27 September 2013 | By Cang Hui

Environmental managers need to understand the factors that control community composition at different spatial and temporal scales to formulate appropriate guidelines for management. To this end, community assemblage rules explain how species are “packed” in a community and how community composition is related to the niche and traits of species. Early theories gave special attention to the role of competition and predation in structuring biotic communities. More recent biogeographical treatments emphasize the regional “top-down” control of unsaturated local communities, as portrayed in the influential “unified neutral theory of biodiversity and biogeography” proposed by Stephen Hubbell, where community assemblage patterns emerge from the interplay of ecologically identical species.

Collaboration between the C·I·B and researchers from the Czech Republic has resulted in an important contribution to understanding how communities are structured. They utilized a unique dataset on the distribution of more than 2000 vascular plant species in 300 nature reserves in the Czech Republic. These species were divided into natives, archaeophytes and neophytes according to their residence time in central Europe. Native vascular plants colonized the region after the last glaciation. Archaeophytes are species introduced to Europe through human activity between the initiation of agricultural activities during the Neolithic period (ca. BC 4000) and the European exploration of the Americas (ca. AD 1500). Neophytes are species introduced to Europe after AD 1500. This extraordinary data set offered the opportunity to amplify the signals of structural changes in regional assemblages that are often weak or unidentifiable in studies conducted over a short period.

Network illustration of neophytes
A network illustration of neophytes (colours indicate modules) and reserves (black dots)

The group built a co-distribution network for each assemblage in terms of species overlaps and turnovers and used a network analytic approach to identify highly connected nodes. Specifically, given a network with nodes connected by edges, this approach partitions nodes into non-overlapping clusters so that the number of within-cluster connections relative to random expectation is maximized; that is, similar nodes are likely connected in a network. From this network analysis, they identified four to six network clusters for each assemblage.

By comparing these network clusters in terms of species composition, phylogenetic structure and habitat characteristics, they tested the following two hypotheses on how biological invasions affect assemblage structures and how a community assemblage evolves.

First, native species should show stronger signals of matching between their habitat requirements and the characteristics of inhabited sites, indicating a “lock-and-key” relationship. In other words, species and sites in older assemblages are expected to belong to distinct functional clusters, with species matching the habitat within the cluster. In contrast, more recent introductions should have a poorer match as many species are initially randomly introduced to sub-optimal clusters of sites. This they named the settling-down hypothesis of diminishing effect of stochasticity with residence time.

Species in “neutral” communities are, by definition, considered ecologically identical, and thus species composition, evolutionary divergence and habitat characteristics of different clusters, if present, should be the result of purely stochastic factors. In contrast, species in niche-based assemblages have different functional roles, leading to modules with distinct taxonomic composition, evolutionary units and habitat characteristics, reflecting a deterministically and/or functionally driven species assemblage. The group argued that species coexistence can be achieved by being either ecologically identical or distinctive, forming niche-differentiated clusters that comprise species with rather similar niche within each cluster. With an increase in residence time, we should see a shift from an initially neutral or stochastic assemblage to a niche-based functional-driven multi-cluster assemblage. This second hypothesis was named the niche-mosaic hypothesis of inlaid neutral clusters in the regional species assemblage.

The intensity of compartmentalization in plants occurring in Czech reserves increased when moving from young to mature assemblages, supporting the settling-down hypothesis. Comparisons between clusters and assemblages revealed fingerprints of over- and under-representation for different plant families. For instance, more true grasses (Poaceae) were introduced before 1500 AD than expected from random draws, while fewer true grasses but more legumes (Fabaceae) were introduced after 1500 AD. Phylogenetic analysis and habitat comparisons gave consistent results, supporting the niche-mosaic hypothesis.

The lock-and-key relationship between species invasiveness and site invasibility identified in the above analysis paves the way for objective prioritization of sites with particular characteristics for management, based on factors that make them particularly susceptible to invasion by species with matching traits. Refined conservation plans should be designed for each functional cluster. For instance, daisies (Asteraceae) prefer reserves with cold winters, whereas legumes (Fabaceae), mustards (Brassicaceae) and mints (Lamiaceae) prefer the western parts of the country with warmer winters. Reserves with cold winters and older establishment seem to resist the invasion of archaeophytes and neophytes. Taken together, reserves should be clustered into groups, and each group should prioritize these potential invaders falling into the same group.

Future research at the C·I·B will seek to build on these results. Ways of using the network methodology for prioritizing management actions will be sought. Further testing of the niche-mosaic hypothesis as a framework for understanding species packing in novel ecosystems is planned for different taxonomic groups and at different spatial scales.

For more information contact, Cang Hui at chui@sun.ac.za

Read the paper Hui, C., Richardson, D.M., Pyšek, P., Le Roux, J.J., Kučera, T. & Jarošík, V. (2013) Increasing functional modularity with residence time in the co-distribution of native and introduced vascular plants. Nature Communications, 4:2454. DOI: 10.1038/ncomms3454.

Examples of natives, archaeophytes and neophytes in central Europe
Examples of natives, archaeophytes and neophytes in central Europe, as well as the four modules identified from species co-distribution networks (four red boxes). Natives (first row): Urtica dioica, Betula pendula, Sorbus aucuparia, Picea abies, Dactylis glomerata, Plantago lanceolata, Hieracium pilosella, Coronilla varia; archaeophytes (second row): Chelidonium majus, Linaria vulgaris, Chenopodium bonus-henricus, Tanacetum parthenium, Cirsium arvense, Plantago major, Arrhenatherum elatius, Echium vulgare; neophytes (third row): Impatiens parviflora, Sarothamnus scoparius, Epilobium ciliatum, Digitalis purpurea, Trifolium hybridum, Aesculus hippocastanum, Robinia pseudacacia, Pinus nigra.