New research from the Centre for Geometric Biology has shown that artificial structures can provide windows for invasion through the mediation of water flow. Lowering water flow increases the prevalence of low oxygen conditions meaning that there is likely to be a higher proportion of habitats that are hostile to native species while invasive species are able to function normally.
Piers and marinas tend to be hot spots for invasive species in marine systems. There have been several hypotheses proposed to explain why artificial structures seem to increase the likelihood of invasion; there may be more propagules from invasive species available to settle on these surfaces due to boat traffic, higher pollution in these areas may facilitate invasion by pollution tolerant species and artificial structures also reduce wave action and water flow.
Marcelo Lagos was particularly interested in how a reduction in water flow might mediate the establishment of invasive species around artificial structures such as piers and marinas and investigated this as part of his PhD research.
Water flow is of direct importance to sessile (immobile) marine invertebrates as a delivery mechanism of both food and oxygen. Where flows are low, oxygen levels can be depleted to concentrations below the physiological tolerance of some members of the community.
Marcelo and colleague Diego Barneche and PhD supervisors, Craig White and Dustin Marshall, were interested in testing whether invasive species are able to maintain higher levels of aerobic metabolism under lower oxygen conditions. If so, then this would give invasive species a competitive advantage over native species facilitating invasion in low flow environments.
In order to explore this question Marcelo had first to measure oxygen availability at a scale that was relevant to the organisms, in this case just millimetres above the organisms themselves. He used fibre optic sensors to do this and, in contrast, used a much older (but still reliable) approach to measuring flow speeds – tracking milk released among the sessile community.
Marcelo then collected a range of native and invasive species in order to estimate tolerance to low oxygen conditions in the lab. He found that invasive species could tolerate significantly lower oxygen levels than native species overall (a) and also for species that had a similar growth form (c), while flat species could tolerate lower oxygen levels than erect species (b).
They recommend that design of artificial structures consider mechanisms to maintain flow rates that will ensure replenishment of oxygen at the relevant scale as this might promote the proliferation of native species and so discourage invasion.