Root contraction does not increase long-term population growth in Pediocactus bradyi, an endangered cactus of Northern Arizona

TL;DR

A study on an endangered cactus in Arizona finds that root contraction does not significantly affect long-term population growth.

Contribution

The study is the first to use long-term demographic data to model the population dynamics of Pediocactus bradyi with and without root contraction.

Findings

Root contraction had no significant effect on long-term population growth rates.

Larger cacti have higher survival and reproductive rates, while growth declines after a certain size.

Survival and growth are the most important factors influencing population growth.

Abstract

Pediocactus bradyi, a semi-globose cactus endemic to northern Arizona, displays a root-contraction mechanism to survive extreme drought: its roots contract, pulling the stem below ground during dry periods, re-emerging once rains return. To quantify how root contraction shapes population dynamics, we developed an integral projection model based on 31 years of demographic data from a P. bradyi population on the Navajo Nation Off-Reservation Trust Land. We explored two scenarios: one including root contraction and one excluding it. We found that, being ∼10% of the individuals and mostly confined to smaller individuals, root contraction did not have an effect on the long-term population growth [λ = 1.041 (1.039, 1.303) with contraction vs. λ = 1.044 (1.035, 1.289) without]. Also, we show that larger individuals have higher survival and reproductive rates, while growth declines beyond 35 mm in diameter. An elasticity analysis confirmed that survival and growth are the main vital rates affecting population growth, followed by root elongation after contraction. Thus, while root contraction may improve individual survival, elongation is in fact more important at the population level. Therefore, as with most cacti species, conservation efforts should focus on ensuring the survival of large individuals irrespective of their root contraction status.

This study assesses whether root contraction affects long-term population dynamics of the endangered cactus Pediocactus bradyi in northern Arizona. Using 31 years of demographic data, models with and without root contraction yielded nearly identical population growth rates. Root contraction occurs infrequently, mainly in small individuals, and has negligible demographic impact. Population dynamics are driven primarily by survival and growth of large individuals, with growth slowing beyond a size threshold. Conservation should prioritize maintaining large individuals rather than root contraction status.

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