Remote analysis of biological invasion and the impact of enemy release

Abstract

Escape from natural enemies is a widely held generalization for the success of exotic plants. We conducted a large-scale experiment in Hawaii (USA) to quantify impacts of ungulate removal on plant growth and performance, and to test whether elimination of an exotic generalist herbivore facilitated exotic success. Assessment of impacted and control sites before and after ungulate exclusion using airborne imaging spectroscopy and LiDAR, time series satellite observations, and ground-based field studies over nine years indicated that removal of generalist herbivores facilitated exotic success, but the abundance of native species was unchanged. Vegetation cover <1 m in height increased in ungulate-free areas from 48.7% +/- 1.5% to 74.3% +/- 1.8% over 8.4 years, corresponding to an annualized growth rate of lambda = 1.05 +/- 0.01 yr(-1) (median +/- SD). Most of the change was attributable to exotic plant species, which increased from 24.4% +/- 1.4% to 49.1% +/- 2.0%, (lambda = 1.08 +/- 0.01 yr(-1)). Native plants experienced no significant change in cover (23.0% +/- 1.3% to 24.2% +/- 1.8%, lambda = 1.01 +/- 0.01 yr(-1)). Time series of satellite phenology were indistinguishable between the treatment and a 3.0-km2 control site for four years prior to ungulate removal, but they diverged immediately following exclusion of ungulates. Comparison of monthly EVI means before and after ungulate exclusion and between the managed and control areas indicates that EVI strongly increased in the managed area after ungulate exclusion. Field studies and airborne analyses show that the dominant invader was Senecio madagascariensis, an invasive annual forb that increased from < 0.01% to 14.7% fractional cover in ungulate-free areas (lambda = 1.89 +/- 0.34 yr(-1)), but which was nearly absent from the control site. A combination of canopy LAI, water, and fractional cover were expressed in satellite EVI time series and indicate that the invaded region maintained greenness during drought conditions. These findings demonstrate that enemy release from generalist herbivores can facilitate exotic success and suggest a plausible mechanism by which invasion occurred. They also show how novel remote-sensing technology can be integrated with conservation and management to help address exotic plant invasions.

Opportunities and Constraints in Characterizing Landscape Distribution of an Invasive Grass from Very High Resolution Multi-Spectral Imagery

“…Comprehensive site-scale analyses of patch distributions are thus important for addressing questions about cross-scale interactions and critical thresholds in the invasion process to inform ecosystem management (George, 1992; Peters et al, 2006; Suding and Hobbs, 2009). However, as such questions continue to emerge in various ecosystems, traditional field approaches become increasingly limited compared to new remote sensing opportunities for assessing these distributions at the whole-site scales and in high spatial detail (Kellner et al, 2011; Bradley, 2014; Asner et al, 2015; Rocchini et al, 2015). Yet, this potential comes at a cost of a higher landscape data complexity that needs to be addressed in order to make reliable inference about the distribution and ecological performance of invaders.…”

Primary Succession on a Hawaiian Dryland Chronosequence

“…atropoides , which accounted for 23.3% of fractional vegetation cover, and the shrub D . viscosa , which was 12.8% [21]. The shrub D .…”

“…Myoporum-Sophora dry forest (MSDF) represents short-stature tree communities dominated by the endemic tree species Myoporum sandwicense or Sophora chrysophylla . MSDF support understories with both native and exotic grass species, including the native Eragrostis atropoides and the exotic Cenchrus setaceus , and occasional native and exotic shrubs and forbs [21]. L .…”

Towards an integration of scale and complexity in marine ecology

“…Sensing ocean color from space is commonly used to investigate of the role of oceanographic processes and bottom-up effects in marine communities (Dayton and Tegner 1984, Menge et al 1997, Navarrete et al 2005, Wieters 2005, Witman et al 2008). The precedent already exists in terrestrial ecosystems where competition for space among trees has been measured (Kellner and Asner 2014) and a manipulative experiment (ungulate removal) was monitored on large spatiotemporal scales (9 years, 4.3 km 2 ) by UAVs (Kellner et al 2011). As the spatial resolution of the airborne sensors increases, more community variables and even response variables in intertidal experiments may be measured over large areas.…”

“…We foresee a significant contribution from remote sensing in the near future in terms of our ability to understand more of the complexity of marine communities across large scales by frequent monitoring of field experiments, particularly in rocky intertidal habitats. The precedent already exists in terrestrial ecosystems where competition for space among trees has been measured (Kellner and Asner 2014) and a manipulative experiment (ungulate removal) was monitored on large spatiotemporal scales (9 years, 4.3 km 2 ) by UAVs (Kellner et al 2011).…”