Many studies of invasive exotic/alien plants have suggested that a "lag" time is a common feature in their population dynamics. Lag times are typically defined as a period of several years to several decades between the introduction and establishment of an exotic and its period of rapid geographic range expansion. Some exotic plant introductions may flourish and occasionally reproduce in an area, but do not form self-replacing populations in the absence of continued reintroductions- these are referred to as “casual” exotic plants by Richardson et al. (2000). Some exotic plant introductions reproduce consistently and sustain populations over many life cycles but do not expand from their site of introduction- these are “naturalized.” Then there are the very small proportion of “invasive” exotic plants that produce large numbers of reproductive offspring that establish and produce their own reproductive offspring at considerable distances from source populations. In a sense, naturalized populations might be considered ticking time bombs, just waiting to break their lag.
Why do we care?
The answer is simple- different stages of invasion have dramatically different economic and ecological effects. Consider the figure below, where we quickly go from a benign introduction that seems to be minding its own business to a full-fledged invasion with exponentially decreasing management options and exponentially increasing costs of management. More accurately, options are quickly reduced to triage, and for some unfortunate situations that involve “transformers” (Richardson et al. 2000), acceptance of the new paradigm or ecological system that has been created. The times involved are all relative of course, but the bottom line is that by the time plant invasions become noticed by the public or policy makers, in most cases the proverbial horse has left the barn and managers are left with diminishingly effective options. When established populations are limited in space complete eradication is still possible and compared to other options, cheap.
What causes these populations to “break lag?”
Multiple potential explanations exist for time lags, including the mathematical/ecological explanation that the population has undergone a bottleneck and is in the lag phase of an exponential growth curve (Sakai 2001). If individuals are few and isolated, more time is required for the population to reproduce and spread than if individuals were abundant. There are also evolutionary implications, and lag time could represent the time required to overcome genetic constraints including the evolution of invasive life history characteristics, the evolution of new adaptations to the new habitat, and the purging of inbreeding depression (Ellstrand and Schierenbeck 2000).
Examples include Pinus strobus in central Europe, which was not noticed to be invasive until 250 years after introduction for forestry (Rejmanek 1996). Acacia nilotica became a problem in Australia sometime between the 1890s and 1950s. Melaleuca was introduced to Florida and Hawaii in the early 1900s, but only began to expand in the last 30-40 years. Bromus tectorum (Mack 1981), B. rubens (Salo 2005), and Pennisetum ciliare (Cox et al. 1988; Olsson et al. 2012)all exhibited significant lags. Kochia scoparia was introduced in Wyoming in the 1880s and didn't expand into other states until the 1950s.
Stochastic events like anomalous rainfall events or seasonal accumulations can influence plants to break their lag periods, as for B. rubens (Salo 2004). During El Niño winters with above average cool season precipitation, the southwestern US often sees a pulse in cool season exotic species (Bromus and Brassica for SE CA, among others), though most are unable to continue to hold the ground they've gained during years of ‘normal' precipitation. A series of warm, wet springs or summers may be required for warm season perennial species to increase, lending inertia to the population that allows it to break out of lag. Allee effects (population size limits growth rates), such as a minimum viable population size for viable pollination, may be required. Intentional or accidental repeated introductions through cultivation as a crop or the ornamental plant trade are likely sources of escape from the lag phase (beware exotic “xeriscape” ornamentals – these plants are pre-screened for adaptation to arid conditions and can sometimes be extremely invasive, especially when natives are stressed during drought conditions). Disturbance events such as fire (B. tectorum, P. ciliare)or flooding (Tamarix chinensis) are also causes. Threshold population levels might be required to facilitate a depression of native species growth and establishment, or increase mortality, opening up niches for the invaders.
So what's to be done?
First, don't be afraid to use exotics in you landscape, your garden, agriculture, or restoration- there are many situations where it makes sense to use these plants, and we'll talk about that in a future post. Just be aware that caution and supervision is necessary. There are now great screening tools out there like the Plant Risk Evaluation (PRE) tool for assessing the invasive potential of ornamentals (Conser et al. 2015). But be aware that we're now dealing with a backlog of potential invasive plants introduced before an awareness of this issue existed. Adding to the burden, climate change has opened up opportunities for potential invaders (Hellmann et al. 2008). The future of plant invasions will be interesting, and for anyone who works in the weed science, invasion ecology, restoration ecology, and related fields, we can all rest assured that we will have job security for a long, long time to come.
References
Conser C, Seebacher L, Fujino DW, Reichard S, and DiTomaso JM (2015) The Development of a Plant Risk Evaluation (PRE) Tool for Assessing the Invasive Potential of Ornamental Plants. Plos One 10
Cox JR, Martin MH, Ibarra FA, Fourie JH, Rethman NFG, and Wilcox DG (1988) The influence of climate and soils on the distribution of 4 african grasses. Journal of Range Management 41:127-139.
Ellstrand NC and Schierenbeck KA (2000) Hybridization as a stimulus for the evolution of invasiveness in plants? Proceedings of the National Academy of Sciences of the United States of America 97:7043-7050.
Hellmann JJ, Byers JE, Bierwagen BG, and Dukes JS (2008) Five potential consequences of climate change for invasive species. Conservation Biology 22:534-543.
Mack RN (1981) INVASION OF BROMUS-TECTORUM L INTO WESTERN NORTH-AMERICA - AN ECOLOGICAL CHRONICLE. Agro-Ecosystems 7:145-165.
Olsson AD, Betancourt JL, Crimmins MA, and Marsh SE (2012) Constancy of local spread rates for buffelgrass (Pennisetum ciliare L.) in the Arizona Upland of the Sonoran Desert. Journal of Arid Environments 87:136-143.
Richardson DM, Pysek P, Rejmanek M, Barbour MG, Panetta FD, and West CJ (2000) Naturalization and invasion of alien plants: Concepts and definitions. Diversity and Distributions 6:93-107.
Salo LF (2004) Population dynamics of red brome (Bromus madritensis subsp rubens): times for concern, opportunities for management. Journal of Arid Environments 57:291-296.
Salo LF (2005) Red brome (Bromus rubens subsp madritensis) in North America: possible modes for early introductions, subsequent spread. Biological Invasions 7:165-180.
Figure from: https://www.eddmaps.org/about/why_plants_invade.cfm