# Impacts of abundance and habitat area weighting in allocating species trends to habitats

## TL;DR

This paper examines how different methods of allocating species abundance trends to habitats affect biodiversity metrics, showing that weighting methods significantly influence results.

## Contribution

The study introduces a novel analysis of how weighting species abundance and habitat area affects habitat-level biodiversity trend assessments.

## Key findings

- Weighting species by abundance and habitat area leads to more accurate habitat-level biodiversity trend detection.
- Unweighted methods overemphasize widespread species, potentially leading to misleading conclusions about habitat health.
- Complex weighting methods reveal more differences in habitat trends, especially for sparsely vegetated areas.

## Abstract

The dynamics of species depend on the management of their habitats. However, in the absence of good habitat monitoring data for many types of species, reliance has been placed on identifying habitats seeing marked changes in biodiversity through combining trends in their associated species into a habitat level metric. Several data sources on species occupancy, abundance within different habitats, and habitat area for two example taxa, bryophytes and lichens, were linked to assess how different methods of allocating existing species’ abundance trends to habitats influenced the habitat statistics. In general, trends through time were similar, but the method of allocation had an impact on the absolute values of the Distribution Index that summarises weighted occupancy. Allowing generalists to contribute equally to specialist species in a habitat gave higher values of habitat level Distribution Index than methods which weighted species according to abundance in that habitat and habitat area. There were also impacts on the analysis of long-term and short-term trend data, with the more complex methods, including abundance within habitats and extent of habitat, detecting more differences between habitats, and, for some habitats, changing positive trends for bryophytes to no significant trend or even negative for sparsely vegetated habitats. If species trend data is to be used for identifying habitats where biodiversity trends are marked, then it is clear that weighting species, such that their total weight across the analysis is the same, is necessary. Developing the precise means to achieve that needs careful thought and the creation of a robust method that works across different species groups, but using unweighted data could lead to erroneous conclusions as they are so dependent on the dynamics of widespread species.