Proportion Curves Research Articles

Alien plants in Chile: inferring invasion periods from herbarium records

We used 71,764 specimens (14,988 alien and 56,776 native) from the herbarium CONC at Universidad de Concepcion, Chile to identify alien invasion periods. We assumed that the pattern of accumulation of specimens can be used for tracing back the distribution in time of alien species introductions in the Chilean territory. To assess this we constructed Invasion Curves (IC) of native and alien species and specimens recorded in the complete territory and we adapted this methodology to draw Proportion Curves (PC). Increments in the proportion of alien vs. native species can be interpreted as expansions in population size of alien species, either locally or by invasion of new areas. To visualize surface expansions consistent with changes in PC we arranged four maps broadly coincident with inflexions in PC: before 1900, 1940, 1980 and 2004. Invasion curves from both native and alien species produced a first step of positive and rapid increment followed by an extended, apparently stable phase. The first expansion phase of alien flora (1910–1940) coincides with a first period of strong growth of Chilean agriculture as indicated by increments in wheat and other cereals production. A more recent second maximum showed by PC (approximately between 1980 and 2000) occurs in a period when: (i) wheat surface goes down but (ii) wheat production increases, and (iii) forestry exports increases. These changes are coincident with increased mechanization making possible more wheat production in fewer surfaces. The expansions of alien plant species in Chile are evident on geographical distribution maps. In only one century alien species expanded to nearly all the territory. Both the North and South extremes however, seem to be an exception to this general trend as shown by the gaps on maps.