Preliminary evidence of the importance of rainfall and ENSO in modify food availibility for white-tailed deer in a Mexican tropical dry forest

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BIOTROPICA 38(5): 695–699 2006 10.1111/j.1744-7429.2006.00184.x

Preliminary Evidence of the Importance of ENSO in Modifying Food Availabilityfor White-tailed Deer in a Mexican Tropical Dry Forest1

Salvador Mandujano2

Departamento de Biodiversidad y Ecologıa Animal, Instituto de Ecologıa A. C., km 2.5 Camino Antiguo a Coatepec No. 351,Xalapa 91070, Ver., Mexico

ABSTRACT

The influence of El Nino/Southern Oscillation (ENSO) on rainfall and its possible effect on availability of food for white-tailed deer (Odocoileus virginianus ) in atropical dry forest in the Pacific coast of Mexico was studied. From 1977 to 2003 there were three significant El Nino and La Nina events. During El Nino yearsrainfall decreased during the wet season ( June to October) and increased during the dry season (November to May), with the opposite effect during La Nina years.Plant diversity was monitored in permanent plots during the wet and dry seasons of 1989–1993. The results provide evidence that ENSO events affect deer foodavailability, particularly in the dry season.

RESUMEN

Este trabajo analiza la influencia de los eventos de El Nino/Oscilacion del Sur (ENSO) sobre el patron de precipitacion y su posible influencia sobre la disponibilidadde alimento para el venado cola blanca (Odocoileus virginianus ) en un bosque tropical seco de la costa Pacıfica de Mexico. En el periodo de 1977–2003 se produjo treseventos claros de El Nino y de La Nina. Durante los anos del El Nino la lluvia en la epoca humeda (junio a octubre) disminuy o, y aumento durante la epoca seca(noviembre a mayo); mientras que durante La Nina se observ o un efecto contrario. La diversidad de plantas en el sotobosque fue monitoreada en parcelas permanentesdurante las epocas de lluvia y sequıa de 1989–1993. Los resultados sugieren que ENSO podrıa tener un efecto adicional sobre la disponibilidad de alimento para elvenado, principalmente en la epoca seca.

Key words: El Nino/Southern Oscillation; food availability; Mexico; Odocoileus virginianus; rainfall; tropical dry forest.

CLIMATIC VARIATION ASSOCIATED WITH THE NORTH ATLANTIC OS-

CILLATION (NAO) and El Nino/Southern Oscillation (ENSO) has

a widespread influence on biosphere primary production and onthe population dynamics of many organisms worldwide (Behren-

feld et al. 2001, Stenseth et al. 2003). These large-scale climate

fluctuations affect local weather through teleconnection patterns

(Stenseth et al. 2003). For example, ENSO episodes are associated

with rainfall patterns in the Pacific region, which affect both marine

and terrestrial ecosystems ( Jaksic 2001). Weather and ENSO effects

can have important consequences for the demography of terrestrial

herbivores (e.g., Owen-Smith 1990, Langvatn et al. 1996, Sæther

1997, McKinney et al. 2001, Marshal et al . 2002, Georgiadis et al.

2003, Oguto & Owen-Smith 2003), because they influence plant

phenology, forage quality, and biomass production, which in turn

affect habitat carrying capacity (Coe et al. 1976, Sinclair et al. 1985,

Langvatn et al. 1996, Sæther1997, Post & Stenseth 1999). Evidence

for these effects on ungulate populations has been generated prin-

cipally in temperate regions and in tropical Africa; in contrast, few

studies exist in Neotropical forests (e.g., Wright et al. 1999, Ticktin

2003).

In the Neotropical region, the white-tailed deer (Odocoileus

virginianus ) is used by indigenous and rural people to complement

their diet and, in some places, as a commercial and game trophy

(Mandujano & Rico-Gray 1991, Escamilla et al. 2000, Naranjo

1 Received 28 April 2005; revision accepted 22 November 2005.2 Corresponding author; e-mail: [email protected]

et al. 2004). Thus, white-tailed deer is an important species from

the management perspective. This study was carried out in a trop-

ical dry forest in Chamela, located on the Mexican central Pacificcoast, with densities of 10–14 deer/km2 (Mandujano et al. 2002).

The seasonality of rainfall is the main factor that influences this

ecosystem’s structure and dynamics (Murphy & Lugo 1986). Plant

communities in Chamela are affected by interannual variation in

trade winds, Pacific hurricanes, and ENSO events (Bullock 1986,

Garcia-Oliva et al. 1991). Along with rainfall, topography influ-

ences the availability of water on the ground, producing extensive

areas of tropical dry forest on slopes andmountaintops while narrow

patches of semideciduous tropical forest establish on riverbanks and

near streams (Lott et al . 1987, Bullock & Solis-Magallanes 1990).

Therefore, this forest is characterized by both temporal and spatial

variations in food availability, nutrients, and water for white-tailed

deer. The first part of this paper analyzes the effect of ENSO on

rainfall patterns in the Chamela study area using data collected over

26 yr. The second part of the paper provides preliminary evidence

for the importance of ENSO in modifying temporal food availabil-

ity for white-tailed deer using data collected over 4 yr.

The study was carried out at the Chamela Biological

Station of the UNAM (National Autonomous University of

Mexico), located on the coast of Jalisco, Mexico (19◦30N,

105◦00 W). The Biological Station (3319 ha) is part of the

Chamela-Cuixmala biosphere reserve (http://www.ibiologia.unam.

mx/ebchamela/HIST2.html). The Station is characterized by slopes

of 21◦ to 34◦ and altitudinal ranges from 30 to 580 m above

C 2006 The Author(s)Journal compilation C 2006 by The Association for Tropical Biology and Conservation

695



696 Mandujano

sea level. Mean annual temperature is 25◦C (Bullock 1986).

The climate is tropical, warm subhumid, with a marked season-

ality (http://www.ibiologia.unam.mx/ebchamela/clima.html). The

yearly minimum and maximum average temperatures are 22.1◦C

and 30.3◦C, respectively. The average number of days with sub-

stantial rain is 52, with approximately five strong rains per year(Bullock 1986). The average annual rainfall from 1977 to 2003 was

755±233 mm, with a total of 1394 mm in the rainiest year (1992)

and 392 mm in the driest (2001). On average, 80 percent of the

rain falls between July and October, whereas 20 percent falls during

the dry season from November to June. Around 90 percent of all

plant species begin to foliate with a minimum of 100 mm of rain

(Bullock & Solis-Magallanes 1990).

The dominant vegetation (covering >80% of the biological

station) is tropical dry forest located on hilly terrain with shallow

soils and low water retention. Many tree and shrub species lose

their leaves during the dry season. Forest height varies between 4

and 15 m and there is a well-developed understory (Lott et al .

1987). Common trees are Cordia alliodora [Ruiz y Pav.] Oken,

Lonchocarpus lanceolatus Benth., and Caesalpinia eriostachys Benth.

The station also has tropical semideciduous forest, ranging from

10 m to 25 m in height. This type of forest is found along the

streams in deep soils with high water retention. A lower percentage

of tropical semideciduous forest tree species lose their leaves during

FIGURE 1. Time series of Southern Oscillation Index values and annual rainfall (wet season plus dry season) for 1977–2003.

the dry season. The most common trees are Astronium graveolens

Jacq., Brosimum alicastrum Sw., and Sideroxylon capiri A. DC.

In this study, the year was considered as beginning in the

month of the first important rain, typically around the third week

of June (Bullock 1986). The probability of having 100 mm of rain

defines a 5-mo wet season ( June to October) and 7-mo dry sea-son (November to May; Garcia-Oliva et al. 1991). Rainfall data

from 1977 to 2003 were obtained from the station’s weather sta-

tion. ENSO information was obtained from the US Department

of Commerce data base (http://www.pmel.noaa.gov/tao/elnino/el-

boy-story.html), as well as from the Climate Diagnostics Cen-

ter (http://www.cdc.noaa.gov./ENSO/enso.different.html) and the

University of Illinois (http://www.ww2010.atmos.uiuc.edu/(G1)/

wwhlpr/guides/mtr/eln/def.rxml). From these data bases, a tempo-

ral pattern of the Southern Oscillation Index (SOI) was obtained

for the period 1977–2003, and SOI was calculated grouping data

from June to May. Negative values of SOI are associated with the

intrusions of El Nino (warm phase), and positive values with those

of La Nina (cold phase; Stenseth et al. 2003). The relationship be-

tween the SOI and annual and seasonal rainfall was analyzed with

linear correlations using the least squares procedure.

To estimate food availability, 25 permanent plots (10×10 m

each) were randomly established in tropical dry forest and 25 in

tropical semideciduous forest. During each sampling period, a 1 m2



Short Communications 697

quadrant was randomly established; only the young leaves and

branches of all species on the understory were collected. Samplings

were done in October for the wet seasons of 1989–1992 and in

May for the dry seasons of 1990–1993. Only the species consumed

by deer at this site according to Arceo et al. (2005) were consid-

ered. Information on food availability was estimated as the standingcrop biomass (g/m2) and species richness (number of species/m2).

However, because these variables were significantly correlated both

in tropical dry (df 7, R 2 = 0.79, P = 0.02) and semideciduous forest

(df 7, R 2 = 0.88, P = 0.004), the Shannon–Wiener diversity index

(H) was used as an estimate of the food available to the deer. The

index was estimated for each quadrant and then averaged to obtain

a seasonal index for each forest type.

Data analysis showed that from 1977 to 1984, annual rain-

fall was near to the average of 755 mm; this was followed by two

dry periods in 1985–1988 and in 2001–2003. The decade from

1989 to 2000 showed high variation in annual rainfall (Fig. 1).

The average rainfall during the wet seasons ( June to October) was

665 ± 222 mm (range: 368–1288 mm), while in the dry seasons

(November to May) it was 90±148 mm (range: 0–721 mm). The

rain that fell during the dry season was independent of the rain-

fall from the previous wet season (df 1,24, R 2 = 0.07, F = 1.73,

P = 0.20).

During the 26 yr analyzed, there were three strong El Nino

events (SOI >−2): 1982–83, 1991–92, and 1997–1998; and five

less evident (SOI: from −1.5 to −1.9) events during 1977–78,

1992–93, 1993–94, 1994–95, and 1998–99 (Fig. 1). La Nina (pos-

itive SOI) was evident during 1988–89, 1998–99, and 1999–2000.

There was no relationship between SOI and annual rainfall for the

1977–2003 period (Fig. 2; df 1,24, R 2 = 0.07, F = 1.83, P = 0.19).

The SOI was positively associated with rainfall during the wet sea-son (Fig. 2; df 1,24, R 2 = 0.40, F = 16.2, P = 0.0005). During the

El Nino years, rainfall was low in the wet season, while during La

Nina years rainfall was high in this season. During the dry season,

the SOI was negatively associated with the rainfall (Fig. 2; df 1,24,

R 2 = 0.28, F = 9.4, P = 0.005). In particular, during the El Nino

years (1982–83 and 1991–92) rainfall was high in the dry season,

while during La Nina years there was no rain in this season.

Monthly rainfall distribution and plant species diversity esti-

mates during the wet and dry seasons of the 1989–1993 period

appear in Figure 3. Species diversity was higher in the wet sea-

son than in the dry season with the exception of 1992 (df 1,12,

F =

30.9, P =

0.0001). Species diversity was similar in both forestsduring the wet season, while in the dry season diversity was higher in

the tropical semideciduous forest than in tropical dry forest (df 1,12,

F = 5.22, P = 0.04). During the wet season diversity increased

slightly in the La Nina year 1989, while during the dry season

the species diversity increased notably in the El Nino year 1992

(Fig. 3).

The analysis of 26 yr suggests a significant association between

ENSO and seasonal rainfall in the tropical dry forest of Chamela.

Unusually high rainfall during the dry season occurred when the

SOI was strongly negative (El Nino years), whereas unusually high

rainfall during the wet season and no rainfall during the dry season

occurred when the SOI was strongly positive (La Nina years). Lima

FIGURE 2. Relationship between annual, wet season and dry season rainfalland Southern Oscillation Index (SOI) in the tropical dry forest of Chamela

during the 1977–2003 period.

et al. (1999) found a similar pattern when analyzing a 99-yr period

in the semiarid region of Chile, as did Marshal et al. (2002) in

the Sonoran desert for a period of 47 yr. Thus, ENSO events

increase the variation of rainfall in the study region and have an

important consequence in the ecosystem function (Garcia-Oliva

et al. 2002).

Rain falling during the wet season controls vegetation growth

and hence annual food production for large ungulates (Coe et al.1976, Owen-Smith 1990, McKinney et al . 2001). The preliminary

evidence of this study suggests that similar to other sites (e.g., Ogutu

& Owen-Smith 2003), ENSO could have an additional effect on

plant species. This effect was evident during the 1992 dry season

when a record 649 mm rain fell in 15 d in January (Garcia-Oliva

et al. 2002). However, this effect depends on the month when

rain falls. For example, during the ENSO years 1983 and 1997,

177 and <100 mm fell in May, respectively. Thus, considering

the notable differences in floristic composition and life-forms, be-

tween the tropical dry and semideciduous forest in the study area

(Lott et al. 1987), we could expect a complex relationship between

ENSO, precipitation, and plant responses, as has been shown in



698 Mandujano

FIGURE 3. Monthly rainfall from June 1989 to October 1993 and seasonal

variation in plant diversity (Shannon index, H) of tropical dry forest (•) and

tropical semideciduous forest (◦).

others studies (e.g., Wright et al . 1999, Connell & Green 2000,

Williamson & Ickes 2002, Rolim et al. 2005).

During the dry season the response to hydric stress depends on

the uptake and control of water loss by plants (Reich & Borchert

1984). For example, in African savannah ecosystems the period of

retention of leaves into the dry season by deciduous trees and shrubs

is dependent upon the preceding rainfall (Owen-Smith 1990). At

the study site, leaffall normally occurs at the beginning of the dry

season, but leaves may remain longer in response to rain at the

beginning of the period (Bullock & Solis-Magallanes 1990). Thus,

in some years ENSO events increase rainfall during the dry sea-

son and the likelihood of plants staying leafy for a longer period

increases the food available to deer. ENSO conditions enhance

fruit production in tropical wet forests (e.g., Wright et al. 1999,

Williamson & Ickes 2002). Considering that white-tailed deer in-

creased the consumption of fruits during the dry season (Arceo et al.

2005), a possible additional effect of ENSO is the increase of foodavailability. However, the temporal dynamics among rain, ENSO,

plant growth, and fruit production are complex and need further

monitoring.

ACKNOWLEDGMENTS

This study was supported by CONACYT and the Department

of Biodiversity and Animal Ecology of Institute of Ecology A. C.

Xalapa, Mexico. F. Garcia-Oliva, S. Gallina, V. Arroyo-Rodriguez,

H. Gomez de Silva, and two anonymous reviewers provided useful

comments on the paper.

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Documents

Preliminary evidence of the importance of rainfall and ENSO in modify food availibility for white-tailed deer in a Mexican tropical dry forest