Paul M. South

Non-native Seaweeds Drive Changes in Marine Coastal Communities Around the World

We conducted a bibliographic survey, adding 69 taxa to a published list of 277 seaweeds, thereby updating the total worldwide list of non-native and cryptogenic seaweeds to 346. Polysiphonia Greville and Hypnea J.V. Lamouroux species were the most common taxa on this list, and the Mediterranean Sea and the NE Atlantic bioregions have received most of the 346 taxa. The most important vectors that carry non-native seaweeds are hull fouling and the transport of aquaculture products including ‘blind passengers’. Once a seaweed has arrived in a new location, it can establish a permanent population and spread through natural dispersal or human activity. Non-native seaweeds have negative impacts on native species through competition, habitat destruction and keystone competition, but also positive impacts through habitat formation, food provision and cascading habitat formation. Quantitative meta-analyses have shown that invasive seaweeds typically have a negative effect on local plants, but neutral or positive effects on animal communities. New meta-analyses presented here indicate that impacts increase with the abundance of non-native seaweeds and that non-native seaweeds may increase sample similarity in invaded plant communities, but not in animal communities. The literature on the impact of non-native seaweeds is extensive, but most studies have focused on a few high-profile species. Comprehensive analyses should be done for more species to allow for better predictions. We conclude that non-native seaweeds have altered shallow coastal communities in most biogeographical regions, and impacts will likely increase along with increases in human populations, transport and associated stressors.

Transient effects of an invasive kelp on the community structure and primary productivity of an intertidal assemblage

Invasive species can have significant impacts on the diversity and productivity of recipient ecological communities. The kelp Undaria pinnatifida (Harvey) Suringar is one of the worlds most successful invasive species but, although itspurported impactsarestrong, thereis littleempirical evidence that itdisplacesnative species.Furthermore, as thisspeciesnaturalisesinlocalcommunities,itspotentialeffectsoncommunitydynamicshavenotbeenwelltested.Here, we test the ecological impacts of Undaria in intertidal communities in southern New Zealand using a combination of surveys, a 2.5-year press-removal experiment and in situ measures of net primary production to gauge its impact on community structure and productivity. Undaria had transient effects on the composition of communities, affecting two seasonally abundant species in 1 year, but these impacts did not persist into the following year. Overall, there were only small effects of Undaria removal on diversity and abundance of native algae and invertebrates at two sites. However, the presence of Undaria more than doubled net primary production of recipient communities during its annual peak abundance when it increased biomass by 606 g DW m � 2 . We conclude that the invasion of Undaria represents an additionalandsubstantialcarbonsubsidytocoastalecosystemswithpotentiallypositiveeffectsonnearshoreproductivity. Additional keywords: intertidal biodiversity, net primary production, NPP, Undaria pinnatifida.

To include or not to include (the invader in community analyses)? That is the question

An increasing number of studies report impacts from invasive species on community metrics or ecosystem functions. We draw attention to an issue arising whenever impact is measured on a community where the invader is an integrated part: should or shouldn’t the attributes of the invader itself be included in the data-analysis? We identify many examples from the published literature showing inconsistency in whether or not data for the invader is included or excluded, and discuss potential implications for ecological interpretations. We also provide a case study to show that the invasive seaweed Undaria pinnatifida can be interpreted to have strong or no impact on seaweed communities, depending on its inclusion or exclusion in the data analysis. We conclude that it is critical for studies to (1) clearly state in the methods section, if the invaders are included or excluded from the data-analysis, (2) acknowledge potential differences in outcomes when comparing results based on different methods, and (3) analyze, if possible, impacts both with and without the invader. Finally, we note that this ‘inclusion versus exclusion’ conundrum is not only relevant to invasion biology, but to any field where the test-object of interest can be an integrated part of the response, such as when impact of seaweed blooms are analysed on community productivity or community effects are quantified over time from ecological pulse-perturbation experiments.

A review of three decades of research on the invasive kelp Undaria pinnatifida in Australasia: An assessment of its success, impacts and status as one of the world's worst invaders

Marine invasive macroalgae can have severe local-scale impacts on ecological communities. The kelp Undaria pinnatifida is one of the most successful marine invasive species worldwide, and is widely regarded as one of the worst. Here, we review research on Undaria in Australasia, where the kelp is established throughout much of New Zealand and south-eastern Australia. The presence of Undaria for at least three decades in these locations makes Australasia one of the longest-invaded bioregions globally, and a valuable case study for considering Undarias invasion success and associated impacts. In Australasia, Undaria has primarily invaded open spaces, turf communities, and gaps in native canopies within a relatively narrow elevation band on rocky shores. Despite its high biomass, Undaria has relatively few direct impacts on native species, and can increase community-wide attributes such as primary productivity and the provision of biogenic habitat. Therefore, Australasian Undaria research provides an example of a decoupling between the success and impact of an invasive species. Undaria will most likely continue to spread along thousands of kilometres of rocky coastline in temperate Australasia, due to its tolerance to large variations in temperature, ability to exploit disturbances to local communities, and the continued transfer among regions via vessel movements and aquaculture activities. However, the spread of Undaria remains difficult to manage as eradication is challenging and seldom successful. Therefore, understanding potential invasion pathways, maintaining native canopy-forming species that limit Undaria success, and effectively managing anthropogenic vectors of Undaria spread, should be key management priorities.

A host-specific habitat former controls biodiversity across ecological transitions in a rocky intertidal facilitation cascade

Few studies have quantified facilitation cascades from rocky intertidal systems, across ecological transition zones, or where the secondary facilitator is an obligate epiphyte. Here, we address these research gaps. We first quantified distributions of the seaweed host Hormosira banksii and its obligate epiphyte Notheia anomala at different tidal elevations in summer and winter at Kaikoura, New Zealand. This analysis showed that the host and the epiphyte were, in both seasons, most abundant at ‘high’ and ‘low’ tidal elevations respectively, probably driven by contrasting responses to competitors and desiccation. We subsequently quantified richness and abundances of mobile invertebrates associated with Hormosira and various levels of epiphytic Notheia. Hormosira fronds were collected from different elevations, seasons, diurnal cycles, with different neighbouring algae and following an epiphyte-removal experiment. All tests showed positive density-dependent effects of Notheia-epiphytism on richness and abundances of invertebrates, with strongest facilitation occurring at the transition from intertidal to subtidal habitats. Our results support a growing number of facilitation cascade studies from different ecosystems and habitats, and suggest that habitat formation–driven facilitation cascades may be particularly common in marine benthic systems where epibiosis can be a dominant life form.

A sixth-level habitat cascade increases biodiversity in an intertidal estuary

Abstract Many studies have documented habitat cascades where two co‐occurring habitat‐forming species control biodiversity. However, more than two habitat‐formers could theoretically co‐occur. We here documented a sixth‐level habitat cascade from the Avon‐Heathcote Estuary, New Zealand, by correlating counts of attached inhabitants to the size and accumulated biomass of their biogenic hosts. These data revealed predictable sequences of habitat‐formation (=attachment space). First, the bivalve Austrovenus provided habitat for green seaweeds (Ulva) that provided habitat for trochid snails in a typical estuarine habitat cascade. However, the trochids also provided habitat for the nonnative bryozoan Conopeum that provided habitat for the red seaweed Gigartina that provided habitat for more trochids, thereby resetting the sequence of the habitat cascade, theoretically in perpetuity. Austrovenus is here the basal habitat‐former that controls this “long” cascade. The strength of facilitation increased with seaweed frond size, accumulated seaweed biomass, accumulated shell biomass but less with shell size. We also found that Ulva attached to all habitat‐formers, trochids attached to Ulva and Gigartina, and Conopeum and Gigartina predominately attached to trochids. These “affinities” for different habitat‐forming species probably reflect species‐specific traits of juveniles and adults. Finally, manipulative experiments confirmed that the amount of seaweed and trochids was important and consistent regulators of the habitat cascade in different estuarine environments. We also interpreted this cascade as a habitat‐formation network that describes the likelihood of an inhabitant being found attached to a specific habitat‐former. We conclude that the strength of the cascade increased with the amount of higher‐order habitat‐formers, with differences in form and function between higher and lower‐order habitat‐formers, and with the affinity of inhabitants for higher‐order habitat‐formers. We suggest that long habitat cascades are common where species traits allow for physical attachment to other species, such as in marine benthic systems and old forest.

Secondary foundation species enhance biodiversity

It has long been recognized that primary foundation species (FS), such as trees and seagrasses, enhance biodiversity. Among the species facilitated are secondary FS, including mistletoes and epiphytes. Case studies have demonstrated that secondary FS can further modify habitat-associated organisms (‘inhabitants’), but their net effects remain unknown. Here we assess how inhabitants, globally, are affected by secondary FS. We extracted and calculated 2,187 abundance and 397 richness Hedges’ g effect sizes from 91 and 50 publications, respectively. A weighted meta-analysis revealed that secondary FS significantly enhanced the abundance and richness of inhabitants compared to the primary FS alone. This indirect facilitation arising through sequential habitat formation was consistent across environmental and experimental conditions. Complementary unweighted analyses on log response ratios revealed that the magnitude of these effects was similar to the global average strength of direct facilitation from primary foundation species and greater than the average strength of trophic cascades, a widely recognized type of indirect facilitation arising through sequential consumption. The finding that secondary FS enhance the abundance and richness of inhabitants has important implications for understanding the mechanisms that regulate biodiversity. Integrating secondary FS into conservation practice will improve our ability to protect biodiversity and ecosystem function.Secondary foundation species, such as epiphytes, form structurally complex habitats on primary foundation species. A meta-analysis shows that they significantly enhance the abundance and richness of inhabitants compared to primary foundation species alone.