A 67-study synthesis finds climate change's effects on marine invaders vary by invasion stage

The Puerto Rico case is an observation, and the authors treat it as one. Writing in phys.org, they say climate change may not have caused Hurricane Maria ^[7]. What the storm did was clear space. Sand and mud buried the native shallow meadows ^[3], and an invader with broad tolerance to a range of environmental factors ^[14] grew back into the opening and reached dominance in some areas ^[5]. The article does not report recovery rates for the natives or for the invader, so it cannot say which of them regrows faster on the same sediment.

The measurement is in the rest of the paper: 67 studies across 30 marine invasive species ^[8], an average of about 2.2 studies per species ^[20]. With that much evidence per species, a site-specific effect size is out of reach for most of the 30. What the synthesis reports is the direction of the effect and the finding that its strength varied with species, environmental conditions and study design ^[9].

One candidate the authors name for that variation is duration. Short-term experiments miss long-term processes such as population growth, which observational studies and models are more likely to capture ^[12]. Some of the disagreement in the literature is therefore a property of study design. The authors call for longer-term experiments, sustained field monitoring and models that project longer-term changes ^[13].

The stage result is the one that bears on planning. Climate change sometimes increases invader abundance, sometimes magnifies establishment success, and rarely promotes spread ^[10]. Their explanation is that the stages come apart: an invader can grow more abundant without becoming established, if conditions remain suboptimal for reproduction ^[11].

The one extreme event in the article carrying an attribution number is the Northeast Pacific marine heat wave, not the hurricane. It persisted almost 1,000 days from 2019 to 2021, averaged about 1.6 C above normal, and reached local peaks of nearly 5.8 C ^[15]. An event of that duration and intensity would have had less than a 1% chance of occurring without elevated greenhouse gas concentrations ^[16]. Almost 1,000 days is roughly two years and nine months ^[21].

There is a precedent in the same paper for management choosing a target and getting a different ecosystem. Smooth cordgrass was introduced deliberately to China in 1979 to stabilize shorelines, and its rapid establishment and spread eventually reconfigured unvegetated tidal mudflats into densely vegetated salt marshes ^[18].