Species-area & species-accumulation curves not the same

30 05 2016

IBI’ve just read an elegant little study that has identified the main determinants of differences in the slope of species-area curves and species-accumulation curves.

That’s a bit of a mouthful for the uninitiated, so if you don’t know much about species-area theory, let me give you a bit of background for why this is an important new discovery.

Perhaps one of the only ‘laws’ in ecology comes from the observation that as you sample from larger and larger areas of any habitat type, the number of species tends to increase. This of course originates from MacArthur & Wilson’s classic book, The Theory of Island Biography (1967), and while simple in basic concept, it has since developed into a multi-headed Hydra of methods, analysis, theory and jargon.

One of the most controversial aspects of generic species-area relationships is the effect of different sampling regimes, a problem I’ve blogged about before. Whether you are sampling once-contiguous forest of habitat patches in a ‘matrix’ of degraded landscape, a wetland complex, a coral reef, or an archipelago of true oceanic islands, the ‘ideal’ models and the interpretation thereof will likely differ, and in sometimes rather important ways from a predictive and/or applied perspective. Read the rest of this entry »

Too small to avoid catastrophic biodiversity meltdown

27 09 2013
Chiew Larn

Chiew Larn Reservoir is surrounded by Khlong Saeng Wildlife Sanctuary and Khao Sok National Park, which together make up part of the largest block of rainforest habitat in southern Thailand (> 3500 km2). Photo: Antony Lynam

One of the perennial and probably most controversial topics in conservation ecology is when is something “too small’. By ‘something’ I mean many things, including population abundance and patch size. We’ve certainly written about the former on many occasions (see here, here, here and here for our work on minimum viable population size), with the associated controversy it elicited.

Now I (sadly) report on the tragedy of the second issue – when is a habitat fragment too small to be of much good to biodiversity?

Published today in the journal Science, Luke Gibson (of No substitute for primary forest fame) and a group of us report disturbing results about the ecological meltdown that has occurred on islands created when the Chiew Larn Reservoir of southern Thailand was flooded nearly 30 years ago by a hydroelectric dam.

As is the case in many parts of the world (e.g., Three Gorges Dam, China), hydroelectric dams can cause major ecological problems merely by flooding vast areas. In the case of Charn Liew, co-author Tony Lynam of Wildlife Conservation Society passed along to me a bit of poignant and emotive history about the local struggle to prevent the disaster.

“As the waters behind the dam were rising in 1987, Seub Nakasathien, the Superintendent of the Khlong Saeng Wildlife Sanctuary, his staff and conservationist friends, mounted an operation to capture and release animals that were caught in the flood waters.

It turned out to be distressing experience for all involved as you can see from the clips here, with the rescuers having only nets and longtail boats, and many animals dying. Ultimately most of the larger mammals disappeared quickly from the islands, leaving just the smaller fauna.

Later Seub moved to Huai Kha Khaeng Wildlife Sanctuary and fought an unsuccessful battle with poachers and loggers, which ended in him taking his own life in despair in 1990. A sad story, and his friend, a famous folk singer called Aed Carabao, wrote a song about Seub, the music of which plays in the video. Read the rest of this entry »

Biogeography comes of age

22 08 2013

penguin biogeographyThis week has been all about biogeography for me. While I wouldn’t call myself a ‘biogeographer’, I certainly do apply a lot of the discipline’s techniques.

This week I’m attending the 2013 Association of Ecology’s (INTECOL) and British Ecological Society’s joint Congress of Ecology in London, and I have purposefully sought out more of the biogeographical talks than pretty much anything else because the speakers were engaging and the topics fascinating. As it happens, even my own presentation had a strong biogeographical flavour this year.

Although the species-area relationship (SAR) is only one small aspect of biogeography, I’ve been slightly amazed that after more than 50 years since MacArthur & Wilson’s famous book, our discipline is still obsessed with SAR.

I’ve blogged about SAR issues before – what makes it so engaging and controversial is that SAR is the principal tool to estimate overall extinction rates, even though it is perhaps one of the bluntest tools in the ecological toolbox. I suppose its popularity stems from its superficial simplicity – as the area of an (classically oceanic) island increases, so too does the total number of species it can hold. The controversies surrounding such as basic relationship centre on describing the rate of that species richness increase with area – in other words, just how nonlinear the SAR itself is.

Even a cursory understanding of maths reveals the importance of estimating this curve correctly. As the area of an ‘island’ (habitat fragment) decreases due to human disturbance, estimating how many species end up going extinct as a result depends entirely on the shape of the SAR. Get the SAR wrong, and you can over- or under-estimate the extinction rate. This was the crux of the palaver over Fangliang He (not attending INTECOL) & Stephen Hubbell’s (attending INTECOL) paper in Nature in 2011.

The first real engagement of SAR happened with John Harte’s maximum entropy talk in the process macroecology session on Tuesday. What was notable to me was his adamant claim that the power-law form of SAR should never be used, despite its commonness in the literature. I took this with a grain of salt because I know all about how messy area-richness data can be, and why one needs to consider alternate models (see an example here). But then yesterday I listened to one of the greats of biogeography – Robert Whittaker – who said pretty much the complete opposite of Harte’s contention. Whittaker showed results from one of his papers last year that the power law was in fact the most commonly supported SAR among many datasets (granted, there was substantial variability in overall model performance). My conclusion remains firm – make sure you use multiple models for each individual dataset and try to infer the SAR from model-averaging. Read the rest of this entry »

Life, death and Linneaus

9 07 2011

Barry Brook (left) and Lian Pin Koh (right) attacking Fangliang He (centre). © CJA Bradshaw

I’m sitting in the Brisbane airport contemplating how best to describe the last week. If you’ve been following my tweets, you’ll know that I’ve been sequestered in a room with 8 other academics trying to figure out the best ways to estimate the severity of the Anthropocene extinction crisis. Seems like a pretty straight forward task. We know biodiversity in general isn’t doing so well thanks to the 7 billion Homo sapiens on the planet (hence, the Anthropo prefix) – the question though is: how bad?

I blogged back in March that a group of us were awarded a fully funded series of workshops to address that question by the Australian Centre for Ecological Synthesis and Analysis (a Terrestrial Ecosystem Research Network facility based at the University of Queensland), and so I am essentially updating you on the progress of the first workshop.

Before I summarise our achievements (and achieve, we did), I just want to describe the venue. Instead of our standard, boring, windowless room in some non-descript building on campus, ACEAS Director, Associate Professor Alison Specht, had the brilliant idea of putting us out away from it all on a beautiful nature-conservation estate on the north coast of New South Wales.

What a beautiful place – Linneaus Estate is a 111-ha property just a few kilometres north of Lennox Head (about 30 minutes by car south of Byron Bay) whose mission is to provide a sustainable living area (for a very lucky few) while protecting and restoring some pretty amazing coastal habitat along an otherwise well-developed bit of Australian coastline. And yes, it’s named after Carl Linnaeus. Read the rest of this entry »

Over-estimating extinction rates

19 05 2011

I meant to get this out yesterday, but was too hamstrung with other commitments. Now the media circus has beat me to the punch. Despite the lateness (in news-time) of my post, my familiarity with the analysis and the people involved gives me a unique insight, I believe.

So a couple of months ago, Fangliang He and I were talking about some new analysis he was working on where he was testing the assumption that back-casted species-area relationships (SAR) gave reasonable estimates of inferred extinction rates. Well, that paper has just been published in today’s issue of Nature  by Fangliang He and Stephen Hubbell entitled: Species–area relationships always overestimate extinction rates from habitat loss (see also the News & Views piece by Carsten Rahbek and Rob Colwell).

The paper has already stirred up something of a controversy before the ink has barely had time to dry. Predictably, noted conservation biologists like Stuart Pimm and Michael Rosenzweig have already jumped down Fangliang’s throat.

Extinction rates of modern biota in the current biodiversity crisis (Ehrlich & Pringle 2008) are wildly imprecise. Indeed, it has been proposed that extinction rates exceed the deep-time average background rate by 100- to 10000-fold (Lawton & May 2008; May et al. 1995; Pimm & Raven 2000), and no rigorously quantification of these rates globally has ever been accomplished (although there are several taxon- and region-specific estimates of localised extinction rates (Brook et al. 2003; Regan et al. 2001; Hambler et al. 2011; Shaw 2005).

Much of the information used to infer past extinction rate estimates is based on  the species-area relationship (e.g., Brook et al. 2003); this method estimates extinction rates by reversing the species-area accumulation curve, extrapolating backward to smaller areas to calculate expected species loss. The concept is relatively simple, even though the underlying mathematics might not be. Read the rest of this entry »