Lag phase

This article is about biological invasions. For the lag phase of a bacterial culture, see Bacterial growth. For the slow years before a boom, see Business cycle.

In invasion biology, a lag phase is the period between the arrival of a species in a new region and the start of its rapid spread. An introduced plant may persist for decades in gardens, or at a handful of sites in the wild, before it explodes.¹ Some delay is inevitable, since a population that grows exponentially looks negligible at first. But many lags last far longer than growth alone would explain, and end only when something changes, either in the invader or in its surroundings.¹

Lags are common but not universal: of 63 alien plants studied in the Czech Republic, about a third showed one.² Giant hogweed is among the best-documented examples, although how long its lag lasted, and when it ended, depend on how it is measured.³

  area
  occupied
     ^
     |                                   ...----'''''  bend: nowhere
     |                              ..-''                 left to go
     |                           .-'
     |                         .'
     |                       .'      exponential spread:
     |                      /        the area doubles
     |                    .'         every 9-14 years 
     |                  .'
     |              ..-'
     |.........---''
     |   lag: decades in gardens
     |   and a few wild sites
     +----+-------------+------------------------------->
       arrival       the lag ends                   time
                     (the landscape changes)

Giant hogweed

Arrival

Giant hogweed appears on the seed list of the Royal Botanic Gardens, Kew, in 1817, and was first recorded in the wild in 1828. Of nineteen European countries with historical records, fourteen had their first before 1900.⁴ The spread across the continent was slow: half of the countries eventually invaded had been reached only after 62 years. Within the Czech Republic the same point was reached in 16 years, and at single sites in 22.⁵

The date of its arrival in the Czech lands has itself been revised. For decades the plant was said to have been planted in the castle garden of Lázně Kynžvart in 1862. In 2020 the record was shown to be a misreading: the plant listed in an 1862 flora of Marienbad was a form of the native common hogweed. The earliest known specimen is now one collected at the spa of Teplice in 1871 by Lajos Haynald, Archbishop of Kalocsa, and the earliest escaped plant was found near Mariánské Lázně in 1877.⁶

A story told in Czech and German sources has Tsar Alexander I present the seeds to Prince Metternich, the owner of Kynžvart, at the Congress of Vienna in 1815, in one of the malachite vases still kept at the castle. No primary source for it has been found, and the castellan knew of none. Tourist guides that name Alexander II are certainly wrong: he became tsar in 1855, four years before Metternich died.⁶

Length of the lag

Estimates of the Czech lag vary with the method:

• a lag of several decades before exponential spread;⁷
• an 80-year lag ending in the 1940s, a figure that counts from the now-discredited date of 1862;⁸
• 60 to 70 years from the first record outside cultivation to the start of exponential spread, in 1936 or 1943 depending on the data;³
• a rapid rise in the number of occupied grid cells only after the 1970s.⁵

In Germany the first escapes date from the middle of the nineteenth century, and exponential spread from about the 1960s.⁹

What ended the lag seems to have been the landscape rather than the plant. European and Caucasian populations differ little genetically, and the authors of the Czech studies conclude that adaptation to a new region was not the bottleneck; the spread began in the second half of the twentieth century, with changes in the landscape as the main trigger.³the plant did not change; the conditions did

Rate of spread

Once under way, the invasion proceeded at a steady exponential rate. Between 1971 and 1995 the number of grid squares occupied by giant hogweed in the Czech Republic doubled every 9.2 years, an average rate among the country’s alien plants.³² Other analyses found doubling times of 13 to 14 years for the number of localities, the number of squares and the invaded area at single sites alike: the plant spread across the country at the same rate as within a single valley.³ A doubling every 13 to 14 years is equivalent to compound growth of about 5 per cent a year.

At the local scale, aerial photographs of ten sites in western Bohemia taken between 1947 and 2000 show the plant advancing on average by 10.8 m, and occupying another 1,261 m², each year. Pastures and fields held 84.7 per cent of its cover.¹⁰ From their first record to their greatest extent, the stands grew on average about 27-fold.³

As it spread, the plant changed its habitat. It left the parks and gardens first for rivers and for linear habitats such as roads and railways, in the 1930s and 1940s, and from the 1970s moved into towns. By the mid-1990s urban and linear sites accounted for nearly three-fifths of its records.³ It also moved downhill: the share of localities above 600 m fell from 28.5 per cent in the early 1970s to 14.7 per cent in 1990.³

Residence time and propagule pressure

Two quantities help to predict whether an introduced species will spread. Residence time is the time since its introduction: the longer a plant has been present, the more widespread it tends to be. In the alien floras of the Azores, the Czech Republic, the Hawaiian Islands and New Zealand, the minimum residence time of a species explained between 4 and 40 per cent of the variation in its range, and the effect can still be detected in plants introduced thousands of years ago.¹¹ Propagule pressure is the number of individuals introduced and the number of occasions on which they arrive.¹²

In giant hogweed both were high. The plant was spread across Europe by the exchange of seed among botanic and private gardens, and within countries from garden to garden.¹³ In twentieth-century Germany it was also propagated as a bee plant.¹⁴ In its native range, the more distant two populations are, the more different they are genetically; in Europe there is no such relationship, the mark of a plant moved about by people.¹⁵ Simulations fitted to fifty years of aerial photographs show that between 0.1 and 7.5 per cent of the seed must travel beyond the usual few metres to explain the spread that was observed.¹⁶

Invasion debt

Many of the most troublesome alien species in Europe arrived decades ago. Across ten groups of organisms in 28 European countries, the number of alien species established today is more closely related to indicators of socioeconomic activity in 1900 than to those of 2000. The authors of the study call this an invasion debt: the consequences of the present high levels of economic activity will probably not be fully realised for several decades.¹⁷the bill arrives later

For management, the lesson of the lag phase is caution. Because invasions can stall for decades and then accelerate, it has been argued that any invader should be assumed capable of harm, and that long periods of seemingly consistent behaviour are poor predictors of what it will do next.¹past performance is no guarantee of future results

See also

• Giant hogweed § History of introduction
• Enemy release hypothesis
• Seed bank
• Sosnowsky’s hogweed