费者红火蚁（Solenopsis invicta）的氮稳定同位素比值（delta-N-15，即δ¹⁵N），探究生境廊道对群落营养结构的影响。鉴于红火蚁的营养级具有灵活性，且其饮食通常能够反映当地猎物的可获得性，我们认为连通斑块与孤立斑块间红火蚁营养级的变化，可用于反映对应生境斑块的营养结构变迁。研究发现，与其他条件一致的连通斑块中的蚁群相比，孤立斑块内的蚁群其δ¹⁵N均值与取值范围均更低，这表明生境廊道或可分别提升红火蚁的营养级与营养广度。此前在本实验景观中开展的研究已证实，连通斑块的植物物种丰富度高于未连通斑块。蚁群δ¹⁵N的斑块均值与植物丰富度呈正相关，提示植物丰富度提升或可解释红火蚁δ¹⁵N的观测变化。综合上述结果，生境破碎化或会降低红火蚁这类广食性消费者的营养级并压缩其营养广度，此类变化大概率反映了孤立斑块内食物网的改变。本研究结果显示，生境廊道或可有效阻止或缓解此类食物网结构改变。