然而,传统的生态位理论在解释热带雨林的物种多样性时遇到了困难:热带雨林的物种多样性太高,无法用传统的生态位分化观点来解释,即没有足够数量的生态位容纳如此众多的雨林树种。以BCI (Barro Colorado Island, 巴拿马) 乔木群落为例,3/4的物种是耐荫树种,它们之间没有明显的生态位分化,无法根据生态位理论予以说明 (Hubbell, 2005a, 2005b, 2006)。与此不同,以美国生态学家Stephen Hubbell为代表的学者 (Hubbell, 1979, 2001, 2005a, 2005b, 2006; Hubbell & Foster, 1983, 1986) 提出了类似于种群遗传学中性理论的解释,认为生态学上相同的物种可以实现共存,物种多度的变化是随机的,而非确定性的;共存的物种数量取决于物种分化(或迁入)和随机灭绝之间的平衡;群落内物种的相对多度随时间表现为随机振荡的波动。之所以被称为中性理论,是因为这一理论假定群落中所有的个体在生态学上都是完全等同的(或者对称的):具有相同的出生率、死亡率、迁移率以及新物种形成的概率。
事实上,中性理论可溯源至MacArthur和Wilson (1967) 首创的岛屿生物地理学理论。岛屿生物地理学认为岛屿上物种的数目取决于物种迁入岛屿的速率和定居在岛屿上物种的灭绝速率,不同岛屿上实际生存的物种数目受到离大陆距离和岛屿面积的强烈影响。该理论没有考虑物种之间的任何差异,也未考虑岛屿上共同出现物种之间的生态位分化问题。Hubbell的群落中性理论将岛屿生物地理学加以扩展:不仅关心“岛屿”上的物种数量,同时还考虑物种的种群动态,并引入了“物种分化”这个进化时间尺度上的生物地理学过程,用于解释生态学时间尺度上局域群落 (Local community) 内以及进化时间尺度上集合群落 (Metacommunity) 内的物种多样性维持问题。这里,局域群落指的是生活在局域空间中竞争相似或相同有限资源的同一营养级的物种集合。而集合群落则是许多局域群落通过迁移连接而成的网络,同时允许在更大的生物地理学空间尺度和进化时间尺度上发生的过程。在大的集合群落尺度上,物种多样性取决于出生、死亡、物种灭绝和新物种分化之间的动态平衡;局域群落通过迁入与集合群落相连,出生、死亡和来自于集合群落的迁入决定了局域群落的结构。在考虑局域群落动态时,因为集合群落动态,相对于局域群落动态而言,发生在更大的空间尺度和更长的时间尺度上,因而集合群落可以看成是相对静止和不变的(Hubbell,2001)。但中性理论并没有明确界定局域群落的范围,只指出局域群落要足够小以至于可以忽略物种分化的影响,而且在实际工作中局域群落和集合群落之间的界线在很多情况下是难以区分的(Hubbell, 2001)。
群落中性理论的两个基本假设是:1)群落内生物个体总的数量固定,某一物种多度的上升必然伴随着其它物种个体数量同等程度的减少;2)所有物种的个体都具有完全相同的出生率、死亡率和扩散率。在这些假定条件下,它预测局域群落内物种相对多度的分布符合“零和多项式分布”(Zero-sum multinomial distribution),而非先前普遍接受的“对数正态分布”。与对数正态分布相比,零和多项式分布曲线在稀少物种一侧的尾部较长,其长度取决于局域群落的大小和个体扩散速率。在解释热带雨林树种的相对多度分布以及种数-面积关系等方面,该理论获得了巨大的成功,尽管在绝大多数情况下不同物种不可能有完全相同的生育率、死亡率和扩散速率。事实上,Hubbell本人也承认他所研究的雨林树种在生长速率和耐阴能力等方面存在着显著差异,但是他认为这些差异对群落结构的形成并不重要,而发生在个体水平上的统计随机性将成为最主要的决定因素 (Hubbell,2001;Bell, 2000, 2001)。
在本文内,我们将系统地介绍中性理论的假设、模型和预测,最新的理论和实验研究进展,以及相应的检验工作,并对今后的发展趋势进行了大致概括,以期能为我国开展有关研究提供一些必要的前沿动态信息。
1 物种多度分布模式和群落中性理论
其中,α>0,0<x<1是常数。由Taylor展开式有:
-ln(1-x)=x+
由(1)式定义的Fα(n)恰好正比于上述对数展开中的各项,故称此分布为对数级数分布或对数分布(Logarithmic distribution)。α即著名的Fisher-α。因为
J=
则x=
S=
这里的多样性指数α曾被用来拟合个体数量大的昆虫和动物的数据,但是它的理论基础并不清楚。而且当应用于热带雨林时,人们发现它经常低估了物种数。
Fisher和Preston等的模型并不依赖于统计学过程。真正把统计理论和统计种群模型连系起来的是种群遗传学中Karlin和McGregor(1967)提出的中性模型。虽然这一模型的背景是种群遗传学,但只需用“物种”、“物种分化和迁移”、“群落”分别代替模型中的“基因型”、“突变”和“种群”,就很容易将它平移到生态学中来。在这个模型中,群落的大小是固定的,即包含J个个体,在每一代,新物种形成的速率为ν。因此,新物种通过迁移或物种分化以速率为ν的泊松过程进入系统。群落中共有S个物种,第i个物种的多度ni在单位时间内只能增加或减少1个单位。Karlin和McGregor (1967) 研究了F(n),即当ni=n时,种i的期望多度,也就是群落生态学中的物种数-多度分布。假定有很多明显不同的物种,并且所有的物种都是小种群,则物种数-多度服从对数级数分布:
其中λ和μ分别是单位出生率和死亡率与种群大小的比值。和公式(1)相比较,νJ相当于Fisher-α,而λ/μ相当于公式(1)中的x。
在大小为J的局域群落中,种i增加一个个体,种j减少一个个体的概率为
Pr{Ni+1,Nj-1,Nk,…,NS|Ni,Nj,Nk,…NS}=
相对物种多度不变的概率为
Pr{Ni,Nj,Nk,…,NS|Ni,Nj,Nk,…NS}=
其中m是单位迁移率,Pi是种i在集合群落中的相对多度。这个转移概率矩阵的特征向量给出了局域群落中各种可能的相对多度组合(假设共有C个)的可能性,记第k个相对多度组合出现的可能性为φ(k),则大小为J的局域群落中按多度排序后物种的期望多度为:
其中ri(k)是相对多度排序为第i的物种在第k次相对多度组合中的多度。局域群落中的物种相对多度分布是一种零和多项式分布,它的形状取决于θ,局域群落的大小J,以及迁移率m。它的优势度-多样性曲线在半对数刻度下是S型的。依赖于与集合群落的隔离度(m的大小)以及物种多度,它的形状可以是似对数级数的、似对数正态的或者似几何形状的(图1)。
图1
图1
不同群落中的物种相对多度分布模式
群落中的物种按照其相对多度从大(左)到小(右)排列。Y轴采用对数刻度,表示物种相对多度的百分值 Species in each community are ranked in percentage relative abundance from the commonest (left) to the rarest (right). The percentage relative abundance is log transformed on the y-axis 1. 亚马逊潮湿的热带雨林 Tropical rain forest in Amazonia 2.哥斯达黎加的热带干燥落叶林 Tropical dry deciduous forest in Costa Rica 3.北太平洋旋涡的海洋桡脚类浮游生物群落 Marine planktonic copepod community from the North Pacific gyre 4.英国的陆地鸟类群落 Terrestrial breeding birds of Britain 5.巴拿马热带蝙蝠群落 (引自Hubbell, 2001) Tropical bat community from Panama (from Hubbell, 2001)
Fig.1
Patterns of relative species abundance in a diverse array of ecological communities
对于集合群落,Hubbell采用Karlin和McGregor (1972) 的方法,假定新物种以点突变的方式形成,则对集合群落大小为J的抽样,其中包含S个物种,且各个物种包含的个体数分别为n1, n2, …, nS,J=
Pr{S,n1,n2,…nS}=
其中θ=2
其中ri是对ni按优势度排序后的结果。C是相对多度组合的总数,ri(k)是相对多度排序为第i的物种在第k次相对多度组合中的多度。Pr{S, r1, r2,…, rS}k是第k次组合的概率。当J趋于无穷大的时候,集合群落的相对物种多度分布将趋于对数级数分布。θ相当于对数级数分布中的α,θ不仅决定了平衡状态下物种丰富度,而且也决定了集合群落中的物种相对多度,Hubbell称其为无量纲的基本多样性指数(Fundamental diversity number)。而如果新物种是以随机裂变的方式形成的,则集合群落的相对物种多度服从零和多项式分布。
在上面的讨论中,我们假定D=1。如果1<D<J,或者D不是一个常数,而是一个随机分布但期望值为D的随机变量,平衡状态下物种相对多度分布模式不受影响,但方差却不相同。大的D意味着群落更快地趋于平衡,也意味着方差更大。
2 群落中性理论的最新进展
继Hubbell (2001)一书出版之后,中性模型从两个途径在理论上取得了很大进展。一种方法是前推法(Forward in time),这种方法采用马尔可夫链来描述状态和转移概率,用控制方程推导出大小为J的局域群落中包含特定个体数的期望物种数(Volkov et al., 2003; Alonso & Mckane, 2004)。其中Volkov等(2003)从刻划种群简单生死过程的随机模型出发,推导出在集合群落中,物种的相对多度曲线符合Fisher的对数级数分布。所不同的是,公式(1)中的x=b/d, 而θ=SMP0v/b相当于Fisher-α,其中P0为灭种的概率,SM为集合群落中的物种数。至此Fisher-α也有了基于生物学的解释。而对于局域群落,包含n个个体的期望物种数为
其中γ=
对于给定的一组参数:J、θ和m,可以求公式(5)的数值解。对于比较大n,上述积分可以用快速下降法快速而有效地求解 (Morse & Feshbach, 1953)。Volkov等就BCI数据对此分析解和对数正态分布的拟合结果做了比较。虽然中性模型的解析解只有1个参数而对数正态分布包含3个参数,中性理论的预测结果比对数正态分布更接近实际观测数据。
其中
记(x)y=
3 中性理论的基本假设
所有物种的个体在生态学上的对等性或相等性是中性理论最基本的假设和出发点,然而同时也是中性理论遭到最多质疑之处。在一个中性群落中,所有的个体,不管属于哪一个物种,在生态学上都是相同的。这不仅意味着它们出生、死亡和迁移的可能性相同,同时意味着种间的竞争也是对称的。在这样的假定之下,物种间在生活史上的差异将不复存在,密度依赖以及其它使得群落趋于稳定的因素也不可能存在。比如就密度依赖来说,死亡率将随着种群大小的增加而增加,即稀有种相对常见种具有一定的优势 (Armstrong, 1989; Chave et al., 2002)。Hubbell (2003) 认为只要密度依赖的强度在各个种间相同,这样的模型就是中性的。然而如果有密度依赖存在,则一个稀有种的个体的死亡可能性就低于一个常见种的个体。因此,密度依赖和个体在生态学上相同的假定实际上是互相矛盾的。
Zhang和Lin (1997) 以及Yu等(1998)先后在中性模型的基础上假定物种间出生率或死亡率有微小差异。他们发现只要物种间竞争能力有微小差异,共存时间就会急剧下降,群落很快被竞争强者(出生率高或死亡率低)所统治。这说明个体在生态学上相同的假设对于中性理论至关重要,同时说明中性理论从这个角度来看是很脆弱的。另外,中性理论完全没有考虑Allee效应,即小种群由于交配机会的限制(或者其它因素)所表现出的种群适合度下降。Allee效应在具有很高物种多样性的群落如热带雨林中肯定是一个不可忽略的因素,因为很多种都是很稀少的。Allee效应对稀有种不利,使得物种共存时间显著下降,并因而导致群落物种多样性和相对多度分布发生显著改变(Zhou & Zhang, 2006)。
然而另一方面,Hubbell (2006) 构造了一个简单的、空间明晰的群落进化模型,并以热带耐荫树种的一组典型的生活史特征为例,说明了生态学上的对等性是很容易进化产生的,尤其在受扩散和更新限制的物种丰富的群落中更是如此。一旦能够进化出生态学上的对等性,则物种可以共存任意长的时间。比如BCI雨林,树种在很大程度上是扩散和更新限制的,一方面因为种子扩散有限,另一方面则是由于捕食者和病原体以及其它非生物因素(比如干旱)的影响使得物种更新受到严重限制。所有这些因素以及群落的高物种多样性(丰富度)很大程度上降低了功能相似物种的竞争排除的可能性,最终使得缓慢的物种分化速率足以补偿物种灭绝。在物种丰富的群落中,一个物种的不同个体的邻居往往大不相同。以BCI为例,一棵树的20个近邻中平均有14个不同的树种,而两个同种个体的20个近邻中平均只有4个物种相同 (Hubbell & Foster, 1986)。在这样的群落中,有方向性的进化不大可能会发生,有效的生态位分化(性状替代)也就很难形成。相反,物种可能集中在相似的常见生活史策略上,以适应在它们的进化周期中最常遭遇的环境。耐荫树种数占总的物种数的3/4,说明物种不是均匀地分布在生活史拓扑空间(Life-history manifold),而是大多数物种集中在耐荫的一端。在这些物种的进化历史中,更多地经历了阴暗生境而非阳光充足的生境,因此大多数物种的生活史都朝着有利于在阴暗生境中生存和生长的方向进化,而不管有多少物种采取了相同的进化路线 (Hubbell, 2006)。
Hubbell (2006) 在最近的研究中还提供了间接的经验证据支持中性假设。如果物种在功能上或生态学上完全或近似地相同,则增加物种多样性不会增加群落的稳定性或生产力,因为增加生态学上相同的物种不会增加功能多样性。如果群落的多样性和稳定性正相关,则多样性高的区域个体周转率应该较低(用绝对多度平均变化百分率来衡量),但Hubbell发现BCI数据并不支持这个预测,多样性高的区域周转速率不仅不低而且还有增大的趋势。Hubbell同时还发现,物种数量和生产力之间不存在正相关关系。
4 检验中性理论的预测
全世界的热带雨林大约有1 700万km2,胸径10 cm以上的树木大约1012棵,共计大约有10 000个树种,是地球上生物多样性的主要组成部分 (Novotny et al., 2002)。这也是用热带雨林检测多样性假设的原因。但也有来自于其它群落的验证。
几乎在同时,Magurran和Henderson ( 2003) 用历时21年对英国布里斯托尔海峡鱼类群落调查数据做了物种多度分布分析。他们发现物种可以分成两大类:在记录中存在10年以上的永久种,以及少于10年的瞬时种。他们还发现这两类物种的物种多度分布不一样:永久种服从对数正态分布,而瞬时种则服从对数级数分布。所以,只有稀有种可能是中性的,而常见种则不是,这与Bell(2000)得出的结论类似。Bell(2000)把物种分成“迁入种”(`Immigrant' species(即在最后100步模拟中进入群落的物种))和“长驻种”(`Long-resident' species)两类。对这两类物种构建物种多度分布的结果与Magurran和Henderson(2003)相一致。
图2
图2
中性模型对沙捞越Lambir山地国家公园内热带树种群落优势度-多样性曲线的拟合
点虚线是θ=310且没有扩散限制(m=1)的集合群落最佳拟合。52 km2样地的树木群落的相对丰富度数据的最佳拟合是θ=310和m=0.15。误差条是±标准差。粗线是观测到的优势度-多样性曲线。中性模型对实测数据的1 197个种拟合得非常好(r2=0.996)(引自Hubbell 2006)
Fig.2
The fit of the UNT to the dominance-diversity curve for the tropical tree community in Lambir Hills National Park, Sarawak (Borneo)
The dotted line extending diagonally down to the right is the best-fit metacommunity curve for θ=310 assuming no dispersal limitation (m=1). The distribution of relative tree species abundance for the 52 ha plot was best fit with θ=310 and m=0.15. The error bars are ± one standard deviation. The heavy solid line is the observed dominance-diversity curve. The agreement between the fitted line and the observed line for 1 197 species excellent (r 2 = 0.996) (from Hubbell 2006)
另一方面,Chave等(2002)对中性模型和非中性模型预测的物种多度分布模式进行了比较,发现这两种模型预言了相似的物种多度分布模式。他们分别对包含和不包含扩散限制的空间结构化中性模型以及6个非中性模型(包括密度依赖和/或种间替换、包含或不包含扩散限制)进行了模拟。结果表明,扩散限制在很大程度上影响物种多度分布,而密度依赖对物种多度分布的影响较小。其它研究也表明非中性模型和中性模型所预测的模式难以区分 (McGill, 2003; Mouquet & Loreau, 2003; Sugihara et al., 2003)。但由此并不能推翻中性理论。用Hubbell (2006)的话说,即使如此,中性模型也是最简约的。从这一角度如果想推翻中性理论,就得证明非中性模型的复杂性是必要的。
Poulin (2004) 研究了脊椎动物肠内寄生虫群落的多样性分布模式。其物种相对多度分布模式与Hubbell的中性理论预测相吻合。但问题在于寄生虫群落却明显不满足中性模型的假设。一方面寄生虫在寄主种群中没有饱和,另一方面也不满足个体在生态学上相同的假定,因为寄生虫的体积存在很大差异。
上述研究针对的是中性模型预测的优势度-相对多度是不是与实际群落的实验数据相吻合。然而Wootton认为,这种检测只能算是一种弱检测。因为实际上对于群落动态至关重要的集合群落大小、物种分化速率以及迁移率和死亡率等参数几乎不可能度量。而且中性模型中决定物种相对多度分布的参数θ和m在很大范围内是可以调节的,这使得中性模型可以与很大范围内的函数形式相一致。一个强检测意味着能从系统动态中得到模型参数的估计值,从而对取得参数值的模型的预测结果与实际数据相比较(Wootton,2005)。
5 新物种形成模式
在Hubbell(2001)的中性理论中,假定了两种新物种形成模式。一种是“点突变“模式(Point mutation mode),也就是一个个体生成一个新物种的概率。这种模式类似于多倍体隔离后的情况。另一种模式叫做“裂变”模式(Fission mode),指的是一个物种突然分裂成两个物种。Ricklefs (2003)考察了物种分化速率为v时中性理论的预测结果。在第一种模式下,新物种的初始个体数为1,从而可能很快灭绝,这意味着将有很多寿命短暂以至分类学家难以识别的物种。中性理论预言物种的平均寿命大约为T~2ln(1/(2v))。用上面我们列举的热带雨林的数据来计算,可得θ大约为100,或者物种分化速率大约为v=θ/N=10-10。从而用上面的公式可以算出物种的平均寿命为44代,或者4 400年(假定一代是100年)。Ricklefs指出这将意味着在BCI每100年会产生25个新树种,即存在大量的用经典的分类方法难以识别的隐性种。另一方面,在裂变模式下,又会导致太多的常见种以及太少的稀有种。为了回应Ricklefs的说法,Hubbell (2003)指出,只有那些古老而又丰富的世系才被分类学家认为是物种。因此,Ricklefs所说的隐性种可能只是种内的变异,而这在种群遗传学中是常见的。Hubbell(2003)认为,他在2001年提出的两种物种分化模式代表了物种分化连续体的两个极端。他提出了一种介于二者之间的物种分化模式,即“周边隔离物种分化”(Peripheral isolate speciation)。在这种模式下,物种的寿命将取中间值,新物种的初始种群大小也将介于前两者之间。并以模拟的方式说明新物种的初始大小对物种多度分布模式有着深刻的影响。
6 总结
生物多样性的分布格局和维持机制一直是群落生态学研究的核心问题。围绕着这一问题,生态位理论和群落中性理论的争论还将继续。中性理论就现在来看也许不是现实群落的一个精确反映。但它的确包含了其它理论所忽略的成分,它强调了随机性的重要作用,把发生在局域尺度上的生态学过程和发生在区域大尺度上的进化和生物地理学过程(比如物种分化和谱系地理学)联系起来了。它至少提供了一个不同时空尺度上群落动态的数值化的零假设。也许我们最需要做的,是在生态位理论和中性理论之间架起一座桥梁,同时发展包含随机性的生态位模型,以及允许种间差异的半中性(Semi-neutral)或者近中性(Nearly neutral)模型1)( Zhou SR, Zhang DY. The nearly neutral theony of biodiversity. (Unpublished))。Volkov等 (2003) 提供的新框架将有助于构建同时包含随机性和确定性过程的综合模型。当前的中性理论并不要求共存物种间有生态位分化,因而生态位在这里没有发挥作用的空间。如果能够把中性理论和生态位理论的关键要素结合起来解释群落多样性模式,那么这将是一次真正的生态学突破(Chase, 2005)。
群落中性理论本身还处在发展初期,它的进一步发展和完善将依赖于理论和实验研究两方面的进展。目前关于中性理论的检验也主要集中在热带雨林,只有少数例外 (Pandolfi, 2002; Magurran & Henderson, 2003; McGill, 2003; Poulin, 2004),未来一个很重要的工作是将对中性理论的检验拓展到其它群落。而关于中性理论的假设、预测以及物种分化模式等都还有很多工作要做。其中包括扩散限制和更新限制在群落装配中的作用,具体地说,即扩散和更新替代限制是不是能够使得种群动态和竞争排除足够缓慢,从而使缓慢的新物种形成能够补偿物种灭绝造成的损失?另外,进化的证据是不是足以解释生态学上的对等性是很容易形成的?而构建合适的统计模型以检验中性模型及其预测也是未来的一个重要方向。
参考文献
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Competition, seed predation, and species coexistence
The distribution of abundance in neutral communities
A spatially explicit neutral model of beta-diversity in tropical forests
To represent species turnover in tropical rain forest, we use a neutral model where a tree's fate is not affected by what species it belongs to, seeds disperse a limited distance from their parents, and speciation is in equilibrium with random extinction. We calculate the similarity function, the probability F(r) that two trees separated by a distance r belong to the same species, assuming that the dispersal kernel P(r), the distribution of seeds about their parents and the prospects of mortality and reproduction, are the same for all trees regardless of their species. If P(r) is radially symmetric Gaussian with mean-square dispersal distance sigma, F(r) can be expressed in closed form. If P(r) is a radially symmetric Cauchy distribution, then, in two-dimensional space, F(r) is proportional to 1/r for large r. Analytical results are compared with individual-based simulations, and the relevance to field observations is discussed.
Comparing classical community models: theoretical consequences for patterns of diversity
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Two hypotheses to explain potentially high forest biodiversity have different implications for the number and kinds of species that can coexist and the potential loss of biodiversity in the absence of speciation. The first hypothesis involves stabilizing mechanisms, which include tradeoffs between species in terms of their capacities to disperse to sites where competition is weak, to exploit abundant resources effectively and to compete for scarce resources. Stabilization results because competitors thrive at different times and places. An alternative, 'neutral model' suggests that stabilizing mechanisms may be superfluous. This explanation emphasizes 'equalizing' mechanisms, because competitive exclusion of similar species is slow. Lack of ecologically relevant differences means that abundances experience random 'neutral drift', with slow extinction. The relative importance of these two mechanisms is unknown, because assumptions and predictions involve broad temporal and spatial scales. Here we demonstrate that predictions of neutral drift are testable using palaeodata. The results demonstrate strong stabilizing forces. By contrast with the neutral prediction of increasing variance among sites over time, we show that variances in post-Glacial tree abundances among sites stabilize rapidly, and abundances remain coherent over broad geographical scales.
Mortality rates of 205 neotropical tree and shrub species and the impact of a severe drought
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The importance of dispersal for biodiversity has long been recognized. However, it was never advertised as vigorously as Stephen Hubbell did in the context of his neutral community theory. After his book appeared in 2001, several scientists have sought and found analytical expressions for the effect of dispersal limitation on community composition, still in the neutral context. This has been done along two relatively independent lines of research that have a different mathematical approach and focus on different, yet related, types of results. Here, we study both types in a new framework that makes use of the sampling nature of the theory. We present sampling distributions that contain binomial or hypergeometric sampling on the one hand, and dispersal limitation on the other, and thus views dispersal limitation as ubiquitous as sampling effects. Further, we express the results of one line of research in terms of the other and vice versa, using the concept of subsamples. A consequence of our findings is that metacommunity size does not independently affect the outcome of neutral models in contrast to a previous assertion (Ecol. Lett., 7, 2004, p. 904) based on an incorrect formula (Phys. Rev. E, 68, 2003, p. 061902, eqns 11-14). Our framework provides the basis for development of a dispersal-limited non-neutral community theory and applies in population genetics as well, where alleles and mutation play the roles of species and speciation respectively.
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Since the publication of the unified neutral theory in 2001, there has been much discussion of the theory, pro and con. The hypothesis of ecological equivalence is the fundamental yet controversial idea behind neutral theory. Assuming trophically similar species are demographically alike (symmetric) on a per capita basis is only an approximation, but it is equivalent to asking: How many of the patterns of ecological communities are the result of species similarities, rather than of species differences? The strategy behind neutral theory is to see how far one can get with the simplification of assuming ecological equivalence before introducing more complexity. In another paper, I review the empirical evidence that led me to hypothesize ecological equivalence among many of the tree species in the species-rich tropical forest on Barro Colorado Island (BCI). In this paper, I develop a simple model for the evolution of ecological equivalence or niche convergence, using as an example evolution of the suite of life history traits characteristic of shade tolerant tropical tree species. Although the model is simple, the conclusions from it seem likely to be robust. I conclude that ecological equivalence for resource use are likely to evolve easily and often, especially in species-rich communities that are dispersal and recruitment limited. In the case of the BCI forest, tree species are strongly dispersal- and recruitment-limited, not only because of restricted seed dispersal, but also because of low recruitment success due to heavy losses of the seedling stages to predators and pathogens and other abiotic stresses such as drought. These factors and the high species richness of the community strongly reduce the potential for competitive exclusion of functionally equivalent or nearly equivalent species.
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The observation that a few species in ecological communities are exceptionally abundant, whereas most are rare, prompted the development of species abundance models. Nevertheless, despite the large literature on the commonness and rarity of species inspired by these pioneering studies, some widespread empirical patterns of species abundance resist easy explanation. Notable among these is the observation that in large assemblages there are more rare species than the log normal model predicts. Here we use a long-term (21-year) data set, from an estuarine fish community, to show how an ecological community can be separated into two components. Core species, which are persistent, abundant and biologically associated with estuarine habitats, are log normally distributed. Occasional species occur infrequently in the record, are typically low in abundance and have different habitat requirements; they follow a log series distribution. These distributions are overlaid, producing the negative skew that characterizes real data sets.
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One of the fundamental questions of ecology is what controls biodiversity. Recent theory suggests that biodiversity is controlled predominantly by neutral drift of species abundances. This theory has generated considerable controversy, because it claims that many mechanisms that have long been studied by ecologists (such as niches) have little involvement in structuring communities. The theory predicts that the species abundance distribution within a community should follow a zero-sum multinomial distribution (ZSM), but this has not, so far, been rigorously tested. Specifically, it remains to be shown that the ZSM fits the data significantly better than reasonable null models. Here I test whether the ZSM fits several empirical data sets better than the lognormal distribution. It does not. Not only does the ZSM fail to fit empirical data better than the lognormal distribution 95% of the time, it also fails to fit empirical data better even a majority of the time. This means that there is no evidence that the ZSM predicts abundances better than the much more parsimonious null hypothesis.
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Two decades of research have not established whether tropical insect herbivores are dominated by specialists or generalists. This impedes our understanding of species coexistence in diverse rainforest communities. Host specificity and species richness of tropical insects are also key parameters in mapping global patterns of biodiversity. Here we analyse data for over 900 herbivorous species feeding on 51 plant species in New Guinea and show that most herbivorous species feed on several closely related plant species. Because species-rich genera are dominant in tropical floras, monophagous herbivores are probably rare in tropical forests. Furthermore, even between phylogenetically distant hosts, herbivore communities typically shared a third of their species. These results do not support the classical view that the coexistence of herbivorous species in the tropics is a consequence of finely divided plant resources; non-equilibrium models of tropical diversity should instead be considered. Low host specificity of tropical herbivores reduces global estimates of arthropod diversity from 31 million (ref. 1) to 4 6 million species. This finding agrees with estimates based on taxonomic collections, reconciling an order of magnitude discrepancy between extrapolations of global diversity based on ecological samples of tropical communities with those based on sampling regional faunas.
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A comment on Hubbell's zero-sum ecological drift model
Long-term permanent plot observations of vegetation dynamics in Budongo, a Ugandan rain forest
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The theory of island biogeography asserts that an island or a local community approaches an equilibrium species richness as a result of the interplay between the immigration of species from the much larger metacommunity source area and local extinction of species on the island (local community). Hubbell generalized this neutral theory to explore the expected steady-state distribution of relative species abundance (RSA) in the local community under restricted immigration. Here we present a theoretical framework for the unified neutral theory of biodiversity and an analytical solution for the distribution of the RSA both in the metacommunity (Fisher's log series) and in the local community, where there are fewer rare species. Rare species are more extinction-prone, and once they go locally extinct, they take longer to re-immigrate than do common species. Contrary to recent assertions, we show that the analytical solution provides a better fit, with fewer free parameters, to the RSA distribution of tree species on Barro Colorado Island, Panama, than the lognormal distribution.
Can high tree species richness be explained by Hubbell's null model
The effects of competitive asymmetry on the rate of competitive displacement: how robust is Hubbell's community drift model
Allee effects and the neutral theory of biodiversity
