Long transient response of vegetation dynamics after four millennia of anthropogenic impacts in an island ecosystem

Abstract

Agents of global change commonly have a higher impact on island ecosystem dynamics. In the Mediterranean region, those dynamics have historically been influenced by anthropogenic impacts, for example, the introduction of invasive species and overharvesting of resources. Here, we analysed the spatio-temporal dynamics of vegetation in sa Dragonera island, which experienced a large environmental change ca. 4000 years ago by the arrival of humans. Anthropogenic impacts, such as herbivory by goats and over-logging, ended in the 1970s, while in 2011 the site became the largest Mediterranean island where rats were eradicated. Invasive rats and goats played the ecological role of two endemic species, the cave goat and the giant dormouse, which inhabited the island for more than 5 million years and were rapidly extinct by humans. We used Landsat imagery to explore NDVI as a proxy of vegetation productivity over the years 1984-2021, orthophotos to assess changes in land and vegetation covers and historical plant inventories to study the dynamics in plant diversity. Results showed that those indicators steadily increased both in spring and in summer, while the noise around the trends was partially explained by climate variability. The regime shifts in the temporal dynamics of vegetation productivity suggested a transient from a perturbed to a non-perturbed stable state. Trends in successional dynamics, spatial self-organization and plant diversity also showed the same type of transient dynamics. Historical perturbations related to harvesting (mainly the synergies between goat browsing, burning and forest over-logging) were more important than rat eradication or the influence of climate to explain the vegetation dynamics. Our study shows the transient nature of this small island ecosystem after 4000 years of perturbations and its current path towards vegetation dynamics more controlled by ecological interactions lacking large herbivores and omnivores, drought dynamics and the carrying capacity of the island.

Keywords: harvesting; invasive species; island ecosystem; long-term anthropogenic impacts; transient dynamics.

FIGURE 1

(a) Schematic reconstruction of the ecosystem state in sa Dragonera since the establishment of the endemic fauna over the Pliocene, ca. 5.3 million years ago and anthropogenic impacts since the arrival of humans, ca. 4 millennia ago. Most palaeontological records come from the island of Mallorca, but due to its close proximity, we assumed that the endemic fauna was similar in sa Dragonera. Extinctions occurring in the small sa Dragonera were likely followed by colonizations from Mallorca, especially during glaciation events and flotsam events. Red arrows show extinctions, grey arrows refer to citations of impacts or their ending and black arrows show the four crucial events to interpret NDVI vegetation dynamics during 1984–2021 (blue arrow). Sparse old data on vegetation features were available for 1921 and 1956 (blue dots). Years correspond to approximate or specific dates recorded in archaeological data and historical and contemporary chronicles (more details are provided in Supplementary Material) (b) Schematic reconstruction of vegetation dynamics over the same temporal window representing the three stable states: herbivory regulated by predation before the arrival of humans (K ₁), harvesting and not regulated herbivory (K ₂), and without herbivory and harvesting (K ₃). Yellow lines represent the expected NDVI major transient dynamics.

FIGURE 2

(a) Vegetation cover types in the study area and locations of the study plots (90 × 90 m) used for vegetation spatial pattern analysis. The map in the upper left corner shows the location of the study area in the Balearic Islands (western Mediterranean). (b) Elevation (meters a.s.l.), (c) slope (%) and (d) orientation maps of sa Dragonera island, all three variables obtained from a 2 × 2 m resolution DEM. Map lines delineate study areas and do not necessarily depict accepted national boundaries.

FIGURE 3

Mean temperature (Tmean, in °C), insolation (INS, in hours/day), evapotranspiration (ETO, in mm), accumulated rainfall (RF, in mm), relative humidity (RH, in %) and drought index SPEI for the season of the year before (SPEI_12) for spring and summer in the study site. A loess smoothing function (black line with standard error shaded in yellow) is fitted to each time series, and the value of r ² for a linear model testing a temporal trend is also shown for each variable but SPEI_12 due to missing values.

FIGURE 4

Temporal dynamics of NDVI (1984–2021) in open shrublands at sa Dragonera (panels a and b for spring and summer respectively) and the control islet of es Pantaleu (panels c and d for spring and summer respectively). We show a loess smoothing function to each time series to ease visualization of the trends. Arrows point to the last year with the presence of rats (and mice and rabbits) in sa Dragonera.

FIGURE 5

Temporal dynamics of NDVI (1984–2021) for open shrublands in sa Dragonera (red lines, a and b for spring and summer respectively) and the control site in es Pantaleu (green lines, c and d for spring and summer respectively). Arrows point to the last year with the presence of rats (and mice and rabbits) in sa Dragonera. Dashed lines show the years when a regime shift occurred; r shows the rate of increase in each segment of the NDVI time series. Orange lines show the trend‐fitting NDVI time series using LandTrendr.

FIGURE 6

Dynamics of plant species richness in sa Dragonera during 1921–2021. Arrows point to the end of harvesting (timber logging and goat herbivory) and rat eradication. A loess regression line with standard errors is shown for the time series data since 1977.