The Galápagos archipelago

14 Pages·2016·0.62 MB·English

FocusArticle
The Galápagos archipelago: a
natural laboratory to examine
sharp hydroclimatic, geologic and
anthropogenic gradients
Madelyn S. Percy,1† Sarah R. Schmitt,2† Diego A. Riveros-Iregui2*
and Benjamin B. Mirus3
Poor understanding of the water cycle in tropical ecosystems has the potential to
exacerbate water shortages and water crises in the region. We suggest that the
Galápagos Islands provide an excellent proxy to regions across the tropics as a
resultofsharphydroclimatic,anthropogenic,andpedohydrologicgradientsacross
the archipelago. Hydroclimatic and pedohydrologic gradients are found across
different elevations on single islands, as well as across the archipelago, whereas
anthropogenic gradients reflect land use and land cover change across islands as
population and growth in tourism have affected individual islands differently.
Thisarticlehighlightsspecificopportunitiestofurtherexamineourunderstanding
of the interactions between water and critical zone processes in tropical ecosys-
tems, making connections between the Galápagos archipelago and much of the
understudied tropics. The Galápagos archipelago offers a natural laboratory
through which we can examine current threats to freshwater security as well as
the dynamics of coupled natural and human systems. ©2016WileyPeriodicals,Inc.
Howtocitethisarticle:
WIREsWater 2016.doi: 10.1002/wat2.1145
INTRODUCTION little research into tropical water cycling is carried
out because of a limited research infrastructure
Inthepast50 years,globalfreshwaterusehasmore
coupled with lack of access to historical data, with
than tripled.1 Currently, the lack of potable water the notable exception of Hawaii.3–5 Projected popu-
resources affects about half of the population in the
lation growth in the tropics combined with climate
tropics, and the proportion of people with access to
change and the lack of conservation efforts in many
safe drinking water is lower in the tropics than in
tropical countries is a growing concern amongst pol-
anyother region oftheworld.2 Despite current water
icymakers, economists, and defense analysts alike.
shortages facing tropical and subtropical latitudes,
However, sites like the Galápagos archipelago pro-
vide an opportunity for researchers to understand
tropical water cycling across a set of inter- and intra-
†Theseauthorscontributedequallytothiswork.
island gradients that cover microclimatic zones repre-
*Correspondenceto:[email protected]
sentative of many zones of the tropics. Furthermore,
1DepartmentofGeologicalSciences,UniversityofNorthCarolina the well-described chronosequence,6 climosequence,7
atChapelHill,ChapelHill,NC,USA
and basaltic bedrock of Galápagos can lend further
2Department of Geography, University of North Carolina at
insight into prior hydrologic studies conducted at
ChapelHill,ChapelHill,NC,USA
3U.S. Geological Survey, Geologic Hazards Science Center, other tropical sites. Compared to Hawaii, for exam-
Golden,CO,USA ple, Galápagos has a much more complex eruption
history, it is located right at the equator, it has a
Conflictofinterest:Theauthorshavedeclarednoconflictsofinter-
estforthisarticle. much narrower range of mean annual precipitation
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than does Hawaii.8 Further, its population density is Many environmental challenges in Galápagos
two orders of magnitude lower than Hawaii,9,10 are also present throughout the tropics, as growing
leading to limited urban and agricultural develop- populations and increasing tourism industries lead to
mentinsomeoftheGalapagosIslands. water crises, or inadequate access to or management
Here, we focus on three gradients across the of freshwater resources.13 This type of water crisis
Galápagosarchipelagothatcouldserveasidealprox- has been observed in numerous tropical systems,
ies for tropical systems around the globe (Figure 1). including agricultural zones,14–17 mountainous
Two of those are natural gradients that affect water areas,18,19coastalzonesandislands.20–23Thesecrises
cycling on theislands: ahydroclimatic gradient and a may be exacerbated by increasing tourism rates,
pedohydrologic gradient. The hydroclimatic gradient which cause issues ranging from soil erosion and
is elevation-dependent and results in microclimate invasionbynon-nativebiota24–26topoorwaterqual-
zonation across the islands. A stark windward– ity and high demands on groundwater systems.27,28
leeward contrast across each island provides for an Weidentify threeparticularchallengesfacingtropical
additional opportunity to compare vertical ground- hydrology that can be explored on the Galápagos:
water systems in wetter and drier environments. The the importance of fog inputs into tropical systems,
second natural gradient is a pedohydrologic gradient, the effects of land use change on soil development
here defined as changing soil–water interactions and evolution, and the role of pedogenesis on
across a chronosequence (islands of different age) hydraulic and hydrochemical fluxes through the soil
and climosequence (temperature and precipitation column. We suggest that the identified gradients in
variations with elevation). The third gradient is an Galápagos, coupled with the increasing human pres-
inter-island, anthropogenic gradient driven by vary- sures, make the archipelago an ideal proxy for not
ing land use and land cover change. Although some just tropical and subtropical ocean islands, but for
islands are well-preserved and do not even allow the continental tropics as well. The Galápagos as a
tourist entry, over 70% of the landscape of San Cris- natural laboratory for earth surface processes has
tóbal and almost 50% of Santa Cruz, the two most been discussed previously29 but the existing opportu-
populated islands, have been dramatically altered by nities to understand the feedbacks between humans,
landusechangeandinvasivespeciessince1987.12 the critical zone, and the water cycle place the
(a) (b) (c)
Windward slope Windward slope Windward slope
800 800 800
ation above amsl (m) 640000 Importance obf ufdogg etto water vTHeraruynm shiitudiom tnoid 640000 Inctrreaanssifnorg immerp ascpt eocifes treasfnMfpeseacfoctjosirem rosefr 640000 cDoencdruecatisivintsygo ilah ynmddr oiiasntucliurrceeasing UmhLniogokihwns to suKwor,einl
v
e
El 200 200 200
High K, rapid infiltration,
Negligible to small impact of
Very arid to arid low soil moisture
transformer species
Sea level Sea level Sea level
FIGURE 1 | Conceptualdiagramsillustratingourhypothesesregardingelevation-controlledgradientsacrosstheGalápagos.(a)Withan
increaseinelevation,theimportanceoffog-waterinputintothewaterbudgetwillincrease,withalargeproportionofannualwaterinputinthe
highlandscomingfromoccultprecipitation.Atthedrycoastalelevationswhichreceivewaterfromconvectiverainfall,theimportanceoffogwater
asaninputintothegroundwatersystemdecreasestozero.(b)Asaresultoflandusechangeonthearchipelago,wesuggestthatinvasive(i.e.,
‘transformer’)speciesintheGalápagoshaveaffectedthehighlandareasmorethantheyhavethelowlands,largelyduetogreaterwater
availabilitywithhigherelevations.11(c)Theincreaseintheamountofwaterinputstothesystemresultsinsoilswithalowerhydraulic
conductivityandhighersoil-moistureretentionduetohighconcentrationsofhydratedclaysathighelevations,andhighhydraulicconductivityand
lowsoil-moistureretentionatlowelevationsduetothelimitedamountofwaterreceivedalongthecoast,thenatureoftheprecipitationatlow
elevations,andthestructuraldifferencesinthesoil.
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Galápagos in a unique position beyond its historical islands, the impact of climate on the hydroclimatic
importance. gradientscanbeconstrained.
The effect of the precipitation–elevation gradi-
ent on island-wide recharge and discharge zones in
PHYSICAL AND ANTHROPOGENIC the Galápagos has been hypothesized, although suc-
VARIABILITY cess in the identification of these zones is varied.
A recent review article comparing the hydrology of
Hydroclimatic Gradients San Cristóbal and Santa Cruz proposed groundwater
Hydroclimatic gradients are significant because they systems that include perched aquifers, impounded
affect the spatial and temporal distribution of fresh- dike aquifers, and a low-gradient basal aquifer3
water cycling. The Galápagos Islands, for example, (Figure 3). A helicopter-borne time-domain electro-
experience strong gradients in temperature and pre- magnetic sounding equipment (SkyTEM) survey car-
cipitation by elevation, and different behavior of pre- ried out on San Cristóbal and Santa Cruz identified
cipitation, runoff, and groundwater–surface water units comparable to those in other basaltic systems,
interactions in each unique microclimatic zone shown in Figure 3.33,34 Additional evidence for the
(Figure 2). In addition to the presence of microcli- hydrogeological units comes from surface water flow
mates, the oscillation of the intertropical convergence patterns35,36 and geochemical studies of the ground-
zone (ITCZ) results in annually different seasons in water.37 This type of work in the Galápagos may be
the archipelago,31 similar to other tropical sites that used as the basis for comparison in other tropical
are also influenced by decadal-scale climate anoma- systems with highly fractured bedrock and high pre-
lies.32 Because of the small spatial extent of the cipitationrates,includingtheHawaiianislands.3
Same scale 91°W Isla pinta 90°W © Australian National University
Isla Base map CAP 12-009
culpepper
Very humid zone
1°30'N
Isla Isla marchena Humid zone
wenman
92°W
Transition zone
Pacific Ocean
Dry zone
0°
Arid zone
Isla santiago
Galapagos Islands
Isla santa cruz
Isla fernandina
Isla
Isla santa cruz Isla san
Pinzon christobal
Puerto
Isla santa fe
ayora
Puerto villamil
Puerto Baquerizo 1°S
Isla isabela Moreno
Main road
Isla santa maria
0 50
Isla espanola
Kilometers
FIGURE 2 | MapofclimatezonesonGalápagos(afterHuttel30)illustratinghowclimatezonesaredistributedbasedonelevationabovesea
levelandwinddirection.NotethatseveralvolcanoesonIsabelaandtheprimaryventonFernandinaarecalderas,andthus,havedifferent
climatesbecauseofanorographiceffect,wherethehighwallsofthecalderashieldtheinsideofthecaldera.Olderislandswithweathered
topographynolongerhavedistinctivecalderas,sotheorographiceffectisfeltacrosstheentireisland.
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800 Santa cruz 800 Windward San cristóbal
Windward side of the side of the
island island
m] 400 m] 400
Elevation [ Well nPoe srcuhrfeadc ea qeuxpifererss swioitnhs Elevation [ Pewrcihthe dsp arqinugifsers
Unkonwn extent
0 Fresh water surface ? 0 of freshwaer lens Sea level (0 m)
?
Salt water surface Unknown extent of
saline intrusion
0 5 10 15 0 5
Distance from coastline [km] Distance from coastline [km]
FIGURE 3 | ConceptualmodelsofthehydrogeologyofSantaCruzandSanCristóbal,themosthighlypopulatedislands,basedonSkyTEM
surveys,3,31,32geochemicalvalidation,33andourinterpretationofworkbyVioletteetal.29
In addition, because of its location in the cen- larger groundwater–surface water systems through-
tral Pacific, changes in the strength and frequency out the tropics.
of El Niño events, and the effects that extreme El
Niño events can have on areas around the tropical
Pacific, can be readily studied in the Galápagos. Anthropogenic Gradients
Recent investigations suggest that global climate Land-based tourism destinations such as the Galápa-
change will result in higher convective precipitation gos, and many other sites in the tropics, rely on a
rates on the islands than that has been observed in healthy natural ecosystem to attract tourists. How-
the past,38 whereas other studies argue that climate ever, strong pressures on native species as increasing
change is already causing more distinct seasons on demands by the growing native population and tour-
the archipelago.39 Climate data from the Galápagos ists can overburden tropical ecosystems. Although
will complement previous work on the El Niño globally tourism does not directly use more than 1%
Southern Oscillation (ENSO) cycle completed of global water resources,43 the residual effects of
throughout the tropics,31,40 with the added advan- tourism in ecologically sensitive areas, including lar-
tage of providing insight into the manifestation of ger native populations to support the tourism indus-
the ITCZ.41,42 try44 necessitates the importation of food, potable
This information highlights several themes to water, and supplies45 to sustain human demand.
be studied in the Galápagos and applied across the Such indirect effects of increased tourism in tropical
tropics. First, understanding inputs into the tropical systems can be studied in a spatially constrained sys-
water cycle is key to understanding how climate tem like the Galápagos, and the implications can be
change may affect surface water and groundwater appliedtoregionsacrossthetropics.
storage and volume, and how precipitation regimes PermanenthumancolonizationoftheGalápagos
may change in the future. As a first step, we com- Islands first took place in the 1800s.46 Between 1985
piled rainfall isotope data (Figure 4) collected on and 2005, the native population of the islands almost
Santa Cruz Island that show a clear seasonal differ- tripled and tourism to the islands exhibited a sixfold
ence associated with convective rainfall (January– increase.47 Both increases haveplacedpressure on the
June) and fog (June–December). However, these natural systems and infrastructure of the islands, as
data highlight our knowledge gaps, as we do not thearchipelagohasbecomeanecotourismhotspotfor
yet understand the effects of ENSO or the spatial itsbiologicalsignificance,30naturalbeauty,andundis-
variability of precipitation inputs across the archi- turbed nature. Because of the population growth on
pelago. In addition, the different microclimates the archipelago, the land use/land cover change is
across the Galápagos provide an opportunity to evolving at higher rates than ever before in the Galá-
investigate how climate influences the partitioning of pagos, as it has in the rest of the tropics.48 There is
surface and shallow subsurface water fluxes into much local concern for the fragile island ecosystem
groundwater systems, thus offering proxies for thatisbeingimpactedinbothdirectandindirectways
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0
‰) –2
O (
18! –4
d
e
ht
g
ei –6
W
–8
January February March April May June July August September October November December
FIGURE 4 | Thirteen-year(1995-2008)averagedmonthlymeansforδ18Oinprecipitation.Datainthisoriginalfigurearederivedfromthe
GlobalNetworkofIsotopesinPrecipitationdatasetatasinglecoastalstationlocated~200M.A.S.L.,SantaCruzIsland,Galápagos.Aseasonal
shiftintheδ18Oishypothesizedtorepresentthedifferenttypesofprecipitationcontributingtotheecosystem.BetweenDecemberandMayof
non-ElNiñoyears,theislandsaredominatedbyconvectiverainfallevents;betweenJuneandNovember,thelowlandsareextremelydryandthe
highlandsexperienceanearlyconstantfog.Notethattherangesofδ18Ovaluesintheprecipitationaresmallerforfoginputsthanforthe
convectiverainfallinputs.
by the growing permanent and tourist populations, anthropogenicgradientsinmanyways(e.g.,mapping
describedas‘themaindriverofchange’ontheislands distinct vegetative zones across the human-inhabited
in recent decades.49 Recent work indicates that stake- islands,53 visualizing the spread of the invasive guava
holders across different sectors of the economy recog- plantontheinhabited islands viatheuseofhighspa-
nize that growing population and increasing tourism tial resolution imagery54). These disturbances all
are driving increasing negative pressures on the envi- have direct and indirect impacts on hydrological pro-
ronment.50 Because Hawaii and other tropical islands cesses and the partitioning of precipitation into evap-
have faced similar challenges,8 the Galápagos can otranspiration,runoff,andgroundwaterrecharge.
serve as a testing ground for best practices applied in Research into human impacts on the Galápagos
othertropicalislandsystems. has implications for other sites across the tropics
Numerous changes to the islands as a result of experiencingincreasesintourismandgrowingperma-
humanpopulationgrowthhavebeenstudiedthrough nent populations. The propagation of invasive species
on-island surveys of land use change and via remote andthespatialpatternsofanthropogenic degradation
sensing.51 One of the current threats to the environ- observed on these islands may reveal patterns that
ment identified by surveys include the expansion of provide insight into land use and land cover change
built area and infrastructure on the inhabited islands acrossthetropics,aswellashowconservation efforts
to accommodate the increase in tourism (Figure 5). mayreversetheeffectsofoveruseofnaturalresources.
Theextension ofbuilt area hasnot only ledto degra- Forexample,severalstudieshaveshowntheeffectsof
dation of the landscape from construction and from remediationeffortsintheGalápagos,53whereasothers
encroachment on native plant habitat but has also haveshownthevalueinactivecontrolofinvasivespe-
led to an increase in the energy and fresh water cies.54 Management practices developed in the Galá-
demands of the islands. As a result of land use pagos may become models for other ecologically
changes, the Galápagos archipelago is severely fragile systems across the globe, especially because
affected by invasive species that threaten native flora entire islands remain off-limits to tourists, rather than
and fauna.52 Because of the growing human popula- tropical systems like Hawaii or continental land-
tion, the importation of food, oil, and water has pro- masses, where humans have left large environmental
vided a means for the introduction of these invasive footprints.
species. Nearly 1500 new species are known to have
been introduced to the Galápagos in the past
40 years, many of which are a direct result of the Pedohydrologic Gradients
increased reliance on imported goods.47 Remote sen- Similar to the Hawaiian islands, the Galápagos
sing studies have quantified degradation along archipelago formed on a hotspot, resulting in a
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0°35´0˝S 0°35´0˝S
2002 Red = 2014 2014
Green = 2002
Green gaps: revegetated
0°40´0˝S 0°40´0˝S
areas between 2014 and
2002
0°45´0˝S 0°45´0˝S
Beach erosion
0°50´0˝S 0°50´0˝S
Airport
0°55´0˝S 0°55´0˝S
Puerto Puerto chino
baquerizo Hotel development
moreno
1°0´0˝S 1°0´0˝S
89°40´0˝W 89°35´0˝W 89°30´0˝W 89°25´0˝W 89°20´0˝W 89°15´0˝W 89°35´0˝W 89°30´0˝W 89°25´0˝W 89°20´0˝W 89°15´0˝W
FIGURE 5 | Preliminarycalculationofthenormalizeddifferencebuilt-upindex(NDBI)fromLandsatimagerycapturedin2002and2014on
SanCristóbalIsland.Redindicatesbuilt-up/barrenareasin2002and2014,respectively,andgreeninthe2014imageindicatesrevegetatedareas
inthe12-yearspan.NotethatcinderconesandyoungerlavaflowsexistintheNWportionoftheislandthathasanotablebarrenappearancein
bothimages.Circlesindicateprincipalareasofchangeofbarren/built-uplandareaovertheintervening12years—suchchangesinNDBIarea
resultofincreasedinfrastructuredevelopmentandbeacherosion.
chronosequence55,56 in which different islands have site in the tropics of spatially intensive soil moisture
well-constrained and different ages over the same observations (e.g., those carried out at the Tarra-
bedrock type. Earth scientists have the opportunity warra catchment in Australia61,62 or across the
to isolate the influence of soil development (pedogen- Appalachians and Piedmont in the United
esis) across different aged bedrock with similar land States63,64). In addition to understanding soil mois-
use histories,57 providing an ideal experimental set- ture distribution, the Galápagos provides a proving
ting with findings pertinent to many parts of the tro- ground for understanding different island aquifer
pics where land use histories are more complex. structures as a result of different eruption and struc-
Several notable studies have already shed light on turalhistories.3,65
‘pedohydrologic’, or soil–water interaction,58 gradi- The limited research on the vadose zone in the
ents across different elevations, which can be typified Galápagos leaves many questions still to be resolved,
and then used to describe systems beyond the Galá- including how changes in the hydraulic properties of
pagosarchipelago. soils across different microclimates affect soil mois-
On windward slopes of San Cristóbal (oldest) ture distribution, and whether the variables affecting
and Santa Cruz (younger), similar distributions of soil moisture change with elevation. Findings could
clays were observed throughout the soil column.59 revealconnectionsbetweenplanttypes,soilmoisture,
However, because of the microclimates present and aquifer recharge, as well as providing tropical
across different elevations, clays at high elevations hydrologists with the opportunity to test pedotrans-
are generally exposed to more meteoric water as a fer functions developed in more temperate cli-
result of the fog, which results in swelling of the mates.66,67 On a theoretical level, the Galápagos
inter-sheet layers and lower hydraulic conductivity provides an opportunity to study pedogenesis across
than soils at lower altitudes.59 On other tropical theaforementionedhydroclimaticgradientsonnearly
islands, like Bali,60 researchers found that the overly- homogeneous bedrock. Understanding soil formation
ing soil type was an excellent predictor of groundwa- processes, especially with regards to sources of soil
ter behavior. The difference in the soil water content material (e.g., atmospheric deposition, chemical
across elevations on Galápagos not only has implica- weathering of bedrock) could aid in better under-
tions for agricultural practices on the islands but also standing bedrock weathering rates and element
provides an excellent opportunity for an analogous depletionacrossarangeofclimates.11,68
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CURRENT CHALLENGES AND variable flow paths within the bedrock. Such studies
OPPORTUNITIES have proven useful on Easter Island,87 Tahiti-Nui,88
La Reúnion,65 Gran Canaria,89 and Hawaii.5 Along
The Galápagos archipelago provides a unique envi- with implications for understanding groundwater
ronment for studying interactions between hydrocli- flow, non-rainfall atmospheric water inputs are a
matic, anthropogenic, and geologic gradients within major source of water for plants throughout the tro-
a limited spatial extent (Figures 1, 2, and 5). Despite pics and into the mid-latitudes.90–92 There have been
the islands’ biological significance, research into the a variety of studies that have used different collectors
critical zone between the groundwater and the to understand the amount of water entering a system
atmosphere is needed to enhance our understanding as a result of fog,93 which is recognized as a signifi-
of how a variety of natural and anthropogenic stres- cant precipitation input into ecosystems in both
ses may impact the islands. The following montane forests81,94–98 and arid climates.82,99–101
section outlines several knowledge gaps in Galápagos The Galápagos archipelago offers potential for
that, when filled, could provide perspective to issues understanding systems across the tropics for which
faced not only in these islands but throughout much fog is an input into the water budget. In addition,
oftheunderstudiedtropics.69 the small area of the islands and the well-defined
microclimatic zones allows for the design of experi-
ments that include high-frequency sampling of
Fog as an Input into the Hydrologic
precipitation.
System
Fog is a principal water source in many tropical and
subtropical ecosystems,70,71 especially those in high Rooting Depth and Soil Development in
topographic positions where soil moisture is low. Stands of Native versus Invasive Species
Studying fog and its relationship with the terrestrial Land use and land cover change in the Galápagos is
system is paramount to understanding overall water mostly driven by the ever-increasing rate of invasion
availability under current conditions, and under by non-native plant species. On San Cristóbal, 71%
future conditions of climate change.72 Understanding of the total agricultural area in 1987 was invaded by
the importance of fog as an input into the water invasive species by 2006; on Santa Cruz, over 50%
budget of the Galápagos will provide insight into not ofagriculturallandhasbeeninvadedbyinvasivespe-
only sources of water in the local groundwater sys- cies when no significant patches of invasive plant
tem but also to understand the behavior of inputs stands were even recorded in 1987.12 The humid
into hydrologic systems across the tropics. The uplands of the Galápagos Islands (>400 m) are par-
advantageofworkingwithfogasaninputintoasys- ticularly impacted by invasive plants.53 The most
tem’s water budget is that fog is often isotopically threatening of these non-native species are the trees
enriched in 2H and 18O compared to rain as a result Psidium guajava (guava) and Cinchona succirubra
offractionationprocesses.73–75 (quinine), the shrub Lantana camara, and the grass
Despite the importance of fog in the Galápa- Pennisetumun purpureum.102 Guava in particular
gos, few studies have sought to understand the input exemplifies characteristics of a ‘transformer species,
of fog into the water budget of the islands. One an invasive species that changes character, condition,
study compared water inputs at humid high altitudes form or nature of ecosystems over substantial areas
and arid low altitudes,76 and the other used isotopes relative to the extent of that ecosystem’103 because it
to trace sources of groundwater by elevation.37 The is both drought- and flood-tolerant, grows well in a
use of stable isotopes of water as a tracer has been range of soil types and light availability regimes, and
successful in tracing sources of groundwater77–79 produces numerous seeds that are dispersed by a
and sources of water for ecosystems.80–82 In Hawaii, numberofanimalspecies.54
for example, evidence for highland fog contributing Despite the knowledge of the extent of invasive
to perched aquifers that discharge into high elevation plantspeciesinGalápagos12,46,50,53,104,105andacross
streams was determined through the use of stable the tropics, the impact of invasive species on rooting
isotopes.4 Similar studies have been carried out in depth, soil development, and unsaturated zone fluxes
Puerto Rico73,83 and mainland South America.84,85 has not been investigated in many places beyond
Because of the complex drainage and perched aqui- recent studies on the island of Maui, Hawaii.106,107
fers in fractured basaltic systems,86 stable isotopic The hydrologic and pedologic implications of inva-
analyses of spring water leaving the groundwater sive species remain poorly characterized across many
system at varying elevations may provide evidence of tropical and subtropical regions, severely impairing
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remediation efforts. Studies of rooting depth in the well-defined precipitation and temperature gradi-
southern Florida demonstrated that Melaleuca quin- ents with elevation, and land-cover changes include
quenervia, an invasive tree, was more competitive patches of diverse native and invasive species.
than the native species due to its ability to develop Because of the stability of three of the five variables
rootsinthesoilsurfaceduringsoil-drying periods,its affecting soil formation, the Galápagos provides a
capacity to develop a deep root system at an early natural laboratory in which watersheds can be used
age, and its effectiveness as a rooter in wet and dry as ‘controls’ and single pedogenetic variables (cli-
soils subject to a fluctuating water table.108 Another mate, biota) can be studied at greater depth than in
study in the semiarid California coastal shrub previous work.
revealed similar competitive capacities of invasive There are very few studies of the Galápagos’
shrubs,astheremovaloftheinvasivespecies resulted soils, but numerous sites across the tropics and
in more efficient water utilization by the native humid subtropics have been the focus of previous
stands.109Theabilityofinvasiveunderstoryplantsto research. The Hawaiian islands, with the well-
change soil functions has been investigated in other constrained chronosequence as one moves from the
systems, and soils beneath the exotic plants were southeast (young) to the northwest (old), have been
found to have higher pH and higher nitrification studied to understand how geologic age affects nutri-
rates than soils below native plants.110 A recent ent cycling and clay development56,68,72 and how
meta-analysis suggests that in terrestrial systems sub- precipitation and temperature gradients in the sub-
ject to plant invasions, litter decomposition rates are tropics affect soil carbon cycling, nutrient use, and
~117% higher in the exotic species than in co- the success of plant growth.32,55,121 In Australia, the
occurring native species, thus having a notable bedrock and climate variables were held ‘constant’
impact on soil nutrient cycling and development.111 and nutrient cycling and isotopic fractionation as a
Furthermore, global climate models often lack infor- result of changing biota were described.122,123 In the
mation regarding rooting depth and root Amazon, studies of soil change and differences in
functioning,112 as do regional primary productivity pedogenesis as a result of human disturbance have
models in the tropics which lack spatially distributed been extensively studied, while climate, bedrock, and
data.113 A better understanding of plant rooting age of the soils are held constant.124,125 Additional
depth in tropical systems and root responses to soil studiesintonutrientcyclingintheAmazonhavebeen
water availability could better inform ecohydrologic successful because three of Jenny’s factors of soil for-
models of the redistribution of soil water by rooting mation have been held constant.126–128 The Luquillo
systems andmakethem morerepresentative ofactual Critical Zone Observatory in Puerto Rico has yielded
water balance parameters.114 Measuring rooting many studies on rates of weathering and erosion in a
depth across hydroclimatic and land use gradients climatic setting comparable to the highlands of the
would be informative of native and invasive vegeta- Galápagos, although the bedrock is less
tion dynamics across a range of climatic conditions, uniform.129–132TheworkdoneatLuquillohasimpli-
many of which are comparable to other areas in the cations for hypothesized weathering rates and modes
tropics. of weathering in the Galápagos, and we suggest that
many of the phenomena observed at Luquillo will be
seen on the archipelago; we can test this hypothesis
Soil Development across Precipitation using a mass balance approach133 to soil elements.
Gradients Additional studies on soil gradients in which some of
Hans Jenny’s classical work115 on pedogenesis intro- the factors of soil formation are held constant have
duced the concept of five soil-formation factors: the been completed in areas undergoing land use change
climate, the underlying parent material, the depth of in China134,135 and Hawaii,136 where changes in
time over which a soil has developed, the biological nutrient concentrations in soils have long-term effects
activity through and above the soil, and the topog- on the success of future generations of native species.
raphy upon which a soil develops. Although the Dust inputs as additional parent material for soils
Galápagos has not been a site of long-term soils have been studied in California,137 the Atlantic coast
research, it is an ideal place to understand how of the Americas,138 and through surveys across the
tropical soils develop across the same geologic globe.139–141 The Galápagos archipelago exhibits an
setting,116–118 with stable topography on shield ideal combination of stages of soil development and
volcanoes,119 and well-constrained bedrock ages,120 precipitation rates across which many gaps in our
but under strong hydroclimatic and land-cover gra- understanding of pedogenesis in tropical regions can
dients. Climate varies greatly across the islands with befilled.
©2016Wiley Periodicals, Inc.
WIREsWater Anaturallaboratorytoexaminesharphydroclimatic,geologicandanthropogenicgradients
CONCLUSION increases.143 Changes in precipitation volume and
type can also impact soils by affecting weathering
Coupling individual, spatially explicit processes with and erosion rates, influencing the hydration of clay
the knowledge of hydrologic processes throughout
minerals, and ultimately changing the hydraulic con-
the understudied tropics is paramount in advancing
ductivity of soils. With changing hydraulic proper-
knowledge of tropical hydrology and datasets that
ties, current water budget inputs of runoff,
can be used as to constrain hydrologic models. There infiltration, and baseflow could change, resulting in
are several biome-wide mechanisms known to influ-
more variable water resources and greater suscepti-
ence water availability in the tropics. Vegetation, for bility to landslides and flooding. Because of the spa-
example, is known to moderate climate in the tropics
tial variability of climates, biota, and soils on the
by cooling the earth’s surface through enhanced
Galápagos, we suggest that the islands can serve a
latent heat fluxes.142 Furthermore, it is established
useful site in concert with Hawaii to investigate the
that the hydrology of nonimpacted (near-natural) influence of hydrologic gradients in the tropics, and
tropical montane forests is characterized by high
can provide valuable insight into how changing cli-
rainfall, potential water input by cloud interception
mates and growing populations may affect water
by plants, low transpiration rates, high streamflow
resourcesaroundtheworld.
and high rainfall interception rates, all of which
could be lowered as forest use intensity by humans
ACKNOWLEDGMENTS
This study was supported by the U.S. National Science Foundation Graduate Research Fellowship Program
(awards to M.P. and S.S.), the NSF-sponsored SAVI International CZO Grant (awards to M.P. and S.S.) and
the Geological Society of America, 2015 Student Research Grant Program (awards to M.P. and S.S.). DARI
acknowledges support from The World Bank and the Administrative Department of Science, Technology, and
InnovationofColombia(COLCIENCIAS)undertheDiasporaProgram(grantno.2012-0692).
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