DOI: 10.5061/dryad.tb2rbp0bh
title: Megaherbivores suppress precipitation-driven plant irruptions in a tropical savanna

description: 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.
description: 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 <em>Acacia [Vachellia]
 drepanolobium</em> and five grass species: <em>Pennisetum
 mezianum</em> Leeke, <em>P. stramineum</em> Peter,
 <em>Themeda triandra</em> Forssk., <em>Lintonia
 nutans</em> Stapf., and <em>Brachiaria lachnantha</em>
 (Hochst.) Stapf. (Young  et al. 1997). Approximately 100 other species of
 plants occur at lower numbers in the plots, including several
 <em>Hibiscus</em> 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 (&gt; 20
 kg) herbivores excluded, W = wild mesoherbivores present, C = cattle
 present (<em>Bos indicus L</em>.), 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 (<em>Loxodonta
 africana</em> Blumenbach) and giraffes (<em>Giraffa
 camelopardalis</em> L.). The mesoherbivores are represented by
 plains zebras (<em>Equus quagga</em> Boddaert), Grevy's
 zebras (<em>E. grevyi</em> Oustelet), elands
 (<em>Tragelaphus oryx</em> Pallas), Grant's gazelles
 (<em>Nanger [Gazella] granti</em> Brooke), cape buffaloes
 (<em>Syncerus caffer</em> Sparrman), oryx (*Oryx beisa
 *Rüppell), and hartebeests (<em>Alcelaphus buselaphus</em>
 Pallas). Non-excluded mammalian herbivores (&lt; 25 kg) include
 steinbucks (<em>Raphicerus campestris</em> Thunberg), bush
 duikers (<em>Sylvicapra grimmia</em> L.), hares
 (<em>Lepus</em> spp.), and several rodent species. 
 <em>Hibiscus</em> 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: <em>H.
 flavifolius</em> Ulbr. and <em>H. aponeurus</em> 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
 <em>H. flavifolius.</em> In each of the 18 KLEE plots, we
 counted the number and measured the heights of all these
 <em>Hibiscus</em> spp. plants in 50 1 × 1 m quadrats, located
 every 20 m along 10 parallel transects.
description: # Megaherbivores suppress precipitation-driven plant irruptions in a
 tropical savanna
 [https://doi.org/10.5061/dryad.tb2rbp0bh](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). ## #### 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 (&gt; 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 (&gt; 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