Invasive rat eradication strongly impacts plant recruitment on a tropical atoll

Abstract

Rat eradication has become a common conservation intervention in island ecosystems and its effectiveness in protecting native vertebrates is increasingly well documented. Yet, the impacts of rat eradication on plant communities remain poorly understood. Here we compare native and non-native tree and palm seedling abundance before and after eradication of invasive rats (Rattus rattus) from Palmyra Atoll, Line Islands, Central Pacific Ocean. Overall, seedling recruitment increased for five of the six native trees species examined. While pre-eradication monitoring found no seedlings of Pisonia grandis, a dominant tree species that is important throughout the Pacific region, post-eradication monitoring documented a notable recruitment event immediately following eradication, with up to 688 individual P. grandis seedlings per 100m2 recorded one month post-eradication. Two other locally rare native trees with no observed recruitment in pre-eradication surveys had recruitment post-rat eradication. However, we also found, by five years post-eradication, a 13-fold increase in recruitment of the naturalized and range-expanding coconut palm Cocos nucifera. Our results emphasize the strong effects that a rat eradication can have on tree recruitment with expected long-term effects on canopy composition. Rat eradication released non-native C. nucifera, likely with long-term implications for community composition, potentially necessitating future management interventions. Eradication, nevertheless, greatly benefitted recruitment of native tree species. If this pattern persists over time, we expect long-term benefits for flora and fauna dependent on these native species.

Fig 1. Study site and two plant species that significantly increased post-eradication (Pisonia grandis and Cocos nucifera).

(A) All research was conducted on the islets of Palmyra Atoll, (B) a tropical island located in the Line Islands Chain in the Central Pacific Ocean. (C) The highest density of P. grandis seedlings was observed one month post- rat eradication, in 2011 and (D) the highest density of non-native C. nucifera seedlings was observed in 2016, five years post- rat eradication. Reprinted under a CC BY license, with permission from Kydd Pollock (1A), Coral Wolf (1C), and Dena Spatz (1D), original copyright 2009 (1A) and 2016 (1C and 1D).

Fig 2. Map of seedling recruitment survey locations on Palmyra Atoll.

Map of Palmyra Atoll with locally rare tree seedling plots shown as colored circles and light green squares indicating seedling transects across the atoll.

Fig 3. Changes in native species seedlings counts pre- and post- rat eradication.

(A) Mean ± SE Pisonia grandis seedlings m^-2 counted on 55 Seedling Transects across Palmyra Atoll pre- (2007) and post- (2011, 2012, 2014, and 2016) rat eradication. (B–E) Seedlings (Mean ± SE) counted per native locally rare tree seedling plot pre- (2004) and post- (2011, 2012, 2014, and 2016) rat eradication. A—C indicate significantly different data within each species grouping (after post-hoc correction; α = 0.05).

Fig 4. Changes in non-native species seedlings counts pre- and post- rat eradication.

(A) Mean ± SE Cocos nucifera seedlings m^-2 counted on 55 Seedling Transects across Palmyra Atoll pre- (2007) and post- (2011, 2012, 2014, and 2016) rat eradication. A–D indicate significantly different data (after multiple comparisons; α = 0.005). (B) Seedlings (Mean ± SE) counted per non-native (Calophyllum inophyllum) locally rare tree seedling plot pre- (2004) and post- (2011, 2012, 2014, and 2016) rat eradication were not significantly different (after post-hoc correction; α = 0.05).

Fig 5. Common native and non-native plant species seedling counts within three forest types.

(A) Mean ± SE Pisonia grandis seedlings m^-2 counted on 55 Seedling Transects distributed across the three forest types (Native, Mixed, Non-Native) found on Palmyra Atoll five years post-eradication (2016). (B) Mean ± SE Cocos nucifera seedlings m^-2 counted on 55 Seedling Transects distributed across the three forest types (Native, Mixed, Non-Native) found on Palmyra Atoll five years post-eradication (2016). A, B indicate significantly different data within each species grouping (after post-hoc correction; α = 0.05).