Why does understanding lag phases help to decipher patterns in plant invasions? 

In the realm of global environmental change, biological invasions stand out as a significant component, wielding considerable environmental and socioeconomic effects, reshaping ecosystems and altering nature and nature's contribution to human livelihoods, with significant economic impacts.  A critical issue is the increasing influx of alien species into various global regions, raising questions about their potential future impact as invaders. Intriguingly, some of these species undergo a 'lag phase' after introduction, a period of dormancy where they occupy a limited area for years, or even centuries, before experiencing rapid expansion. Neglecting to consider the lag times of these 'sleeper' species could significantly underestimate their impact on risk assessments while ignoring a time window for management.  Understanding the frequency and scale of these lag phases is vital for better comprehending invasion timelines and effective species management.

The causes of these prolonged lags are diverse and interconnected, encompassing intrinsic eco-evolutionary processes and extrinsic environmental conditions. Factors like long generations, the need to adapt to new environments, or arrival in marginally suitable habitats could contribute to these lags. Even without long-distance dispersal, environmental shifts such as changes in land management, fire frequency, mutualists or climate can disrupt this phase. However, attributing a specific cause remains challenging due to the complexity of interacting factors.

Should we link an unexplored pattern to a much-debated one?

While niche theory and its applicability to the modelling of species distributions is under debate, it provides a valuable framework for exploring ecological phenomena. If we navigate the assumptions properly, the niche concepts help us better understand invasion dynamics. In an abiotic context, climate significantly dictates the geographic limits of species, although other factors like soil and land use also play roles. Yet, niche theory can also shed light on the role of climate in population dynamics within invaded regions. For species displaying biphasic population dynamics, their climatic existence in the invaded range can be dissected into pre- and post-lag phases, offering insights into whether their climatic range has changed post-introduction.

With more than one-third of all assessed species being recorded with an initial stagnant population growth rate, we wonder what other bi-or multiphasic scenarios are common. A fraction of the non-lagging species may remain dormant or have been ‘promoted’ to impactful invaders in the public spotlight. Also, virtual species are an excellent toy in quantitative ecology for assessing how patterns, such as the lag phase, can be reproduced parametrically. This is particularly useful in cases where the underlying biodiversity data spans centuries and inherits aggregated spatial and temporal biases. Those simulations rely on several assumptions (dispersal mechanism, reproduction traits), simplifying nature to isolate a pattern of interest despite gaps. 

Where to next?

Using climate matching as the underpinning mechanism of correlative distribution models, we more or less explicitly assume that those abiotic tolerances are conserved over space and time. In addition to the heated debate around this, climate change provides novel combinations and opportunities for invaders. So, if absolute tolerances may be static, exposure to previously unavailable combinations may increase a population’s growth rate and invasion or at least dispersal success. Increased frequency and intensity of extreme events accompany climate change, potentially aiding dispersal, changing disturbance regimes, and generating novel climate combinations.

So, every time we set out to explore invasion patterns or processes, will we return home with the message of context-dependency and idiosyncratic evidence?

* The title was borrowed from Cousens R. D. (2023): Effective ecology. In: Cousens R. D. (ed.) Effective ecology: seeking success in a hard science. CRC Press, Taylor & Francis, Milton Park, United Kingdom, pp. 173-182. Doi:10.1201/9781003314332-10

Photo by Jan Huber