Megaherbivores suppress precipitation-driven plant irruptions in a tropical savanna
Data files
Mar 19, 2025 version files 1.06 MB
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1_DataPrep.R
19.02 KB
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Bliss.R
10.77 KB
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Hibiscus_DRC_Ht.csv
67.37 KB
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HibRain.xlsx
10.45 KB
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KLEE_DrepsHibiscus_2007-2024_HW.xlsx
729.65 KB
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KLEE_rain_prev1-12mo_2001-2024_plot.xlsx
223.06 KB
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README.md
3.40 KB
Abstract
Irruptions in plant and animal populations are not uncommon in plants and animals. It has been suggested that these irruptions may be reduced by predators or herbivores, but there is a paucity of controlled experimental evidence. In addition, the factors underlying irruptions are rarely explored quantitatively. Using data from the Kenya Long-term Exclosure Experiment (KLEE), we show that populations of perennial Hibiscus spp. (primarily H. flavifolius) show multiple short-term irruptions a year after rainy periods, increasing in abundance in some cases by more than an order of magnitude before declining in ensuing months and years. We demonstrate that these irruptions are largely limited to experimental plots from which large mammalian herbivores have been excluded, particularly megaherbivores (elephants and giraffes). This represents the only controlled replicated experimental demonstration of top-down regulation of irruptions in either plants or animals. African elephants and giraffes are often at greater risk of local extinction than other large mammals, and their absence appears to destabilize this African savanna ecosystem, providing additional support for their conservation.
https://doi.org/10.5061/dryad.tb2rbp0bh
Description of the data and file structure
Files and variables
File: 1_DataPrep.R
Description: The R script to visualise the data and prepare the data for Bayesian functional Linear regression with Sparse Step functions (BLiSS) analysis.
File: Bliss.R
Description: The R script to conduct the BLISS analysis.
File: HibRain.xlsx
Description: The three-month cumulative rainfall.
Variables
- Months = month triads (JFM = January/February/March, AMJ = April/May/June, JAS = July/August/September, OND = October/November/December)
- Year = year
- PPTmm = total precipitation in mm
- blank cells indicate that no plants were found in the quadrat (as opposed to not sampled).
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File: Hibiscus_DRC_Ht.csv
Description: Hibiscus spp. diameter at root collar (DRC, cm) and height (cm) data.
Variables
- Date = sampling date.
- Plot = sampling plot (Block-Treatment)
- Block = replicate block (N=north, C=central, S=south).
- Treatment = herbivore treatments: O = all large (> 20 kg) herbivores excluded; W = wild mesoherbivores present; C = cattle present; WC = cattle and wild mesoherbivores present; MW = megaherbivores (giraffes, elephants) and wild mesoherbivores present; MWC = all herbivores present. Ch indicates high cattle grazing (see ‘Cattle’ variable).
- Cattle = cattle grazing intensity (None/Moderate/High).
- Mega = megaherbivore (giraffes, elephants) accessibility (Yes/No).
- Meso = Mesoherbivore accessibility (Yes/No).
- Belt = belt transect identity (1-4).
- Species = plant species (all are Hibiscus spp.).
- SD = stem diameter (cm; e.g., SD-3 = diameter of 3rd stem of multi-stemmed plant).
- Height = plant height (cm).
- DRC = diameter at root collar (cm).
- SDmax = maximum stem diameter (cm).
- StemCount = total number of stems.
- Notes = any notes related to the observations.
- Latin = Latin binomial of the plant species.
- Wildlife = accessibility of the treatment plot to wildlife (None/Mega+Meso; see ‘Mega’ and ‘Meso’ variables).
- Height.grp = height class categories.
- NAs indicate no data (as opposed to not sampled).
File: KLEE_rain_prev1-12mo_2001-2024_plot.xlsx
Description: rainfall data.
Variables
- Date = year and month of rainfall.
- Block = replicate block (N = North, C = Central, S = South).
- PPT = monthly precipitation (mm).
- cum = cumulative and mo = month (e.g., cum3mo = cumulative 3 month antecedent rainfall; PrevYr = previous year).
File: KLEE_DrepsHibiscus_2007-2024_HW.xlsx
Description: The heights (cm) of Acacia drepanolobium and Hibiscus spp. plants in 1x1 m quadrats.
Variables
- DATA = whether there are data for this row (YES/NO).
- DATEog = vegetation sampling date.
- SEASON = season (WET/DRY)
- BLOCK = replicate block (NORTH/CENTRAL/SOUTH)
- TREATMENT = herbivore treatments: O = all large (> 20 kg) herbivores excluded; W = wild mesoherbivores present; C = cattle present; WC = cattle and wild mesoherbivores present; MW = megaherbivores (giraffes, elephants) and wild mesoherbivores present; MWC = all herbivores present.
- SPECIES = plant species sampled.
- Sampling locations (1A to 10A).
Code/software
R version 4.3.2
This research was carried out from 2007 to 2023 in the Kenya Long-term Exclosure Experiment (KLEE), located on the property of the Mpala Research Centre (0°17’ N, 36°52’ E; 1,800 m asl) in Laikipia, Kenya. Annual rainfall at the site is highly variable interannually, and during the study period averaged 597 + 196 mm/year (mean + SD; range 330–1,009 mm yr-1; Fig. 1c). The rainfall pattern is weakly trimodal, peaking in March–May. The study site is a semi-arid savanna ecosystem underlain with black cotton soils and dominated by the tree Acacia [Vachellia] drepanolobium and five grass species: Pennisetum mezianum Leeke, P. stramineum Peter, Themeda triandra Forssk., Lintonia nutans Stapf., and Brachiaria lachnantha (Hochst.) Stapf. (Young et al. 1997). Approximately 100 other species of plants occur at lower numbers in the plots, including several Hibiscus species. Mpala is managed for both livestock production and wildlife conservation and supports a diverse variety of herbivores.
The KLEE experiment was established in 1995 and uses semi-permeable barriers to create six 200 × 200 m herbivore exclosure treatments replicated in three blocks, for a total of 18 plots. Treatments and abbreviations are as follows: O = all large (> 20 kg) herbivores excluded, W = wild mesoherbivores present, C = cattle present (Bos indicus L.), WC = cattle and wild mesoherbivores present, MW = megaherbivores and wild mesoherbivores present, MWC = all herbivores present (Young et al. 1997). In this ecosystem the megaherbivores are elephants (Loxodonta africana Blumenbach) and giraffes (Giraffa camelopardalis L.). The mesoherbivores are represented by plains zebras (Equus quagga Boddaert), Grevy's zebras (E. grevyi Oustelet), elands (Tragelaphus oryx Pallas), Grant's gazelles (Nanger [Gazella] granti Brooke), cape buffaloes (Syncerus caffer Sparrman), oryx (*Oryx beisa *Rüppell), and hartebeests (Alcelaphus buselaphus Pallas). Non-excluded mammalian herbivores (< 25 kg) include steinbucks (Raphicerus campestris Thunberg), bush duikers (Sylvicapra grimmia L.), hares (Lepus spp.), and several rodent species.
Hibiscus spp. surveys were carried out in June of each year from 2007 to 2023 (approximately 2–3 months after peak rainfall). Two species are represented in this data set: H. flavifolius Ulbr. and H. aponeurus Sprague & Hutch, distinguishable only in flower, and are lumped here. Both are single- to few-stemmed, semi-woody perennial herbs that can attain heights of 2–3 m. Based on proportions of these two species flowering in the area (pers. observ.), the great majority of sampled plants were H. flavifolius. In each of the 18 KLEE plots, we counted the number and measured the heights of all these Hibiscus spp. plants in 50 1 × 1 m quadrats, located every 20 m along 10 parallel transects.