Data from: The influence of herbivores on primary producers can vary spatially and interact with disturbance

Identifying the major drivers of ecosystem change remains a central area
of ecological research. Although top–down drivers of change have received
particular focus, we still have little understanding of how consistently
these factors control an ecosystem's shift in both directions,
between different ecosystem states. Using a crossed experiment in a
shallow embayment in southeastern Australia, we investigated the roles of
disturbance (kelp removal) and sea urchin herbivory (via increased
density) to determine their contributions to shifts away from a
kelp-dominated community. In a second experiment, done in urchin barren
areas at two sites, we tested whether reductions in ambient sea urchin
densities allowed an algal shift in the reverse direction. In both
experiments, we observed that high densities of sea urchins could
negatively influence kelp and macroalgal abundance. However, in the kelp
bed, a moderate or severe disturbance resulted in a comparable algal
response, irrespective of urchin density. The influence of sea urchins
also varied dramatically between the two urchin barren sites. Here,
reducing urchin densities resulted in algal recovery at one site, but at
the other site, substantial colonisation of barren areas by canopy-forming
brown algae and Ulvales occurred across all (low, medium, and high) urchin
density treatments. Our findings illustrate multiple pathways of urchin
barren creation and algal recovery, and reveal that shifts both to and
from an urchin barren state can occur irrespective of herbivore pressure.
These alternate pathways can operate over short spatial distances or with
different regimes of disturbance.

厘清生态系统变化的核心驱动因子，始终是生态学研究的核心议题之一。尽管下行控制驱动因子（top-down driver）已受到学界重点关注，但我们仍难以明晰这些因子能否稳定驱动生态系统在不同状态间实现双向转换。本研究依托澳大利亚东南部一处浅海湾开展交叉设计实验，探究了干扰（移除海带）与海胆牧食（通过提升海胆密度）的作用，以明确二者对海带主导群落向外转换的贡献。在第二项实验中，我们于两处海胆荒（urchin barren）区域开展研究，以验证降低原位海胆密度是否可促使藻类群落发生逆向转换。两项实验均观测到，高密度海胆会对海带及大型藻类的丰度产生负面影响。但在海带床中，无论海胆密度如何，中度或重度干扰均可引发相似的藻类群落响应。海胆的影响在两处海胆荒区域间也存在显著差异：其中一处区域在降低海胆密度后实现了藻类恢复，但另一处区域中，无论海胆密度处于低、中、高何种处理水平，冠层型褐藻与石莼目（Ulvales）藻类均已大量定植于荒地区域。本研究结果揭示了海胆荒形成与藻类恢复的多条潜在路径，并表明无论牧食压力如何，生态系统均可向海胆荒状态转换或从中脱离。这些差异化的群落转换路径可在短空间尺度内存在，或伴随不同的干扰制度发生。