Range-extending tropical herbivores increase diversity, intensity and extent of herbivory functions in temperate marine ecosystems

1. Climate change is modifying species distributions around the world,
forcing some species poleward, where they can alter trophic interactions.
Many tropical herbivorous fishes have successfully expanded their ranges
into temperate ecosystems, and while it is clear they drive increases in
herbivory rates in specific localities, little is known about how they
might affect the diversity of herbivory functions across large spatial
scales, considering their interaction with assemblages of native
herbivores in temperate habitats. 2. We assessed the spatial overlap and
habitat associations of native temperate and range-expanding tropical
herbivorous fishes in six sub-regions of south-western Australia to
determine how incursions of tropical species may have affected the
diversity, redundancy (index of uniqueness) and the ‘spatial extent’
(addition of functions in new areas) and ‘intensity’ (increasing density
of functional groups) of specific herbivory functions in recipient
ecosystems. 3. Tropical herbivores were more abundant in temperate
ecosystems, forming schools from 40 (parrotfish) to 200 (rabbitfish)
individuals strongly associated with seagrass meadows and reefs with high
cover of turf algae. Overlap with temperate herbivores was highest in the
northern sub-regions, forming unique assemblages, with no apparent species
displacements. The addition of tropical species increased functional
diversity and uniqueness (the complement of redundancy), introducing novel
herbivory functions to many locations. Seagrass browsing increased in
spatial extent (27%) and intensity (15 x), while seaweed browsing and
grazing increased in intensity by up to 2.5 x in regions with high
abundances of tropical herbivores. 4. Our results suggest that the
diversity, intensity and spatial extent of different herbivory functions
can change as tropical species with different habitat affinities,
behaviors and diets shift their distributions poleward. Changes in
functional redundancy are likely to be heterogeneous in space and might
not increase initially because the diversity of herbivory functions is
relatively low in some temperate marine ecosystems. However, there is the
potential for greater redundancy as further tropical species arrive, their
abundances increase and the spatial and functional overlap of communities
rises.

1. 气候变化正在改变全球物种分布，迫使部分物种向两极迁移，在此过程中它们可能改变营养相互作用（trophic interactions）。许多热带草食性鱼类已成功将活动范围扩展至温带生态系统。尽管已有研究明确它们在特定区域提高了草食作用速率，但考虑到它们与温带栖息地本地草食动物群落的相互作用，其如何在大空间尺度上影响草食功能多样性仍知之甚少。2. 我们评估了澳大利亚西南部六个子区域中本地温带草食性鱼类与范围扩展的热带草食性鱼类的空间重叠及栖息地关联，以确定热带物种的入侵可能如何影响受体生态系统中特定草食功能的多样性、冗余度（独特性指数，index of uniqueness）、“空间范围”（在新区域增加功能）和“强度”（功能群密度增加）。3. 热带草食性鱼类在温带生态系统中更为丰富，形成由40条（鹦嘴鱼）至200条（兔鱼）个体组成的鱼群，且与海草床及草皮藻（turf algae）覆盖率高的礁石密切相关。它们与温带草食性鱼类的重叠度在北部子区域最高，形成独特群落，未观察到明显的物种替代现象。热带物种的加入提高了功能多样性与独特性（冗余度的互补面），并向许多区域引入了新的草食功能。海草啃食的空间范围增加了27%，强度提高了15倍；而在热带草食性鱼类丰度高的区域，海藻啃食与牧食（grazing）强度最高提升了2.5倍。4. 我们的结果表明，随着具有不同栖息地偏好、行为模式及食性的热带物种向两极迁移，不同草食功能的多样性、强度及空间范围可能发生变化。功能冗余度的变化在空间上可能具有异质性，且初期未必增加——这是因为部分温带海洋生态系统的草食功能多样性相对较低。然而，随着更多热带物种抵达、其丰度增加以及群落空间与功能重叠度上升，未来可能出现更高的冗余度。