Different mammals, same structure: Co-occurrence structure across the Plio-Pleistocene transition

A central goal in ecology is investigating the impact of major
perturbations, such as invasion, on the structure of biological
communities. One promising line of inquiry is using co-occurrence analyses
to examine how species’ traits mediate co-existence and how major
ecological, climatic, and environmental disturbances can affect this
relationship, and underlying mechanisms. However, present communities are
heavily influenced by anthropogenic behaviors and may exhibit greater or
lesser resistance to invasion than communities that existed before human
arrival. Therefore, to disentangle the impact of individual disturbances
on mammal communities, it is important to examine community dynamics prior
to humans. Here, we use the North American fossil record to evaluate the
co-occurrence structure of mammals across the Great American Biotic
Interchange. We compiled 126 paleocommunities from the late Pliocene
(4–2.5 Ma) and early Pleistocene (2.5–1 Ma). Genus-level co-occurrence was
calculated to identify significantly aggregated (co-occur more than
expected) and segregated (co-occur less than expected) genus pairs. A
functional diversity analysis was used to calculate functional distance
between genus pairs to evaluate the relationship between pair association
strength and functional role. We found that the strength distribution of
aggregating and segregating genus pairs does not significantly change from
the late Pliocene to the early Pleistocene even with different mammals
forming the pairs, including immigrant mammals from South America.
However, we did find that significant pairs, aggregations, and
segregations, became more similar in their functional roles following the
Plio-Pleistocene transition. Due to different mammals and ecological roles
forming significant associations and the stability of co-occurrence
structure across this interval, our study suggests that mammals have
fundamental ways of assembling that may have been altered by humans in the
present.

生态学的核心目标之一，是探究诸如生物入侵（invasion）等重大扰动对生物群落结构的影响。一条颇具前景的研究路径，是借助共现分析（co-occurrence analyses），检验物种性状如何介导物种共存，以及重大生态、气候与环境扰动会如何影响这一关系及其背后的潜在机制。然而，当前的生物群落深受人类活动（anthropogenic behaviors）的影响，相较于人类定居前存在的群落，其对生物入侵的抵抗力或强或弱。因此，为厘清各类单一扰动对哺乳动物群落的影响，研究人类活动前的群落动态至关重要。本研究依托北美化石记录，对大美洲生物交流（Great American Biotic Interchange）期间的哺乳动物共现结构展开评估。我们整合了上新世晚期（4.0–2.5 Ma）与更新世早期（2.5–1 Ma）的126个古群落（paleocommunities）数据。通过计算属级（Genus-level）共现情况，识别出显著聚集（aggregated，共现频率高于预期）与显著分离（segregated，共现频率低于预期）的属对。我们采用功能多样性分析（functional diversity analysis），计算属对间的功能距离（functional distance），以评估配对关联强度（pair association strength）与功能角色（functional role）之间的关系。研究发现，即便组成属对的哺乳动物类群（包括来自南美洲的移入哺乳动物）存在差异，聚集型与分离型属对的强度分布从上新世晚期至更新世早期均未发生显著变化。不过，我们发现在上新世-更新世过渡期（Plio-Pleistocene transition）之后，显著属对、聚集组与分离组在功能角色上的相似性有所提升。鉴于本研究区间内形成显著关联的哺乳动物类群与生态功能存在差异，且共现结构在此期间保持稳定，本研究表明哺乳动物群落存在固有的组装方式，而现代人类活动或已改变了这一方式。