Effect of Disturbance on Population Dynamics of

Ecology C h a p t e r 12

Effect of Disturbance on Population Dynamics of Mexican Cycads Pablo Octavio-Aguilar1, 7, Andrés Rivera-Fernández2, Lourdes Georgina Iglesias-Andreu3, Andrew P.Vovides4, Miguel Ángel Pérez-Farrera5, Manuel Martínez-Melendez5, and Jorge González-Astorga6 1

Laboratorio de Genética, Instituto de Ciencias Básica e Ingeniería, Universidad Autónoma del Estado de Hidalgo. Ciudad del Conocimiento Carretera Pachuca-Tulancingo Km. 4.5. C.P. 42184, Mineral de la Reforma, Hidalgo, México 2 Facultad de Agronomía, Universidad Veracruzana. Circuito Gonzalo Aguirre Beltrán s/n, Zona Universitaria. C.P. 91090, Xalapa Enríquez, Veracruz, México 3 Instituto de Biotecnología y Ecología Aplicada, USBI-U V. Av. de las Culturas Veracruzanas No.101 Col. E. Zapata. C.P. 91090, Xalapa, Veracruz México 4 Laboratorio de Biología Evolutiva de Cycadales, Red de Biología Evolutiva, Instituto de Ecología, A.C. Apdo. Postal 63. C.P. 91070, Xalapa, Veracruz, México 5 Herbario Eizi Matuda, Universidad de Ciencias y Artes de Chiapas. Libramiento Norte Poniente, Col. Lajas-M aciel. C.P. 29039, Tuxtla Gutiérrez, Chiapas, México 6 Laboratorio de Genética de Poblaciones, Red de Biología Evolutiva, Instituto de Ecología. A.C. Km 2.5 Antigua Carretera a Coatepec No. 351, C.P. 91070, Xalapa, Veracruz, México 7 Author for correspondence; email: aguilpo@yahoo.com.m x Keywords: demography, projection matrices, life t able response experiment, disturbance, conservation, Mexican cycads

Abstract Demographic models based on projection matrices can infer the effect of disturbance on natural cycad populations by human activities. However, owing to the longevity of plants, -1—

it is difficult to determine long-term population dynamics. Fortunately, today, several

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population studies have allowed us to determine the overall effects of disturbance on these

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D i stur banc e and P op ulati on D y nam i c s i n M e x i can C ycad s

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long-lived species, thus facilitating efficient decision making for their conservation and management. In general, disturbance decreases the reproductive potential of the populations through elimination of adult plants and increase the sensibility of early stages, therefore underpinning the importance of reproductive adult permanence to maintain healthy populations and survival.

Resumen Los modelos demográficos basados en matrices de proyección pueden inferir el efecto del disturbio por actividades humanas sobre las poblaciones naturales de cícadas. Sin embargo, debido a la longevidad de las plantas es difícil determinar su dinámica poblacional a largo plazo. Afortunadamente, hoy en día; se han realizado varios estudios poblacionales para determinar el efecto del disturbio sobre estas especies de larga vida, facilitando la toma de decisiones eficaces sobre su manejo y conservación. En general, el disturbio disminuye el potencial reproductivo de las poblaciones al eliminar plantas adultas además de incrementar la sensibilidad de etapas tempranas del ciclo de vida, por lo tanto; se sustenta la importancia de la permanencia de los adultos reproductivos para el mantenimiento saludable y la sobrevivencia de las poblaciones.

Introduction Projection matrix models can be a guide for management and conservation of plant populations. Many demographic studies have been published to date with dozens appearing annually (Crone et al., 2011). These models are based on observation of multiple events through the lifecycle of the species such as: (1) mortality by category, (2) growth of individuals, (3) fecundity and permanence, or (4) stasis in any given category. The majority of demographic studies, in plant species, are performed over a limited period of a few years, with results being adequate for short-term conservation strategies (Stott et al., 2010), but with limited application for long-l ived species, such as cycads that show long transit periods through their life-cycle stages. Population growth rates and survival of seeds and seedlings, which are extremely sensitive to desiccation, depends largely on environmental conditions of the population’s location (Donaldson, 2003). Per-

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forming short-term demographic studies on single locations is partial and unreliable,

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Cycad Biology and Conservation since a short period extract a low variability. However, several research groups have applied bi-or tri-annual models on different cycad species and contrasted different management conditions, and in some cases, these models result in more robust conclusions (Negrón-Ortiz et al., 1996; Raimondo & Donaldson, 2003; Pérez-Farrera et al., 2006; Octavio-Aguilar et al., 2008; Cabrera-Toledo, 2009; Álvarez-Yepiz et al., 2011; Lázaro- Zermeño et al., 2011). In general, the illegal trade reduces both reproductive components and persistence of natural populations.

Matrix Analyses Matrix projection in the majority of cycads showed a population growth rate (λ) that was positive or near unity, even when distinct periods and contrasting conditions have been averaged (Table 12-1). This shows plant persistence in spite of environmental adversities. However, this apparent stability maybe the result of insufficient sampling periods that could not detect important changes (Caswell, 2001). With respect to population growth of Zamia loddigesii, the decrement of 21.8% could be explained by a sampling of three periods over 10 years, thus registering mortality and loss of individuals that could otherw ise go undetected in studies of shorter duration, even

able 12-1 Population growth rate (λ) calculated by matrix T analysis for Mexican cycads. Species Ceratozamia mirandae C. norstogii C. mexicana Dioon caputoi D. edule D. merolae D. sonorense Encephalartos cycadifolius E. villosus Zamia amblyphyllidia Z. loddigesii Mean ± s.d.

λ 0.955 1.075 1.081 1.007 0.996 1.000 1.098 1.000 1.047 0.983 0.782 1.005 ± 0.082

Citation Pérez-Farrera et al., 2006 Martínez-Melendez, 2012 Rivera-Fernández,2012 Cabrera-Toledo, 2009 Octavio-A guilar et al., 2008 Lázaro-Zermeño et al., 2011 Álvarez-Yepiz et al., 2011 Raimondo & Donaldson, 2003 Raimondo & Donaldson, 2003 Negrón-Ortiz et al., 1996 Octavio-A guilar unpublished data

-1— s.d.= standard deviation.

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with different sampling periods. In this respect, sampling does not need to have the same periodicity given that each matrix represents an event and not necessarily units of time (Caswell & Shyu, 2012). Since long-term studies require greater resources, it is desirable to find an alternative method to evaluate effects of stochastic events on populations without multiple sampling periods on any given site. The present study compares results from previous studies to determine the effects of disturbance on the population dynamics of some Mexican cycads using life t able response experiment (LTRE) analysis, with aims to establish a general conservation and management strategy for their populations, but keeping in mind that higher number of samplings results in greater confidence, nevertheless sampling needs to be optimized.

Life Table Response Experiment Method In this paper, previous data were taken up to four Mexican cycads: Ceratozamia mexicana (Rivera-Fernández, 2012), C. mirandae (Pérez-Farrera et al., 2006), C. norstogii (Martínez- Melendez, 2012), and Dioon edule (Octavio-Aguilar et al., 2008). The analysis considered sites located in contrasting disturbed conditions or data generated by modeling in the case of D. edule. An estimate of the sensitivity of the population growth rate to concrete perturbations is LTREij = Vij xδλ / δ aij |mean, where Vij is the standard deviation of the transition matrix elements, and δλ/δa ij is the sensitivity evaluated at the mean matrix. Following Caswell (2001), the method considers T … T treatments with population growth rates λ … λ . We choose a reference N

1

1

N

(r)

matrix A as a baseline against which to measure treatment effects, where A(r) is the mean matrix A(r ) =

1 ∑A(i ). N i

With this mean matrix, we estimated λr. Similarity, with the mean matrix A(m) from treat-

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ment without disturbance, we estimated λ m.

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Cycad Biology and Conservation Expanding λ as a function of aij, around A(r), gives the growth rate in treatment m as:

(

)

λ m ≈ λ r + ∑(aij(m) − aij(r ) ) δλ δ a AE , ij ij where A = (A(m) + A(r)/2. The terms in the summation are the contributions of the aij to the effect of treatment m on population growth. The difference between λm (estimated from the matrix A(m)), and λr (estimated as a function of the aij, around A(r)), determine how the treatments contribute to differences in lambda (Caswell, 2001).

Effects of Disturbance Disturbances are events that cause positive or negative changes in the natural development of a population (Sánchez-Velásquez & Pineda-López, 2010). If the event persists, the population dynamics must be modified as an adaptive response; however, if this is inadequate, the population growth rate diminishes, leading to extinction. Elasticity analysis may be used to identify lifecycle stages susceptible to management for conservation biology studies. In this sense, transitions of high elasticity values have an important impact on λ (Caswell, 2001). Decreasing populations show high elasticity for survival of individuals and a lesser elasticity in growth and reproductive components (Lehtilä et al., 2006). Other comparative analyses such as LTRE may be used to compare sensitivity matrixes of different populations since they are based on transition rate variability and their total variance contribution to the population growth rate (Caswell, 2001). At least four comparative studies on Mexican cycads with contrasting disturbance conditions have been done to date. Based on these matrixes, an LTRE analysis was made to determine if disturbance effect is significant in the transitions for explaining variance of λ (Table 12-2). In all cases, disturbance diminishes population size or λ values, but significance levels are unknown. In accordance to what has been reported for long-lived species, survival of individuals (L) was expected to have been an important factor in that its elasticity value increases response to disturbance (Lehtilä et al., 2006). This has not been observed with the Ceratozamia spp. analyzed. -1—

In the case of C. mirandae, the matrix size classes w ere more precise for recording

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between stage transitions, making growth elasticity values (G) higher than the permanence

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able 12-2 Elasticity values of four Mexican cycads in T contrasting disturbance conditions. Elasticity values λ

L

G

F

Ceratozamia mexicana (undisturbed) C. mexicana (disturbed)

1.136 1.027

0.534 0.491

0.382 0.417

0.085 0.092

C. mirandae (undisturbed) C. mirandae (disturbed)

1.039 0.860

0.195 0.048

0.499 0.783

0.306 0.169

C. norstogii (undisturbed) C. norstogii (disturbed)

1.74 1.23

0.399 0.579

0.452 0.318

0.148 0.072

Dioon edule (undisturbed) D. edule (disturbed)

0.999 0.995

0.839 0.966

0.161 0.034

0 0

Species

LTRE

λ r: 1.169; λ m: 0.98* λ r: 1.024; λ m: 0.95* λ r: 2.025; λ m: 1.48* λ r: 1.002; λ m: 0.99

λ = population growth rate; λ m = mean population growth rate between treatments; λr = maximum population growth rate obtained from mean matrix analysis; F = reproductive component elasticity; G = individual growth elasticity; L = permanence elasticity of individuals (stasis); LTRE = values of the life t able response experiment. *p